Read Hydrogeological characterization and Groundwater protection in a tropical mountainous karst, NW Vietnam text version

Hydrogeological Characterisation and Groundwater Protection of Tropical Mountainous Karst areas in NW Vietnam

by

Vu Thi Minh Nguyet

Department of Hydrology and Hydraulic Engineering

V U B ­ HYDROLOGIE (48) 2006

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This publication is Nr. 48 of the series "V U B ­ Hydrologie" edited by the Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel. Orders should be sent by letter to the Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels. c 2006 Dienst Uitgaven VUB Wettelijk depot 1885

This dissertation is dedicated to Mr. Thai Duy Ke, our respectful, good-hearted, beloved colleague and great friend, who is always alive in our memory.

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Acknowledgments

This thesis owes much to the help and support of many people, all of whom have contributed in different ways. First of all, I would like to express my sincere gratitude and appreciation to my promotors, Prof F. De Smedt and Dr. N. Goldscheider, for their valuable guidance, fruitful discussions and consistent support that made it possible for me to finish this work. I am grateful to Dr. O. Batelaan for his suggestions, practical help and support on my work over many years. I would like to thank the Directorial Board of the Research Institute of Geology and Mineral Resources (RIGMR) for the strongly support on my work; to Prof. Duong Duc Kiem, Pham Binh, Nguyen Tam, Dang My Cung and many senior researchers and colleagues at RIGRM for the professional advice, supporting data and for their assistance in fieldtrips; to the local people in the Son La and Tam Duong areas who helped me with spring monitoring, tracer sampling and other help during the fieldtrips. I am grateful to jury members: Prof. J. Wastiels, Prof. J. Vereecken, Prof. F. De Smedt, Prof. W. Bauwens, Prof. E. Keppens, Dr. O. Batelaan (Vrije Universtiteit Brussel), Dr. N. Goldscheider (Université de Neuchâtel, Switzerland), Prof. R. Swennen (Katholieke Universiteit Leuven) and Dr. M. Dusar (Belgian Geological Survey) for their willing review, valuable and helpful suggestions to improve this thesis. I thank to Dr. Michael Whitburn for his help on English correction. Special thanks to the Belgian Technical Cooperation, the Vietnamese-Belgian Karst Project, and the Swiss Commission for Scholarship for partly provided financial support of my work. Thank to Mrs. Daphnée Windey, Dr. Paul Verle (Belgian Technical Cooperation in Brussels) and all people at the Belgian Technical Cooperation in Hanoi for the willing logistic support. A special thanks goes to Dr. Koen Van Keer for his fully support and practical help on my work from initial stage and during difficult moments. I want to thank the professors and assistants working at the Centre of Hydrogeology, University of Neuchâtel, for giving their knowledge on karst hydrogeology and laboratory

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experience to me. Thanks to Mariona, Alessandro and many other friends for the unforgettable time we spent together in Switzerland. Many thanks to my Vietnamese friends in VUB and other Universities/cities for the pleasant time we spent together to study in Belgium. Special thanks to my dear friends, whom I cannot mention here for understanding and support. Many thanks to my parents for their patience and give me encouragement during my study.

Abstract

In NW Vietnam, karst areas cover nearly 18% of the land surface and have substantial socioeconomic importance as groundwater resources, as well as zones for forestry, agriculture and tourism. In many areas, however, both the karst landforms and the groundwater resources have recently come under pressure in response to urbanisation, economic development and increase of population. Karst aquifers are particularly vulnerable to contamination resulting from human activities. Karst groundwater consequently requires special protection. A sound knowledge of the hydrogeological system is a precondition for any protection strategy. Such understanding, however, is presently lacking in Vietnam. This work aims at better understanding the hydrogeological characteristics of the tropical karst regions in Vietnam and providing a scientific basis for groundwater protection. The study focuses on two major mountainous areas that belong to the NW karst belt: Son La and Tam Duong, which mainly consist of thick Middle Triassic carbonate-rock formations. An investigation methodology has been applied and adapted to the conditions of the remote areas, for which little information is available. The employed methods included tracer tests, hydrodynamic, hydrochemical and microbiological spring monitoring, as well as stable isotope and rare earth elements studies. Tracer tests proved underground connections between several swallow holes and springs in the two test site areas. The NW-SE and SW-NE faults have a great influence on the underground drainage patterns. The flow paths run either across the folds along the SW-NE faults or follow the NW-SE faults; these flow paths coincide with the preferential directions of cave development. Groundwater mixing effects can be observed in both areas. Hydrochemical data from Son La show a significant difference in the Mg2+ and Ca2+ contents between a swallow hole and a connected spring, which can be explained by mixing effects. Stable isotope results further support this observation. The high stability of 18O of karst springs in the Nam La valley (Son La) compared with meteoric water also indicates that this karst system contains well-mixed groundwater. The hydrochemical results from the Tam Duong area show a difference in Mg2+ and Ca2+content between a swallow hole and a connected spring, which also can be explained by the mixing effect. The little variation in chemical content along the flow path compared to the Son La area may reflect the reduced water­rock interaction in this karst system.

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Large karst springs are observed in Son La, while smaller karst springs occur in Tam Duong. The results obtained from this study suggest that concentrated recharge prevails in the Tam Duong area, while the recharge processes and groundwater flow in the Son La area appear to be more complicated. There is evidence for point recharge and conduit flow on one hand, but also for significant diffuse recharge and flow through small joints and fractures on the other hand. Tracer tests in the Son La area gave groundwater flow velocities ranging from 75 to 166 m/h. These are typical values for karst aquifers and indicate low-resistance flow paths. The flow velocities in the Tam Duong area are up to 700 m/h, which is one of the highest values recorded in the literature. The two investigated springs near Tam Duong show a different hydrological and physical-chemical response on precipitation events. A dilution effect was observed at one karst spring, while the other spring displayed a piston effect. The physical-chemical parameters of all sampled karst water in both areas meet the WHO standards for drinking water. The REE concentration levels found in spring water from Tam Duong are higher than those from other karst areas reported in the literature but still safe for the health of the consumers. In contrast, the microbial investigation revealed that all karst water contain high levels of thermotolerant coliforms (TTC). The contamination shows high temporal fluctuations and mainly results from untreated domestic wastewaters, agriculture and other human activities. In order to protect the valuable groundwater resources in Vietnamese karst areas, a simplified methodology for mapping groundwater vulnerability and contamination risk was developed and first applied in the test sites. It is based on a conceptual framework proposed by the European COST Action 620. The vulnerability map takes into account the overlying layers (O) and the flow concentration (C). The risk map is obtained by a combination of the vulnerability map and a simplified hazard assessment. The maps provide a basis for land-use planning and groundwater protection zoning. Groundwater protection should be a priority in vulnerable zones such as swallow holes and along sinking streams. The work gives details and an insight into the understanding of karst hydrogeological characterization in the Son La and Tam Duong areas. The methods applied in this work constitute useful tools for the hydrogeological investigation of remote and mountainous tropical karst areas in Vietnam and made it possible to provide a scientific basis for sustainable groundwater management.

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Table of contents

Acknowledgments ...............................................................................................................i Abstract..............................................................................................................................iii Table of contents ................................................................................................................v List of figures ....................................................................................................................ix List of tables ....................................................................................................................xiii

1

Introduction .................................................................................................. 1

1.1 1.2 Karst in tropical regions ..........................................................................................1 Karst hydrogeological research in Vietnam...........................................................2 Overview of karst in Vietnam ............................................................................2 Importance of karst hydrogeology study ............................................................3

1.2.1 1.2.2 1.3 1.4

Objectives and structure of the study .....................................................................4 Research collaboration.............................................................................................6

2

Study area-the NW karst belt ..................................................................... 7

2.1 Geography .................................................................................................................7 Location and topography ....................................................................................7 Climate ...............................................................................................................7 Social and economic conditions .........................................................................8 Overview of geological setting...........................................................................9 Tectonics...........................................................................................................10

2.1.1 2.1.2 2.1.3 2.2 2.2.1 2.2.2 2.3 2.4

The geology of the NW karst belt............................................................................9

Principles of hydrogeological characterization of karst aquifers ......................11 Karst landform .......................................................................................................14 Definition of tropical karst landforms ..............................................................14 Karst landscapes in NW Vietnam.....................................................................15

2.4.1 2.4.2

3

Methods and techniques ............................................................................ 19

3.1 Tracing experiment ................................................................................................19 Tracing in karst study .......................................................................................19

3.1.1

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3.1.2 3.1.3 3.1.4 3.2 3.2.1 3.2.2 3.3 3.4 3.5

Tracer breakthrough curve ............................................................................... 20 Traci95 Programme.......................................................................................... 22 Tracing tests in Son La and Tam Duong areas ................................................ 23 Overview.......................................................................................................... 24 Hydrochemical investigation in the test sites................................................... 25

Hydrochemical investigation................................................................................. 24

Microbiological investigation ................................................................................ 26 Stable isotope study................................................................................................ 27 Rare earth elements study..................................................................................... 29

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Hydrogeology of the Son La karst area.................................................... 31

4.1 4.2 4.3 Location, landscape and climate........................................................................... 31 Overview of previous studies ................................................................................ 32 Geology.................................................................................................................... 33 Geological framework and stratigraphy........................................................... 33 Stratigraphy...................................................................................................... 34 Tectonics .......................................................................................................... 36 Hydrogeology, spring and surface water ......................................................... 38 Tracer tests ....................................................................................................... 41 Tracer sampling and analysis........................................................................... 43 Results.............................................................................................................. 43 Discussion ........................................................................................................ 47 Hydrochemistry and karst water quality .......................................................... 50 Oxygen isotope ................................................................................................ 53 Hydrogeology and underground flow paths..................................................... 57 Hydraulic properties and groundwater quality................................................. 57 Groundwater mixing ........................................................................................ 58

4.3.1 4.3.2 4.3.3 4.3.4 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.5 4.5.1 4.5.2 4.6 4.6.1 4.6.2 4.6.3

Tracer tests ............................................................................................................. 41

Hydrochemistry...................................................................................................... 50

Conclusion .............................................................................................................. 57

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5

Hydrogeology of Tam Duong karst area.................................................. 61

5.1 5.2 5.3 Location, topography and climate ........................................................................61 Overview of previous studies .................................................................................62 Geology ....................................................................................................................63 Geological framework ......................................................................................63 Stratigraphy ......................................................................................................63 Tectonics...........................................................................................................65 Hydrogeology, spring and surface water..........................................................67 Overview ..........................................................................................................69 Injection and sampling points...........................................................................70 Tracer analysis..................................................................................................71 Results ..............................................................................................................71 Discussion.........................................................................................................74 Overview ..........................................................................................................76 Sample collection .............................................................................................76 Sample analysis ................................................................................................76 Results ..............................................................................................................77 Discussion.........................................................................................................80 Sampling and analytical techniques .................................................................84 Results and discussion ......................................................................................85 Point recharge, fault tectonics and underground flow path ..............................92 Dynamics and interaction of the hydrochemical and microbiological Groundwater quality.........................................................................................94

5.3.1 5.3.2 5.3.3 5.3.4 5.4 5.4.1 5.4.2 5.4.3 5.4.4 5.4.5 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.5.5 5.6 5.6.1 5.6.2 5.7 5.7.1 5.7.2 5.7.3

Tracer experiment ..................................................................................................69

Hydrochemistry and microbiology .......................................................................76

Rare earth elements (REE) study..........................................................................84

Conclusion ...............................................................................................................92

parameters.........................................................................................................................93

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Karst Groundwater Vulnerability and Risk Mapping........................... 97

6.1 The European approach: COST 620 ....................................................................97 Introduction ......................................................................................................97

6.1.1

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6.1.2 6.1.3 6.2 6.2.1 6.2.2 6.2.3 6.3 6.3.1 6.3.2 6.4 6.4.1 6.4.2 6.4.3 6.5

Definitions of groundwater vulnerability, hazard and risk .............................. 97 The origin-pathway-target model..................................................................... 99 (General) proposed methodology................................................................... 100 Groundwater vulnerability ............................................................................. 101 Hazard and risk .............................................................................................. 103 Introduction.................................................................................................... 104 Groundwater vulnerability mapping .............................................................. 106 Introduction.................................................................................................... 107 Groundwater vulnerability mapping .............................................................. 108 Hazard assessment, risk mapping and validation .......................................... 110

Methodology adaptation...................................................................................... 100

Application in the Tham Ta Toong area............................................................ 104

Application in Tam Duong area ......................................................................... 107

Discussion on applicability of the methodology ................................................ 111

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Conclusions ............................................................................................... 113

7.1 Karst hydrogeological characterisation ............................................................. 113 Groundwater flow path and groundwater mixing effect................................ 113 Hydraulic properties....................................................................................... 114 Karst water quality ......................................................................................... 115

7.1.1 7.1.2 7.1.3 7.2 7.3 7.4

Groundwater protection...................................................................................... 115 An investigation methodology............................................................................. 116 Recommendations ................................................................................................ 117

References ........................................................................................................ 121

viii

List of figures

Fig. 1.1: Karst areas of Vietnam (modified after Dusar et al. 1994) with location of the test sites ...............................................................................................................................3 Fig. 2.1: Tectonic framework of NW Vietnam (modified after Tran Van Tri et al, 1979) and location of the test sites..........................................................................................9 Fig. 2.2: Shallow and deep karst systems with regard to the position of the base level (Bögli, 1980)...........................................................................................................................12 Fig. 2.3: Recharge into carbonate aquifers (Gunn, 1986) ........................................................12 Fig. 2.4: Conductivity scale-effect in karst system (Kiraly, 1975) ..........................................13 Fig. 2.5: Interpretation of a karst spring hydrograph and chemograph (Ford and Williams, 1989)...........................................................................................................................14 Fig. 2.6: Geographical location of main cities/towns in Northern Vietnam ............................16 Fig. 2.7: Peak cluster depression karst landscape in NW Vietnam ..........................................17 Fig. 2.8: Peak forest karst landscape in NW Vietnam..............................................................17 Fig. 3.1: Tracer breakthrough curve and residence times.........................................................21 Fig. 3.3: The portable microbial Lab Oxfam-DelAgua with main consumables (Photo by Oxfam-DelAgua) ........................................................................................................27 Fig. 4.1: Son La karst landscape, view from Son La pass to the SW.......................................31 Fig. 4.2: Monthly rainfall (mm) in Son La (collected data in Son La station from 1974-1998) ....................................................................................................................................32 Fig. 4.3: Geological map and geological cross sections of the Son La area (modified after Vibekap, 2003). ..........................................................................................................37 Fig. 4.4: Karst aquifers, springs and surface water in the Son La area. ...................................38 Fig. 4.5: Spring hydrograph of Nam La River measured at Ban Toong village and Hang Doi spring in 2000 (VIBEKAP data). ...............................................................................40 Fig. 4.6: Tracer location and proven groundwater flow connections.......................................44 Fig. 4.7: Measured tracer concentrations at Ban Sang spring for the February 2000 test and theoretical breakthrough curves modelled using Traci95 (left uranine, right salt). ...45 Fig. 4.8: Measured tracer concentrations at the Long Ngo spring for the test in October 2000 and theoretical breakthrough curves modelled using Traci95....................................46

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Fig. 4.9: Measured uranine concentrations at the Hang Doi spring during the October 2001 test, and theoretical breakthrough curves modelled by using Traci 95...................... 46 Fig. 4.10: Measured electrical conductivity (EC) at the Long Ngo spring and rainfall recorded at the Son La station during the October 2000 tracer test.......................................... 49 Fig. 4.11: Piper diagram of karst rivers systems in the Son La area ; the black triangle symbol presents for the Nam La River water system; the grey cycle symbol is for the Suoi Muoi River water system. .......................................................................................... 51 Fig. 4.12: Species of dissolved inorganic carbon as function of pH (Fetter, 2001)................. 52 Fig. 4.13: Location of sampling stations for isotope study in the Nam La River area, Son La province...................................................................................................................... 53 Fig. 4.14: Oxygen isotope composition of rainfall, river and spring water at the Nam La valley, Son La (July-October, 2002).................................................................... 56 Fig. 4.15: Influence of oxygen isotope composition of rainfall water on the Nam La River water........................................................................................................................... 56 Fig. 4.16: The Mg2+ versus Ca2+ concentrations at swallow holes and connected springs in the Son La area; the dot lines indicate the existence of underground flow connections, which was proven by the tracer test. Flow connection 1: Ban Lay-Long Ngo, flow connection 2: Nha Tu-Hang Doi, flow connection 3: Tham Han-Ban Sang. ............ 58 Fig. 5.1: View of the test site from NE (left) and from SW (right). ........................................ 61 Fig. 5.2: Measured precipitation and temperature in the Tam Duong area from 1996 to 2000 (reference data: Japanese Mining Project, 2002). ...................................................... 62 Fig. 5.3: Geological map and geological cross section in the Tam Duong area (modified after VIBEKAP, 2003). The number 1 represents Dau Nguon Sin Ho spring; and number 2 represents Nha May Che spring. The symbols I, II and III represent Nam So, Lan Nhi Thang-Hong Thu Man and Yen Chau faults respectively. ................................. 65 Fig. 5.4: Karst aquifer, springs and surface water at the Tam Duong area (same area as Fig. 5.3); number 1, 2 as on Fig. 5.3; the Lo Gach, Nam Loong, C320 and Lai Chau army springs are represented by the number 3, 4, 5 and 6 respectively. The Tam Duong and Nung Nang streams are mapped on the basis of the field observations.............. 68 Fig. 5.5: Tracer location and proven groundwater flow connections (detail from Fig. 5.4)... 72 Fig. 5.6: Measured tracer concentrations at spring 1 (left) and spring 2 (right) and theoretical breakthrough curves simulated using Traci 95. ......................................................... 73

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Fig. 5.7: Plan view and vertical profile of Suoi Thau cave (Belgian­Vietnamese Caving Expedition, 2002). ......................................................................................................75 Fig. 5.8: Piper diagram of karst springs in the Tam Duong area; the cycle symbol represents the Dau Nguon Sin Ho spring, the cross symbol is the Nha May Che spring. The triangle symbol represent other karst springs, which are used for drinking water in the area........................................................................................................................79 Fig. 5.9: Precipitation, conductivity, water level and hydrochemical and microbiological parameters at the Dau Nguon Sin Ho spring (spring 1). The large symbols and the bold underlined numbers represent a sample taken at the Nung Nang cave at the 29.08.04. .....................................................................................................................82 Fig. 5.10: Precipitation, conductivity, water level, hydrochemical and microbiological parameters at the Nha May Che spring. The large symbols and bold underlined numbers represent a sample taken at the Suoi Thau swallow hole at the 23.08.04....83 Fig. 5.11: Location of the various sampling sites in the Tam Duong area...............................85 Fig. 5.12: Shale ­ normalized REE patterns of carbonate rocks from the Tam Duong and Nam Son areas. ...........................................................................................................90 Fig. 5.13: Shale-normalized REE patterns for water from the Tam Duong area. ....................91 Fig. 5.14: The Mg2+ and Ca2+ concentrations measured in all water samples from the Tam Duong area; the dot symbol represents Dau Nguon Sin Ho spring; the cross symbol is Nha May Che spring and triangle symbol is other springs. .......................................94 Fig. 6.1: The intrinsic vulnerability mapping is based on the origin-pathway-target model (Goldscheider and Popescu, 2004). ..........................................................................100 Fig. 6.2: Proposed methodology for groundwater vulnerability and risk mapping (for explanation see the text). ..........................................................................................101 Fig. 6.3: Illustration of the proposed method of groundwater vulnerability mapping. The O factor takes into account the protectiveness of the overlying layers, the C factor considers the concentration of flow towards swallow holes (allogenic recharge), the vulnerability map is created by overlying the O and C maps (Nguyet and Goldscheider, in press). ............................................................................................103 Fig. 6.5: Geology of the Tham Ta Toong area, Son La province (Nguyet et al., 2004b). .....105 Fig. 6.6: O map, C map and vulnerability map of the Son La karst area, and legend for the three maps (Nguyet et al., 2004b). ......................................................................107

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Fig. 6.7: Hydrogeological map of the test site. The estimated catchemnt area of two main karst springs, groundwater flow paths and other karst features are also presented in the figure. ................................................................................................................. 108 Fig. 6.8: O and C map of the test site. The resulting vulnerability map is shown in Fig. 6.9. .................................................................................................................................. 109 Fig. 6.9: Vulnerability, hazard and risk maps for the Tam Duong test site. Both the tracer test results and the high contents of bacteria in spring 1 confirm the vulnerability and risk assessment near swallow hole 1............................................................................... 111

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List of tables

Table 4.1: Stratigraphic table of the Son La area corresponding to mapsheet in Fig. 4.3........34 Table 4.2: Summary of tracer experiments in the Son La karst area, Son La province ...........42 Table 4.3: Overview obtained tracer results and estimated hydraulic parameters of karst groundwater flow paths in the Son La area. ...........................................................47 Table 4.4: Physical properties and major ions content (mg/l) of karst water in the Son La area (VIBEKAP data) ....................................................................................................50 Table 4.5: The 18O of meteoric water, river and karst spring water at the Nam La valley, Son La (July-October 2002) (location: Fig. 4.13) ..................................................54 Table 4.6: The molar [Mg2+]/[Ca2+] ratios for swallow hole and connected spring waters from the Son La area ..............................................................................................59 Table 5.1: Stratigraphical table of the Tam Duong area..........................................................64 Table 5.2: Tracer results and estimated hydraulic properties from tracer experiments at the Dau Nguon Sin Ho spring (spring 1) and Nha May Che spring (spring 2)............74 Table 5.3: Microbial contamination and major ions content in 15 karst springs which are used for drinking water in the Tam Duong area, and the WHO standards. The bicarbonate was calculated by using AquaChem 4.0. ............................................77 Table 5.4: The molar [Mg2+]/[Ca2+] ratios for swallow hole and connected spring waters from the Tam Duong area.......................................................................................81 Table 5.5: REE, Sc and Y concentrations (ppb) of Triassic limestone from the Tam Duong and Nam Son areas. ................................................................................................86 Table 5.6: Average of 9 rare earth elements concentration (ppb) in Triassic carbonate rocks from Tam Duong, and Nam Son in compared to other carbonate rocks from Dinant and southern Nevada; Dinant data are from D. Nuyens (1992), and Nevada data is from Guo et al .(2005).................................................................................86 Table 5.7: Field parameters, and Sc, Y and REE concentrations (ppb) of water from carbonate, granite and conglomerate in the Tam Duong area. ...............................88

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Introduction

1 Introduction

1.1 Karst in tropical regions

Karst is a term used to describe a landscape and/or a type of aquifer made of hard rock and characterised by surface and underground phenomena of chemical dissolution. Carbonate terrains cover about 7-12 % of planet's dry and ice-free surface. About 25 % of the global population's water requirements is supplied by karst water (Ford and Williams, 1989). There are three consistent factors influencing the nature of karst landscape and development. The first factor is the rocks in which karst landforms are formed, the second factor is climate, and the third factor is the drainage system or base-flow of the area. Areas of differing climate produce different landforms or karst topography: e.g., Caribbean karst, temperate karst and tropical karst (Ford and Williams, 1989). Karst landforms are best developed in the tropical regions where high rainfall, warm temperatures and thick vegetation result in high concentration of CO2, and large quantities of groundwater flows. In tropical regions, there are others landforms in addition to those found in temperate karst zones. Features such as dolines, poljes, dry valleys, caves, etc., are found in all karst regions, but residual hills as tower karst is specifically characteristic of tropical karst. The tower karst occurs in Papua New Guinea, Australia, Honduras, Cuba, Jamaica, Puerto Rica and Southeast Asia including Malaysia, Indonesia, Thailand and Vietnam (Gunn, 2004). The tower karst in Guilin of southern China is regarded as one of the most spectacular landform in the world. The group of karstifiable rocks are not restricted to evaporites and carbonates, which are distributed abundantly in all continents. Under tropical conditions, quartzitic rocks are also karstifiable (Ford and Williams, 1989). The best silicate karst developments have been reported from Venezuela, Brazil, northern Australia and southern Africa (Williems et al., 2002; Gunn, 2004). Quartz sandstone landscape in northern Australia is similar to tower karst developed on limestone (Young, 1986). The understanding of karst hydrogeology in tropical regions is generally less common in comparison to other karst zones. Many previous karst studies in tropical regions have focused more on origin and evolution of karst towers and on karstifiable rocks than on karst

1

Chapter 1

hydrogeology. To date, the number of publications on karst hydrogeology, karst groundwater, and karst modelling as well as karst groundwater protection in tropical regions are still relatively limited. It is necessary to understand karst hydrogeology in order to protect the spectacular karst landscape and its sustainable development in tropical regions.

1.2 Karst hydrogeological research in Vietnam

1.2.1 Overview of karst in Vietnam

Karst is a widespread phenomenon in Southeast Asia. This region contains some of the most spectacular surface karst in the world. The countries of Myanmar, Thailand, Laos, Cambodia, Malaysia and Vietnam all have important limestone karst areas. Large areas of sandstone and buried evaporite karst are also present in this region. The total karst areas cover about 10 % of the region, around 215, 000 km2 (Mouret, 2004). In Vietnam, karst areas cover approximately 18% of the land surface or about 60,000 km2 (Dao Trong Nang, 1979). Figure 1.1 shows the occurrence of karst areas in Vietnam. The areas are located on a tropical humid belt and have typical tropical karst characteristics. Geographically, the Vietnamese karst areas are divided into four main karst regions: the Tay Bac, the Dong Bac, the Viet Bac and the Centre of the country (Fig. 1.1). The NW karst belt is stretching over 300 km from the Chinese border to the coastline (Gulf of Bac Bo) and coves about 8,190 km2 (Tuyet , 1998). This karst belt is closely related with the well-known tropical karst regions of South China. This is the karst area studied in this work. Karst areas have a substantial socio-economical importance as groundwater resources (for drinking and irrigation water supply and hydropower generation), as well as zones for forestry, agriculture and aquaculture, for extraction of limestone and mineral resources, and for tourism. They generally also have a great local and global ecological significance, being sanctuaries for the last primary forests of Vietnam, as well as for numerous endemic plant and animal species. Several karst areas in Vietnam are listed as World Heritage Site, such as the Ha Long Bay and Phong Nha.

2

Introduction

Fig. 1.1: Karst areas of Vietnam (modified after Dusar et al. 1994) with location of the test sites

1.2.2 Importance of karst hydrogeology study

In many karst areas of Vietnam, the landform and groundwater recently have come under high pressure in response to urbanization, economical development and increase of population. Moreover, karst landscapes and aquifers are extremely fragile. Karst aquifers are particularly vulnerable to contamination resulting from human activity. Contaminants can easily reach groundwater through thin soils and via swallow holes where they are rapidly transported over large distances (Vesper et al., 2001). Unsound management or protection can trigger problems such as water depletion, water pollution (with sediments and chemical or microbial contaminants), and accelerated erosion. These problems already manifest themselves in various karst areas of Vietnam.

3

Chapter 1

A sound knowledge of the hydrogeological system is a precondition for any protection strategy. It is essential to integrated and sustainable management and conservation of the regions. Such understanding is presently lacking in Vietnam. The existing knowledge on karst hydrogeology is of a general, descriptive and fragmentary nature. Earlier studies mainly reported on karst geomorphology or surface karst, and descriptive karst landform and its classification (Dao Trong Nang, 1979; Rozycki, 1984; Tuyet et al., 1996). Several other karst projects focused on exploited mines in karst areas. In these projects, different karst landforms were located on maps (Nguyen Quang My, 1992). Sub-surface karst and karst hydrogeology were only general mentioned, such as the existence of cave or depth of caves based on geological observations and theoretical descriptions. Other studies focused on stratigraphical and paleotological investigation in carbonate rocks. During the past years, several karst areas in Northern Vietnam have been studied within the framework of the Vietnamese-Belgian Karst Project (VIBEKAP), which includes speleological, geomorphological and hydrological investigations, remote sensing, GIS, flooding prediction and other aspects and methods (Hung et al., 2002; VIBEKAP, 2003; Tam, 2003; Liu et al., 2005; Tam et al., 2005). However, many questions on karst hydrogeology are still not considered. Information on water quality at karst springs, which are used for drinking water, is still missing. Groundwater flow on karst and fluctuations in quantity and quality of groundwater resources, as well as contamination sources have not been investigated. A karst groundwater protection scheme is still not established and applied in any of studies Vietnamese karst areas. Hence, through building knowledge on karst (hydrogeology) the situation and living condition in the karst areas of the country can be improved.

1.3 Objectives and structure of the study

This work is a contribution to the knowledge on karst hydrogeology and groundwater protection in the tropical karst regions of NW Vietnam. The objectives of the study are: · To test and adapt chemical-microbiological, stable isotope, and tracing techniques in karst hydrogeology.

4

Introduction

·

To map underground water flow paths and characterize the properties of underground water transport.

·

To characterize the dynamics of karst underground water flow in reaction to precipitation events.

·

To provide scientific information about groundwater quality and to assess the degree of pollution as well as to identify the processes of contaminant transport in karst systems.

·

To develop and apply an approach for karst groundwater vulnerability mapping in remote mountainous areas

The study deals with two major areas that belong to the NW karst belt: Tam Duong and Son La (Fig. 1.1). The economic development, urbanisation and increase of population in those areas has recently put more and more direct and/or indirect pressure on karst groundwater demand, groundwater environment, and its related problems. It is, therefore, necessary to have a comprehensive understanding of karst hydrology and groundwater protection in these areas. Chapter 2 presents an overview of the regional geography, geology and karst landscape. Applied methods and their modification for local conditions are discussed in chapter 3. The next chapters present the two studied areas: Son La (chapter 4) and Tam Duong (chapter 5). The geology, hydrology and hydrogeology of the area are described and the obtained results of the tracer tests, hydrochemical and microbial investigation, stable isotope composition and rare earth elements study are discussed in detail. This is followed by conclusions on hydrogeological characteristics. Chapter 6 focuses on groundwater vulnerability, hazard and risk mapping. An overview of the European approach (COST 620) and how it has been adapted and applied to the Son La and Tam Duong areas are presented in this chapter. The last chapter 7 gives a discussion on hydrogeology, groundwater protection, and investigated methods and recommendations for the future research in the areas.

5

Chapter 1

1.4 Research collaboration

This thesis was prepared at the Department of Hydrology and Hydraulic Engineering (HYDR), Vrije Universiteit Brussels, in collaboration with the Research Institute of Geology and Mineral Resources (RIGMR), Hanoi, Vietnam, and the Centre of Hydrogeology (CHYN), University of Neuchâtel, Switzerland. The Belgian Technical Cooperation (BTC), the Vietnam Belgian Karst Project (VIBEKAP) and the Swiss Commission for Scholarship partly provided financial support for these research activities. The first stage of this study was done in the Son La area. The field work activities here were directly linked to the VIBEKAP project, except for stable isotope investigation. In a next stage, our work was focused to the Tam Duong area. The field work activities in this area were supported by CHYN and RIGMR; senior researchers, colleagues of RIGMR and friends gave good support to this study in the Tam Duong area. The fieldtrips would not have been successful without the support and collaboration of local people. A huge number of observations were needed in the field and in remote area like the Tam Duong and Son La, where geographic information and infrastructure are limited, it would take a lot of time to localise and access the swallow holes and springs without the help of local people. During the tracer tests, water samples were taken manually by the people from the Thai, H'Mong and Kinh ethnic groups. All samples were collected at the correct time and gave good results; only very few samples gave aberrant values. Children helped spontaneously to do the flow measurements, the water sampling, and the microbiological colony counting in the area of Tam Duong.

6

The NW karst belt

2 Study area-the NW karst belt

2.1 Geography

2.1.1 Location and topography

The NW karst belt is one of four main karst regions in Vietnam. It is located within the coordinates 20o00' to 24o00' north and 102o15' to 106o10' east. This karst belt extends in a NW-SE direction from the Chinese border to the Gulf of Bac Bo (Fig.1.1). The belt is 500 km long and has an average width of 20 to 30 km, to maximum 50 km in some places. Topographically, the regional relief decreases from NW to SE. Tuyet et al. (1996) noted that some places in the NW part have altitudes approximately of 3000 m, and decrease to 2000 m and 1000 m in the centre part. The altitude steps-down to 500 or even to 200 m in the SE part. Several studies have described the topography of this region, which is characterized by linear belt-shaped, strongly folded, bedrocks of the Ma River anticlinorium and Da River synclinorium. The surface is divided into narrow and elongated mountain belts, separated from each other by tectonic faults, that are expressed on the ground surface in the form of streams, river valleys, elongated troughs, etc,. The mountainous relief of the area has both step-wise characteristics and linear forms, at the same time changing alternately from high mountain ranges surrounding plateaus to low mountains with valleys and tectonic-denudation depressions. On the other hand, the regional relief is strongly dissected with relative height differences from 30-50 m to 1500-2000 m and drainage density 1-1.2 km/km2, strongly affected by exogenous processes (erosion, gravity movement, landslides, rock fall, etc.), which intensively occur due to the humid tropical climate with high rainfall intensity (Tuyet et al., 1996; Van et al., 2003).

2.1.2 Climate

The region has a tropical monsoon climate with cold, dry winters and hot, humid summers. The climate is variable due to the complicated regional variation in relief. The region is divided into two climate zones: the mountainous climate zone in the NW part and Central Vietnam climate zone in the SE part.

7

Chapter 2

The first climate zone covers mostly the karst belt including all of the Son La, Lai Chau (Tam Duong), and Dien Bien provinces and a part of the Lao Cai, Yen Bai and Hoa Binh provinces, while the second climate zone covers only the low relief area that is close to the coast. The average rainfall in NW part is 1500 mm, ranging from 1438 mm in Son La to 2145 mm in Tam Duong. The rainfall is very variable between the years and is unevenly distributed in two seasons. The rainy period begins in May and usually ends in September or October with rainfall amounts of about 82% of the total annual rainfall. The greatest monthly rainfall occurs in June, July or August. The dry period is from October through April. In the SE part the rainfall period starts usually one month later. The average rainfall there is 1720 mm. The average annual air temperature is 21.5oC in the NW part and 23.5oC in the SE part. The temperature decreases with altitude and varies about 10oC between winter and summer. The average relative humidity is often higher than 80%, and even more than 85% in the rainy period. Data published by the National Meteorological Centre show that the average annual evaporation for the whole area is 875 mm, and this evaporation increases with decreasing altitude.

2.1.3 Social and economic conditions

Various ethnic minority groups are living in the NW area, mainly including the Thai, H'Mong, Kinh, Dao, Muong, and Tay ethnic groups. The H'Mong and Dao often inhabit the high mountains above 1000 m, while the Thai and Tay often settle along the rivers, valleys, and the lowland areas. The Kinh lives along the roads, the town centers and the lowlands. The Muong lives in the SE of the karst belt. The area has one of the highest population growths in the country. Agricultural production of the area is not abundant and consists of mainly rice, maize, tea, while industrial activities are minimal. This zone has a backward economic development and difficult living conditions. In recent years the Vietnamese Government focuses on improving the living conditions in this zone but the living conditions are highly depending on the natural resources and its produce.

8

The NW karst belt

2.2 The geology of the NW karst belt

2.2.1 Overview of geological setting

North Vietnam can be divided into two main tectonic units: the Bac Bo Fold Belt and the Indochina Fold Belt separated spatially and structurally by the Ma River Suture zone or Ma River fault (Tran Van Tri et al., 1979). The Bac Bo Fold Belt is composed by three fold systems Tay Bac, Viet Bac and Dong Bac (Tran Van Tri et al., 1979). The NW karst belt is located within the structural framework of the Tay Bac fold system. Figure 2.1 shows the tectonic framework of NW Vietnam and the location of the test sites.

Fig. 2.1: Tectonic framework of NW Vietnam (modified after Tran Van Tri et al, 1979) and location of the test sites

The Tay Bac has general a NW orientation and it is situated between two deep-seated faults, so-called the Ma River and Hong River (Red River). Structurally, this system forms part of the Hong River anticlinorium, the Da River rift and the Ma River anticlinorium (Tran Van Tri et al., 1979).

9

Chapter 2

The NW region consists of different formations formed in periods from Late Proterozoic to Cainozoic, including terrigenous rocks, carbonate rocks and metamorphic siliceous rocks. Carbonate formations were formed in a large period from Proterozoic to Cretaceous and consist mainly of limestone and dolomite (Tuyet et al., 1996). However, only the carbonate formations of Middle Devonian Ban Pap Formation, Carboniferous-Permian Bac Son Formation and especially Middle Triassic Dong Giao Formation are widespread and favourable for karstification.

2.2.2 Tectonics

The mountain range of NW Vietnam is located in an active tectonic region with different tectonic cycles. The Ma River suture is related to the active Himalaya uplift and South China Sea rift. This results in ongoing uplift of NW Vietnam and rejuvenation of the karst landscape. Detailed regional and local tectonic characteristics have been studied by many geological projects. The following sections only briefly describe the tectonic characteristics of the area on a regional scale. 2.2.2.1 Folding characteristics As mentioned above, the Tay Bac mountain ranges in NW Vietnam mainly consist of the Hong River (Red River) anticlinorium, the Da River rift - formed as syclinorium, and the Ma River anticlinorium. From general structural point of view, series of folds, including series of syclinoriums and alternating anticlinoriums, are described in this region (Tran Van Tri et al., 1979; Tuyet et al., 1996). The carbonate rocks located in the tectonic units above were affected by different folding processes at different stages. The oldest carbonate rocks formed in Early Proterozoic were controlled by the folding process during the later Proterozoic. Such carbonate rocks metamorphosed to marble, and formed in the core of the Ma River and Red River anticlinoriums. The slight folds are often observed in carbonate rocks formed in Middle Devonian. The carbonate formations formed in the later Paleozoic are generally defined by folds plunge of about 50o. In contrast, the carbonate rocks formed in the Middle Triassic are characterized by a series of open anticlinoriums and syclinoriums trending NW-SE like a wave system.

10

The NW karst belt

2.2.2.2 Faulting characteristics Fault systems in the NW Vietnam are classified into 4 different orientations: the NW-SE, the NE-SW, E-W and N-S (Tran Van Tri et al., 1979). The Hong River (Red River), the Da River and the Ma River faults are major fault systems in the area. These deep-seated faults form in a NW-SE direction, with a 20o to 40o dip. Basic magmatic intrusive-effusive formations such as Vien Nam and Cam Thuy are observed along these faults. Other dominant faults systems in the upper part of the Ma River and the fault system in the lower part of the Da River are also developed in a NW-SE direction. On a regional scale, NE-SW faults are relatively short and discontinuous. Most of the faults are of thrust type and originated under the compressive state of the regional stress fields and may be tight; thus not favourable for the development of large valley, sinkhole and cave systems (Van, 2003). However, due to uplifted erosion, shallow decompression phenomena are widening fractures and joints, and local stretches of the active fault system may be in tensional regime.

2.3 Principles of hydrogeological characterization of karst aquifers

A "karst aquifer is an aquifer in which the flow of water is or can be appreciable through one or more of the following: joints, faults, bedding plane partings, and cavities- any or all of which have been enlarged by dissolution" (Field, 2002). The karst aquifer may have primary (intergranular) and secondary (fracture) porosity openings, which are saturated with water when below the water table. Karst aquifers are subdivided in two main types depending on their position compared with the relevant (hydrologic) base level (Bögli, 1980 (Fig. 2.2). Shallow karst aquifers have their karst basis above the base level of the system. Deep karst is present when the base of karst aquifer is below the base level of the system. A karst system can be mixed of shallow and deep karst types.

11

Chapter 2

Fig. 2.2: Shallow and deep karst systems with regard to the position of the base level (Bögli, 1980)

The karst aquifers receive recharge through autogenic and allogenic systems. An autogenic system is one composed entirely of karst rocks and derives its precipitation water through the soil and unsaturated zone. By contrast, an allogenic system derives water from an adjacent non-karst area via dolines or swallow holes. Ford and Williams (1989) noted that the mixed autogenic and allogenic intermediate case is the most common in practice. Figure 2.3 illustrates the recharge into a karst aquifer from both concentrated and diffuse sources.

Fig. 2.3: Recharge into carbonate aquifers (Gunn, 1986)

12

The NW karst belt

Karst aquifers are heterogeneous and anisotropy results in variation and directional difference of hydraulic conductivity. Kiraly (1975) demonstrated that the hydraulic conductivity in karst systems varies considerably with the scale of estimated samples (Fig. 2.4). For instance, rock samples (pore and micro-fissures) have hydraulic conductivity values of 10-9 to 10-5 m/s, while the hydraulic conductivity ranges from 10-4 to 100 m/s at aquifer catchment scale. The highly different conductivities/permeabilities of fissured systems and conduit systems complicate the hydrogeological characterization of karst aquifers. Groundwater flow in karst aquifers, consequently, is significantly different from that of other aquifers. In karst aquifers, flow in conduit networks is fast and often turbulent, while flow through the matrix of the rock (fissures and pores) can be exceedingly low. The residence times in any karst aquifer vary considerably with the flow path that the water has followed (Smart and Worthington, 2004a).

Fig. 2.4: Conductivity scale-effect in karst system (Kiraly, 1975)

Variations in discharge at a karst spring are often accompanied by changes in spring water temperature and electrical conductivity. Figure 2.5 shows an idealized separation of spring hydrograph and chemograph data (Ford and Williams, 1989). In conduit systems, there is often first an increase in conductivity together with an increase in discharge followed by a decrease in conductivity and temperature. This type of reaction on hydrologic events is called piston effect and will be further discussed in the section 5.5.

13

Chapter 2

Fig. 2.5: Interpretation of a karst spring hydrograph and chemograph (Ford and Williams, 1989)

2.4 Karst landform

2.4.1 Definition of tropical karst landforms

Several terms such as cockpit karst, cone karst, kegel karst, tower karst, fengcong and fengling are usually used in order to describe variant morphology of residual hills of tropical karst land form. Because the tropical karst terms are used as synonyms in different areas or countries, it is therefore useful to explain the origin of above terms: · Cockpit karst is a Jamaican term that is used to describe tropical karst topography containing many closed depressions surrounded by steep conical hills (Field, 2002). · Cone karst is a karst landscape dominated by low conical (or hemispherical) hills that form only in wet tropical climates (Field, 2002). The generally conical carbonate hills may be isolated from each other visually or share lower ground surfaces such as pedestals or ridge remnants (Day and Tang, 2004).

14

The NW karst belt

·

Kegel karst is a German term that is used to describe several types of tropical humid karst characterized by numerous closely spaced cones, hemispherical or tower-shaped hills having intervening closed depressions and narrow steep-walled karst valleys or passageways.

·

Tower karst is a landscape of residual (carbonate) hills scattered in a plain, even though the "towers" may not necessary be steep (Ford and Williams, 1989). The residual hills display a variety of shapes from tall sheer sided towers to cones or even hemispheres. Others are asymmetric, reflecting the influence of dip or erosional processes. Although some rise directly from the plain, many surmount pedestals. Some towers are isolated, while others are in groups rising from a common base.

·

Chinese researchers have identified two types of tropical karst landscapes: fenglin (peak forest) and fengcong (peak cluster). The peak forest consists of individual isolated residual hills rising from floodplains. The peak cluster comprises a group of residual hills emerging from a common bedrock basement and incorporating closed depressions between the clusters of peaks.

There is no definitive distinction between cone karst and tower karst (Day and Tang, 2004) and the cockpit karst is one kind of kegel karst (or tower karst). To avoid confusing, we prefer to use only tower karst to describe the general residual carbonate hills, and peak forest or peak cluster for the karst landform features in this study.

2.4.2 Karst landscapes in NW Vietnam

The karst landscape in Southeast Asia, particular in Vietnam, is one of the most notable and spectacular landscapes in the world (Mouret, 2004; Waltham and Hamilton-Smith, 2004). The NW Vietnamese karst belt is closely related to the karst belt of southern China, but also represents a typical Vietnamese karst landscape due to the distinctive characteristics of stratigraphy, tectonic, climate and geomorphology in this region. The karst landscape of peak cluster­depression stretching in the NW-SE alternate with dry, narrow, and steep valleys are widely developed in the area (Fig. 2.7). Major canyons often cut through these karst terrains (Mouret, 2004). This type of karst landscape is not only observed in the high altitude areas of the center part and NW part of the belt, as for instance in Tam

15

Chapter 2

Duong and Son La; it also occurs in the SE part in relatively low relief areas such as Cuc Phuong and Moc Chau (Fig. 2.6).

Fig. 2.6: Geographical location of main cities/towns in Northern Vietnam

The peak forest karst landscape is also observed in this belt (Fig. 2.8). Such peak forest exist in relative low and moderate altitude areas, as for instant in Hoa Lu, Son La, Moc Chau (Fig.2.6); and also in high altitude areas like Tam Duong, Tua Chua, Sin Ho, etc. These peak forests have many different forms of vertical slope towers, or conical or pyramid towers. The steep slope towers have only a minimal soil cover; the conical and pyramid towers are covered by residual soil. Most invidual towers are asymmetrical, reflecting structural or other influences. It is observed that the invidual towers usually rise from a continuous carbonate surface covered by alluvium, while other towers rise from a surface of non-carbonate rocks. The most impressive karst towers rise from the sea in the Ha Long Bay (World Heritage Site). Several big karst plateaus and karst fields are present in the area. Van et al. (2003) mentioned that the plateaus can clearly be distinguished from other type of landforms by their high altitude due to neotectonic uplift. The Sin Ho (Tam Duong) karst plateau is located at an altitude of about 2000 m, the Moc Chau karst plateau at an altitude of about 1000 m. The Mai Chau karst ­ erosion valley and Cuc Phuong karst field are beautiful and are high potential areas for tourism.

16

The NW karst belt

Fig. 2.7: Peak cluster depression karst landscape in NW Vietnam

Fig. 2.8: Peak forest karst landscape in NW Vietnam

Cave systems are numerous in this karst belt. Subhorizontal caves are dominant. Vertical caves, however, are often investigated in the NW part of the belt where relief is uplifted because of neotectonic activities. In addition, due to the neotectonic uplift, ancient caves with multilevel systems uplift are found in area. The Cong Nuoc cave, situated in the Tam Duong area with the depth of -600 m, is known as the deepest cave in Southeast Asia. High density vegetation and typical tropical forest cover the karst belt. The "green karst landscape" is often observed in the SE part and in high mountainous areas of this karst belt.

17

Methods and techniques

3 Methods and techniques

Worldwide, karst hydrogeological research is conducted on different scales and using different methods. Some of the methods - even the essential methods - are less used in lowincome countries and/or remote areas because of the difficult technical and operational conditions. The test sites are located in one of the most remote and poorest regions of Vietnam. The following methods are applied to the research areas. · · · · · · Tracer experiments Hydrochemical study Microbiological investigation Stable isotope study Rare Earth Elements (REE) study Karst groundwater vulnerability, hazard and risk mapping

These methods are not employed separately in the concerned areas. A method and its results are applied and interpreted in combination with those of other techniques to achieve effective inFormation of the test sites. Details of the vulnerability, hazard and risk mapping applied in this study are presented in chapter 6.

3.1 Tracing experiment

3.1.1 Tracing in karst study

Smart and Worthington (2004b) defined water tracing (tracing experiment) as the use of natural or induced properties in the water, allowing detection of that water at some point downstream and gaining insight in the character of the flow path followed by the water. Tracer techniques are powerful tools with many applications in hydrological investigations. The tracing technique in hydrogeology and related issues have been described in detail by Käss (1998), Smart and Worthington (2004b), Crawford (2004), Divine and McDonnell (2005), etc. This section only briefly presents the tracing in karst research in accordance with above references.

19

Chapter 3

`The tracing test is a primary tool of the karst hydrogeologists. General speaking, the technique is often used in karst studies to determine an underground water flow path, groundwater travel times, catchment boundaries and recharge areas. Tracer tests have also been applied to define contamination problems and to assess the vulnerability and determination of protection zones in karst area. The first tracing tests using chaff were applied to solve problems in karst groundwater in 10 A.D. (Crawford, 2004). Recently, the technique has been developed and applied in all type of aquifers. Tracers can be divided into physical, chemical, isotopic and biological tracers; and two tracing types: natural tracing and artificial tracing. Three tracing methods are applicable: qualitative, semi-quantitative and quantitative. Qualitative tracing simply detects the tracer in the water, while semi-quantitative tracing includes defining the concentration of the tracer in the water over the time. Quantitative tracing includes tracer concentration measurements with flow determinations. Natural tracing includes any substance naturally occurring in the water that is used to follow flowing water. Unlike that, artificial tracing deliberately introduced into the water to follow flowing water. Artificial tracing is widely used in karst hydrology because it allows control over the magnitude of the tracer concentration and the specificity of the site to be traced. Fluorescent dyes are the predominant tracers currently being used. Tracer tests in karst are widely used in "point-to-point" mode to define the trajectory taken by underground water. Typically, this enables to identify the destination spring (resurgence) of a sinking stream. The detection of an injected tracer at the spring means that there is a connection between the point of injection and the point of recovery. A series of point-to-point tracer tests can be used to establish a regional network of karst underground flow paths. Depending upon the natural conditions and the aim of the experiment, tracer(s) can be injected into swallow holes, boreholes, closed depressions, fissures, or even spread over the ground surface either by instantaneous or continuous injection. The sampling method is selected on the basis of the tracer used, the application and local conditions. Qualitative tracing, semi-quantitative and quantitative tracing is applied depending on the same considerations.

3.1.2 Tracer breakthrough curve

The curve generated by plotting measured tracer concentration versus time (after injection) is the so-called tracer breakthrough curve. The shape of the tracer breakthrough curve depends

20

Methods and techniques

upon the character of the tracer, flow conditions and structure of the aquifer. The interpretation of tracing test is based on a detailed evaluation of the tracer breakthrough curve. It is useful, therefore, to introduce the main parameters that are often used in analysis of a tracer breakthrough curve (Fig. 3.1). · Time to first arrival (t1) is the time when the first tracer is detected at the sampling point · Time to maximum concentration (t2) is the time when the maximum tracer concentration is detected at the sampling point · Mean travel time (t3) is the time when the centroid of the tracer mass traverses the sampling point

Fig. 3.1: Tracer breakthrough curve and residence times

A simple way to evaluate tracer velocities is to calculate "linear" velocities, using the distance between the injection and sampling points. The fastest flow velocity, dominating flow velocity and median flow velocity are calculated using the time to first arrival, the time to maximum concentration and the mean travel time respectively. A computer programme to analyse tracer breakthrough curves, Traci95, for instance, allows more parameters of breakthrough curves to be determined, as for example: the dispersion coefficient, Peclet number, etc.

21

Chapter 3

3.1.3 Traci95 Programme

Traci for Windows 95 (Traci95) is a user-friendly computer program used to evaluate artificial tracer tests. It contains several analytical solutions for different groundwater aquifer types and hydraulic situations. Traci95 can be used for tracer analysis in karst aquifers (Käss, 1998; Werner et al., 1997). The mathematical evaluation of the breakthrough curve from a tracing test is possible using analytical and numerical processes. The simple explicit form of the analytical solutions allows quick determination of transport parameters. The analytical solution below applied for tracing test in conduit system of a karst aquifer (Werner et al., 1997) is given by equation 1.2. Many karst breakthrough curves, however, are characterized by tailing or multiply peaks. Such breakthrough curves can be evaluated with the multi-dispersion-model, MDM (Werner et al., 1997). This model is an extension of the solution in equation 1.2 above. The obtained breakthrough curve is taken to be a superposition of different flow paths. The parameters of the individual curve are determined step by step in the evaluation.

22

Methods and techniques

3.1.4 Tracing tests in Son La and Tam Duong areas

As mention above, the tracing test provides a major tool in the hydrogeological characterization of karst aquifers. However, before this work, tracing tests were not used in any karst study in NW Vietnam. Within the framework of this thesis, the tracing experiments were first applied in the Son La and Tam Duong karst areas. A detailed account of the tracing experiment is presented in section 4.3 and 5.3. Dyes and common salt (sodium chloride) were selected as tracers. A quantitative tracer method that combines determination of the tracer concentration over time with flow (discharge) measurements was mainly employed. This method allows to characterise the groundwater flow path, and the total mass of tracer collected at the sampling site to be determined. The discharge was measured either by a water level logger (tests in Suoi Muoi, Nam La), or by flow velocity multiplied to cross-section area (tests in Bon Phang, Tam Duong), or by the salt-dilution method (test in Tam Duong). In addition, the qualitative tracer method that provides point-to-point and catchment information was also applied in the areas. This method involves only the identification of the injected tracer in water samples. Collected water samples of the experiment in the Bom Bay area and charcoal bags samples of the experiment in Tam Duong were analysed to confirm the presence of tracer in the water. In both the Son La and Tam Duong areas, the selected tracers were injected by instantaneous injection method. The tracers were dissolved in water prior to injection. The active swallow holes or caves were selected for injection, and springs and caves were selected for sampling. The sampling technique, however, was slightly modified. As automatic-sampling equipment was not available, local peoples took the samples manually. The water samples were collected

23

Chapter 3

in small volumes and protected against the light; the used charcoal bags were dried and also protected against the light. The fluorescent dyes were detected by spectrofluorometer in Belgium and Switzerland, and by digital fluorometer in Vietnam. Both sodium and chloride concentrations were detected in Vietnam. The obtained tracer flow velocities were calculated as "linear" velocities as mentioned in section 3.1.2. The hydraulic properties of the underground water flow paths are determined by the computer programme Traci95. Depending on the different tracer experiments in the Son La and Tam Duong areas, either the single fissure-dispersion-model (SFDM) or the multidispersion-model are chosen to evaluate the tracer breakthrough curves. Unknown parameters as the mean transit time (t0) and the dispersion parameter (PD) that need to be determined for the advection-dispersion transport process in karst conduit are estimated by inverse evaluations. The results of the tracer test are iteratively fitted through a minimization procedure on the analytical solution function. The method of moments is selected to estimate the initial values. A least squares method is used to check which distribution of the fitting parameters provides the best fit.

3.2 Hydrochemical investigation

3.2.1 Overview

Hydrochemical investigations are often carried out in the context of water quality studies and to investigate water pollution. The chemical studies can provide inFormation about contaminations. There is considerable literature on groundwater geochemistry, environments and flow systems based on hydrochemical data (Glynn and Plummer, 2005). The hydrochemical data is used to gain insight into the flow pattern of water and the interaction between groundwater and surface water, and water-rock interaction. The hydrochemical parameters then serve as natural tracers to distinguish between waters of different origins. Furthermore, a good understanding of hydrochemistry is required to design and operate systems for drinking water preparation and wastewater treatment. The goals of hydrochemical investigations applied in karst aquifer study are to understand the structure and function of the aquifers. Hydrochemical investigations, for instance, would introduce the degree of karstification, model of infiltration and transit time of water in the

24

Methods and techniques

studied karst aquifers. On the other hand, hydrochemical investigations are also applied to study contaminant processes for management and protection of karst aquifers. The hydrochemical investigations in karst studies mainly focus on the variability of spring water chemistry as a means of characterising the karst aquifer, particularly the interpretation of plots of chemical variables as a function of time (chemographs) either on seasonal time scales or during individual storm rainfall events. Simultaneously, the discharge rates as a function of time (hydrographs) are recorded (Vesper and White, 2004).

3.2.2 Hydrochemical investigation in the test sites

The hydrochemical study in the Son La area was carried out to determine water quality, and to characterize spatial distribution of karst water in the area. The VIBEKAP data from the January 2000 campaign is used in the present study. The water samples were collected at most important springs, swallow holes and streams that are located in different geological formations. Physical parameters such as pH, temperature, TDS were measured in the field. Major ions such as Ca2+, Mg2+, Na+, K+, SO42-, Cl-, NO3- and F- were analysed at the Laboratory of Physical­Chemical Geology, Catholic University Leuven, Belgium. The obtained data are interpreted by using the software AquaChem 4.0. Details of results are presented in section 4.4. The hydrochemical investigation in the Tam Duong area was carried out in a rainy season of 2004 to determine the water quality, and to understand how chemical parameters vary during and after precipitation events. This study required continuously sampling over short time intervals in several karst springs. In order to avoid that a huge volume of water samples should be transported, the samples were taken in small 13 ml plastic tubes. These samples were analysed at the Centre of Hydrogeology, University of Neuchâtel, Switzerland by ionic chromatograph method using ionic chromatograph dionex DX-120. The major cations Na+, K+, NH4+, Ca2+, Mg2+, and anions F-, Cl-, NO3- and SO42- were measured. In principle, the equipment performs isocratic ion analysis applications using conductivity detection. To measure the conductivity, two different columns of eluents (one for anions and other for cations) are respectively used. Only 3 ml of sample is injected for measurement of the major anions; and another 3 ml of acidified water sample is injected for the major cations. The standard samples and blank samples are first calibrated before starting the analysis of each

25

Chapter 3

series of water sample. The ion contents are then calibrated as a function of detected conductivity. Details of sampling and results are presented in section 5.4.

3.3 Microbiological investigation

Many types of micro-organisms are found in groundwater, including bacteria, archaebacteria and protozoans as well as viruses. Most of these microorganisms occur naturally and permanently in groundwater and are harmless while some are pathogenic (Chapelle, 2001). Microbial contamination often results from human activities. In many studies, bacteria including Escherichia coli, faecal streptococci and Clostridium are commonly used as an indicator to know if water is contaminated and may contain other pathogenic microorganisms. Thermotolerant coliform analysis carried out at 44°C indicate bacteria of faecal origin. Bacteriological problems in karst, particular in karst groundwater, are mentioned by many authors (Ford and Williams, 1989; Drew and Hötzl, 1999; Auckenthaler et al., 2003; Pronk et al., in press). In karst systems, the infiltration and flow conditions favour the transport of micro-organisms. In recharge periods, contaminants from the land surface can be washed into aquifer either diffusely by infiltration and subsequent percolation through the soil, epikarst, unsaturated zone, or concentrated via swallow holes (Goldscheider, 2004). The high velocity leads to a fast transport of the contaminants through the whole karst aquifer system. Therefore, using faecal bacteria as contamination indicator in karst aquifers has been applied in many areas. The faecal bacteria analysis, particularly thermotolerant coliform, using the culture-counting techniques was applied for the first time in the Tam Duong area. The samples for microbiology were collected at all important springs which are used for drinking water in the area. At the same time, samples were collected at short time intervals, ranging from 8 to 14 h, at two main springs. In normal conditions, water samples for microbial study should be analysed in the laboratory on the same sampling day. However, in case of the Tam Duong area it was impossible to transport the water samples to a microbial laboratory in the required time. Hence, the analyses were done in situ using the portable microbial Lab OXFAMDELAGUA (Fig. 3.3).

26

Methods and techniques

Fig. 3.3: The portable microbial Lab Oxfam-DelAgua with main consumables (Photo by OxfamDelAgua)

In principle, the analysis is carried out by passing a measured quantity of water through a sterile filter (0.45 µm pore size). Any bacteria present in the water are caught in the filter. The filter is then placed onto a paper pad soaked in a liquid growth medium which feeds coliform bacteria but inhibits the growth of any bacteria caught on the filter. The filter is kept in 440C at the kit's incubator during 18 h (1-2 h in room temperature and 15-16 h in incubator). During that time the coliform bacteria multiply many times to form colonies that can be seen or counted with the naked eye. To avoid errors during counting, different volumes of the samples (from 0.1 ml to 100 ml) are analysed depending on the level of contamination. Most samples were analysed twice with different volumes of water (1 or 2 ml and 10 ml or even larger volume). The contaminant level is presented by both minimum and maximum counted colonies per unit quantity of water. Details of sampling and results are presented in section 5.4

3.4 Stable isotope study

Stable isotope ratios of oxygen and hydrogen have been applied extensively in hydrological investigations and complementing geochemistry over the past few decades. The use of stable isotopes, in particular the isotope ratios of oxygen and hydrogen as conservative tracer, has improved our understanding of problems related to catchment and groundwater studies (Barnes and Allison, 1988; Darling and Bath, 1988; Schramke et al., 1996; Clark and Fritz, 1997; Vitvar and Balderes, 1997; Genereux and Hooper 1998; Katz, 2002). Coplen et al. (2001) have provided some relevant examples using stable isotopes to solve practical

27

Chapter 3

hydrologic problems of recharge and discharge processes, mean transit time, mixing fraction between surface-groundwater interactions, and climatic conditions during the recharge process Several researchers addressed the problems on karst hydrology investigations by using stable isotopes. The studies of 18O and 2D seepage water of karst caves (Yonge et al., 1985; Caballero et al., 1995) show that the isotopic compositions of those waters, just as in groundwater, can be considered as the mean isotopic value for precipitation water in the surrounding area. Other studies of 18O and 13C in speleotherms found that the isotopic behavior strongly depended on the cave environment (Verheyden, 2000). Results from isotopic and hydrochemical studies to evaluate the regional recharge as well as the importance of point-source recharge to karst aquifers in sub-humid to semi-arid regions with low topography are presented by Leaney and Herczeg (1995) and Herczeg et al. (1997). Researchers have also used stable isotopes as tracers for determining the component mixing in groundwater in karst area (Lakey and Krothe, 1996; Nativ et al., 1999; Lee and Krothe, 2001; Long, 2002). Nguyen Tam (2004) presented the use of oxygen and carbon isotopes measured in a stalagmite from the Ta Chinh cave (NW Vietnam) as palaeo-climatic indicators to provide preliminary paleoenviromental information of the area. However, the application of stable isotopes in hydrogeologial study has not been carried out in this NW karst belt. This study reports stable isotope measurements, particular oxygen isotopes, of meteoric and surface and spring water in attempt to use as natural tracer for determining hydrogeological functioning of the area. The study also provides the information of 18O as valuable reference data for any further stable isotope application in hydrogeological, geological and speleological studies in Vietnam. The oxygen isotope samples (96 samples) were prepared and analyzed at Laboratory of Stable Isotopes, Department of Geology, VUB, by isotope ratio mass spectrometry using the CO2/H2O equilibration method. In principle, a small quantity of CO2-gas is brought in contact with the water sample so that isotopic exchange can take place (C16O2 + H218O C16O18O + H216O). If isotopic equilibrium is attained at a constant known temperature, the oxygen isotope composition (18O) of the water can be calculated from the 18O of the CO2 measured in a mass spectrometer (after Epstein and Mayeda, 1953). The 18O values then are reported in units of parts per mil (0/00) with reference to Standard Mean Ocean Water (Equation 1.3).

28

Methods and techniques

Details of the water sampling and the obtained results are presented in section 4.4.2

3.5 Rare earth elements study

The rare earth elements (REE) are a group of 17 chemical elements composed of scandium, yttrium and the lanthanides. The acronym REE, which is used in this study, is commonly used in geochemistry to signify the lanthanides series. Useful reviews of the aqueous geochemistry of the REE have been published by Henderson (1984) and Brookins (1989). In natural water, the REE occur in general in trivalent oxidation state. They have similar geochemical behavior, except for Ce and Eu that can have a valence of 4+ and 2+ respectively, depending on the redox potential. The REE concentration in groundwaters is controlled by several factors including release from weathering phases, pH and redox status, adsorption, complexing ligands in groundwater, and hydrogeological conditions (Shand et al., 2005). Early investigations of REE in the aqueous system were focused on sea water. The REE have been applied in many studies to investigate geochemistry of natural water (Smedley, 1991; Johanneson et al., 1996, Johanneson, 2005). The REE can also be useful tracers of groundwater-rock interaction. Today, REE geochemistry has been widely used as a powerful tool for tracing geochemical processes within the earth (Johannesson, 2005). In Vietnam, the REE have been applied in several geological studies. The elements were mainly used as tracers in petrogenetic studies magma rocks (Tam et al., 1996). The most detailed investigation of REE in the geological study was carried out by a Japanese team in the Dong Pao area (30 km from Tam Duong town) in 2002 (Japanese Mining Project, 2002). This project mentioned the occurrence of the rare earth minerals in carbonate rocks of the Dong Giao Formation. The study of REE in carbonate rocks and carbonate aquifers is still uncommon however. The aim of this investigation is to study the distribution and chemical behavior of the REE in the carbonate aquifer in the Tam Duong area, so as to achieve a better understanding of waterrock reaction, particularly in the karst areas. The current study is also to provide information

29

Chapter 3

about water quality and to shed some light on whether the REE have a toxicological impact on drinking water from groundwater in the area. The rock and water samples were transported to and analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at the Institute of Geology (University of Neuchâtel, Switzerland), following an analytical technique described in detail by Steinmann and Matera (2002). Details of the water sampling and the obtained results are presented in section 5.5.

30

The Son La karst area

4 Hydrogeology of the Son La karst area

4.1 Location, landscape and climate

The Son La area is situated in the mountainous setting of the Da River catchment, more specifically to the west of this river in the Son La province, about 300 km NW of the capital Hanoi. The Son La town, centre of Son La province, is one of the largest centers in the mountainous areas of NW Vietnam. The area is densely populated by different ethnic groups, including the Kinh, Thai, H'Mong. The economic and living standards in the area are low, except in the main town and immediate surrounding areas. The main agricultural production is maize, rice and cassava; the yield from agricultural production is highly depending on the natural conditions that prevail in the area. The karst landscape is characterized by tropical mountainous karst landforms. The peak cluster-depression stretching in a NW-SW direction alternates with blind valleys, as for example the Suoi Muoi and Nam La valleys. Deep dolines and chained sharp peaks are observed in the NE part of the area. Surface streams such as the Suoi Muoi and Nam La Rivers sink underground via swallow holes and caves. In the SW part (Fig. 4.1) peak forests are widespread in an environment dominated by dissolution-erosion valleys and sharp peaks forming asymmetrical pyramid towers.

Fig. 4.1: Son La karst landscape, view from Son La pass to the SW

The area has a tropical monsoon climate, characterized by two seasons: a hot and rainy summer, and a cold and dry winter. The annual mean temperature is 21°C. The absolute

31

Chapter 4

maximum temperature reaches 40°C, while the minimum recorded temperature is 1.1°C. The average humidity of the air attains 80%, but may drop in winter to a 60% low. The mean annual rainfall is 1413 mm, but precipitation is unevenly distributed over the year (Fig. 4.2). The rainy season ranges from April to September with a rainfall higher than 100 mm/month, while the dry season starts in October and ends in March with a rainfall lower than 50 mm/month.

Fig. 4.2: Monthly rainfall (mm) in Son La (collected data in Son La station from 1974-1998)

In this area, karst has an important impact on living conditions. In fact, many karst springs are used for drinking water and agricultural activities. Large populations of Son La town (about 50%) rely on one single karst spring (Tham Ta Toong spring) for drinking water. Problems such as flooding in the Suoi Muoi and Nam La valleys and water pollution severely affect the living conditions of local people.

4.2 Overview of previous studies

Previous studies on hydrogeology were carried out to establish the hydrogeological map and geological - hydrogeological characterization of the Son La area on a regional scale (Hop, 1994; Tuyet et al., 1996). During the past years, the hydrological-hydrogeological study in the Son La area was preliminary investigated by several diploma theses within the frame work of the VIBEKAP project (Van den Bossche, 2000; Nguyet, 2000, Tin 2001). A Ph.D work on "characterization of a karstic system by an integrative and multi-approach study", within the

32

The Son La karst area

framework of the VIBEKAP project, was carried out (Tam, 2003). This study aimed to investigate the hydrodynamic properties of a cavern conduit system by integrative analyses. The work also proposed a simple method to interpret the available pumping test data in the area. Relationships between lineaments and borehole specific capacity in the area were presented. However, the study was focused mainly on the Nam La valley. The karst hydrogeological characterization of the area has not been undertaken. A sound knowledge of structure and functioning of the karst system on the catchment scale and a local scale was still lacking. Within the framework of this thesis, the hydrogeological characterization of the Son La area is further investigated. Karst groundwater flow connections and information of hydraulic parameters of conduit flow will be identified and mapped. Furthermore, the recharge processes and different flows through karst aquifer will be identified. Groundwater mixing effect will be characterized in order to better understand the structure and functioning of the whole karst system.

4.3 Geology

4.3.1 Geological framework and stratigraphy

As mentioned in section 2.2.1, the Son La region lies on the Tay Bac fold system, which belongs partly to the Bac Bo fold belt. More precisely, this region is situated between the WSW part of the Ma River anticlinorium and the E-NE part of the Da River rift (Fig. 2.1). These two structures are separated by the Son La deep-seated fault in a NW-SE direction. VIBEKAP (2003) referred to this region, more specifically, as the transition zone between the Song Ma anticlinorium (SW), the Son La synclinorium (centre), and the Da River rift (NE). The NE limb of the Ma River anticlinorium is formed by rocks of the late Proterozoic and early-middle Cambrian Formations. The late Cambrian and Carboniferous-Permian rocks are well exposed in the Son La synclinorium. The younger rocks including late Permian to late Triassic formed in the Da River rift. In particular, the metamorphic rocks of the Nam Co Formation outcrop at the core of the Ma River anticlinorium. Terrigenous rocks, terrigenous-carbonate rocks and metamorphic siliceous rocks of the Song Ma, Ham Rong, Dong Son and Nam Pia Formations, as well as

33

Chapter 4

the carbonate rocks of the Ban Pap and Chieng Pac Formations are located further to the NE limb of the anticlinorium. The Da River rift is composed of different rocks formed during the Permian-Late Triassic period; it includes the Cam Thuy, Co Noi, Dong Giao and Nam Tham Formations, which are located in the central part of the study area. The younger terrigenous molasse rocks of the Suoi Bang, Yen Chau Formations and the young Paleogene volcanic rocks are situated in the E ­NE part of the Son La region along the Da River fault.

4.3.2 Stratigraphy

Many different formations ranging from Late Precambrian to Cretaceous and Quaternary deposits are present in the Son La region (Table 4.1 and Fig. 4.3). Some formations are well exposed and act as one of the controlling factors of the hydrogeological characteristics of the area. The Ordovician Dong Son Formation outcrops in the Ban Lay area. The formation has a thickness of 400 to 1000 m. It contains shale, siltstones and sandstone.

Table 4.1: Stratigraphic table of the Son La area corresponding to mapsheet in Fig. 4.3.

34

The Son La karst area

The Devonian Nam Pia Formation consists of quartzitic sandstone, sericite schist with total thickness of 400 m. This formation conformably overlies the Dong Son Formation. The Devonian Ban Pap Formation outcrops in a NW-SE direction along the the Bon Phang area. It comprises siltstones and shales about 190 m thick in the lower part, then changing to a limestone about 1000 m thick in the upper part. The Carboniferous-Permian Chieng Pac Formation consists of light-coloured bedded to massive limestone. The total thickness attains 600-800 m. This formation outcrops in Chieng Pac are to the west of Son La town The Permian Cam Thuy Formation is encountered on the Son La pass and north of the Thuan Chau town. It mainly consists of dark grey to black basaltic rocks with agglomerates. Siltstone and tuffaceous siltstones occur in the lower part. The total thickness attains 500 m. The Permian Yen Duyet Formation consists of sandstones, siltstones, shale and calcareous shale. The total thickness attains 250-500 m. The most important rocks outcrop in the Son La area is the Triassic Dong Giao Formation. It consists of a dark grey, thin bedded limestone with frequent interlayers of grey shale and marls at the base, changing upward to a light grey or pinkish massive limestone (Tuyet et al., 1996). The Dong Giao Formation is among the purest limestone of the area (from 93-98% CaCO3) and, thus, it is very favorable for karstification. This formation attains a thickness of 1200 m The Triassic Nam Tham Formation outcrops along the Dong Giao Formation, in a NW-SE direction (Fig. 4.3). It is composed of thin bedded yellowish to whitish grey shale, siltstones and sandstone in the lower part, changing to thin bedded clayey limestone, siliceous limestone with interbeds of calcareous shale. The maximum thickness of this formation is about 8001000 m. The Quaternary deposits include the recent alluvium of the river flood plains, a mixture of boulders, sand, and clay ranging in thickness from 1 m to 10 m. It mainly outcrops along two blind valleys of the Suoi Muoi and Nam La Rivers. Quaternary deposits also occur in several big dolines in the area.

35

Chapter 4

4.3.3 Tectonics

4.3.3.1 Folding characteristics The area is generally characterized by a series of folds in a NW-SE direction (Fig. 4.3). The carbonate rocks of the Ban Pap, Chieng Pac and Dong Giao Formations are formed in simple folds in amplitudes of several kilometres. Small folds with wavelengths and amplitudes of only several meters are observed in the non-carbonate rocks. In particular, it is observed that the carbonate rocks of the Ban Pap and Chieng Pac Formations usually expose in longitudinal folds with average wavelengths of several hundred meters trending NW-SE. Because of plunge of main NW-SE folds, long wavelength and low amplitude transversal folds in a NE-SW direction are frequently exposed in the limestone of the Dong Giao Formation. 4.3.3.2 Faulting characteristics The Son La area is traversed by a fault network, which can be classified into 4 systems: the NW-SE, NE-SW, E-W and N-S (Fig. 4.3). · The NW-SE faults trend 285o to 300o or 320o to 330o; some even trend 330o to 345o. These faults are the most developed faults systems (VIBEKAP, 2003). The faults often dip 60o-80o toward the SW or NE. Many faults play an important role in the geological structure on a regional scale, while other faults act as a separation factor in structural zones or strata on a local scale. · The NE-SW fault system is characterized by short and discontinuous faults. These faults often run across longitudinal the folds in the area. VIBEKAP (2003) noted that these faults mainly trend 20o-30o or 60o-70o, with a dipping variation from 30o to 70o. The faults control the orientation of river valleys, as for instance the Suoi Muoi and Nam La valleys. · The E-W and N-S faults dip relatively vertically (VIBEKAP, 2003). These two faults systems are less obvious in the area. The large tectonic shear zone near the Dong Hung villages is the most significant of the E-W faults exposed in the area.

36

The Son La karst area

Fig. 4.3: Geological map and geological cross sections of the Son La area (modified after Vibekap, 2003).

37

Chapter 4

4.3.4 Hydrogeology, spring and surface water

4.3.4.1 Hydrogeology The hydrogeology of the Son La karst area is characterized by two karst aquifers: the Dong Giao (aquifer 1) and Chieng Pac-Ban Pap aquifers (aquifer 2) (Fig. 4.4). The non-carbonate rocks of the Nam Tham, Co Noi, Tan Lac, Yen Duyet and Cam Thuy Formations separate these two karst aquifers.

Fig. 4.4: Karst aquifers, springs and surface water in the Son La area.

The aquifer 1 consists of mainly Middle Triassic limestone, which has a high permeability. Geological map and geological cross sections (Fig. 4.3) show that this aquifer is underlain by low permeable layers of the Tan Lac Formation that consist mainly of shales, sandstone and calcareous shales. Most of the Dong Giao Formation belongs to the zone of open karst; forming a karst groundwater unconfined aquifer. This karst aquifer receives water mainly by surface runoff (the Suoi Muoi and Nam La Rivers) through swallow holes, caves and by recharge from infiltration. The productive units of the aquifer 2 in the area are the Chieng Pac and Ban Pap Formations. This aquifer consists of limestone and interbeds of only a small portion of siltstone and shale in lower part. The aquifer is highly permeable and underlain by low permeable layers of the Nam Pia and Dong Son Formations. Because of the tectonic structure, the rocks of the karst

38

The Son La karst area

aquifer are laterally separated from other rock units by strike-slip faults. The aquifer, is also unconfined. Both the Dong Giao and Chieng Pac-Ban Pap Formations have an important thickness. Geological cross sections (Fig. 4.3) show that the base of two large rivers is higher than the karst rock (aquifer) base. These two aquifers, thus, are deep karst systems. 4.3.4.2 Spring and surface water The surface water in the Son La area is relatively abundant in comparison with other karst areas within the NW karst belt. The surface water is unevenly distributed over the land surface. The streams with high flow rates originate in the non-carbonate area; they continuously flow to carbonate areas located downstream of impermeable layers and exposed along the fault network, and then sink to the sub-surface through swallow holes and caves. Other lower flow runoffs emerge as in the discharge from springs and then flow to noncarbonate areas. The Suoi Muoi River and Nam La River are the two largest surface water systems in the area The Suoi Muoi River consists of two branch surface systems in the SW and NW (Fig. 4.4). The first branch starts from many small surface runoffs in non-carbonate rocks of the Nam Co Formation. The second branch comes from many small streams formed in the Cam Thuy and Dong Son Formations. These two branch systems are combined at the Nga Ba Chieng Pac and become the so-called Suoi Muoi River. The river flows toward the NE-SW until the Ban Phe village. At the end of the Suoi Muoi valley, the river entirely sinks into the limestone of the Dong Giao Formation mainly through the Tham Han cave and several small swallow holes. The average discharge measured at the Ban Phe village is 4.67 m3/s (VIBEKAP data), but varies greatly between the dry and wet periods. During the rainfall period, the Suoi Muoi "blind" valley often floods because of big rainfall storms. The main Nam La River (Fig. 4.4) comes from the S-SW direction; a small part flows from the discharge of several springs on both sides of the Nam La valley. This river ends at the Khau Pha pass in several swallow holes such as the Ban Ai cave, Khau Pha cave and the Lom Co Co cave. The average river discharge is 5.43 m3/s, but also varies considerably between the summer and winter seasons (Fig. 4.5). Flooding problems frequently occur in the Nam La valley during the rainfall period.

39

Chapter 4

Fig. 4.5: Spring hydrograph of Nam La River measured at Ban Toong village and Hang Doi spring in 2000 (VIBEKAP data).

Many springs occur within this carbonate area. The large springs are indicated on the map of Fig. 4.4. The Hang Doi and Ban Sang springs are the two largest springs in the area with average measured discharges of 8.58 m3/s and 7.92 m3/s respectively (Tam, 2003). Other springs have smaller discharge rates. The karst springs are used to meet the drinking and agriculture needs of large populations in the area. The major Tham Ta Toong spring supplies drinking water to 50% of the population of Son La town. 4.3.4.3 Caves Almost all the caves in the Son La karst area have developed in a direction that coincides with the faults in the NW-SE and NE-SW orientations. The inflow caves are often in a NE-SE and E-W direction, while the outflow caves have usually developed in a NE-SW direction (VIBEKAP, 2003). The caves have developed in different stages; the change from one stage to another often goes together with the change in direction of the cave. A summary by VIBEKAP (2003) determined that most caves (84%) in the area have a horizontal conduit structure, which combines many different small conduits caves with various forms and sizes. Details of the cave structure and it characteristics are described in the reports of the Belgian­ Vietnamese Cave Expedition (1996; 1998; 2000) and VIBEKAP (2003).

40

The Son La karst area

4.4 Tracer tests

Tracer tests were carried out in the Son La area during 2000-2003 in the framework of the VIBEKAP project and with the support of the Belgian Technical Cooperation. Fluorescent dyes and common salt were used as tracers. These were the first tracer tests applied in a karst study in NW Vietnam. The aim of the tests was a better understanding of subsurface movements of karst water in the area. The main objectives were: · To map underground water flow paths and to characterize the properties of underground water transport. · To investigate the relation between the stratification, geological structures and karst groundwater flow. · To test and adapt tracing techniques for application under the practical conditions of the test site.

4.4.1 Tracer tests

Both qualitative and quantitative tracer experiments were carried out in the Son La area. The tests are summarized in the following: February 2000: Suoi Muoi valley. This was a preliminary experiment carried out in the study area (Table 4.2). Uranine and sodium chloride (salt) tracers were used. Uranine was injected at two different branches of an active swallow hole at the northern end of the Suoi Muoi River valley. Sodium Chloride was injected just before the swallow hole outside the Tham Han cave (end of Suoi Muoi valley, Fig. 4.6). The Ban Sang spring/cave, one of the largest springs (average discharge of 7.92m3/s) located about 2 km on the other side of a high mountain range was selected for sampling. The tracer samples were taken during 3 days at time intervals of 0.5-1h with the help of local people. October 2000: Bon Phang area. Uranine and salt tracers were also used in this experiment (Table 4.2). The main goal of this test was to provide inFormation of contamination transport in the Long Ngo spring, where a drinking water supply station for several villages in the Bon Phang commune was funded by UNICEF. This spring is located in the limestone of the Ban Pap Formation, which belongs to the Chieng Pac-Ban Pap aquifer. The spring discharge flows to the Suoi Ban stream, and then towards to the Suoi Muoi River. Field observation and cave

41

Chapter 4

data (Belgian-Vietnamese Caving Expedition, 1996) point towards the existence of an under flow path between the Ban Lay swallow hole and Long Ngo spring. The swallow hole at Ban Lay village was thus selected as the injection site. This swallow hole is situated on the boundary between the impermeable rocks of the Dong Son and the limestone of the Ban Pap Formation. With the help of local people, samples were taken at the Long Ngo spring in short intervals of 30 min for two days. October 2001: Nam La valley. The main goal of this test was to trace where the Nam La River would reappear and to provide information on karst sub-surface flow to better understand and remedy the serious regular flooding problems in the Nam La valley. The main swallow holes where the Nam La River entirely sinks into subsurface were selected for injecting the tracers. Uranine was injected at the last stretch of the Nam La River, at the Nha Tu cave; sulforhodamine B was injected at Ban Ai swallow hole just 1.5 km before the Nha Tu cave. The Hang Doi spring/cave, located about 5 km on the other side of the Khau Pha pass (towards the east), was selected for sampling during the experiment. This is the main spring in the Son La area (average discharge of 8.58 m3/s), and is located on the boundary of the limestone of the Dong Giao Formation and non-carbonate rocks of the Nam Tham Formation. The water samples were taken at short time interval of 30 min to 60 min for 4 days.

Table 4.2: Summary of tracer experiments in the Son La karst area, Son La province.

42

The Son La karst area

February 2003: Nam La valley. This qualitative tracer test had the same goal as the previous test carried out in October 2001. Uranine and sulforhodamine B were also used in this experiment. Again, the Ban Ai swallow hole was selected for injecting the uranine. Sulforhodamine B was injected at the Lom Co Co sink hole/cave (Fig. 4.4 and 4.6). The Hang Doi spring was observed for 4.5 days.

4.4.2 Tracer sampling and analysis

The tracer samples were collected in 50 ml plastic bottles. To protect the samples from the light, sample bottles were covered by aluminium paper and stored in dark conditions. The discharge of the tests on February 2000 and October 2001 were obtained by measuring water levels in function of the existent stage-discharge curves. The discharge of the October 2000 experiment was measured by estimated flow velocity multiplied by spring cross-section area. During this test, the electrical conductivity (EC), which indicates approximate concentration of total dissolved solids, was measured in situ by an EC - meter. The tracer samples of the Suoi Muoi test were transported and analyzed in Belgium. The dye tracer samples of the Bon Phang and Nam La tests were detected by Quantech digital filter fluorometer FM 109510-33 at RIGMR. This equipment is designed to perform analytical quantitative fluorescence measurements of various fluorescent materials. Salt tracer samples were analyzed by atomic absorption spectrometry at the Geological Analysis Centre, Vietnam Geological Survey.

4.4.3 Results

Figure 4.6 maps the proved karst groundwater connections in the Son La area resulting from the tracer tests. The uranine and salt injected at swallow holes of the Suoi Muoi River were detected at the Ban Sang spring. The injected tracers (uranine and salt) at the Ban Lay cave also reached the Long Ngo spring. The injected uranine at the Nha Tu cave and injected sulforhodamine B at the Lom Co Co were detected at the Hang Doi spring. Only the injected tracers at the Ban Ai cave produced negative results because of difficulties in monitoring. The measured tracer concentrations at the sampling points versus time are shown in Fig. 4.7, 4.8 and 4.9.

43

Chapter 4

Fig. 4.6: Tracer location and proven groundwater flow connections.

4.4.3.1 The Suoi Muoi valley test The uranine injected at the Tham Han cave first reached the Ban Sang spring after 17 h; while the salt injected at the end of Suoi Muoi River arrived in the spring after 20.5 h. The maximum tracer concentration was attained 24 h and 26 h after injection for uranine and salt respectively. The fastest trace velocity ranges from 95 m/h to 118 m/h, and the dominant tracer velocity ranges from 75 m/h to 81 m/h. The percentage recovery is 96% of injected uranine and 71% of injected salt. The Traci95 program is used for modeling the tracer breakthrough curves and for determining the hydraulic properties of the underground flow paths. The theoretical tracer breakthrough curves are simulated by best-fit method on the basis of measured concentrations. The multidispersion-model was applied. This model hypotheses that an injected tracer splits up into several tracer clouds, which are transported simultaneously through spatially separated flow paths characterized by different flow velocities and dispersions. For these experiments, the tracer breakthrough curves can be modeled with two peaks/tracer clouds (Fig. 4.7). The Peclet numbers vary form 33 to 147; and the longitudinal dispersivities range between 13 and 58 m (Table 4.3)

44

The Son La karst area

Fig. 4.7: Measured tracer concentrations at Ban Sang spring for the February 2000 test and theoretical breakthrough curves modelled using Traci95 (left uranine, right salt).

4.4.3.2 The Bon Phang test Tracers injected at the Ban Lay cave were found at the Long Ngo spring. Figure 4.8 shows measured tracer concentrations versus time. The first appearance of salt at the Long Ngo spring occurred 7 h after injection; the uranine arrived at this spring 7.5 h after injection. Maximum tracer concentration was reached 2 and 3 h after the first appearance of salt and uranine respectively. The calculated fastest tracer velocity ranges from 200 to 214 m/h, and the dominant tracer velocity ranges from 143 to 166 m/h. Percentage injected tracer recovery is 29 % for uranine and 44 % for salt. The Traci95 is also performed to simulate theoretical tracer breakthrough curves and the hydraulic properties of the groundwater flow connections. A single fissure-dispersion-model was applied in this case. This model was originally made for fissured aquifers with a porous matrix. Tracer transport is restricted on a single fissure or a series of parallel fissures. This implies that, if the mean transit time of water is sufficiently short, the tracer has no time to diffuse into the matrix deep enough to be affected by adjacent fissures. The model can also be applied for karst aquifers. The main conduits correspond to the single fissure in the model (Goldscheider, 2002). The Traci95 program simulated a single peak tracer breakthrough curve (Fig. 4.8). The obtained hydraulic properties are shown in Table 4.3.

45

Chapter 4

Fig. 4.8: Measured tracer concentrations at the Long Ngo spring for the test in October 2000 and theoretical breakthrough curves modelled using Traci95.

4.4.3.3 The Nam La valley tests The uranine injected at the Nha Tu cave in October 2001 reached the Hang Doi spring. The injected sulforhodamine B at the Lom Co Co swallow hole in February 2003 was also detected at the Hang Doi spring. The injected tracers at the Ban Ai swallow hole, however, were not detected at the Hang Doi spring either in the October 2001 or February 2003 experiments.

Fig. 4.9: Measured uranine concentrations at the Hang Doi spring during the October 2001 test, and theoretical breakthrough curves modelled by using Traci 95.

The uranine first arrived at the sampling point (Hang Doi spring) after 17 h. Figure 4.9 presents the measured uranine concentrations versus time. The time to maximum uranine concentration is 25.5 h after injection. The uranine recovery rate is 42%. As in the test in the Bon Phang area, a single fissure-dispersion-model was applied to simulate the theoretical

46

The Son La karst area

tracer breakthrough curve (Fig. 4.9). The obtained hydraulic properties of the sub-surface water flow path are shown in Table 4.3. During the test in February 2003, the sulforhodamine B reappeared at the Hang Doi spring in the second day of observations. This was the qualitative tracing test; however, in the present case, the detected sulforhodamine B tracer at the sampling point further indicated the flow connection. The result of this test did not only prove the existence of the karst groundwater flow connection between the Lom Co Co swallow hole and the Hang Doi spring; it also provided information about the tracer transition time at the spring after more than 24 h.

Table 4.3: Overview obtained tracer results and estimated hydraulic parameters of karst groundwater flow paths in the Son La area.

4.4.4 Discussion

4.4.4.1 The tracer recovery rate The obtained tracer recovery rates vary from 29% to 96 % for different tests (Table 4.3). It is clear that all the tests have tracer losses. There are several possible explanations for apparent loss of tracer as well as for the major difference in recovery rates between the experiments. It seems very obvious that the losses could be due to the presence of some unmonitored spring(s) in the system, since the tracer tests were monitored at only one expected outlet. The remaining reasons are then that: (1) mass balance error due to crudely estimated injected tracer mass, (2) the missing tracers were adsorbed by some substances within the system, (3) sampling and analytical errors. Naturally, one, two or even all possibilities can occur at the same time.

47

Chapter 4

Salt and especially dye tracer are affected by some substances within the system that causes losses in tracer mass. Such losses are due to either adsorption of dye on suspended sediment, karst conduits wall, or suspended organic matter in the system, or to the diffuse or flow of some of the tracer from the cave passages into water-filled voids in the surrounding bedrock (Atkinson et al., 1973). Cations of salt are subjected to a loss due to ion exchange; anions are subject to a marked sorption in aquifers with organic layers also (Käss, 1998). Using tracer in tropical karst regions like the Son La area could strongly be affected by the above considerations; the tracer recovery mass would be always lower than the injected mass. For the October 2000 test, because the fluoresent analytical equipment was not available, the dye water samples were stored at room temperature (in dark condition) for 8 months before being analyzed; while salt samples were analyzed only a few days after sampling. The uranine recovery rate is low, even lower than the salt recovery rate in this case (Table 4.3); this could be due to of microbiological decay. 4.4.4.2 Hydraulic properties Worthington et al. (2000) mentioned that the karst conduit velocities are usually between 4 m/h and 416 m/h, with averages of about 70 m/h. The estimated velocities in the Son La area vary from 75 to 166 m/h (Table 4.3). These velocities are relatively fast even for karst groundwater flow and indicate a low resistance flow route. The Peclet numbers vary from 95 to 147.5, which indicate an advection mass transport controlled karst conduit system in the area. The tracer breakthrough curves of the February 2000 experiment (Suoi Muoi test) have two tracer peaks, which indicate the existence of two karst conduits in the underground system. The first peak stands for the main conduit with high concentration passed at high flow velocity, and the second corresponds to a branch conduit with lower flow velocity. The long longitudinal dispersivity in the branch conduit can be due to the existence of a reservoir in this conduit. The simulated tracer breakthrough curves in the Bon Phang test shows only one peak. In addition, the dominant tracer velocity and short breakthrough curve suggest a single karst conduit without major bifurcation.

48

The Son La karst area

A one peak tracer breakthrough curve was also observed in the October 2001 test in the Nam La valley. The tracer breakthrough curve suggests the existence of a single karst conduit between the Nha Tu cave and the Hang Doi spring. 4.4.4.3 Salt (sodium chloride) tracer and measured Electrical Conductivity As mentioned in the tracer section (4.3.1), the electrical conductivity (EC) was measured in situ in conjunction with the water sampling at the Long Ngo spring during the October 2000 test. Figure 4.10 plots the measured EC and rainfall recorded at the Son La station versus time. It clearly shows that the behaviour of measured EC at the Long Ngo spring matches the salt tracer arrival, and in this case, could be considered as an "EC-breakthrough curve". The time of the first EC increase and maximum EC values attained (Fig. 4.10) coincide almost perfectly with the times of first and of maximum salt arrived (Fig. 4.8 and Table 4.3). This observation is useful for further testing using salt tracer and could reduce analysis cost. Instead of analyses salt samples, the measurement EC could be used to determine the salt content in this karst area.

Fig. 4.10: Measured electrical conductivity (EC) at the Long Ngo spring and rainfall recorded at the Son La station during the October 2000 tracer test.

Additionally, a comparison of the measured EC at the Long Ngo spring with rainfall data also provides useful information on hydrological function of this karst system. The EC and rainfall

49

Chapter 4

plot on Fig. 4.10 can be considered as a "simple chemograph". The decrease in conductivity is due to immediate influx of rainfall at this spring. The dilution effect indicates the arrival of newly infiltrated rainwater; it also suggests that this karst spring flow is following concentrated (allogenic) recharge at swallow hole.

4.5 Hydrochemistry

4.5.1 Hydrochemistry and karst water quality

The physical properties and major ions content of karst water in the Son La area vary considerably. The pH values range from 5.3 to 7.9; the average temperature is 22oC. The ions Ca2+, Mg2+ and HCO3- are dominant in all sampling points in the area (Table 4.4). The water chemistry in the area can be classified into two chemical types: Ca-Mg-HCO3 and Ca-HCO3 (Fig. 4.11).

Table 4.4: Physical properties and major ions content (mg/l) of karst water in the Son La area (VIBEKAP data).

50

The Son La karst area

Fig. 4.11: Piper diagram of karst rivers systems in the Son La area ; the black triangle symbol presents for the Nam La River water system; the grey cycle symbol is for the Suoi Muoi River water system.

Within the karst groundwater flow path, which was proven by the tracer test, the water type at the swallow hole(s) is similar to the water type at the connected spring. For instance, the karst water at the Ban Lay swallow hole and the corresponding connected Long Ngo spring has a chemical type of calcium-bicarbonate; the water at the Tham Han swallow hole and the connected Ban Sang spring has a chemical type of calcium-magnesium-bicarbonate. Calciummagnesium-bicarbonate water is also present at the Nha Tu swallow hole and connected Hang Doi spring. The two water types (calcium-bicarbonate and calcium-magnesium-bicarbonate) confirm the presence of limestone and dolomite in the carbonate rocks of Dong Giao formation, which was suggested by Tuyet et al. (1996). The calcium-bicarbonate water resulted mainly from dissolution of pure limestone rocks, while the calcium-magnesium-

51

Chapter 4

bicarbonate resulted either from dissolution of dolomite or/and from the basalt rocks in catchment area of swallow holes and springs. The major Tham Ta Toong spring, which supplies 50% of the drinking water to the Son La town, has a water chemical type of calcium-bicarbonate. This spring water has a TDS value of 344 mg/l, a pH value of 5.8 and a temperature of 22oC (sample 7 in the Table 4.4). The concentration of NO3- is 1.9 mg/l, which is lower than the WHO standards for drinking water (NO3- < 50 mg/l). The concentrations of Na+, K+, SO42- are low (< 4.5 mg/l). In terms of major hydrochemistry, this karst spring meets WHO standards for drinking water. Figure 4.12 shows the different dissolved carbonates species (H2CO3, HCO3- and CO32-) as a function of the pH (Fetter, 2001). At a pH of 6.3, the activities of HCO3- and H2CO3 are equal. With pH>6.3, HCO3- becomes the predominant species, and at pH< 6.3 there is more H2CO3. The same relation for the CO32- and HCO3-, the two species have equal activity at pH of 10.3. The Tham Ta Toong spring (and several other springs) in the Son La area has pH value smaller than 6.3, which indicates the carbonic acid (H2CO3) is the predominant species. The more carbonic acid is available at this spring, thus, the more carbonate dissolves as reaction below: CaCO3 + H2CO3 Ca2+ + 2 HCO3-

Fig. 4.12: Species of dissolved inorganic carbon as function of pH (Fetter, 2001).

52

The Son La karst area

4.5.2 Oxygen isotope

4.5.2.1 Water sampling and analysis The water was sampled at the Nam La River area in the rainy season from July to October 2002. All samples were instilled with a few drops of HgCl2 and stored in 50 ml plastic bottles.

Fig. 4.13: Location of sampling stations for isotope study in the Nam La River area, Son La province.

To collect rainfall samples, two rainfall stations were installed. Figure 4.13 shows the location of the sampling stations. The first station (station 1) was installed at the Ban Hin village, while the second station (station 2) was set up at the Cong Doan hotel in Son La town. Station 1 is located on the small hill near the Son La pass at an altitude of 850 m. Station 2 is sited on the roof of the Cong Doan hotel in the centre of Son La town, about 3 km from station 1 at an altitude of 610 m. The meteoric samples were collected on a weekly schedule at both stations. The Nam La River water was sampled directly in the river near the Ban Ai swallow hole/cave (Fig. 4.13). Karst spring water samples were collected at the major Tham Ta Toong spring and Hang Doi spring. The river and spring water samples were taken at 4 days intervals. The oxygen isotope samples were prepared and analyzed at Laboratory of Stable Isotopes, Department of Geology, VUB, by isotope ratio mass spectrometry using the CO2/H2O

53

Chapter 4

equilibration method (section 3.4). Standard samples and duplicate samples are used to ensure accuracy of results. The calculated reproducibility (precision) for the samples is 0.147 0/00, and the standard error on a sample is estimated as 0.3 0/00. 4.5.2.2 Results and discussion The oxygen isotope results are displayed in Table 4.5 and Fig. 4.14 for all sampling sites. The results show that the 18O of meteoric water (from July to October 2002) varies considerably. The 18O of station 1 (Ban Hin village) and station 2 (Cong Doan hotel) vary from -12.3 to -6.4 (mean ­9.5), and from -9.8 to -4.13 (mean -6.9) respectively. This strong variation is encouraging for using 18O to investigate mixing of water in the karst aquifer.

Table 4.5: The 18O of meteoric water, river and karst spring water at the Nam La valley, Son La (July-October 2002) (location: Fig. 4.13).

54

The Son La karst area

Precipitation at higher altitude will be isotopically depleted, representing the so-called altitude effect or elevation effect. The 18O depletion generally varies between -0.15 and -0.5 per 100 m rise in altitude (Clark and Fritz, 1997). The 18O results of meteoric water show that an altitude effect is clearly observed in this area. The meteoric water collected from two stations at altitudes of 610 m and 850 m, respectively, shows an altitude effect of (18O) ­1.1 per 100 m rise. Surprisingly, there is no correlation between the measured oxygen composition and the rainfall volume. The amount effect, that the water during heavy rainstorms is more depleted than during moderate intensity rainfall, is not observed in this case. This may be due to the low sampling resolution. On the other hand, the 18O of samples collected at the Nam La River and at the two karst springs (July­October, 2002) shows less variation (Fig. 4.14). The average 18O of the Nam La River is -7.5, with a variation of ­8.1 to ­6.2. The average is -7.5, varying from -7.7 to -6.9 for the major Tham Ta Toong spring, and the average for the Hang Doi spring is -7.6, varying from -7.9 to -7.1. The 18O values of karst springs are quite constant during the rainy season in comparison with the meteoric water, which could explain that this karst system has well-mixed groundwater and that the rainfall water (from station 1 and station 2) has no direct significant effect on those karst springs. Recharge and groundwater flow in karst aquifers are typical because of a generally present dual porosity (Ford and Williams, 1989; Clark and Fritz, 1997; Lakey and Krothe, 1996). Diffuse recharge and point (concentrated) recharge are possible co-existing even in a single karst aquifer; conduit flow and diffuse flow are also similarly present. Consequently, in this case, rainfall water may not enter the karst springs rapidly through conduit flow, but it can occur after long circulation through diffuse flow, where water seeps in slowly along small joints and fractures. Nevertheless, Fig. 4.15 shows the influence of the oxygen isotope composition of meteoric water on the Nam La River water. Figure 4.15 shows that the 18O peaks at the two stations had a significant effect on the Nam La River after a delay of one week. This observation supports the idea that the Nam La River water has been mixed with rainfall water, and that the resident time for this mass transport of rainfall water to appear in the river is about one week. This observation also seems to confirm other results published by Tam (2003) that were carried out in this valley.

55

Chapter 4

Fig. 4.14: Oxygen isotope composition of rainfall, river and spring water at the Nam La valley, Son La (July-October, 2002).

Fig. 4.15: Influence of oxygen isotope composition of rainfall water on the Nam La River water

56

The Son La karst area

4.6 Conclusion

4.6.1 Hydrogeology and underground flow paths

The hydrogeology in the area is characterized by the Dong Giao (aquifer 1) and the Chieng Pac-Ban Pap (aquifer 2) karst aquifers. The non-carbonate Nam Tham Formation occurs in a small surface in a NW-SE direction acting as a barrier that divides the two karst aquifers. Faults, joints and caves systems have a great effect on the groundwater flow system. The geological structure extensively presents faults systems in the NW-SE, NE-SW, E-W and N-S orientations, in which many of the fault structures may form barriers restricting or diverting the lateral movement of groundwater. Cave expedition data (Belgian­Vietnamese Cave Expedition) show that many swallow hole caves often developed in the NW-SE, while the spring caves developed in the NE-SW direction. Tracer tests in the Suoi Muoi and Nam La valley (in the Dong Giao karst aquifer) proved the flow connection between swallow hole caves and springs caves in this karst aquifer. It also indicates underground drainage patterns that run across folds in the direction of the cave development. On the other hand, the tracer test in the Bon Phang area (in the Chieng Pac-Ban Pap karst aquifer) confirmed the underground water flow path inflow in the NW to SE direction along the major NW-SE fault. The results that were obtained from the tracer test also suggest that concentrated (point) recharges input to karst springs via conduit karst systems are present in the area. Nevertheless, the stable isotopes results suggest the diffuse recharges input to karst springs through joint and fractures systems are also occurring. The measured discharge (by VIBEKAP project) at the Suoi Muoi and Nam La River systems indicates that the swallow holes discharge rate accounts for 60% of the connected spring discharge rate. The results indicate that point and diffuse recharges in addition to karst water stored within the aquifer (in vadose and phreatic zones) all contribute to the large yield of the springs in the area.

4.6.2

Hydraulic properties and groundwater quality

The groundwater flow velocities as measured in the Son La tracer tests are relatively high for tropical karst. The Pelect numbers vary from 33 to 147, which indicate an advection mass transport control in the area's karst sub-surface flow system. Fitted tracer breakthrough curves point to the existence of main and tributary karst conduits in the case of Suoi Muoi and a single karst conduit without major bifurcation in the Bon Phang and Nam La areas.

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Chapter 4

The chemical content shows a high variation between the karst springs, but their values are still lower than recommended limit for drinking water. The chemical parameters at the Tham Ta Toong spring matches to the standards for drinking water. Hence, microbiological monitoring at short time interval in the main karst springs that supply drinking water should be considered.

4.6.3 Groundwater mixing

Figure 4.16 shows the relation between Ca2+ and Mg2+ contents of the swallow hole and the connected spring. The concentrations generally increase along karst groundwater flow paths due to chemical dissolution of carbonate rocks. However, only carbonate dissolution processes along the studied karst conduits can not offer such high increasing concentrations in the short residence time (proved by tracer tests) and in present physical conditions of the underground water systems. Therefore, the remarkable difference can be explained by a groundwater mixing effect in the underground water systems. The karst flow conduits coming from the Ban Lay, Nha Tu and Tham Han swallow holes respectively to the Long Ngo, Hang Doi and Ban Sang springs could be mixed to the deep karst water that have a much higher chemical content.

Fig. 4.16: The Mg2+ versus Ca2+ concentrations at swallow holes and connected springs in the Son La area; the dot lines indicate the existence of underground flow connections, which was proven by the tracer test. Flow connection 1: Ban Lay-Long Ngo, flow connection 2: Nha Tu-Hang Doi, flow connection 3: Tham Han-Ban Sang.

58

The Son La karst area

The oxygen isotopic results further support the interpretation of mixing effect. The stable isotope composition at the Hang Doi and Tham Ta Toong karst springs during the rainy period (Fig. 4.14) also suggests that karst springs are evidently composed of mixed groundwater.

Table 4.6: The molar [Mg2+]/[Ca2+] ratios for swallow hole and connected spring waters from the Son La area.

Moreover, table 4.6 shows that along a flow path, Mg2+, Ca2+ and total dissolved solid (TDS) content increase, while the pH slightly decreases. The molar [Mg2+]/[Ca2+] ratios for swallow hole and connected spring waters are rather stable, and vary around 0.3-0.4. This observation could suggest that water-rock interaction with dolomite may have occurred or that the process of dedolomitization, which is the simultaneous dissolution of dolomite and precipitation of calcite (Saunders and Toran, 1994; Appelo and Postma, 2005), may take place in carbonate aquifers in this area. The process of dedolomitization is given by the following chemical reaction: CaMg(CO3)2 (dol.) + Ca2+ 2CaCO3 (cal.) + Mg2+

According to Appelo and Postma (2005), the increasing Ca2+ concentration due to gypsum dissolution induces calcite to precipitate. The CO32- concentration decreases because of calcite precipitates, and this stimulates the dissolution of dolomite and an increase of the Mg2+ concentration. In case of equilibrium with calcite and dolomite, the [Mg2+]/[Ca2+] ratio gets about 0.8. The ratio of [Mg2+]/[Ca2+] becomes 0.3 when groundwater is supersaturated with respect to calcite (Appelo and Postma, 2005). One of the reactions, proposed by the later authors, is given below: 1.3 CaSO4 + 0.3 CaMg(CO3)2 0.6 CaCO3 + Ca 2+ 0.3 Mg2+ + 1.3 SO42-

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Chapter 4

The constant slope of increase in Mg2+ and Ca2+ along the groundwater flow paths, which exhibit by parallel lines in Fig. 4.16, can be explained by the mixing of the same original deep groundwater in studied karst system. It is possible that the dedolomitization process occur in Chieng Pac- Ban Pap and Dong Giao carbonate aquifers, which indeed contain homogeneous dolomite.

60

The Tam Duong karst area

5 Hydrogeology of Tam Duong karst area

5.1 Location, topography and climate

The Tam Duong area is situated in NW Vietnam, about 500 km in a NW direction of the capital Hanoi. The area (Tam Duong town) has been the centre of the Lai Chau province since 2003. The area has spectacular karst tropical landforms (Fig. 5.1) characterised by peak clusters-depression peak forests, swallow holes and karst springs. The topographical elevation ranges from 800 to 1400 m. A karst valley is located in the central area. To the SW, the area is limited by the Nung Nang mountain range, to the NW, by new Phong Tho town and to the NE, by the Nam So River. The central part of the area is the Tam Duong town.

Nung Nang

Sung Phai

Tam Duong Tam Duong

Fig. 5.1: View of the test site from NE (left) and from SW (right).

The area has a semitropical climate, heavily influenced by the monsoon regime with a wet season from May to August and a dry season from November to February. The months from March to April and from September to October are the transition time from one season to another; when precipitation and temperature can be variable. The average annual air temperature is 20°C. Humidity in the dry season is between 70 and 80 %, and between 80 and 90% in the wet season. The mean annual rainfall is 2622 mm (Fig. 5.2). This area is one of the most poorest and remote regions of Vietnam. Ethnic minority groups, including H'Mong, Dao, Man, etc., live in the area. Maize, rice and tea planning are the most important agricultural products in the area. Transport conditions are difficult; it takes more than one day to go from Hanoi to the test site.

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Karst plays an importance role for local economic and living conditions. The local population is largely dependent on water from the karst springs, which is used for drinking and for agriculture activities. The spectacular karst landscape and ethnic culture in the area are potential attractions for tourism.

Fig. 5.2: Measured precipitation and temperature in the Tam Duong area from 1996 to 2000 (reference data: Japanese Mining Project, 2002).

5.2 Overview of previous studies

Studies were carried out (Bui Phu My et al., 1978; Tran Van Tri et al., 1979; To Van Thu et al., 1996) to draw the geological maps (scale 1:200 000 and 1:50 000) and to investigate geological characteristics of the area on a regional scale. Several research projects aimed at setting up hydrogeological maps and hydrogeological schematic maps at scale 1:200 000 and 1:50 000 were also carried out in the area. The information on hydrogeology was mentioned in general and descriptive terms only. The most detailed geological investigation in this area was executed by the VIBEKAP project in 2000 -2003. This project also focused on speleological investigations. The hydrological-hydrogeological investigation in the area was only briefly considered with little observation. The knowledge on karst hydrogeology in the study area is still limited.

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The Tam Duong karst area

This work presents the detailed observation and an insight into understanding of karst hydrogeological characterization of the area. The information on karst water quality in terms of hydrochemistry and microbiology, degree of pollution and source of contamination will be provided. Karst groundwater flow connections will be mapped. The characterization of underground water transport and the dynamics of the karst flow system in reaction to precipitation events will be considered. The work also discusses the adaptation of investigated methodology to the typical conditions of the study area.

5.3 Geology

The following geological characteristics are described in accordance with the results of the previous studies in the area.

5.3.1 Geological framework

The Tam Duong area is situated on the Da River rift, which is one of the main structural units of the Tay Bac fold system. This structure is bounded by the Phan Si Pang anticline to the NE, and by the Ma River anticlinorium to the SW (Fig. 2.1). The test site, in particular, is located at the NW end of the Da River rift. The Da River Rift formed during the Upper Palaeozoic by the extension of the continental crust along NW-SE striking faults. The rift is composed of 3 stages: pre-rift, rift and post ­ rift. The rocks of pre-rift stage do not occur in the study area. The volcanic, terrigeneous, carbonate rocks of the Vien Nam, Tan Lac, Dong Giao Formations, which are mainly present at the test site, belong to the Rift stage. The terrigenous molasse rocks of the Suoi Bang, Yen Chau Formations were formed at the post-rift stage.

5.3.2 Stratigraphy

The area mostly consists of Triassic sedimentary rocks of the Tan Lac and Dong Giao Formations (Table 5.1). The Middle Triassic Dong Giao Formation covers a major part of the test site, while the early Triassic Vien Nam Formation and the late Cretaceous Yen Chau Formation and Quaternary deposits cover only a small part (Fig. 5.3).

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The Vien Nam Formation consists of basalt, alkaline tholeitic basalt, and alkaline olivine basalt about 1150 m thick. This Formation outcrops on the right side of the Nam So River from Ta Leng to Then Xin. The formation is overlain by the Tan Lac Formation. The Tan Lac Formation consists of sandstone, tuffogeneous sandstone, siltstone, and claystone with interbeds of calcareous sandstone, claystone and limestone lenses. The rocks are grey, pinkish grey or light grey. The total thickness ranges from 300 to 600 m. At the test site, the Tan Lac Formation outcrops in the Sung Phai anticline, trending to the NW-SE for about 10 km in length and 2 km in width. This formation also outcrops in Nung Nang (Fig. 5.3).

Table 5.1: Stratigraphical table of the Tam Duong area.

The Dong Giao Formation covers a major part of the investigation area, forming a continuous belt from the Muong So and Lan Nhi Thang karst plateau through the Tam Duong, Nung Nang and Ban Giang in a NW-SE direction (Fig. 5.3). This formation is underlain by the Tan Lac Formation. The Dong Giao Formation is divided into a lower and an upper subformation. The lower sub-formation contains grey calcareous shale, marl, and thin-bedded limestone. The upper sub-formation contains fine to medium grained, clean, thin bedded (5-10 cm), thick bedded (40-50 cm) or even massive limestone changing from dark to light grey. The total thickness ranges from 300 to 800 m. The Yen Chau Formation consists of continental, fluvial rocks, e.g. red-coloured conglomerate, sandstone, gridstone, siltstone, claystone with some interbeds of calcareous conglomerate and gridstone. The formation has a total thickness of 1500 m. The formation outcrops along the SW boundary of the study area (Fig. 5.3).

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The Tam Duong karst area

Several massifs of the Pu Sam Cap intrusive complex outcrop along the right side of the Nam So River. This alkaline complex consists of crystalline syenite, quartz syenite and granosyenite. Quaternary deposits are present in a small part of the study area (Fig. 5.3). The Quaternary deposits crop out along the Nam So valley and Ban Giang valley. Quaternary deposits consist of gravels, sand and clay with a thickness of only 1-2 m.

Fig. 5.3: Geological map and geological cross section in the Tam Duong area (modified after VIBEKAP, 2003). The number 1 represents Dau Nguon Sin Ho spring; and number 2 represents Nha May Che spring. The symbols I, II and III represent Nam So, Lan Nhi Thang-Hong Thu Man and Yen Chau faults respectively.

5.3.3

Tectonics

5.3.3.1 Folding characteristics The folds in this area were described by Bui Phu My et al. (1978) as the "Lan Nhi Thang" fold system. These folds are formed in a NW-SE direction and characterised by the

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Chapter 5

terrigeneous rocks of the Tan Lac Formation in the core, and the Dong Giao limestone in the limbs. The VIBEKAP geologists described this fold system in more detail: it is a large anticline, complicated by higher rank folds in the shape of an overturned fold. Axial planes are, in general, inclined to the NE, with the SW limb steep and overturned, and the NE limb gentler, while the fold axis gradually plunges to the SE. The main anticline occurs in the non-carbonate rock of the Tan Lac Formation. To the SE of this main anticline, the area is characterised by a series of anticlines and synclines. It has been observed that, near the Lan Nhi Thang and Suoi Thau villages, the Dong Giao limestone has been removed (see geological cross section); the area is formed by the non-carbonate of Tan Lac Formation at altitudes higher than 1000 m. This may indicate that the main anticline has been considerably uplifted in the area. Lower down, near the Nam Loong and central town, the Tan Lac Formation is also uplifted but is not yet wholly exposed on the ground surface (VIBEKAP, 2003). From Tam Duong town, the fold axis plunges gradually to the SE and sharply to the NW (Fig. 5.3). 5.3.3.2 Faulting characteristics On a regional scale, the faults are dominated by NW-SE trends (Fig. 5.3). Within the investigation area, the main faults are described as follows: · The Nam So fault (I) is easily recognisable in the field as the NE tectonic boundary of the Dong Giao limestone. The fault runs parallel to Nam So valley from Ban Hon to Dong Pao, dipping to the NE and acting as a thrust fault. · The Lan Nhi Thang-Hong Thu Man fault (II) also forms in the NW-SE direction. This fault only cuts within the limestone of the Dong Giao Formation. The fault dips about 40o to the NE. · The "Yen Chau" fault (III) runs along to the border of the Dong Giao and Yen Chau Formations and is displayed as vertical rock faces on the Dong Giao limestone. Faults in a NE-SW direction are also observed at the test site. The faults show either along the river valleys or as a linear line between the shallow holes and springs. The lineament study based on the enhanced image and geophysical data in the area presented two main directions of lineaments: the NW-SE direction (300°-320°) and NE-SW direction (30°-50°) (Binh, 2003).

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The Tam Duong karst area

5.3.4 Hydrogeology, spring and surface water

5.3.4.1 Hydrogeology The hydrogeology of the Tam Duong area is characterised by the main karst aquifer of the Dong Giao Formation, which is underlain by the low permeable layer of the Tan Lac Formation (Fig. 5.3). The Dong Giao consists of limestone covered by shallow soils. The lineaments and caves are well developed in this Formation. Many springs (Fig. 5.4) occur within this formation and are the main drinking water source in the area. The Tan Lac Formation consists of clay and shale, and is characterised by low permeability. This Formation acts as an aquiclude on the base of the Dong Giao karst aquifer. The Yen Chau Formation consists of sandstone, claystone and conglomerate. This Formation is characterised by low permeability and high surface runoff. However, this Formation outcrops only in the SW border and plays no significant role in the hydrogeology of the area. The Vien Nam Formation and the Quaternary deposits outcrop further to the NE side and along the Nam So River respectively. These Formations, therefore, play no significant role in the hydrogeological characterization of the study area. About 75% of the study area is formed by the karstified Dong Giao limestone and belongs to the zone of open karst. The groundwater is unconfined. To the NE side of the main anticline, the base of the karst aquifer is below the Nam So River and forms a deep karst. To the SW side of the main anticline, the base of karst aquifer above the valley base or sometimes below the valley depends on the folds structure. Mixed forms of shallow and deep karst are characteristic in this case. 5.3.4.2 Springs and surface water Field observations show that the surface water in the Tam Duong area can be separated into two different flow systems: one from the Hong Thu Man passes to the north, and another, from Hong Thu Man passes to the south (Fig. 5.4). The Nam So River occurs between the karst limestone and non-carbonate on the NE side. This River flows along the Nam So fault to the Nam Xe; it then changes to a NE-SW direction and flows to the Muong So town (new Phong Tho town).

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Chapter 5

At the centre of the study area, only the Nam Loong stream is indicated on the topographic map (scale 1:50 000). However, the field observations show that the source of surface waters is either in the non-carbonate areas and disappear in limestone areas through swallow holes, or from karst springs discharge flow to non-carbonate areas. These streams are initially mapped in Fig. 5.4 on the basis of field observations.

Fig. 5.4: Karst aquifer, springs and surface water at the Tam Duong area (same area as Fig. 5.3); number 1, 2 as on Fig. 5.3; the Lo Gach, Nam Loong, C320 and Lai Chau army springs are represented by the number 3, 4, 5 and 6 respectively. The Tam Duong and Nung Nang streams are mapped on the basis of the field observations.

The most significant stream is the Tam Duong, which is fed by several small streams coming from the slope of the mountains on both sides of the Tam Duong valley; but the main inflow comes from the Dau Nguon Sin Ho spring (spring 1) and Nha May Che spring (spring 2). This stream flows along the Lan Nhi Thang- Hong Thu Man fault and combines with the Ban Giang stream and flow into the Nam Mu River. In contrast, the Nung Nang stream occurs in non-carbonate rocks near the Nung Nang village and ends at the Con Voi cave (Nung Nang cave). The Dau Nguon Sin Ho spring (spring1) is situated at an altitude of approximately 850 m on the Tam Duong valley. The spring has the form of a big pool surrounded by the Dong Giao limestone. This spring is located at the core of the anticline, where the base of the karst aquifer underlain by the impermeable layer is uplifted. The spring discharge is estimated about 200 to 400 l/s during the observation period. Before 2003, this spring was mainly used

68

The Tam Duong karst area

for drinking water for almost all town people. Now the spring is still used for drinking water but only for part of the inhabitants of the town. The Nha May Che spring (spring 2) is situated at the same altitude of the Dau Nguon Sin Ho spring, but on the other side of the Tam Duong valley on the border of non-carbonate rock and karst limestone. This spring supplies drinking water for part of the population of Tam Duong town and for the Tea processing factory. The spring discharge is estimated between 50 to 100 l/s during the observation period. Other springs are situated along the faults and/or at the place where the base of the karst aquifer is shallow, as for instance the Lo Gach and Lai Chau army spring, and the C320 and Hang Nam Loong springs. These springs generally have small discharge rates. The two biggest springs in the area are the Ban Giang and Thuy Dien springs, which are located respectively towards the NW and SE parts of the investigation area. The springs have an estimated discharge rate of approximately 400-600 l/s during the observation period. 5.3.4.3 Caves Caves are well developed in the Tam Duong area. A total length of about 15000 m of caves has been investigated by Belgian speleologists. The caves are generally developed in horizontal direction (Belgian ­ Vietnamese Caving Expedition, 2000 and 2002). Caves located around the central valley are relatively shallow, while the caves that were investigated in the Lan Nhi Thang to the NW are much deeper. The Cong Nuoc cave has a depth of -600 m, which is mentioned as the deepest cave in SE Asia (Gunn, 2004). Several caves in the area have a good potential for tourism.

5.4 Tracer experiment

5.4.1 Overview

On the 9th September 2004, a multi-tracer test using fluorescent dyes was carried out in the area. It was the first tracer test applied in this area. The main objectives of this tracer tests are: · To determine underground water flow paths and to characterise the hydraulic properties of the karst aquifer

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Chapter 5

·

To investigate the relation between stratification, geological structures and the underground flow pattern, and to provide information about contaminant transport compared with hydrochemical and microbiological data.

·

To evaluate and adapt the tracer technique for application under local conditions.

5.4.2

Injection and sampling points

The Nung Nang cave (Con Voi cave) and Suoi Thau swallow hole were selected for injecting tracers in the area. The geological data and field observation hypothesize that these swallow holes could be connected with the two main springs which are used for drinking water in Tam Duong town. At 14:30, 500 g of uranine (U) was injected at the Nung Nang swallow hole/cave. Three hours later, 500 g of rhodamine B (Rh) was injected at the Suoi Thau swallow hole. Both tracers were dissolved in water prior to injection. The Nung Nang swallow hole/cave is situated at the boundary between the impermeable rock of the Tan Lac Formation and the limestone of the Dong Giao Formation. This swallow hole/cave is located at an altitude of 1000 m. The swallow hole water has a pH of 7.9 and an electrical conductivity of 262 µS/cm. The estimated discharge is 70-120 l/s. The Suoi Thau swallow hole is also located at the boundary between the Tan Lac marl and Dong Giao limestone, but on the other side of the main anticline. This swallow hole water has a pH value of 8.1 and an electrical conductivity of 175 µS/cm. The estimated discharge is 30-60 l/s. Water samples were manually taken at the two main springs in short time intervals with the help of local people. The water was sampled continuously for 14 days at spring 1, and for 1.5 day at spring 2. The samples were collected in plastic bottles and stored in dark and cold conditions to prevent photolytical and microbiological decay. It was impossible to take water sample at all springs; hence, charcoal bags were installed at other karst springs during 14 days.

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5.4.3 Tracer analysis

The water samples and the charcoal bags were analysed at CHYN laboratory by spectrofluorometer Perkin-Elmer LS 50B. This equipment has extremely low detection limits; for instance, an uranine concentration value of 0.005 µg/l can still be detected. In principle, the equipment measures the fluorescent tracer density with excitation monochromator and fluorescence monochromator. A Xenon lamp is used as light source, which is passing the excitation monochromator at given wavelength through the water sample. Each fluorescent trace has specific absorption and emission wavelengths. The value of emission wavelength of fluorescence in a water sample passing the fluorescence monochromator is recorded as the fluorescence intensity. The different tracers have different excitation wavelength, i.e. a value of 485 nm and 550 nm for uranine and rhodamine B respectively. The fluorescence concentration then is calibrated as a function of fluorescence intensity. The equipment can detect a maximum intensity of less than 1000 mV with very small error (only 5 mV 0.05µg/l). The tracer samples have intensity higher and should be diluted. In order to improve precision during the dilution, the samples are diluted and measured at least twice; the final intensity value is then taken as the average value of the measurements.

5.4.4 Results

The uranine (U) injected at the Nung Nang cave was found at the Dau Nguon Sin Ho spring (1), while the rhodamine B (Rh) injected at the Suoi Thau swallow hole was observed at the Nha May Che spring (2) (Fig. 5.5). The tracer-breakthrough curves generated by plotting measured tracer concentration at the two springs versus time are presented in Fig. 5.6. Both uranine and rhodamine B tracer were not detected in any other karst springs in the area.

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Fig. 5.5: Tracer location and proven groundwater flow connections (detail from Fig. 5.4).

5.4.4.1 The Dau Nguon Sin Ho spring (spring 1) The uranine first arrived after 41.5 h of injection on 11th Sep at 8:00. The maximum uranine concentration reached 21.3 µg/l, and was attained on 11th Sep at 20:00, after 12 h of the first arrival. The tracer concentration was below the detection limit after 200 h. The distance between the Nung Nang cave and Dau Nguon Sin Ho spring is about 3000 m, so the calculated maximum trace velocity is 72 m/h or 1728 m/day, and the dominant tracer velocity is 56 m/h or 1345 m/day. The total discharge at the Dau Nguon Sin Ho spring measured at the 10 September 2004 by the salt-dilution method is 224 l/s. The uranine recovery at the spring corresponds to 365 g or 73%. This recovery rate is high but still lower than the 100% theoretical recovery. The discharge was estimated by salt dilution method and was used as a constant in the calculation. This estimated discharge value possibly was not very precise, and spring discharge was not steady during the tracer experiment. These missing values could influence the tracer recovery rate. It is also possible that a small amount of the injected uranine was stored and temporary delayed in the deeper karst system. Traci95 was applied to evaluate the tracer breakthrough curves and to determine the hydraulic properties. The single fissure-dispersion-model was used (Fig. 5.6). The obtained hydraulic properties are presented in Table 5.2.

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Fig. 5.6: Measured tracer concentrations at spring 1 (left) and spring 2 (right) and theoretical breakthrough curves simulated using Traci 95.

5.4.4.2 The Nha May Che Spring (spring 2) The rhodamine B arrived at spring 2 between 2.0 and 2.5 h after injection on 9th Sep at 20:00. As the sampling interval was 30 min, the time of first arrival cannot be determined more precisely. The maximum rhodamine B concentration reached 1242 µg/l, was measured in a water sample taken after 3.0h; the maximum must have occurred between 2.5 and 3.0 h. After 10.5 h the concentration was below the detection limit. The distance between the Suoi Thau swallow hole and the Nha May Che spring is approximately 1750 m; hence, the calculated maximum tracer velocity ranges from 700 m/h to 875 m/h; and the dominant tracer velocity is 583 m/h. The discharge at the Nha May Che spring is about of 70 l/s, which was calculated by the estimated flow velocity multiplied by the spring cross-section. The rhodamine B recovery at this spring corresponds to 370 g or 74%. There are probably some reasons cause the loss of mass recovered in this case. Firstly, the estimated discharge could not be precise. Secondly, the rhodamine B is a type of dye-fluocent that is more influenced by the system and loss of mass may occur (Käss, 1998). The hypothesis is that the adsorption of rhodamine B on suspended sediment in this karst system could influence the obtained recovery mass. Similar to spring 1, the Traci95 program is also used to simulate the breakthrough curve (Fig. 5.6). The obtained results are presents in Table 5.2

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5.4.5 Discussion

The tracer results have proven the existence of underground drainage passage between the Nung Nang cave and the Dau Nguon Sin Ho spring, and between the Suoi Thau swallow hole and the Nha May Che spring (Fig. 5.5). However, the tracer results also show that the two proven flow connections have different hydraulic properties (Table 5.2). The conduit velocities in karst are usually between 4 m/h and 416 m/h with averages about of 70 m/h (Worthington et al., 2000). The highest velocity noted in literature is 1450 m/h (Aley, 1975). The maximum flow velocity at the Dau Nguon Sin Ho spring is 72 m/h, which is a typical velocity for karst aquifers. The maximum flow velocity of 700 to 875 m/h at the Nha May Che spring is extremely fast for karst aquifers. Apparently, the flow from the Suoi Thau swallow hole runs directly to the Nha May Che spring as an open conduit (see Fig. 5.7).

Table 5.2: Tracer results and estimated hydraulic properties from tracer experiments at the Dau Nguon Sin Ho spring (spring 1) and Nha May Che spring (spring 2).

There was no precipitation during the experiments, the karst aquifers therefore could not be affected by surface runoff. There are probably some factors that may influence the hydraulic properties of karst drainage pattern as follow: · The Suoi Thau cave and Nha May Che spring are situated on the fold limb close to the core of the main anticline, where the non-carbonate rocks of Tan Lac Formation are uplifted. The geological cross-section (Fig. 5.3) shows that this zone belongs to the zone of shallow karst. In the zone of shallow karst the underground flow takes place near the base of the karst aquifer (Goldscheider, 2002). The underground flow pattern consequently takes place near the impermeable layer of the Tan Lac Formation. Belgian­Vietnamese Caving Expedition (2002) showed that this swallow hole/cave has a length of 1282 m, and a depth of ­102 m (Fig 5.7). This cave consequently acts

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as an open fluvial channel, largely accessible for speleologists, through which the karst water flow runs with high velocity. · In contrast to the Nung Nang area, the situation between the injection point and sampling point is quite different. The Nung Nang cave is situated on the limb of a anticline, where the base of the karst aquifer is uplifted on a local scale. There are several anticlines and synclines between the Nung Nang cave and the Dau Nguon Sin Ho spring, and the karst zone in this area is a mixture of shallow and deep karst. There are two faults system in a NW-SE and a SW- NE direction passing the area (section 5.2.3.2). The groundwater flow path in this case does not run parallel with the NW-SE faults, but crosses to the limestone along the faults in a NE-SW direction.

Fig. 5.7: Plan view and vertical profile of Suoi Thau cave (Belgian­Vietnamese Caving Expedition, 2002).

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5.5 Hydrochemistry and microbiology

5.5.1 Overview

The hydrochemical in combination with the microbiological investigation was carried out for the first time in this study area. The aim of the study is (1) to test the water quality and degree of contamination, (2) to investigate the temporal fluctuation of contamination in response to the precipitation events, (3) to characterise the karst aquifer system and to compare with tracer test data. Furthermore, the goal of this study is to test and adapt the hydrochemical and microbiological techniques in local area conditions.

5.5.2 Sample collection

Sampling was carried out every day from 17th of August to 10th of September 2004 at the main springs (the Dau Nguon Sin Ho and Nha May Che springs). Only one sample was taken at the two swallow holes and the other springs in the area. At the same time, two samples per day were taken at the two main karst springs. At all springs, electrical conductivity (EC), water temperature and pH data were also measured in situ with the water collection. Precipitation data for August and September 2004 (of the Tam Duong station) at time intervals of 6 h was collected from the National Meteorological Centre. The hydrochemical samples were collected in two small plastic bottles (13 ml). The samples for anions were filtered by Millipore filter (0.45 µm pore-size); the samples for cations were filtered and acidified by nitric acid (1.05 M). The microbial samples were collected in 500 ml plastic bottles that were sterilised by immersion in boiling water for 10 - 15 minutes.

5.5.3 Sample analysis

The hydrochemical samples were analysed at the CHYN Laboratory by ionic chromatograph method using Ionic Chromatograph Dionex DX-120 (section 3.2). The major cations Na+, K+, NH4+, Ca2+ and Mg2+, and anions F-, Cl-, NO3- and SO42- are measured. The bicarbonate was not measured in situ due to the titration was not available; then it was calculated by using AquaChem 4.0. Therefore, it is not possible and not meaningful to set up an ion balance and to calculate the analytical error in this case.

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The microbial samples were analysed at the same day of sampling by portable microbial Lab OXFAM-DELAGUA (section 3.3). The result is given in colony forming units (CFU) per 1 ml of water

5.5.4 Results

5.5.4.1 Overview

Table 5.3 presents the microbial and major ion content in the karst springs. The physical parameters of karst springs in the area are relatively variable. The average pH value is 7.6, ranging from 6.7 to 8.4; the average temperature is 20.8 o C, ranging from 19.6 to 23.9 o C and the average EC is 281 µS/cm, ranging from 120 to 460 µS/cm. The microbial analysis results show that the thermotolerant coliforms in the karst springs are common and variable; from 1 to 192 CFU were detected in 1 ml of water. The Dau Nguon Sin Ho spring (spring 1) has the highest coliforms content. The coliforms in all karst springs show a temporal variation and exceed the WHO standards for drinking water, which state that thermotolerant coliforms must be absent in a 100 ml water (Table 5.3)

Table 5.3: Microbial contamination and major ions content in 15 karst springs which are used for drinking water in the Tam Duong area, and the WHO standards. The bicarbonate was calculated by using AquaChem 4.0.

The hydrochemical results show that the ions Ca2+, Mg2+ and HCO3- are dominant in all springs in the area (Table 5.3). The water chemistry can be subdivided into two chemical types: Ca-HCO3 and Ca-Mg-HCO3 (Fig. 5.8). For all springs, the average concentrations of NO3- and SO42- are 1.8 mg/l and 4.16 mg/l respectively. These anion contents are lower than the WHO threshold standard for drinking water. Most springs have low (< 1mg/l)

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concentrations of Na+, K+ and Cl-. The concentrations of F- are often very low (average of 0.19 mg/l), and below the WHO limit (Table 5.3). The concentrations of NO3-, SO42-, Na+, K+ and Cl- in several springs are significantly higher than in other springs, but these concentrations are still below the WHO standards (NO3- < 50 mg/l). 5.5.4.2 The Dau Nguon Sin Ho spring (spring 1) Figure 5.8 presents the hydrographs and chemographs together with the results of the microbiological investigation. The water at the Dau Nguon Sin Ho spring is chemically characterised by calcium-magnesium-bicarbonate (Fig. 5.8) with conductivity values of 320380 µS/cm. The spring monitoring results (Fig. 5.9) show that, after strong rainfall events, the discharge measured as water level at spring 1 increases and the EC increases almost simultaneously. This observation indicates the first arrival of higher mineralised water from deeper zones of the aquifer due to increasing hydraulic pressure in the aquifer. About 40 h after the discharge increases in this spring, the EC decreases significantly which indicates the arrival of fresh infiltrated water. This type of behaviour is known as piston effect (Ford and Williams, 1989). The time lag between the precipitation event and the arrival of fresh water at the spring roughly corresponds to the tracer travel time from the swallow hole to the spring. The chemographs show the temporal variability of the major ion concentrations at this spring. The Ca2+ and Mg2+ concentrations are rather stable, ranging from 44.5 to 51.7 mg/l for Ca2+, and from 20.6 to 24.7 mg/l for Mg2+ respectively. The concentrations of Na+, K+, and Cl- are quite stable and below 1 mg/l. The maximum content of NO-3 is 2.28 mg/l, which is below the standards (50 mg/l) for the drinking water. The bacterial contamination at this spring was much higher than the limit with temporal fluctuations, ranging from 1 to 192 CFU/ml (Fig 5.8). Within the sampling point, there was also high difference between the maximum and minimum contamination level. The temporal contamination displayed no clear systematic variation with precipitation and also with chemical parameters at the spring. The contamination seems slightly higher after storms and also high during periods of no precipitation.

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Fig. 5.8: Piper diagram of karst springs in the Tam Duong area; the cycle symbol represents the Dau Nguon Sin Ho spring, the cross symbol is the Nha May Che spring. The triangle symbol represent other karst springs, which are used for drinking water in the area.

5.5.4.3 The Nha May Che spring (spring 2) Figure 5.9 presents the hydrochemical results together with the results of microbiological investigation in the Nha May Che spring. This spring has a chemical type of calciumbicarbonate (Fig. 5.8) with conductivity values of 120-245 µS/cm. The dilution effect, in which discharge increases almost immediately after a precipitation event while EC decreases, is observed in this spring. Figure 5.9 also shows that the temporal fluctuations of ions display no systematic variation with precipitation or with the bacteria contamination. The magnesium content at this spring was much lower than that at the Dau Nguon Sin Ho spring. The high correlation between Ca2+ and SO42- in this spring can be explained by the dissolution of gypsum present in aquifer

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rocks (CaSO4.2H2O Ca2+ + SO42- + 2H2O). The concentration of other ions such as Na+, Cl- and NO3- are similar as at the Dau Nguon Sin Ho spring: the concentrations are low, quite stable and far below the limits for drinking water. The bacterial contamination at this spring was generally lower than that at the Dau Nguon Sin Ho spring, but still higher than the limit for drinking water. The contamination showed temporal fluctuations, ranging from 1 to 39 CFU/ml (Fig. 5.10). The difference between the maximum and minimum contamination level within a sample was less than in the first spring. The temporal contamination in this spring also displays no clear systematic variation with precipitation and chemical parameters of the spring.

5.5.5 Discussion

5.5.5.1 Sources of contaminant Several studies suggest that the temporal fluctuations of bacterial contamination in karst areas are correlated with turbidity (Pronk et al., in press), discharge rate (Auckenthaler et al., 2002), or with concentrations of nitrate of springs (Scanlon, 1990). In the Tam Duong area, the temporal fluctuation of bacteria is not clearly in correlation either precipitation or hydrochemical parameters (Fig. 5.9 and 5.9). It is, therefore, impossible to predict the bacteria content based on monitoring those parameters. The temporal fluctuations of contamination at both springs are complicated because several processes may affect these fluctuations independently. The high contamination of bacteria could be due to untreated domestic waste water or agricultural activities. Bacterial contamination may come from waste dumping at swallow holes. A hospital and a tea factory in the central town are also potential sources of contamination. For instance, the rapid change in contamination level from 8 to 192 CFU/ml (Fig. 5.9) occurred in a very short time (10 h) at the Dau Nguon Sin Ho spring could have resulted only from human activities directly passing this spring, from a point-source contamination (swallow holes, dolines) entering the spring, and from surface contamination such as waste transport after a storm event. A similar situation occurs at the Nha May Che spring; the rapid change from 2 to 39 CFU/ml within 12 h (Fig. 5.10) could have resulted from human activities directly passing the spring, and from the contamination at swallow holes.

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5.5.5.2 Calcium, magnesium and resident time at two springs The difference in total Ca2+ and Mg2+ content and the molar ratios of [Mg2+]/[Ca2+] at the two springs are remarkable. The total Ca2+ and Mg2+ at the Dau Nguon Sin Ho spring (spring 1) are evidently higher than that at the Nha May Che spring (spring 2) ( Fig. 5.9 and 5.10). The main difference between the two springs is the Mg2+ content. At spring 1, the Mg2+ concentration ranges between 21.3 to 24.8 mg/l, while at spring 2 this is only 1.6 to 3.4 mg/l. These differences between the two springs can be explained by: · The difference in Mg2+ content of the two swallow holes. In swallow hole 1, 15.4 mg/l Mg2+was measured, while only 2.4 mg/l was found in swallow hole 2. The different concentrations at the springs can partly be attributed to the geochemical characteristics of the sinking stream catchments. · Mixing effect and dedolomitization process in the Dong Giao karst aquifer. The karst flow conduits from the swallow holes to the springs could be mixed with deeper groundwaters that have higher chemical content. The increase in EC, Ca2+ and Mg2+ and decrease in pH suggest that dedolomitization process may take place in this karst aquifer. The molar ratios of [Mg2+]/[Ca2+] for flow system 1 (Table 4.6) is 0.7 to 0.79, which suggests the waters have equilibrated with calcite and dolomite (Appelo and Postma, 2005). By contrast, the much lower [Mg2+]/[Ca2+] ratios for shallow flow system 2 suggest that a little water-rock interaction with dolomite has occurred in this karst system.

Table 5.4: The molar [Mg2+]/[Ca2+] ratios for swallow hole and connected spring waters from the Tam Duong area.

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Fig. 5.9: Precipitation, conductivity, water level and hydrochemical and microbiological parameters at the Dau Nguon Sin Ho spring (spring 1). The large symbols and the bold underlined numbers represent a sample taken at the Nung Nang cave at the 29.08.04.

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Fig. 5.10: Precipitation, conductivity, water level, hydrochemical and microbiological parameters at the Nha May Che spring. The large symbols and bold underlined numbers represent a sample taken at the Suoi Thau swallow hole at the 23.08.04.

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The differences between the water types can be due to hydraulic conductivity, seasonal chemistry fluctuations and TDS concentration, bearing effect on residence time (Scanlon, 1990). The short residence time at the Nha May Che spring is pointed out by the low electrical conductivity (average 196 µS/cm) and large seasonal fluctuations of Ca2+ (from 27.4 to 48.8 mg/l). In contrast, the longer residence time at the Dau Nguon Sin Ho spring is indicated by higher electrical conductivity (average 352 µS/cm) and lower seasonal chemistry fluctuations of Ca2+ and Mg2+ (from 65.9 to 76.5 mg/l). 5.5.5.3 Comparison of tracer test, and hydrochemical and microbiological results The tracer tests proved the existence of underground connections in the area. The results show an extremely rapid flow velocity at the Nha May Che spring (spring 2), and a typical karst flow velocity at the Dau Nguon Sin Ho spring (spring 1). The spring monitoring shows correlated results: the dilution effect and the short residence time in spring 2, and piston effect and the longer residence time in spring 1. In spring 1, the time duration roughly corresponds to the tracer transit time that was obtained from tracer tests. The bacteria content in a sample from swallow hole 1 was similar to what was measured at spring 1 at the same day, and the bacteria content in a sample from swallow hole 2 was similar to the content measured in spring 2 (Fig. 5. 9 and 5.10 ). This observation is consistent with the tracer test results that proved a direct connection between the swallow holes and the springs. It is also consistent with the hydrochemical data that show a high degree of similarity between the swallow holes and the springs.

5.6 Rare earth elements (REE) study

5.6.1 Sampling and analytical techniques

Carbonate rocks of the Dong Giao Formation in the Tam Duong and Nam Son (Hoa Binh province) areas were collected. Figure 5.11 shows the location of the various sampling sites in the Tam Duong area. Water from most springs located in the NW, SE and Tam Duong town were sampled. Water samples of the Yen Chau conglomerate and sandstone formation were collected from several permanent surface runoffs located in the SW margin of the area. To the NE, water samples were taken in several surface streams in the area of the Pu Sam Cap

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intrusive rock. Groundwater samples of the Dong Giao limestone Formation in the Nam Son area were also collected.

Fig. 5.11: Location of the various sampling sites in the Tam Duong area.

The pH, temperature, electrical conductivity and redox potential (Eh) were measured directly in the field at each sampling site. Water samples for REE were filtered through 0.45 µm Millipore and acidified in situ to pH<2 with ultra-pure nitric acid. The samples were analyzed at the Institute of Geology (University of Neuchâtel, Switzerland) by ICP-MS (Section 3.5). The analysis results of standard samples show that the detection limits for both rock and water samples were mostly low. The precision of REE measurements was generally lower than the 10% relative standard deviation (RSD).

5.6.2 Results and discussion

5.6.2.1 REE in karst groundwater and Dong Giao limestone The REE concentrations (in ppb) for 9 samples of Triassic limestone, which belongs to the Dong Giao Formation, are listed in Table 5.5. Concentrations (in ppb) of the REE in water from the Tam Duong area are presented in Table 5.7. The sum of concentrations of the REE, Sc and Y in the Dong Giao limestone in the Tam Duong area varies considerably between 3.45 and 139.6 ppm, while the limestone in the Nam Son area has relatively stable concentrations of < 9 ppm. Groundwaters from the Tam Duong area have concentrations between 0.76 and 6.2 ppb, while the concentrations of Nam Son karst groundwater are often below the detection limit.

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Table 5.5: REE, Sc and Y concentrations (ppb) of Triassic limestone from the Tam Duong and Nam Son areas.

Compared to some other carbonate rocks in Dinant, Belgium (Nuyens, 1992), within 9 elements in table 5.6, carbonate rocks from Tam Duong have much higher REE concentrations. Concentrations of the LREE and MREE for carbonate rocks in Tam Duong exhibit evidently higher than REE reported by Nuyens (1992). However, the REE concentrations in Tam Duong are relatively similar to REE concentrations in carbonate rocks from southern Nevada (Guo et al., 2005).

Table 5.6: Average of 9 rare earth elements concentration (ppb) in Triassic carbonate rocks from Tam Duong, and Nam Son in compared to other carbonate rocks from Dinant and southern Nevada; Dinant data are from D. Nuyens (1992), and Nevada data is from Guo et al .(2005).

Within the Tam Duong area, the REE concentrations of karst groundwater are similar to concentrations of water from granite and conglomerate (Table 5.7). These REE concentrations

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are higher than the REE in carbonate waters from the literature (e.g. Johanneson, 2000; Tang and Johanneson, 2005). These REE values, however, are also much smaller than those found in the drinking water stations that were reported by de Boer (1995); it is also lower than the REE data in the water from granite reported by Smedley (1991). De Boer et al. (1995) estimated the indicative admissible concentrations (iAC) for drinking water for Pr, Nd, Sm, Eu, Dy, Ho, Er and Lu were 1500 ppb, and for Gd and Tm were 10.5 ppb. Other elements (La, Ce, Tb, Yb, Sc and Y) had indicative admissible concentrations of 2 ppb. The REE concentrations in the Tam Duong are lower than the iAC levels. These levels were slightly exceeded with a factor of less than 2 for Sc only in few locations. Thus, it can be concluded that the REE concentrations in the Tam Duong area are generally still safe for humans who drink the water. The REE solubility in terrestrial waters is strongly controlled by pH and Eh. For the present study, however, the pH values are not much different between sampling sites (7.2<pH< 8.3). No relation was found between the pH/Eh and REE concentrations in water. Therefore, it is possible that in this case other factors have a more controlling role in REE concentrations than pH and Eh. 5.6.2.2 Shale normalized REE patterns In order to best evaluate the geochemical processes responsible for the fractionation of the REE in natural waters, it is most appropriate to normalize the waters to the rocks with which they react. In fact, natural waters may react with many different rock types that have different REE signatures. Hence, it is impossible to develop normalizing standard for every system under study (Johannesson, 1996). The concentrations of the REE in natural waters, therefore, are commonly normalized to shale. The composite shale used to normalize the rock and water REE concentrations, in this study, is the shale standard reported in Sholkovitz (1988). Figure 5.11 presents the shale­normalized REE patterns for Triassic limestone in the Tam Duong and Nam Son area. Figure 5.12 shows the shale­normalized patterns of groundwater from the Tam Duong area.

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Table 5.7: Field parameters, and Sc, Y and REE concentrations (ppb) of water from carbonate, granite and conglomerate in the Tam Duong area.

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In general, the carbonate rocks in both Tam Duong and Nam Son have flat REE patterns when normalized to shale. The patterns have slight enrichment in the MREE relative to the LREE and HREE (Fig. 5.12). For example, the shale-normalized Gd/La ratios [(Gd/La)SN] vary from 1.4 to 2.4, and the Gd/Yb from 1.6 to 3.7. The shale-normalized Sm/La ratios vary from 1.1 to 2.1, and the Sm/Yb from 1.3 to 3.3. In contrast, the groundwaters are distinctly enriched in the MREE over both the LREE and HREE. The shale-normalized ratios for Sm are greater than for all other REE, followed by Eu, Gd (Fig. 5.13). The shale-normalized Sm/La ratios range from 3 to 31, and the Sm/Yb from 6 to 79. Similar shale­normalized REE patterns (Fig. 5.13) are observed in water from granite and conglomerate rocks in the area. The patterns also have enrichment in the MREE over the LREE and HREE. These shale-normalized patterns of carbonate waters in the Tam Duong are different from those in other carbonate areas. Previous studies have presented groundwaters in carbonate aquifers exhibit either a relative flat shalenormalized patterns with slight HREE enrichments or negative Ce anomalies similar with seawater patterns (Johannesson et al., 1997; 2000). Many previous investigations noted that groundwater samples exhibiting REE patterns similar to the REE patterns of the rock through which they flow (Smedley, 1991; Banks et al., 1999). However, other authors also noted that groundwater samples showing REE patterns that differ dramatically from those of the aquifer rocks (e.g. Johannesson and Hendry, 2000; Leybourne et al., 2000). In the present case, comparing the Fig. 5.12 and Fig. 5.13 shows that shalenormalized patterns of groundwaters from karst aquifers are significantly different from the shale-normalized patterns of the host rocks. This observation suggests that the REE signatures of aquifer rock may control aqueous REE patterns by water-rock interaction processes; it also suggests that other factors such as the solution/surface complexation may also play substantial roles in controlling the aqueous REE signatures in this system.

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Fig. 5.12: Shale ­ normalized REE patterns of carbonate rocks from the Tam Duong and Nam Son areas.

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The Tam Duong karst area

Fig. 5.13: Shale-normalized REE patterns for water from the Tam Duong area.

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The origins of shale-normalized MREE enrichments in natural water were discussed by Gosselin et al.,(1992), Sholkovitz (1995), Johannesson et al., (1996), Hannigan and Sholkovitz (2001). These authors suggested a number of possible processes that maybe responsible for MREE enrichments in natural water including organic colloids, solid-liquid exchange reactions, secondary minerals and REE dissolution complexes. Depending on the particular conditions of the systems, either the phosphate, the carbonate or the sulfate complexes dominate REE speciation. Shand et al. (2005) concluded that the carbonate species predominate in water with neutral pH. However, Johannesson et al. (1996) also pointed out that sulfate complexation probably controls MREE enrichment in low pH waters. Due to lack of data, the model calculation to demonstrate the dominant species of REE in waters cannot be tested in this study. Nevertheless, the waters in the Tam Duong have a pH between 7 and 8, and total phosphate concentration cannot compete with carbonate ions concentration, suggesting that the carbonate complexes may dominate over phosphate complexes. Henderson (1984) noted that the REE distribution for the carbocernaite mineral (Ca,Ce,Na,Sr)CO3 in dolomite-calcite carbonatite has enrichment in Ce and an anomalous enrichment in Sm, while tengerite mineral [CaY3(CO3)4(OH)3.3H2O] has enrichment in Sm and other MREE. The shale-normalized REE patterns of waters in our study evidently show anomalous enrichment in Sm and also in Ce. Consequently, it may suggest the presence of carbocernaite and tengerite in carbonate rocks in the aquifer materials at the test site.

5.7 Conclusion

5.7.1 Point recharge, fault tectonics and underground flow path

The tracer experiment has proven that a spring is point-to-point connected by a swallow hole: the Nung Nang cave is connected to the Dau Nguon Sin Ho spring, and the Suoi Thau swallow hole is connected to the Nha May Che spring. A high recovery rate was obtained at both sampling points. The results obtained suggest that the area is dominated by point recharges. There is not much difference in estimated discharge between springs and swallow holes in the area, and the discharge at the springs is controlled mainly by the discharge at the swallow holes. The tracer injected in the Nung Nang area has proven there is an inflow from the SW to NE. It determines the underground drainage patterns running across to folds and along faults in a

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NE-SW direction. On the other hand, the tracer injected at the Suoi Thau swallow hole and cave data have proven that the underground flow path mainly flows from the SW to the NE and then changes slightly in a SE direction. Both NW-SE and NE-SW faults control the underground drainage patterns in the area, but the second faults dominate in this case. Nevertheless, the underground flow paths in the Tam Duong karst area are controlled by the stratigrahy in the area, hydrologic base level, and topographic conditions. The Dong Giao karst aquifer has a thickness of 300-600 m. It is underlain by an impermeable layer of the Tan Lac Formation that acts as an aquiclude. The main anticline has divided the karst aquifer into the NE and SW part. The NE area belongs to the zone of deep karst. The surface runoff from the main anticline runs to the karst aquifer via swallow holes to Nam So River. The SW area belongs to a zone of mixed deep and shallow karst. The underground water level in this area is controlled by geological structure (the series of anticlines) and hydrologic base level. The shallow groundwater base level in the Nam Loong and centre of Tam Duong results from traversal of the Tan Lac Formation in the area. The groundwater velocities in Tam Duong area calculated using the tracer test range extremely, from 72 to 700 m/h. Groundwater velocities at the Son La karst area range from 95 to 235 m/h (Table 4.3, section 4.3). The groundwater velocity at the Nha May Che spring is 700 m/h, which is the highest velocity ever recorded for a tropical karst in NW Vietnam. The single and simplified peak in the breakthrough curve at the Dau Nguon Sin Ho spring and Nha May Che spring indicates a single karst conduit without major bifurcation.

5.7.2 Dynamics and interaction of the hydrochemical and microbiological parameters

The hydro-chemographs of the two springs react differently on precipitation events. Spring 2 shows a pure dilution effect, while a piston effect can be observed at spring 1. The freshly infiltrated rainwater arrives with roughly 40 h delay, which corresponds to the transit time between swallow hole 1 and spring 1 proven by the tracer test. The water from swallow hole and spring 1 can clearly be differentiated from water from swallow hole and spring 2. The first type of water contains significantly more magnesium, which suggests the increased water-rock interaction with dolomite in this karst flow system. In the Mg2+/Ca2+ plot, water samples from flow system 1 form a cluster, which is clearly

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separated from the cluster representing flow system 2 (Fig. 5.14). The plot also shows that the Mg2+ and Ca2+ concentrations are increasing during the underground passage from the swallow holes to the springs, which can be explained by additional mineral dilution, and mixing processes within the aquifer.

Fig. 5.14: The Mg2+ and Ca2+ concentrations measured in all water samples from the Tam Duong area; the dot symbol represents Dau Nguon Sin Ho spring; the cross symbol is Nha May Che spring and triangle symbol is other springs.

Due to the temporal variability of the contamination release, the bacteria contents often show sudden and strong variations, e.g. 1­2 CFU/ml in one sample and 34­39 CFU/ml in the next sample that was taken 12 h later. The bacteria contents do not correlate with any other parameters and thus appear to be largely unpredictable. On the basis of the existing data, in the test site, it is thus not possible to use hydrological, physical or chemical data as indicators for microbial groundwater quality.

5.7.3 Groundwater quality

Groundwater in the Tam Duong area is contaminated in terms of microbiology. Contamination levels at all the springs are far too high to meet water quality standards. The

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contents of thermotolerant coliforms at spring 1 varied between 1 and 192 CFU/ml, while 1­ 39 CFU/ml were measured at spring 2. Contamination was influenced by one or many independent processes. Swallow holes, dolines, surface contamination sources and human activities may all influence contamination levels in the area. On the other hand, the chemical parameters at two of the main springs in the area were quite stable and well under the limits. The hydro-geochemistry does not vary and substantially indicates that a single measurement at other karst springs in the area could provide some information on water quality. It showed that some of the chemical parameters at other springs were higher than average values in the area, but still below the limits for drinking water. The nitrate concentrations, which were often high in the agricultural zones, were low (average of 1.8 mg/l) in this area. At two springs located close to the rice field, nitrate concentrations (11.4 and 13.1mg/l NO3-) were much higher than at other springs, but still below the WHO standard for drinking water (< 50 mg/l). The average fluorite content in the area is 0.19 mg/l (ranging from 0.018 to 0.78 mg/l), which is moderate for general karst groundwater and well below the WHO standard (<1.5 mg/l). Other physical parameters at all springs are moderate for general karst groundwater and meet the standards for drinking water.

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6 Karst Groundwater Vulnerability and Risk Mapping

6.1 The European approach: COST 620

6.1.1 Introduction

Groundwater is an important resource for human life in many regions in the world. The impact from human activities may cause to pollute groundwater resources. Carbonate aquifers are particularly more vulnerable to contamination because of typical aquifer properties. Karst groundwater thus needs special protection. In developed countries, the groundwater is considered a valuable resource that must be protected. A priority attention is given to the groundwater that is used for drinking water. However, it is not practical to demand maximum protection for large areas, as the resulting land-use restrictions would not be acceptable. Vulnerability, hazard and risk maps are valuable tools for land-use planning and groundwater protection. Various methods for vulnerability mapping have been developed and applied in recent decades: hydrogeological complex and setting methods, index models and analogical relations, parametric system models, mathematical models and statistical methods. However, the application of different methods often leads to contradicting results. Furthermore, many of the common methods do not adequately consider the specific nature of karst (Gogu and Dassargues, 2000; Goldscheider, 2002; Gogu et al., 2003; Vias et al., 2005). Therefore, the European COST Action 620 was established in order to develop a pan-European approach to "vulnerability and risk mapping for the protection of carbonate (karst) aquifers" (Daly et al., 2001; Zwahlen, 2004). The methodology or parts of it have successfully been applied in several karst areas in Europe.

6.1.2 Definitions of groundwater vulnerability, hazard and risk

The term vulnerability of groundwater to contamination was introduced by Margat (1968). Vrba and Zaporozec (1994) addressed the background concept of groundwater vulnerability based on the assumption that the physical environment provides some natural protection to groundwater against human impacts, especially with regard to contaminants entering the subsurface environment. They also mention that vulnerability is a relative, non-measurable and dimensionless property.

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There are various vulnerability definitions proposed by different authors. However, there is still no commonly agreed understanding of the term vulnerability (Goldscheider, 2002) or the definition is very similar (Brouyère, 2004). In COST 620 the following vulnerability definitions are given: · The intrinsic vulnerability of groundwater to contaminants takes into account the geological, hydrological and hydrogeological characteristics of an area, but is independent of the nature of the contaminants. · The specific vulnerability additionally takes into account the properties of a particular contaminant and its relationship to the hydrogeological system. Even though such qualitative definitions are easy to understand and practical to use, the quantitative aspects should be still considered for validation and comparison purposes (Goldscheider, 2004). The definition of vulnerability should reflect the capacity of the aquifer to reduce any type of contamination. The following questions should be answered when the pollution or contamination occurs in the catchment area: (1) where does the pollution start, (2) to which level, and (3) for how long. More details of quantitative point of view are presented by Brouyère (2004). Within the framework of groundwater protection, a hazard can be defined as a "potential source of contamination resulting from human activities" (De Ketelaere et al., 2004). Three aspects are important when evaluating hazards: the quality (toxicity, pathogeneity) of the contaminants, the quantity (mass, load) of contaminants that may be released, and the likelihood of contaminant release (chronically or accidental). A detailed rating, weighting and ranking system can be used for the evaluation of groundwater hazards. The risk of groundwater contamination takes into account different aspects: the presence and harmfulness of hazards, the groundwater vulnerability, and, in some cases, the importance or value of the groundwater. COST Action 620 proposed different types of risk assessment, including risk intensity, risk sensitivity and total risk assessment (Hötzl et al., 2004). These risk maps can be prepared for the groundwater resource or for a particular source, either for specific contaminants or on the basis of the intrinsic vulnerability map.

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6.1.3 The origin-pathway-target model

The concept of groundwater vulnerability, hazards and risk mapping for groundwater protection is based on an origin-pathway-target model. Origin is the term used to describe the location of a potential contaminant release. The pathway includes the passage of contaminants from the origin to the target. Target is groundwater surface in aquifer or water in the well or spring. There are two approaches to groundwater protection: resource protection aims on protecting the whole groundwater body, while source protection aims to protect a particular spring or well only. The two approaches are closely related to each other; protecting a source usually involves providing protection for the resource as well. For questions related to resource protection, the groundwater surface is defined as the target, and the pathway consists mostly of the vertical passage through the layers above the groundwater surface. For source protection, the well or spring is the target, and the pathway includes additionally the mostly horizontal flow route in the saturated zone (Goldscheider, 2004) The COST 620 approach to intrinsic vulnerability mapping based on the origin-pathwaytarget model, considers four factors (Fig. 6.1): the overlying layers, the concentration of flow, the precipitation regime and karst network development. According to Goldscheider and Popescu (2004), the overlying layers (O factor), i.e. the layers between the land surface and the groundwater table, may provide some degree of protection to the groundwater. In karst areas, however, allogenic recharge via swallow holes may entirely bypass these layers. Therefore, the concentration of flow (C) in the catchments of sinking streams has to be considered when assessing karst groundwater vulnerability. The precipitation regime (P) influences runoff and recharge and thus impacts groundwater vulnerability. This factor is important when comparing groundwater vulnerability in different climatic regions but less relevant for vulnerability mapping within an individual catchment. The K factor describes the hydraulic properties of the karst aquifer. Resource vulnerability maps are created by a combination of the first three factors; source vulnerability maps additionally consider the K factor. Following the origin­pathway­target model, there are three elements that identify the risk of contamination of groundwater: (1) the hazard posed by a potential activity (equivalent to origin); (2) the intrinsic vulnerability of groundwater to contamination (equivalent to pathway), and (3) the potential consequences of a contamination event (the target is the groundwater) (Daly, 2002).

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Fig. 6.1: The intrinsic vulnerability mapping is based on the origin-pathway-target model (Goldscheider and Popescu, 2004).

6.2 Methodology adaptation

6.2.1 (General) proposed methodology

The COST 620 approach has been successfully applied in several karst areas in Europe (Zwahlen, 2004). The methodology was developed by European scientists having in mind the hydrogeological conditions and data availability in their countries. In order to apply this approach to Vietnamese karst areas, it is necessary to adapt it to the local conditions. The proposed vulnerability, hazard and risk mapping are based on the concepts that are described in section 6.1.3. However, the methodology consists only of one type of vulnerability map, one type of hazard map, and one type of risk map. The number of factors was reduced and the assessment schemes were strongly simplified (Fig. 6.2) Therefore, it can be applied in areas with limited data availability and economic resources.

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Fig. 6.2: Proposed methodology for groundwater vulnerability and risk mapping (for explanation see the text).

6.2.2 Groundwater vulnerability

The first element of the proposed methodology is the groundwater vulnerability map, which is an intrinsic groundwater resource vulnerability map. The land surface is defined as the origin, the groundwater table is the target, and the pathway includes the passage from the land surface to the groundwater. Only two factors are considered: overlying layers (O) and flow concentration (C). The O factor applies for all types of hydrogeological environments, while the C factor is specific for karst (Fig. 6.2). The assessment scheme for the O factor is simple: A low protective function is assigned to situations where the aquifer is covered by less than 30 cm of soil. Thick layers of low to

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moderate permeability provide high natural protection, e.g. > 5 m of loam. A moderate protection is assigned to intermediate situations, e.g. 1 m of soil overlying the aquifer. The C factor expresses the degree to which the overlying layers are bypassed by flow concentration in the catchments of swallow holes. The first step is to determine the dominant flow process (modified after Goldscheider et al., 2000): · · · Type A: direct infiltration and percolation takes place in high permeable Formations. Type B: intermediate situations. Type C: frequent surface runoff takes place on low permeable Formations.

Surface runoff in the catchment of a sinking stream may rapidly transport contaminants from the land surface into the aquifer, while runoff towards an ordinary stream poses no threat to groundwater quality. Therefore, the second step is to subdivide the land surface into four types of zones: · Zone 1: swallow holes, sinking streams up to 1 km upstream from the swallow holes, and 20 m buffer zones on both sides of the sinking streams. · · · Zone 2: the rest of the catchment areas of sinking streams. Zone 3: areas outside the catchments of sinking streams but within the karst area. Zone 4: areas that drain laterally out of the karst hydrogeological system.

The C map is obtained by combining the map showing the dominant flow type and the surface catchment map. For porous and fissured aquifers, the O map can directly be translated into a vulnerability map. For karst aquifers, the vulnerability map is obtained by overlying the O and C maps (Fig. 6.3). In areas that generate runoff toward swallow holes, the C factor overrides the O factor, i.e. such areas are always classified as highly to extremely vulnerable, independent from the overlying layer properties. Four classes of vulnerability are distinguished, symbolised by colours ranging from dark red for extreme vulnerability to light blue for low vulnerability. In a black-white printout, these colours appear dark grey to light grey.

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The maps showing the dominant flow process and the surface catchments, as well as the O and C maps, can be considered as intermediate steps to generate the vulnerability map. Only this latter represents an important element of the final groundwater protection scheme.

Fig. 6.3: Illustration of the proposed method of groundwater vulnerability mapping. The O factor takes into account the protectiveness of the overlying layers, the C factor considers the concentration of flow towards swallow holes (allogenic recharge), the vulnerability map is created by overlying the O and C maps (Nguyet and Goldscheider, in press).

6.2.3 Hazard and risk

The hazards can be classified on the basis of three aspects: quality, quantity, and likelihood of a potential contaminant release. These aspects are difficult to quantify, both for conceptual reasons and in terms of data availability. The proposed methodology simply classifies hazards

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into high, moderate and low. Urbanisations and industry, waste disposal sites, main roads, petrol stations, intensive agriculture and untreated domestic wastewater release are classified as highly dangerous hazards. Small roads, villages with wastewater treatment systems, and low-intensity agriculture are moderate hazards. Natural vegetation, forest and very lowintensity agriculture (e.g. organic fruit-tree-growing) are low hazards. Overlying the groundwater vulnerability map and the hazard map creates the risk map (Fig. 6.2). The importance of the groundwater could be included as an additional element. The risk map thus includes two aspects: the vulnerability of the groundwater to contamination and the harmfulness of the hazards.

6.3 Application in the Tham Ta Toong area

6.3.1 Introduction

The test site is the catchment area of the major Tham Ta Toong spring (Fig. 6.4), which contributes 50% to the drinking water to Son La town. This area belongs to the Son La karst area, of which the geological and hydrogeological conditions are described in chapter 4.

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The catchment consists of rocks formed in late Precambrian to Triassic, including limestone (Dong Giao, Ban Pap, Chieng Pac), claystone, sandstone, basalt and tuff. Surface water comes from non-carbonate areas and sinks in the sub-surface through the swallow holes and caves. Within the framework of the Vibekap Project, some of these caves were explored. Speleological data suggests an existence of under ground flow paths between the swallow holes and the springs. However, this hypothesis still needs to be proved by tracer tests.

Fig. 6.5: Geology of the Tham Ta Toong area, Son La province (Nguyet et al., 2004b).

To protect the spring against contamination from human activities, simple barriers setting around the spring can be used (Fig. 6.4). However, in general for karst areas, the contamination may directly reach the karst spring through swallow holes and caves. In this case, the major spring can still be highly polluted. Therefore, it is necessary to protect and prevent pollution. A simplified method based on the COST Action 620 for groundwater vulnerability mapping is applied to this test site in order to improve the situation.

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6.3.2 Groundwater vulnerability mapping

6.3.2.1 Overlying layer (O factor) For the Tham Ta Toong area, the O factor (Fig. 6.6) was determined on the basis of the geological map (Fig. 6.5) and field observations using the table in Fig. 6.2. The limestone of the Ban Pap, Chieng Pac and Dong Giao Formations covered by shallow soil was assigned a low degree of protection. A moderate protection was assigned to the areas with sandstone, siltstone intercalated with limestone, and quaternary deposits. A high protection cover is assigned in areas with siltstone and basalt 6.3.2.2 Flow Concentration (C factor) The C factor was assessed in two steps. First, the dominant flow processes were determined using the geological map and field observations. Direct infiltration (A) takes place on limestone formations with shallow soils; frequent surface flow (C) occurs on quaternary deposits and clayey formations; an intermediate situation (B) was assigned to all other settings. The swallow holes were mapped during fieldwork; their catchments were delineated based on a topographic map. Overlaying the O and C map then created the vulnerability map. Only three classes of vulnerability are present in the test site: extreme, high and moderate. Extreme vulnerability is restricted to small zones near sinking streams. High vulnerability is present both on karst limestone (low protection of the overlying layers) and on quaternary deposit (surface flow near sinking streams). The rest of the area was classified as moderately vulnerability. These results are reasonable in terms of karst hydrogeology, and at the same time applicable, as strict land use restrictions will only be required on relatively small zones.

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Fig. 6.6: O map, C map and vulnerability map of the Son La karst area, and legend for the three maps (Nguyet et al., 2004b).

6.4 Application in Tam Duong area

6.4.1 Introduction

The test site is the catchment area of the Dau Nguon Sin Ho and Nha May Che karst springs (Fig. 6.7), which are used for drinking water of part of the habitants in Tam Duong town. The area belongs to the Tam Duong karst area, of which the geological and hydrogeological characterizations are described in chapter 5.

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Fig. 6.7: Hydrogeological map of the test site. The estimated catchemnt area of two main karst springs, groundwater flow paths and other karst features are also presented in the figure.

The simplified method (for groundwater vulnerability, hazard and risk mapping) is applied to this test site. A geological map, a topographic map (scale: 1:50.000), a land use activities map as well as cave data and direct field observations served as basis data and information for the vulnerability, hazard and risk mapping.

6.4.2 Groundwater vulnerability mapping

The O map of the Tam Duong area reflects the lithology and the soil pattern (Fig. 6.8). Large parts of the test site consist of karst limestone covered with shallow soils, which provide a low degree of protection, i.e. a low O factor (which corresponds to a high vulnerability). A moderate protectiveness was assigned to the thick loamy deposits that are locally present in the main valley. The O factor is high in areas made of marl, conglomerate and sandstone.

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Fig. 6.8: O and C map of the test site. The resulting vulnerability map is shown in Fig. 6.9.

The first step toward the C map was to identify the dominant flow processes based on the geological map and direct field observations. Limestone areas are predominantly drained by infiltration and percolation (type A). Surface runoff occurs frequently on marl (type C), while the conglomerates and sandstones represent an intermediate situation (type B). The second step consisted of preparing the surface catchment map. The swallow holes, sinking stream and buffer zones were classified as zone 1. Their catchment areas, which are essentially formed of marl, were classified as zone 2. The rest of the karst area belongs to zone 3. The steep conglomerate and sandstone slopes that dip towards the karst area also belong to this zone. The area SW of the topographic watershed was classified as zone 4. The C map was prepared by overlying the dominant flow process map and surface catchment map (Fig. 6.8). The vulnerability map (Fig. 6.9) reflects both the overlying layers and the concentration of flow. The swallow holes and the sinking streams with buffer zones were classified as zones of extreme groundwater vulnerability. A high vulnerability was attributed both to karst areas covered by shallow soils, and to sinking stream catchments formed of marl. The areas in the main valley, where thick loamy soils cover the karst aquifer, were classified as zones of moderate vulnerability, as well as the conglomerate slopes that drain towards the adjacent karst aquifer system. The SW corner of the test site is a zone of low groundwater

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vulnerability, as the karst aquifer is covered by hundreds of metres of conglomerate and sandstone, which drain laterally out of the karst hydrogeological system towards a large river.

6.4.3 Hazard assessment, risk mapping and validation

The hazards in the Tam Duong area concentrate in the main valley in and around the town. The urbanisation itself, a hospital, a tea factory, some gasoline stations and the locally intense agriculture represent the most important hazards. Low-intensity agriculture and the roads were classified as moderate hazards. Some hazards resulting from untreated domestic wastewater, rice paddies and other agricultural activities are also present in the small side valleys, often near the sinking streams. Large parts of the mountainous areas, however, are overgrown by natural or quasi-natural vegetation, or used by very low-intensity agriculture. The risk map was prepared by overlying the hazard map and the groundwater vulnerability map. The risk map helps to identify zones where action is required to reduce the risk of groundwater contamination. In the test site area, extreme risk is present in small zones, where important hazards are directly located at a swallow hole or near a sinking stream. The main valley and the side valleys are a mosaic of zones of high and moderate risk, which reflects the patterns of the vulnerability map and the hazard map. No significant risk is present in most parts of the mountainous areas, particularly in the SW corner of the area, where thick conglomerates and sandstones cover the deep karst aquifer. The tracer tests and microbiological investigations in the area can be used to validate the vulnerability and risk assessment. The tracer experiments show that tracers injected into the swallow holes reappeared at the springs 41.5 h (spring 1) and 2.5 h (spring 2) after the injection at high concentrations and recovery rates exceeding 70 %. These findings confirm the extreme vulnerability that was assigned to the sinking streams and their catchments. Both springs show highly variably microbial contamination (see chapter 5). The bad microbial water quality of both springs can be attributed to the zones of elevated risk in their catchments. The very high contamination at spring 1 corresponds to the zone of extreme risk near swallow hole 1 and thus confirms the risk assessment (Fig. 6.9).

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Fig. 6.9: Vulnerability, hazard and risk maps for the Tam Duong test site. Both the tracer test results and the high contents of bacteria in spring 1 confirm the vulnerability and risk assessment near swallow hole 1.

6.5 Discussion on applicability of the methodology

The proposed methodology was first applied in high mountainous karst areas with limited data availability. The geological, topographic and hydrological characteristics of the test sites are relatively complicated. Nevertheless, the methodology was easily applicable in the areas, as it uses basic data, which are available for most areas or can easily be assessed in the field. For vulnerability mapping in both application areas, the basic data include geology, topography, and the location of swallow holes and sinking streams. For the Tam Duong case, hazards are grouped in three simple classes, and the risk map is obtained by overlying the two other maps. The use of a GIS facilitates the creation of the maps. The vulnerability, hazard and risk maps of the test site are plausible and were confirmed by tracer tests and

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microbiological data. The method can thus be applied to other Vietnamese karst test sites and appears to be well adapted to be used in other developing countries, particularly in tropical karst regions. The vulnerability map can be used to find a balance between human activities and economic interests on one hand and groundwater protection on the other hand. Groundwater protection should be the priority in the most vulnerable zone. Strong land-use restrictions are recommended in these zones. Less restriction is required in the less vulnerable zones. However, contaminant release should clearly be reduced as much as possible on the entire land surface. In some cases, additional criteria should be considered for the delineation of groundwater protection zones and the definition of adequate land-use responses, such as the importance of the groundwater.

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7 Conclusions

7.1 Karst hydrogeological characterisation

7.1.1 Groundwater flow path and groundwater mixing effect

The hydrogeological characterization of the Tam Duong and Son La areas has been investigated using different methods. Tracer experiments have proven the existence of underground drainage patterns between the swallow holes and the springs in those areas. The NW-SE and SW-NE faults have greatly affected underground water flow systems. The SWNE faults seem to dominate in the test sites. The geological analysis, speleological data and tracer results suggest that, in both areas, the groundwater flow paths run either across to folds along the SW-NE striking faults or follow the NW-SE faults; these flow paths could also be followed in the direction of cave development. The groundwater mixing effect in karst aquifer is well indicated in both areas by hydrochemistry, stable isotope results. In the case of Son La, there is a remarkable difference in Mg2+ and Ca2+ content between a swallow hole and a connected spring within a groundwater flow path, which could be explained by the mixing effect. The karst conduit flow system could be mixed with the deep flow system, which has a much higher chemical content. Also stable isotope results further support this observation. The high stability of 18O of karst springs in the Nam La valley compared with meteoric water could explain that this karst system contains well-mixed groundwater. The hydrochemical behaviour along a flow path suggests that water-rock interaction with dolomite may have occurred or that the process of dedolomitization may take place in carbonate aquifers in this area. The constant slope of increase in Mg2+ and Ca2+ along the groundwater flow paths can be explained by the mixing of the same original deep groundwater in studied karst system. In the Tam Duong area, the difference in Mg2+ and Ca2+ content between a swallow hole and connected springs as well as the hydrochemical behaviour along a flow path also can be explained by the by additional mineral dilution, and mixing processes within the aquifer. However, the hydrochemical difference between the swallow hole and connected spring is not very high as in the Son La area. The little variation in chemical content along the flow path may reflect the reduced water­rock interaction in this karst system.

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Karst springs, because of their differences in origin (size, catchment), show differences in hydrogeological characteristics. Large yield karst springs are often observed in Son La, while much smaller karst springs often occur in Tam Duong. Information from the tracer tests and results from other methods in the Tam Duong area suggest that concentrated recharges prevail in the area. In contrast, the results obtained from this study in the Son La area suggest that the recharge process and groundwater flow are more complicated. Conduit flow and diffuse flow could exist in a karst aquifer. The point recharges input to karst spring via conduit karst systems, and diffuse recharge seeps slowly along small joints and fractures. The recharge water and water stored in the phreatic zone all contributed to the large yield springs in the area.

7.1.2 Hydraulic properties

The linear groundwater flow velocities measured in the tracer tests are highly variable. Groundwater velocities in the Son La area range from 75 m/h to 166 m/h, which are typical velocities for karst groundwater flow and indicate a low resistance flow route. The estimated Peclet numbers are much higher than 6, which indicate an advection mass transport control karst conduit flow system in this area. The observed tracer breakthrough curves of the Suoi Muoi tracer test (Son La area) presents two tracer peaks, which suggests the existence of two karst conduits in the groundwater system. Tracer breakthrough curves of the Bon Phang and Nam La test (Son La area) present one tracer peak only, which may suggest a single karst conduit without major bifurcation in the system. Groundwater velocities in the Tam Duong area range from 72 m/h to 700 m/h, which is high for groundwater flow in a karst aquifer. The groundwater velocity as estimated from the tracer test in the Nha May Che spring (spring 2, Tam Duong area) is the highest velocity ever recorded for a tropical karst in NW Vietnam. The estimated hydraulic properties indicate an advection mass transport controlled karst conduit flow. Tracer breakthrough curves also suggest a single karst conduit without bifurcation in the system. The hydraulic (characterization) effect varies from one karst spring to another within this karst aquifer (system) in the Tam Duong area. The hydrograph and chemograph of the Nha May Che spring show the dilution effect. In contrast, a piston effect is observed at the Dau Nguon Sin Ho spring. The time lag of the piston effect fairly coincides with the transit time obtained in the tracer experiment.

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7.1.3 Karst water quality

The karst water quality in the Son La area is assessed by measurement of physical and chemical parameters of all the main drinking water karst springs and karst swallow holes in the area. The physical and chemical contents show a high variation between the measured points, but their values are all lower than recommended limits of WHO for drinking water. Both physical and chemical parameters at the major Tham Ta Toong spring are also lower than the WHO standards for drinking water. Information about the quality of the groundwater in the Tam Duong area is provided by a hydrochemical method in combination with a microbiological investigation, and by a rare earth elements (REE) study. The chemical parameters at all karst springs and swallow holes were below the standards limits for drinking water. Other physical parameters are moderate for general karst groundwater and also meet the standards for drinking water. The REE concentration levels found in spring water from Tam Duong are higher than those from other karst areas reported in the literature but still safe for the health of the people who consume drinking water in the area. The microbial investigation shows that all karst water contains high levels of thermotolerant coliforms. The contamination reflected temporal fluctuations and mainly results from untreated domestic waste water and human activities.

7.2 Groundwater protection

Land use activities in the Son La and Tam Duong are dominated by agriculture. This study provides information about the impact of agriculture on karst groundwater in these areas. The agricultural pollutant sources obviously produce an impact on the groundwater quality in the test sites. Moreover, rise of population and rapid urbanization also produce impacts on karst groundwater in these areas. Domestic waste water contributes a main source of bacterial pollution in the karst groundwater in urban areas. The pollution of karst aquifers in terms of microbiology is one of the environmental problems in the areas. Most karst springs have very high bacteria contamination levels. The waste deposits dumped at the big doline and pollution at swallow holes may produce contamination. The hydrogeological study (tracer results) in the area showed that surface water sinks into the swallow holes and rapidly arrives at the springs. Furthermore, because of the rapid velocities of groundwater in the karst aquifer, the contaminants from those sources may travel and spread over several kilometres through the aquifer in only a few hours.

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To protect karst groundwater, the proposed method of vulnerability, hazard and risk mapping was first applied in the Son La and Tam Duong area. This method uses basic data available for most areas; therefore, it is also applicable to the two test sites. The vulnerability, hazard and risk maps also provide a valuable basic tool for land-use planning and sustainable groundwater management of the area. Groundwater protection should be a priority in vulnerable zones such as swallow holes and along sinking streams.

7.3 An investigation methodology

Karst aquifers, as mentioned by many authors, have typical characteristics that are different from those of other aquifers. Some classical hydrogeology methods, like pumping tests and piezometric maps, are therefore invalid and cannot be applied. In addition, a single hydrogeological method can not give comprehensive answers of karst system (Bakalowicz, 2005). A specific investigation methodology that has recently been used by karst hydrogeologists is summarised by Bakalowicz (2005). The methods are generally grouped into 5 types: methodological approaches, the structure approach, the functional approach, an investigation methodology and modelling approaches. An individual approach or group approach to be used depends on the aim of the study and the conditions of the test site. The investigation methodology, considered as "an integrating medical approach of karst systems", includes a set of methods to study the structure and function of karst aquifers at catchment and local scale. Within the framework of this study, an investigation methodology has been applied to study karst aquifers and karst systems. It is impossible and unnecessary to apply all available methods in the case of Son La and Tam Duong, but several have been used and adapted to typical conditions of the test sites. These include: · Geological analysis and mapping technique considered as preliminary investigations of the test sites. Available geological information of the Son La and Tam Duong areas has been collected and analysed to characterise and delineate the karst system. · Artificial tracing tests were carried out in these areas for the first time, although the technique is used as one of the most common karst study tools in the world. Tracer tests at both areas (Son La and Tam Duong) were successful and provided valuable information to characterize groundwater flow paths. Both fluorescent dye and

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common salt tracers can be usefully applied in the NW karst belt. Tracer (monitoring) sampling had to be adapted to local conditions. Tracer samples were manually taken with the help of local people, and by using charcoal bags. To reduce transport problems, water samples were collected in small light proof bottles. · The spring hydrograph was monitored by water level loggers in the Son La area and was manually measured in the Tam Duong. Main karst springs were monitored in the short time interval during a rainy season in order to better characterise the dynamic functioning of the karst systems. · Hydrogeochemical and stable isotope methods were applied as natural tracer to determine the characterization of the karst systems. Meteoric, surface and spring water were collected for isotope measurements to determine the interaction between karst groundwater and surface water. During a rainy season, karst spring waters were collected for hydrogeochemical study in order to characterise the dynamic of karst groundwater flow in reaction to precipitation events. · Microbiological investigation was carried to characterise the karst water quality. The water from main karst springs were collected and analysed for thermotolerant coliforms. The results of this investigation were interpreted with the spring hydrographs and chemographs in order to try a better understand the variability of microorganisms in the karst aquifers.

7.4 Recommendations

The hydrogeological characterization and karst groundwater protection of the Son La and Tam Duong areas have been investigated in this study. However, several questions on the karst drainage pattern, characterization of karst aquifers as well as karst groundwater environment remain open. The methods applied in this study constitute useful investigation tools in karst hydrogeology research in mountainous areas. Although there are some limitations related to methods applied at the test sites, the results clearly provide reliable and valuable information. To further increase the power of the methods used in karst study in NW Vietnam, the following recommendations are made: · Tracer test should be considered as one of the most powerful techniques in karst study as well as a common tool in hydrogeological investigation. Although many authors

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have referred to the use of the tracer techniques in karst studies around the world, tracer tests are still a novel investigation in karst research in NW Vietnam. The results of this study show that the tracing technique is an applicable and useful tool that should be applied in any further karst hydrogeological and environmental research project. Both fluorescent dye and common salt can be used as a tracer. The fluorescent dye can be injected at the sites that have a high flow rate, and/or sampling sites that have a high discharge rate. Dye tracers are especially useful for tests that have to be carried out in difficult transport conditions, where a small amount of tracer can easily be transported to the injection sites. In contrast, common salt (sodium chloride) is a low cost tracer, but often requires a large amount of injected tracer. Our experiments show that this tracer is more practical for cases where injection sites are easily accessible, and sampling sites have a small discharge rate. · Spring monitoring and hydrochemical and microbiological investigations should be carried out continuously over short time steps in different hydraulic events. The spring represents the dynamic function of the whole karst system (Ford and Williams, 1989; Bakalowicz, 2005). Therefore, further work should be focused on the main karst springs at the test sites. · The stable isotope method is a significant potential tool in karst hydrologicalhydrogeological study. The 18O can be used as a natural tracer for determining the component mixing (i.e. the "new" and "old" components) in karst groundwater. Water samples of meteoric, stream and karst spring water as well as epikarst zone should be collected in high sampling resolution during rainfall or over long time periods. · Hydrogeological investigation should be carried out in combination with speleological surveys. The active caves, which can be easily accessed by speleologists, are good places for groundwater monitoring and hydrochemical water sampling. Tracers could be injected and/or sampled in deep active caves with the help of speleologists. Karst groundwater information, tracer results and cave data should be interpreted with respect to the whole karst system in order to have appropriate answers on karst hydrogeological characterisation of the studied areas. In response to urban development and environmental problems associated with this development, it is necessary to have a deeper understanding of karst hydrogeology and to

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protect karst groundwater. Therefore, in the Son La area, the following activities are recommended: · More detailed hydrogelogical investigation of the major Tham Ta Toong spring and other karst springs, which are used as drinking water in the urban area of the Son La is highly recommended. The aim of this further work would be to provide information about water quality and to characterize the structure and dynamic function of the karst aquifer. Spring monitoring and hydrochemical and microbiological investigations should be carried out over time intervals of several months and for different flow conditions. Tracer tests to map underground water flow paths connected to the Tham Ta Toong spring would be highly interesting. · A tracer experiment at the Ban Ai sinkhole is recommended to determine whether there is an underground water flow path between the sinkhole and the Hang Doi spring. Tracer experiments at the big dolines at Ban Giang (Fig. 4.4) would also be useful in order to delineate the catchment area of the Suoi Muoi and Nam La Rivers. For the follow up work, dye tracers are best selected, but the tracer monitoring (and tracer test design) can be improved. Manually and integrative tracer sampling techniques applied during several weeks at all possible springs are highly recommended. · The stable isotope results obtained in this recent work were collected in a rainy season. Further detailed work on stable isotope in combination with chemical investigation is proposed. The expected results may help to separate the mixing component in karst system. · Groundwater vulnerability mapping of the Tham Ta Toong catchment area was established on the basis of the geological data and field observation. So far, information is still missing about karst drainage pattern flow paths that could be connected to the major Tham Ta Toong spring. Updating of the groundwater vulnerability map should be done as soon as possible after information about karst groundwater flow paths becomes available. Establishing hazard and risk maps are also highly recommended.

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For the Tam Duong area, further investigation is recommended such as: · Tracer experiments carried out in the central part of the area (Tam Duong town or Nam Loong village) would be useful. There is still no information about the regional groundwater flow system and catchment area of the big karst springs further to the NW (Nam So River) and SE (Ban Giang village). Future work should make it possible to characterize the karst flow system on a regional scale. · Further hydrogeological investigation in the area will provide more detailed information about the geology and hydrogeology of the karst system. Land-use practices are also rapidly changing in urbanization areas like the Tam Duong. The vulnerability, hazard and risk maps should therefore be frequently updated.

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References

Aley, T., 1975. A predictive hydrogeological model for evaluating the effects of land use on the quantity of quality of water from Ozark Springs, Ozark Underground Laboratory, Missouri. Andreo, B., Carrasco, F., Bakalowicz, M., Mudry, J. and Vadillo, I., 2002. Use of hydrodynamic and hydrochemistry to characterise carbonate aquifers. Case study of the Blanca-Mijas unit (Malaga, southern Spain). Environmental Geology, 43(1-2): 108-119. Appelo, C.A.J. and Postma, D., 2005. Geochemistry groundwater and pollution. Balkema, Amsterdam, 649 pp. Atkinson, T.C., Smith, D.I., Lavis, J.J. and Whitaker, R., J, 1973. Experiments in tracing underground waters in limestones. Journal of Hydrology, 19: 323-349. Auckenthaler, A., Raso, G. and Huggenberger, P., 2002. Particle transport in a karst aquifer: natural and artificial tracer experiments with bacteria, bacteriophages and microspheres. Water Science and Technology, 46(3): 131-138. Bakalowicz, M., 2005. Karst groundwater: a challenge for new resources. Hydrogeology Journal, 13(1): 148-160. Banks, D., Hall, G., Reimann, C. and Siewers, U., 1999. Distribution of rare earth elements in crystalline bedrock groundwaters: Oslo and Bergen regions, Norway. Applied Geochemistry, 14: 27-39. Barnes, C.J. and Allison, G.B., 1988. Tracing of water movement in the unsaturated zone using stable isotopes of hydrogen and oxygen. Journal of Hydrology, 100(1-3): 143-176. Barnes, S. and Worden, R.H., 1998. Understanding groundwater sources and movement using water chemistry and tracers in a low matrix permeability terrain: the Cretaceous (Chalk) Ulster White Limestone Formation, Northern Ireland. Applied Geochemistry, 13(2): 143153. Belgian-Vietnamese Speleogical Expedition, 1996. Belgian-Vietnamese Speleogical Expedition Son La 1995-1996: Cave investigation, a start for research on sustainable development. Belgian Vietnamese Karst and Caves Association-Speleoklub van de Universiteit te Leuven-Research Institute of Geology and Mineral Resources. Belgian-Vietnamese Speleogical Expedition, 1998. Belgian-Vietnamese Speleogical Expedition Son La 1997-1998: Report of the third expedition in the Northern provinces of

Son La and Lai Chau. Belgian Vietnamese Karst and Caves Association-Speleoklub van de Universiteit te Leuven-Research Institute of Geology and Mineral Resources. Belgian-Vietnamese Speleogical Expedition, 2000. Belgian-Vietnamese Speleogical Expedition Son La 2000: Report of the fourth expedition in the northern provinces of Son La and Lai Chau. Belgian Vietnamese Karst and Caves Association-Speleoklub van de Universiteit te Leuven-Research Institute of Geology and Mineral Resources. Belgian-Vietnamese Speleogical Expedition, 2002. Belgian-Vietnamese Speleogical Expedition 2002: Report of the sixth expedition in the province of Lai Chau. Belgian Vietnamese Karst and Caves Association-Speleoklub van de Universiteit te LeuvenResearch Institute of Geology and Mineral Resources. Binh, N.D., 2003. Intergrating remotely sensed and geophysical data for geological study in Tam Duong -NW Vietnam. Master Thesis, Vrije Universiteit Brussel, Brussels. Birk, S., Liedl, R. and Sauter, M., 2004. Identification of localised recharge and conduit flow by combined analysis of hydraulic and physico-chemical spring responses (Urenbrunnen, SW-Germany). Journal of Hydrology, 286(1-4): 179-193. Bögli, A., 1980. Karst Hydrology and Physical Speleology. Springer-Verlag, 284 pp. Brookins, D., 1989. Aqueous geochemistry of rare earth elements. In: P.R. Lipin and G.A. McKay (Editors), Geochemistry and Mineral of rare earth elements. Reviews in mineralogy, pp. 201-225. Brouyère, S., Jeannin, PY., Dassargues,A., Goldscheider, N., Popescu IC., Sauter, M., Vadillo, I., Zwahlen, F, 2001. Evaluation and validation of vulnerability concepts using a physically based approach, Proc. of the 7th Conf. on Limestone Hydrology and Fissured Media, J. Mudry & F. Zwahlen (Eds.). Sciences et Techniques de l'Environnement, Université de Franche-Comté Mémoire, No13: 67-72. Brouyère S, 2004. A quantitative point of view of the concept of vulnerability. In: F. Zwahlen (Editor), Vulnerability and risk mapping for the protection of carbonate (karst) aquifers, final report COST Action 620. European Commission, Directorate-General for Research, Luxemburg, pp. 10-15. Bui Phu My, 1978. Geological map of Vietnam 1:200 000, the Lao Cai-Kim Binh sheet, Geological Survey of Vietnam. Caballero, E., De Cisneros, C.J. and Reyes, E., 1996. A stable isotope study of cave seepage waters. Applied Geochemistry, 11(4): 583-587. Chapelle, F.H., 2001. Ground - water microbiology and geochemistry. John Wiley & Songs, INC, 477 pp.

122

Clark, I. and Fritz, P., 1997. Environmental Isotopes in Hydrogeology. Lewis, Boca Ration, 328 pp. Cook, P. and Herczeg, A.L., 2001. Environmental tracers in subsurface hydrology. Kluwer Academic Publishers, 532pp pp. Coplen, T.B., Herczeg, A.L. and Barnes, C., 2001. Isotope engineering-Using stable isotopes of the water molecule to solve practical problems. In: P. Cook and A.L. Herczeg (Editors), Environmental tracers in subsurface hydrology. Kluwer Academic Publishers, pp. 79-110. Crawford, N.C., 2004. Water tracing:history. In: J. Gunn (Editor), Encyclopedia of Cave and Karst Science. Fitzroy Dearborn. Daly et al., 2002. Main concepts of the "European approach" to karst-groundwatervulnerability assessment and mapping. Hydrogeology Journal, 10(2): 340-345. Dao Trong Nang, 1979. Karstic landforms in Vietnam. Science-Techn. Publishing House, Hanoi. Darling, W.G. and Bath, A.H., 1988. A stable isotope study of recharge processes in the English Chalk. Journal of Hydrology, 101(1-4): 31-46. Day, T. and Tang, T., 2004. Tower karst. In: J. Gunn (Editor), Encyclopedia of Cave and Karst Science. Fitzroy Dearborn. De Boer, J.L.M., Verweij, W., van der Velde-Koerts, T. and Mennes, W., 1996. Levels of rare earth elements in Dutch drinking water and its sources. Determination by inductively coupled plasma mass spectrometry and toxicological implications. A pilot study. Water Research, 30(1): 190-198. De Ketelaere D, Hötzl H, Neukum C, Civita M and Sappa G, 2004. Hazard analysis and mapping. In: F. Zwahlen (Editor), Vulnerability and risk mapping for the protection of carbonate (karst) aquifers, final report COST Action 620. European Commission, Directorate-General for Research, Luxemburg, pp. 86-105. Divine, C.E. and McDonnell, J.J., 2005. The future of applied tracers in hydrogeology. Hydrogeology Journal, 13(1): 255-258. Doerfliger, N. and Zwahlen, F., 1998. Practical Guide: Groundwater Vulnerability Mapping in Karstic Regions (EPIK). Swiss Agency for the Environment, Forests and Landscape (SAEFL), Bern, 56 pp. Drew, D. and Hötzl, H. (Editors), 1999. Karst Hydrogeology and Human Activities. Impacts, Consequences and Implications. International Contributions to Hydrogeology. A.A Balkeman, 321 pp.

123

Dusar, M., Masschelein, J., Tien, P.C. and Tuyet, D., 1994. Belgian-Vietnamese Speleological Expedition, Son La 1993. Belgian Geological Survey. Professional Paper, 1994/4-N.271, 60 pp. Dussart-Baptista, L., Massei, N., Dupont, J.P. and Jouenne, T., 2003. Transfer of bacteriacontaminated particles in a karst aquifer: evolution of contaminated materials from a sinkhole to a spring. Journal of Hydrology, 284(1-4): 285-295. Epstein, S. and Mayeda, T., 1953. Variation of 18O content of waters from natural sources. Geochimica et Cosmochimica Acta, 4: 213-224. Fetter, C.W., 2001. Applied Hydrogeology. Prentice Hall, NJ, 598 p. Field, M.S., 2002. A Lexicon of Cave and Kars Terminology with Special reference to Enviromental Karst Hydrology. EPA/600/R-02/003. Ford, D.C. and Williams, P.W., 1989. Karst Geomorphology and Hydrology. Chapman & Hall, 601 pp. Frederickson, G.C. and Criss, R.E., 1999. Isotope hydrology and residence times of the unimpounded Meramec River Basin, Missouri. Chemical Geology, 157(3-4): 303-317. Genereux, D.P. and Hooper, R.P., 1998. Oxygen and Hydrogen Isotopes in Rainfall-Runoff Studies. In: C. Kendall and J.J. McDonnell (Editors), Isotope tracers in catchment hydrology. Elsevier, pp. 319-346. Glynn, P.D. and Plummer, L.N., 2005. Geochemistry and the understanding of ground-water systems. Hydrogeology Journal, 13(1): 263-287. Gogu, R.C. and Dassargues, A., 2000. Current trends and future challenges in groundwater vulnerability assessment using overlay and index methods. Environmental Geology, 39(6): 549-559. Gogu, R., Hallet, V. and Dassargues, A., 2003. Comparison of aquifer vulnerability assessment techniques. Application to the Néblon river basin (Belgium). Environmental Geology, 44(8): 881-892. Goldscheider, N., Klute, M., Sturm, S. and Hötzl, H., 2000. The PI method, a GIS-based approach to mapping groundwater vulnerability with special consideration of karst aquifers. Z. angew. Geol, 46: 157-166. Goldscheider, N., 2002. Hydrogeology and Vulnerability of Karst Systems-Examples from the Northern Alps and the Swabian Alb. Ph.D Thesis, Schriftenreihe Angewandte Geologie Karlsruhe, 236 pp.

124

Goldscheider, N., 2004. The concept of groundwater vulnerability. In: F. Zwahlen (Editor), Vulnerability and risk mapping for the protection of carbonate (karst) aquifers, final report COST Action 620. European Commission, Directorate-General for Research, Luxemburg, pp. 5-9. Goldscheider, N. and Popescu, I.C., 2004. The European Approach. In: F. Zwahlen (Editor), Vulnerability and risk mapping for the protection of carbonate (karst) aquifers, final report COST Action 620. European Commission, Directorate-General for Research, Luxemburg, pp. 17-21. Gosselin, D., Smith, M.R., Lepel, E.A. and Laul, J.C., 1992. Rare earth elements in chloriderich groundwate, Palo Duro Basin, Texas, USA. Geochimica et Cosmochimica Acta, 56: 1495-1505. Gunn, J. 1986. Modelling of conduit flow dominated karst aquifers. In G. Günay and A.I. Johnson (Editors), Karst water resources. IAHS, Publication 161: 587-596. Wallingford, UK. Gunn, J. (Editor), 2004. Encyclopedia of Caves and Karst Sciences. Fitzroy Dearborn, London, 902 pp. Guo, C., Stetzenbach, K.J. and Hodge, V.F., 2005. Determination of 56 trace elements in three aquifer-type rocks by ICP-MS and approximation of the relative solubilities for these elements in a carbonate system by water-rock concentrations ratios. In: K.H. Johannesson (Editor), Rare Earth Elements in Groundwater flow systems. Water Science and Technology Library, pp. 39-65. Hannigan, R.E. and Sholkovitz, E.R., 2001. The development of middle rare earth element erichments in freshwaters: weathering of phosphate minerals. Chemical Geology, 175: 495-508. Henderson, P. (Editor), 1984. Rare Earth Element Geochemistry. Elsevier, AmsterdamOxford-New York-Tokyo. Herczeg, A.L., Leaney, F.W.J., Stadter, M.F., Allan, G.L. and Fifield, L.K., 1997. Chemical and isotopic indicators of point-source recharge to a karst aquifer, South Australia. Journal of Hydrology, 192(1-4): 271-299. Hess, J.W. and White, W.B., 1993. Groundwater geochemistry of the carbonate karst aquifer, southcentral Kentucky, U.S.A. Applied Geochemistry, 8(2): 189-204. Hoefs, J., 1997. Stable Isotope Geochemistry. Springer-Verlag Berlin, 201 pp.

125

Hop, N.D., 1994. Geology of Thuan Chau sheet, scale 1:50 000. Research Institue of Geology and Mineral Resources, Hanoi. Hötzl H, Delporte C, Liesch T, M.P., Neukum C and Svasta J, 2004. Risk mapping. In: F. Zwahlen (Editor), Vulnerability and risk mapping for the protection of carbonate (karst) aquifers, final report COST Action 620. European Commission, Directorate-General for Research, Luxemburg, pp. 113-120. Hung, L.Q., Dinh, N.Q., Batelaan, O., Tam, V.T. and Lagrou, D., 2002. Remote sensing and GIS-based Analysis of Cave Development in the Suoi Muoi catchment (Son La, Vietnam). Journal of Cave and Karst Studies, 64(1): 23-33. Japanese Mining Project, 2002. Ore mining and enviromental impact in the Dong Pao area, Vietnam Geological Survey. Jenkins, A., Ferrier, R.C., Harriman, R. and Ogunkoya, Y.O., 1994. A Case-Study in Catchment Hydrochemistry - Conflicting Interpretations from Hydrological and Chemical Observations. Hydrological Processes, 8(4): 335-349. Johannesson, K.H., Berry Lyons, W., Yelken, M.A., Gaudette, H.E. and Stetzenbach, K.J., 1996. Geochemistry of the rare-earth elements in hypersaline and dilute acidic natural terrestrial waters: Complexation behaviour and middle rare-earth element enrichments. Chemical Geology, 133(125-144). Johannesson, K.H., Stetzenbach, K.J., Hodge, V.F., Kreamer, D.K. and Zhou, X., 1997. Delineation of ground-water flow systems in the southern great basin using aqueous rare earth element distributions. Ground Water, 35(5): 807-819. Johannesson, K.H. and Hendry, M.J., 2000. Rare earth element geochemistry of groundwater from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada. Geochimica et Cosmochimica Acta, 64: 1493-1509. Johannesson, K.H., Zhou, X., Stetzenbach, K.J. and Hodge, V.F., 2000. Origine of rare earth elements signatures in groundwaters of circumneutral pH from southern Nevada and eastern California, USA. Chemical Geology, 164(239-257). Johannesson, K.H. (Editor), 2005. Rare earth elements in groundwater flow systems, 51. Water Science and Technology Library, 293 pp. Katz, B.G., 2002. Demystifying Ground-water flow and contamination movement in karst system using chemical and isotopic tracers. U.S. Geological Survey .Water-Resources Investigations Report 02-4174, Atlanta,Georgia, pp. 13-19. Käss, W., 1998. Tracing technique in Geohydrology. A.A Balkema/Rotterdam, 581 pp.

126

Kiraly, L., 1975. Rapport sur l'etat actuel des connaissances dans le domaine des caractères physiques des roches karstiques. In: Burger A, Dubertret L (Editors), Hydrogeology of karstic terrains. IAH, International Union of Geological Sciences, Series B, 3: 53-67. Lakey, B. and Krothe, N.C., 1996. Stable isotopic variation of storm discharge from a perennial karst spring, Indiana. Water Resources Research, 32: 721-731. Leaney, F.W. and Herczeg, A.L., 1995. Regional recharge to a karst aquifer estimated from chemical and isotopic composition of diffuse and localised recharge, South Australia. Journal of Hydrology, 164(1-4): 363-387. Lee, E.S. and Krothe, N.C., 2001. A four-component mixing model for water in a karst terrain in south-central Indiana, USA. Using solute concentration and stable isotopes as tracers. Chemical Geology, 179(1-4): 129-143. Leybourne, M.I., Goodfellow, W.D., Boyle, D.R. and Hall, G.M., 2000. Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in groundwater associated with Zn-Pb massive sulfide deposits. Applied Geochemistry, 15: 695-723. Liu, Y., Batelaan, O., Smedt, F., Huong, N. and Tam, V., 2005. Test of a distributed modelling approach to predict flood flows in the karst Suoimuoi catchment in Vietnam. Environmental Geology, 48(7): 931-940. Long, A.J. and Putnam, L.D., 2002. Evaluating travel time and transient mixing in a karst aquifer using time-series analysis of stable isotope data, U.S. Geological Survey. WaterResources Investigations Report 02-4174. Margat J, 1968. Vulnérabilité des nappes d'eau souterraine à la pollution (vulnerability of groundwater to pollution). BRGM-Publication 68 SGL 198 HYD, Orléans. McCarthy, J.F. and Shevenell, L., 1998. Processes controlling colloid composition in a fractured and karstic aquifer in eastern Tennessee, USA. Journal of Hydrology, 206(3-4): 191-218. Mouret, C., 2004. Karst in Southeast Asia. In: J. Gunn (Editor), Encyclopedia of Cave and Karst Science. Fritzroy Dearborn, pp. 100-104. Nativ, R. et al., 1999. Separation of groundwater-flow components in a karstified aquifer using environmental tracers. Applied Geochemistry, 14(8): 1001-1014. Neal, C. et al., 1992. Stable hydrogen and oxygen isotope studies of rainfall and streamwaters for two contrasting holm oak areas of Catalonia, northeastern Spain. Journal of Hydrology, 140(1-4): 163-178.

127

Nguyen Quang My, 1992. Karst in Vietnam. Hanoi University. Internal Reprot, Dept. Geography and Geology. Nguyen Tam, 2004. The stable isotope in karstic system study. Case study: Son La-Vietnam. Master Thesis, Vrije Universiteit Brussel. Nguyet, V.T.M., 2000. Design of karst web-based database and Hydrological Analysis for Thuan Chau-Son La catchment, Vietnam. Master Thesis, Vrije Universiteit Brussel. Nguyet, V.T.M., Batelaan, O. and De Smedt, F., 2004a. Contribution to the karst hydrogeology of Son La, Vietnam by artificial tracer experiments, International Transdisciplinary Conference on Development and Conservation of Karst Regions, Hanoi, Vietnam, pp. 13-18. Nguyet, V.T.M., Goldscheider, N. and Batelaan, O., 2004b. Adaptation and application of the pan-European approach to groundwater vulnerability mapping to the Son La karst area, International Transdisciplinary Conference on Development and Conservation of Karst Regions, Hanoi, Vietnam, pp. 165-168. Nguyet, V.T.M. and Goldscheider, N., 2006. Tracer tests, hydrochemical and microbiological investigations as a basis for groundwater protection in a remote tropical mountainous karst area, Vietnam. Hydrogeology Journal. DOI: 10.1007/s10040-006-0038-z Nguyet, V.T.M. and Goldscheider, N., in press. A simplified methodology for mapping groundwater vulnerability and contamination risk, and its first application in a tropical karst area, Vietnam. Hydrogeology Journal. Nuyens, D., 1992. Geochemie van de Lanthaniden in carbonaatsedimenten van de Bahamas (Post-Aptiaan) en Belgie (Dinantiaan). Ph.D thesis Thesis, Katholieke Universiteit Leuven. Panagopoulos, G., Lambrakis, N., Katagas, C., Papoulis, D. and Tsolis-Katagas, P., 2005. Water - rock interaction induced by contaminated groundwater in a karst aquifer, Greece. Environmental Geology, 49(2): 300-313. Perrin, J., Jeannin, P.-Y. and Zwahlen, F., 2003. Epikarst storage in a karst aquifer: a conceptual model based on isotopic data, Milandre test site, Switzerland. Journal of Hydrology, 279(1-4): 106-124. Perry, E., Velazquez-Oliman, G. and Marin, L., 2002. The hydrogeochemistry of the karst aquifer system of the northern Yucatan Peninsula, Mexico. International Geology Review, 44(3): 191-221. Pronk, M., Goldscheider, N. and Zopfi, J., in press. Dynamics and interaction of organic carbon, turbidity and bacteria in a karst aquifer system. Hydrogeology Journal.

128

Rozanski, K., Araguas-Araguas, L. and Gonfiantini, R., 1993. Isotope Patterns in Modern Global Precipitation. Geophysical Monograph, 78: 1-36. Rozycki, S.Z., 1984. Some tropical karst processes in North Vietnam. Quaternary Studies in Poland, 5:137-151. Saunders, J.A. and Toran, L.E., 1994. Evidence for dedolomitization and mixing in Paleozoic Carbonates Near Oak Ridge, Tennessee. Ground Water, 32(2): 207-214. Scanlon, B.R., 1990. Relationships between groundwater contamination and major-ion chemistry in a karst aquifer. Journal of Hydrology, 119(1-4): 271-291. Schramke, J.A., Murphy, E.M. and Wood, B.D., 1996. The use of geochemical mass-balance and mixing models to determine groundwater sources. Applied Geochemistry, 11(4): 523539. Shand, P., Johannesson, K.H., Chudaev, O., Chudaeva, V. and Mike Edmunds, W., 2005. Rare earth element contents of high pCO2 groundwaters of primorye, Russia: mineral stability and complexation. In: K.H. Johannesson (Editor), Rare Earth Elements in Groundwater flow systems. Water Science and Technology Library, pp. 161-186. Shevenell, L. and McCarthy, J.F., 2002. Effects of precipitation events on colloids in a karst aquifer. Journal of Hydrology, 255(1-4): 50-68. Sholkovitz, E.R., 1988. Rare earth elements in the sediments of the North Atlantic Ocean, Amazon delta, and east China sea: reinterpretation of terrigenous input patterns to the oceans. Am. J. Sci, 288: 236-281. Smart, C. and Worthington, S.R.H., 2004a. Groundwater in karst. In: J. Gunn (Editor), Encyclopedia of Cave and Karst Science. Fitzroy Dearborn. Smart, C. and Worthington, S.R.H., 2004b. Water tracing. In: J. Gunn (Editor), Encyclopedia of Cave and Karst Science. Fitzroy Dearborn. Smedley, P., 1991. The geochemistry of rare earth elements in groundwater from the Carnmenellis area, southwest England. Geochimica et Cosmochimica Acta, 55: 27672779. Spangler, L.E., 2002. Use of dye tracing to determine conduit flow paths within sourceprotection areas of a karst spring and wells in the Bear river range, northern Utah, U.S. Geological Survey.Water-Resources Investigations Report 02-4174. Steinmann, P. and Matera, V., 2002. Minéralisation micro-ondes avec le rotor moyenne pression MPR, GEA-analyses géochimiques et environnementales. Institut de Géologie, Université de Neuchâtel.

129

Tam, B.M., Thu, T.V., Hoa, T.X., Nguyen, N.V., 1996. Petrogenesis of granitoids in Phong Tho - Lai Chau area. Geology and Mineral 5: 54-71. Hanoi. Tam, V.T., 2003. Characterization of a karstic system by an intergrative and multi-approach study. A case study of Suoi Muoi and Nam La catchments in northwest Vietnam. Ph.D thesis, Vrije Universiteit Brussel, Brussels, 152 pp. Tam, V.T., De Smedt, F., Batelaan, O., Hung, L. and Dassargues, A., 2005. Study of cavernous underground conduits in Nam La (Northwest Vietnam) by an integrative approach. Hydrogeology Journal, 13(5 - 6): 675-689. Tang, J. and Johannesson, K.H., 2005. Rare earth element concentrations, speciation, and fractionations along groundwater flow paths: the Carrizo sand (Texas) and upper Floridan aquifers. In: K.H. Johannesson (Editor), Rare Earth Elements in Groundwater flow systems. Water Science and Technology Library, pp. 223-251. Tin, Q.D., 2001. Hydrochemical analyses of Suoi Muoi catchment, Vietnam. Master Thesis, Vrije Universiteit Brussel, Brussels. To Van Thu. et al., 1996. Report on Geology and Mineralogy deposits of the Phong Tho sheet, Intergeo, Hanoi. Tran Van Tri., Nguyen Xuan Tung and Nguyen Dinh Uy., 1979. Geological of Vietnam (north part). Research Institute of Geology and Mineral Resources, 80 pp. Turner, J.V. and Barnes, C.J., 1998. Modeling of Isotope and Hydrogeochemical Responses in Catchment Hydrology. In: C. Kendall and J.J. McDonnell (Editors), Isotope tracer in catchment hydrology. Elsevier, Amsterdam, pp. 723-759. Tuyet, D., Tuy, P.K., Ke, T.D., Quyet, Q.X., Tuan, L.C., Thang, D.V., Xuyen, C.S and Quynh, D.V., 1996. Karst geology investigation of the northwest region, Research Institute of Geology and Mineral Resources, Hanoi. Tuyet, D., 1998. Summary of the report on karst geology investigation of the northwest region, Research Institute of Geology and Mineral Resources, Hanoi. Van, T.T., Ke, T.D., Tuy, P.K., Trung, N.D. and Thang, D.V., 2003. A report on geological characteristics of the Pu Luong area, Research Institute of Geology and Mineral Resources, Hanoi. Van den Bossche, P., 2000. Evaluation of karst hydrological modelling techniques for tropical areas. Master Thesis, Vrije Universiteit Brussel, Brussels. Van den Driessche, K., 2001. Potentials of stable water isotopes as a natural tracer in Schelde basin. Doctor thesis Thesis, Vrije Universiteit Brussel, 206 pp.

130

Vaute, L., Drogue, C., Garrelly, L. and Ghelfenstein, M., 1997. Relations between the structure of storage and the transport of chemical compounds in karstic aquifers. Journal of Hydrology, 199(3-4): 221-238. Verheyden, S., 2000. Speleothems as paleoclimatic archives. Isotopic and geochemical study of the cave environment and its Late Quaternary records. Ph.D Thesis, Vrije Universiteit Brussel. Veselic, M., 2003. Protection of groundwater in classical Karst systems. Critical Reviews in Analytical Chemistry, 33(4): 327-332. Vesper, D.J., Loop, C.M. and White, W.B., 2001. Contaminant transport in karst aquifers. Theoretical and Applied Karstology, 13-14: 101-111. Vesper, D.J. and White, W.B., 2004. Storm pulse chemographs of saturation index and carbon dioxide pressure: implications for shifting recharge sources during storm events in the karst aquifer at Fort Campbell, Kentucky/Tennessee, USA. Hydrogeology Journal, 12(2): 135-143. Vias, J.M., Andreo, B., Perles, M.J. and Carrasco, F., 2005. A comparative study of four schemes for groundwater vulnerability mapping in a diffuse flow carbonate aquifer under Mediterranean climatic conditions. Environmental Geology, 47(4): 586-595. VIBEKAP, 2003. Vietnamese-Belgian Karst Project: final report on research results. Catholic University Leuven-Research Institute of Geology and Mineral Resources. Vitvar, T. and Balderer, W., 1997. Estimation of mean water residence times and runoff generation by 18O measurements in a Pre-Alpine catchment (Rietholzbach, Eastern Switzerland). Applied Geochemistry, 12(6): 787-796. Voss, C.I., 2005. The future of hydrogeology. Hydrogeology Journal, 13(1): 1-6. Vrba J and Zaporozec, A (Editors), 1994. Guidebook on Mapping Groundwater Vulnerability. International Contributions to Hydrogeology (IAH), 16, 131 pp. Waltham, T. and Hamilton-Smith, E., 2004. Ha Long Bay, Vietnam. In: J. Gunn (Editor), Encyclopedia of Caves and Karst Science. Fitzroy Dearborn, pp. 413-414. Wels, C., Cornett, R.J. and Lazerte, B.D., 1991. Hydrograph separation: A comparison of geochemical and isotopic tracers. Journal of Hydrology, 122(1-4): 253-274. Werner, A., Hötzl, H., Maloszewski, P. and Käss, W., 1997. Interpretation of tracer tests in karst systems unsteady flow conditions. Karst Hydrology. IAHS Publ.No. 247, pp. 15-26. WHO,2004. Guidelines for drinking-water quality, 3rd edition, Volume 1, Recommendations. World Health Organisation, Geneva, 515 pp.

131

Willems, roches

L.,

Pouclet,

A.

and

Vicat, non

J.P.,

2002. en Bull.

Existence Afrique Soc.

de Géol.

karsts

en et

cristallines

silicatées

carbonatées

sahélienne

équatoriale,

implications

hydrogéologiques",

France,

173, n° 4, 337-345. Worthington, S.R.H., Davies, G.J. and Frod, D.C., 2000. Matrix, fracture and channel components of storage in a Paleozoic limestone aquifer. In: C. Wicks and I. Sasowsky (Editors), Groundwater Flow and Contaminant Transport in Carbonate Aquifer. Balkema, Rotterdam. Yonge, C.J., Ford, D.C., Gray, J. and Schwarcz, H.P., 1985. Stable isotope studies of cave seepage water. Chemical Geology (Isotope Geosc. Section), 58: 97-105. Young, R.W., 1986. Tower Karst in Sandstone-Bungle Massif, North-western Australia. Zeitschrift Für Geomorphologie, 30(2): 189-202. Zwahlen, F. (Editor), 2004. Vulnerability and Risk Mapping for the Protection of Carbonate (Karst) Aquifer, final report COST Action 620. EUR 20912. European Commission, Directorate-General for Research. Luxemburg.

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