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Hydrochemistry of waters from five cenotes and evaluation of their suitability for drinking-water supplies, northeastern Yucatan, Mexico

Javier Alcocer 7 Alfonso Lugo 7 Luis E. Marín Elva Escobar

Abstract Waters from five cenotes that are currently being used for aquatic recreational activities and that lie along the Cancun­Tulum touristic corridor, Mexico, were evaluated hydrochemically to determine whether the cenotes may be considered as potential drinkingwater sources. Several parameters exceed the Mexican Drinking Water Standards (MDWS), but since they do not pose a significant health threat, four of the five cenotes may be used as drinking-water sources. The common contaminants in the Yucatan Peninsula, fecal coliforms and nitrate, are in most cases below the MDWS (0­460 MPN/100 ml and 0.31­1.18 mg/L, respectively). Although these four cenotes meet the MDWS, a careful groundwater management policy needs to be developed to avoid contamination (fecal and nitrates) and saltwater intrusion. Résumé Les eaux de cinq cénotés, qui sont normalement utilisées pour des activités de plein air, dans la région touristique de Cancun­Tulum (Mexique), ont été soumises à analyses chimiques pour savoir si les cénotés peuvent être considérés comme des sources d'eau potable. Plusieurs paramètres dépassent les normes mexicaines en matière d'eau potable; mais comme ceux-ci ne posent pas de problème réel de santé, quatre des cinq cénotés peuvent être captés pour l'eau potable. Les contaminants habituels dans les eaux de la presqu'île du Yucatan, coliformes fécaux et concentraReceived, October 1996 Revised, June 1997; March 1998 Accepted, July 1997 Javier Alcocer (Y) 7 Alfonso Lugo Limnology Laboratory, Environmental Conservation and Improvement Project, UIICSE, Universidad Nacional Autonoma de Mexico, Campus Iztacala, Av. de los Barrios s/n, Los Reyes Iztacala, 54090 Tlalnepantla, Estado de Mexico, Mexico Fax: c52-5-277-1829 e-mail: jalcocer6servidor.unam.mx Luis E. Marín Geophysics Institute, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, 04510 Mexico, D.F. Mexico Elva Escobar Benthic Ecology Laboratory, Institute of Marine Sciences and Limnology, Universidad Nacional Autonoma de Mexico, Apdo Postal 70-305, Ciudad Universitaria, 04510 Mexico, D.F. Mexico Hydrogeology Journal (1998) 6 : 293­301

tions élevées en nitrate, sont la plupart du temps audessous des normes (respectivement 0 à 460 germes/ 100 ml et 0,31 à 1,18 mg/l). Bien que ces quatre cénotés satisfassent aux normes, il est nécessaire de mettre en place des règles précises de l'utilisation de l'eau souterraine, afin d'éviter la contamination par les germes fécaux et par les nitrates, ainsi que l'intrusion marine. Resumen Se analizó hidroquímica y bacteriológicamente el agua de algunos cenotes localizados a lo largo del corredor turístico Cancun­Tulum, que actualmente se utilizan para diversas actividades recreativas, para determinar su potencial de uso como fuente de abastecimiento de agua potable. La mayor parte de los parámetros excedieron los criterios establecidos en la Norma Mexicana para Agua Potable (NMAP), sin embargo, como éstas no representan una riesgo para la salud, el agua de cuatro de los cinco cenotes puede ser emplada como fuente de abastecimiento de agua potable. Los contaminantes comúnes del agua subterránea de la península de Yucatán, coliformes fecales y nitratos, se encuentran en la mayoría de los casos por debajo de la NMAP (0­460 NMP/ 100 ml y 0.31­1.18 mg/l, respectivamente). A pesar de que estos cuatro cenotes cumplen con la NMAP, es necesario desarrollar una política de manejo adecuada del agua subterránea para evitar la contaminación de este recurso (fecal y por nitratos), así como la intrusión de agua salina. Key words Contamination 7 hydrochemistry 7 karst 7 Mexico 7 water supply

Introduction

The Yucatan Peninsula, southeastern Mexico, is a limestone plain with a significant proportion of evaporites. Nearly the entire peninsula is underlain by porous and fissured limestone with a veneer of soil and xerophytes. The southernmost part is covered by a typical tropical rain forest; temperature variations are small, and seasonality is, therefore, defined by the rainy/dry season. Quintana Roo, at the eastern part of the Yucatan Peninsula, is characterized by two climatic periods that last six months each; the rainy season is from March/April to October/November, and the dry season

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294 Fig. 1 Location of study area, northeastern Yucatan Peninsula, Mexico

is from October/November to March/April. Few surface-water bodies exist, and the rivers are short. Water flow occurs primarily underground. Locations are shown in Figure 1. Solution lakes, locally known as "cenotes," are common features of the Yucatan Peninsula. "Cenote" is a somewhat loosely defined term that refers to various types of water bodies contained in limestone cavities. Pearse et al. (1936) describes four types of cenotes according to their shape, i.e., jug-shaped, vertical-walled, aguada-like, and cave-like. Cenotes are further classified into two types according to their water characteristics. The most common has clear water, a clean sandy or rocky bottom, and a homogeneous well oxygenated water mass. In contrast, some cenotes are stagnant, turbid, and stratified thermally. In these, the upper layer is alkaline and oversaturated with dissolved oxygen, whereas the bottom layer is acid, lacking dissolved oxygen, with H2S in the deeper waters. Historically, cenotes served as the only sources of water supply and as important ceremonial places for the ancient Maya culture. Without them, the Mayans would have been without sufficient water (Back 1995). For the growing urban and tourist industry of the Mexican Caribbean region (e.g., the Cancun­Tulum touristic corridor, which is the coastal area between the cities of Cancun and Tulum, Fig. 1), cenotes play an important role as potential drinking-water resources and recreational sites, such as swimming and cave-diving. Although Quintana Roo is one of the least populated states in Mexico (29th place out of 32 states), its population increased by 43% from 1990 to 1995, and the number of houses increased by 55%. Population and housing statistics are given in Table 1. In addition,

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Quintana Roo is today one of the most highly urbanized Mexican states (5th urbanization level out of a maximum of 7, where Mexico City is at level 7; INEGI 1995). This accelerated urbanization and increase in tourism (Table 1) require large amounts of fresh water. Seemingly, Yucatan characteristics "should provide abundant supplies of water; however, factors of climate and hydrogeology have combined to form a hydrologic system in which fresh water is scarce and whose chemical environments ­ seawater intrusion and groundwater pollution ­ decrease even that restricted supply" (Ward et al. 1985). Water is scarce throughout the Yucatan Peninsula, even though rainfall is sometimes as great as 1500 mm/ yr (mean is 1050 mm/yr). The rainy season is brief, and 85% of the precipitation returns to the atmosphere by evapotranspiration. In addition, the peninsula is underlain by extremely permeable carbonate rocks that have been faulted and folded. The insoluble fraction of the limestones produces thin residual soils, and bedrock outcrops are common. Soils and bedrock do little to impede the rapid infiltration of meteoric water. Even during intense storms, surface retention is rarely observed. The high infiltration characteristics of the surface, large permeability of the rocks, and low relief of the area combine to produce a regional aquifer with a very small hydraulic gradient. For example, the gradient in northwestern Yucatan is about 7­10 mm/km (Marín 1990). Thus, Yucatan's only source of potable water is a thin, fragile aquifer that underlies the peninsula (Doehring and Butler 1974) and is underlain by denser, saline water that occurs more than 100 km inland (Steinich and Marín 1996). High sulfate concentrations

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295 Table 1 Population and housing statistics from Quintana Roo state and some of the largest cities along the Cancun-Tulum touristic corridor near the study area, 1990 (INEGI 1991), and 1995 (INEGI 1996). Parameter 1990 Population Houses Potable water 1 Drainage 2

1 2

Quintana Roo 1995 703 536 163 894 127 228 129 576 1990 167 730 39 866 29 358 27 145

Cancun 1995 297 183 75 445 50 895 71 333

Playa del Carmen 1990 3098 711 441 346 1995 17 621 4646 2325 3205 1990 2111 481 312 145

Tulum 1995 3603 806 632 487

493 277 106 094 82 588 58 906

Number of houses with potable-water pipe line Number of houses with drainage

occur in waters of some cenotes in the Yucatan Peninsula, resulting from solution of gypsum deposits that underlie the region (Perry et al. 1996; Velázquez 1995; Steinich et al. 1996). The aquifer is highly vulnerable to contamination due to its hydrogeologic characteristics and to anthropogenic activities, including the discharge of both domestic and industrial wastes into the aquifer and the indiscriminate use of pesticides and fertilizers (Marín and Perry 1995). The objective of this paper is to evaluate the hydrochemistry of water in five cenotes along the Cancun­Tulum touristic corridor to determine whether the waters (1) meet the Mexican Drinking Water Standards (Diario Oficial de la Federación 1989) for drinking-water supplies, and (2) meet the Mexican Standards for use as aquatic recreational centers.

Methodology

The five cenotes that were studied are (1) Carwash (20716.48bN, 87729.74bW); (2) Cristal (20712.50bN, 87728.98bW); (3) Mayan Blue (20711.61bN, 87729.74bW); (4) the main entrance of the Nohoch (20717.93bN, 87724.20bW); and (5) Casa (20715.97bN, 87723.41bW). Locations are shown in Figure 1, and data are given in Table 2. Locations were determined using a Magellan Field Pro V GPS (Global Positioning System) instrument calibrated at Puerto Aventuras (Fig. 1). The Nohoch cenote is cave-like, and the other four are welllike (Table 2).

To detect maximum possible chemical variations during the dry/wet periods, sampling was conducted at the end of the dry season (March 1995), when maximum concentrations were expected, and at the end of the rainy season (October/November 1995), when maximum dilution was anticipated. At each cenote pool, a midday profile of temperature, pH, conductivity (K25), dissolved oxygen, and percentage of dissolved oxygen was made using a calibrated Hydrolab Datasonde3/Surveyor3 multiparameter water-quality datalogger and logging system. Temperature, conductivity, and dissolved-oxygen vertical profiles were checked in situ for possible stratification (thermo-, halo-, or oxyclines). When the water column was homogeneous, a mid-depth sample was collected with a 5 l Van Dorn water bottle for further chemical and microbiological analyses; otherwise, two or three samples were taken, one from each stratum. At each site, three splits were taken, for alkalinity, anions, and cations (acidified). Water samples were refrigerated until analyzed in the laboratory. Anions analyzed include carbonates and bicarbonates (through alkalinity), sulfate, and chloride; cations include calcium, magnesium, sodium, and potassium. The anions were measured with a HACH DREL/2000 water-quality portable laboratory. Nutrients (NO2, NO3, PO4) were analyzed following the techniques described by Strickland and Parsons (1972), Parsons et al. (1984), and Stirling (1985). Total dissolved solids (TDS) were estimated by adding the individual species. Microbiological analyses (fecal coliform) were done by using the technique described by the APHA et al. (1985).

Table 2 General characteristics of the five cenotes. (mbslpmeters below surface level; Recreational usepswimming, scuba-diving, etc.; Domestic usepdrinking water, bathing, laundry, etc.) Parameter Size (m!m) Maximum depth (m) Water level (mbsl) Present water use Bottom characteristics Vegetation Carwash cenote 50!15 6 0 Recreation Rocky Sparse Cristal cenote 15!9 5 0 Recreation Sand to Muddy Cabomba, Benthic algae Mayan Blue cenote 50!10 5 0 Recreation Rocky Nymphaea, Sagittaria Nohoch cenote 75!5 7 0 Recreation, Domestic Rocky Sparse Casa cenote 25!20 7 0 Recreational Rocky with detritus Mangroves

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Results

The results of each of the physical, geochemical, and biological parameters, including temporal and spatial variations, are shown in Tables 3 and 4, and in Figure 2, and they are discussed below.

Temperature The water temperature of the cenotes is fairly constant (Table 3). Temperature differences between dry and rainy seasons are negligible. Water in the Carwash cenote exhibited the maximum temperature variation (26.40B1.03 7C) in the rainy season. However, this maximum variation was associated with an ephemeral thin lens of rainwater that had formed over the "standard" pool water prior to sampling. This stratum was alTable 3 Minimum-maximum concentrations of physical, chemical, and biological parameters of five cenotes, compared with the Mexican Legislation standards. Concentrations in mg/L, exParameter Alkalinity Chloride Phosphate Nitrate nitrogen Nitrite nitrogen Dissolved oxygen Sulphate Aesthetic Oil and grease Floating matter Odor pH Total dissolved solids Temperature Fecal coliform

1 2

kaline, more dilute and warmer than the pool water that it overlay. The average temperature for most springs (except thermal springs) is nearly equal to the average mean air temperature in the area (van der Kamp 1995). Temperature of the water in the cenotes (24.65­28.29 7C) is similar to the mean air temperature calculated by Ward et al. (1985) and ranges from 23 7C in January to 28 7C in May. The data are similar to the mean water temperature reported by Pearse et al. (1936) of 25 7C (21.9­28.5 7C) and Herrera-Silveira et al. (1997) of 26.4 7C (22.0­33.5 7C).

pH Pearse et al. (1936) report pH values of 6.8­8.6 for water in cenotes in northwestern Yucatan state, and Hercept pH (pH units), total dissolved solids (g/L), temperature ( 7C), and fecal coliforms (MPN/100 mL) Cristal cenote 350­696 936­1070 0.02­0.03 0.982­1.222 ND­0.003 0.93­2.62 110­162.5 OK ABS ABS ABS 6.76­6.83 2.0­2.5 25.2­26.2 0­460 Nohoch cenote 332­342 689­1050 ND­0.01 0.703­1.03 0.003­0.006 1.88­2.345 82.5­145 OK ABS ABS ABS 6.88­6.95 1.7­2.0 24.7­26.4 15­30 DWS 1 400 250 0.1 5.0 0.05 4.0 500.0 * ABS ABS ABS 5­9 0.5 NC 6c2.5 1000** RPC 1 NE 3 NE NE NE NE NE NE * ABS ABS ABS NE NE NE 200

Casa cenote 134­292 6237­16 200 ND 4­0.05 0.314­1.023 ND­0.006 2.17­7.02 900­2400 NOT ABS 5 ABS ABS 6.76­7.92 12.5­34.6 25.6­28.3 8­110

Carwash cenote 348­440 207­689 0.007­0.01 0.627­1.09 0.0025­0.003 2.98­4.68 30­80 NOT ABS ABS ABS 6.73­7.47 0.5­1.7 25.1­28.1 0­174

Mayan Blue cenote 344­696 880­1273 ND­0.004 0.492­1.000 ND­0.0122 0.64­4.07 145­170 NOT ABS ABS ABS 6.77­7.31 1.7­2.9 25.2­26.6 8­46

DWS, Drinking-water supply standards RPC, Recreational use with primary or direct contact 3 NE, Not established 4 ND, Not detected 5 ABS, Absent 6 NC, Natural conditions

* Water body must be free of substances that: (1) adversely modified the water physical characteristics or make deposits, (2) hold floating matter which produces nasty appearance, (3) impart odor, taste, or turbidity, (4) favors nasty or undesirable aquatic life ** With conventional treatment

Table 4 Mean ionic salinity of the five cenotes during the dry and rainy seasons. Values are in % meq/L Parameter Casa cenote (top) DS 1 Sodium Potassium Calcium Magnesium Bicarbonate Carbonate Chloride Sulfate

1 b

Casa cenote (bottom) DS 78.4 2.4 3.2 16 0.3 0 90.2 9.5 RS 83 2.6 3.2 11.1 0.6 0 93.4 5.8 DS 38.7 0.6 39.8 20.9 40 0 54 5.6

Carwash cenote RS 55.1 0.7 25.9 18.3 13.5 0 77.1 6.4

Mayan Blue cenote DS 59.5 1.4 20.3 18.7 14.5 0 75.9 9.4 RS 67.3 1.2 18.2 18.3 7.8 0 83.5 7.9 DS 62.4 1.2 19.7 16.7 19.2 0 74.3 6.2

Cristal cenote RS 63.9 1.4 18.8 16 9.1 0 80.1 8.6 DS 53.1 0.5 12.1 34.3 9.1 0 82.6 8.2

Nohoch cenote RS 55.2 0.7 26.1 17.9 13.4 0 78 6.6

RS 2 75.8 2.4 5.9 15.9 1.4 0 89.2 9.1

78.6 2.4 4.5 14.5 0.9 0 88.3 10.8

DS, Dry season RS, Rainy season Q Springer-Verlag

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Fig. 2 Main physical, geochemical, and bacteriological characteristics of the five cenotes compared with the Mexican Drinking Water Standards (MDWS). Dotted lines or an arrow indicate values of MDWS

298

rera-Silveira et al. (1997) report a mean pH value of 7.5 (7.2­8.6) from a variety of aquatic ecosystems, mostly in the Yucatan state. The pH of water in the five cenotes sampled is constant and slightly acid (i.e., ~7; Table 3; Fig. 2). The pH differences between dry and the rainy season are insignificant (6.73­7.66 and 6.74­7.92 in the dry and the rainy seasons, respectively). The highest pH values ( 1 7) correspond to the water in the lower water stratum of the Casa cenote, which is seawater. The remaining samples, including the upper brackish stratum of the Casa cenote, were acid (pHp6.73­6.95). During the rainy season, the pH of water from the Mayan Blue and Carwash cenotes is slightly alkaline. This condition is probably related to particulate CaCO3 material that probably washes into the cenotes. The presence of these layers in both cenotes was confirmed by their lower conductivity and salinity values and higher temperature. For both cenotes, the deeper water is acid (pHp6.74­6.82).

son) to 696 mg/L CaCO3 (Cristal and Mayan Blue cenotes in the rainy season; Tables 3 and 4; Fig. 2). Waters in the Carwash, Nohoch, and the upper stratum of the Casa cenote have small variations in alkalinity between the two periods (3­27%), whereas waters in the Cristal and Mayan Blue cenotes and water in the lower stratum of the Casa cenote have large fluctuations (98­102%). The alkalinity in the water of the Mayan Blue, Cristal, and Carwash cenotes in the rainy season is higher than in the dry season, in contrast to waters in the Nohoch and Casa cenotes, where the dry-season alkalinities are higher than in the rainy season. The lowest alkalinities (i.e., ~300 mg/L CaCO3) occur in water in the Casa cenote, especially in the deep stratum.

Total Dissolved Solids (TDS) Concentrations of TDS are uniform and characteristic of fresh water. An exception is water from the Casa cenote, which varies temporally from brackish to saline, and where surface brackish water overlies seawater (Tables 3 and 4; Fig. 2). In the rainy season, the brackish-water layer of the Casa cenote is thicker (almost 6 m) than in the dry season (about 4 m). The brackish surface stratum is about 12.5­13.5 g/L in both seasons. In the rainy season, a thin, diluted stratum overlies the main water bodies of the Mayan Blue and Carwash cenotes (1700 and 500 mg/L, respectively). Dissolved Oxygen (DO) Characteristically, cenote waters are undersaturated in dissolved oxygen (i.e., DO ~50% saturation; Pearse et al. 1936). This undersaturation is associated with biological (respiration and microbial oxidation of organic matter) and chemical oxidation of the groundwater. The uppermost layer has the highest DO concentrations, and DO diminishes gradually with depth (Table 3; Fig. 2). Dissolved oxygen crosses the atmosphere­water interface, which explains the higher concentrations in the top layer. In addition, photosynthetically-produced DO (i.e., aquatic macrophytes and periphyton) increases DO concentrations in the pools when compared with nearby groundwater. Microbiological (respiration) and chemical DO consumption reduce the DO content of groundwater, which explains the occurrence of a lower DO concentration in the bottom layer. Alkalinity Alkalinity of cenote waters ranges from 134 mg/L CaCO3 (Casa cenote bottom stratum in the rainy seaHydrogeology Journal (1998) 6 : 293­301

Sulfate (SO4) The lowest concentration of sulfate (30 mg/L) occurs in water in the Carwash cenote in the rainy season; the highest value (2400 mg/L) occurs in the lower water layer of the Casa cenote in the rainy season (Tables 3 and 4; Fig. 2). The largest seasonal fluctuation in sulfate concentration was measured in the Casa cenote (900­2400 mg/L SO4) and the lowest in the Mayan Blue cenote (145­170 mg/L SO4). Waters in the other cenotes have fresh-water concentrations and lower sulfate concentrations (170 mg/L). Chloride (Cl) The highest measured chloride concentration (6237­16,200 mg/L) occurs in the lower water stratum of the Casa cenote (Tables 3 and 4; Fig. 2), because of the proximity of this cenote to the sea (about 200 m). Although dilution was expected during the rainy season, the highest surface chloride concentrations in water from the Casa and Nohoch cenotes were observed in this season. The salinity of the upper water layer of the Casa cenote remains homogeneous throughout both seasons, probably because of the continuous mixing between the upper brackish and the lower saline strata. Waters in the other cenotes have lower concentrations in the rainy season, as expected. The chloride concentration fluctuates between the two seasons. The largest seasonal variation in concentration is in water from the Casa cenote (6237­16,200 mg/L), and the smallest is in water from the Cristal cenote (936­1070 mg/L). Pearse et al. (1936) report lower chloride concentrations (70­560 mg/L) from waters in cenotes located farther inland in the Yucatan state. Nitrite Nitrogen (N-NO2) The nitrite concentrations in water of all five cenotes are insignificant compared to the other nitrogen species, including NH4, which is the second-most abundant form of nitrogen after nitrates (Table 3; Fig. 2). Waters from the Cristal, Nohoch, and Casa cenotes have higher nitrite values in the dry season, and waters in the

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Mayan Blue and Carwash cenotes have higher nitrite values in the rainy season.

Nitrate Nitrogen (N-NO3) The most abundant nutrient in waters from all five cenotes is nitrogen in the form of nitrate (Table 3; Fig. 2). Waters from the Mayan Blue and Nohoch cenotes have higher nitrate concentrations in the rainy season; the remaining cenotes have higher concentrations in the dry period. Nitrate concentrations of these five cenotes are substantially less than groundwater nitrate values reported from Yucatan state ( 1 45 mg/L N-NO3) by Pacheco and Cabrera (1997). Nonetheless, as these authors mention, large differences in nitrate concentration in waters from adjacent wells in Yucatan suggest local rather than regional contamination.

Phosphate (PO4) Phosphate concentrations of waters in the cenotes are low (n.d.­0.013 mg/L PO4), due to the presence of high concentrations of calcium. This condition has been observed in similar karstic systems elsewhere (Margalef 1983, Sánchez et al. 1998). No significant differences exist between seasons (Table 3; Fig. 2). The lower water stratum of the Casa cenote has dissolved phosphate concentrations that are below detection limit in both the rainy and dry seasons. The highest concentration of phosphate occurs in water from the Mayan Blue cenote (0.0130 mg/L PO4), and the lowest concentrations are in waters from the Mayan Blue, Casa, and Nohoch cenotes (not detected).

Aesthetic Characteristics The cenotes have clear and oxygenated waters most of the time, and rocky or sandy bottoms that are covered to a variable degree with submersed aquatic macrophytes (e.g., Cabomba sp., Nymphaea sp., Sagittaria sp.). Diverse biota (e.g., fish, crayfish, and turtles) are easily observed through the transparent crystal-blue waters in the cenotes. During the rainy season, waters of the Casa, Carwash, and Mayan Blue cenotes have a dark-red coloration that is produced by tannic acid leached from the surrounding area. In addition, a large quantity of suspended matter (i.e., particulate organic matter) adds turbidity to the reddish water. However, this reddish layer disappears within a few days after rainfall stops. The presence of tannic acid diminishes the aesthetic attraction and increases the cost of making the water potable. No oil, grease films, or matter was observed floating on the water surfaces. Stagnant cenotes develop brightgreen floating algal blooms, and sometimes leaves or small tree branches fall on the surface. However, during this study, no anthropogenic floating matter was observed. Water in the cenotes is odorless.

Discussion and Conclusions

The five cenotes in this study correspond to the "common" cenote, with clear water, a clean sandy or rocky bottom, and a homogeneous, oxygenated water mass. No horizontal differences (physical or chemical) were observed in the cenote pools. This characteristic is probably related to their small surface areas, constant water flows, and, thus, short retention times. Moore et al. (1992) calculated maximum groundwater flow velocities of 1 cm/s in the Carwash cenote and 3 cm/s in the Mayan Blue cenote. In the Cristal cenote and in the fast-flowing current in the Casa cenote, horizontal water movement from the springs, where water emerges to form the pools, and from the pools into the caves, where water goes back underground, was observed by following the trajectory of disturbed fine silt. Currently, all five cenotes are being used for water sports, such as swimming and diving, which are designated as "Recreational Use with Primary or Direct Contact." In the foreseeable future, these cenotes may be considered as sources for drinking water. The Casa cenote is the only one that does not meet the MDWS, due to its high TDS content; thus, it will not be considered as a potential source for drinking water. The MDWS and the "Standards for Recreational Use with Primary or Direct Contact" are given in Table 3. Groundwater salinity in Yucatan Peninsula is 400­2900 mg/L, as measured by Doehring and Butler (1974) from various wells throughout the peninsula. Data from this study, excluding the Casa cenote, are within this range. The bottom water stratum of the Casa cenote is almost pure seawater (Tables 3 and 4); it

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Fecal Coliform Bacteria Waters from the five cenotes tested contain small amounts (fewer than 500 bacteria per 100 mL) of fecal coliform bacteria, and most samples had counts that were fewer than 50 bacteria per 100 mL (Table 3; Fig. 2). Other studies (e.g., Doehring and Butler 1974) report that counts of fecal coliforms fluctuate from 0­4200 bacteria per 100 mL (1255B1086 bacteria per 100 mL). Results of the present study indicate that counts are 3­460 bacteria per 100 mL. In general, the number of fecal coliform bacteria is higher in the rainy season. Only water from the Cristal cenote has more bacteria in the dry season than in the rainy season (460 and 186 bacteria per 100 mL, respectively). Waters from the Nohoch cenote (15 and 30 bacteria per 100 mL, respectively) and the Maya Blue cenote (19 and 46 bacteria per 100 mL, respectively) have about double the number of fecal bacteria in the rainy season compared to the dry season. The most striking difference in bacteria count is in water from the Carwash cenote, where fecal coliform bacteria increased from 3­174 bacteria per 100 mL from the dry to the rainy season.

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has the highest dissolved-oxygen concentration and DO saturation values ( 1 70%). Water in the Cristal cenote has a slightly higher bottom DO concentration, because it has a larger population of aquatic macrophytes (i.e., Cabomba sp.), compared to the other cenotes. A freshwater input to the Mayan Blue and Carwash cenotes in the rainy season accounts for the higher DO concentration of the shallow water. Dissolved-oxygen concentrations of the cenotes are below the desirable level (4 mg/L), as defined by the MDWS. Oxidation processes, including chemical and microbiological, tend to remove most or all of the DO from the groundwater (van der Kamp 1995), which explains the low DO concentrations in the pools. Sinkholes along the northeastern coast of Yucatan act as local basins for the collection of organic debris (Stoessell et al. 1993). DO levels in the pools are higher than in the springs, because of the series of chemical changes that occur as oxygen-poor groundwater emerges at the ground surface and is suddenly exposed to the oxygenrich atmosphere, to temperature changes, and to biological activity (e.g., photosynthesis). Escobar-Briones et al. (1997) and J. Alcocer (unpublished data) observed that DO levels in groundwater of the cave systems of the cenotes average 1.5­2.0 mg/L, which is 0.5­1.0 mg/L lower than in the pools. Herrera-Silveira (1994) reports DO concentrations less than 1 mg/L in groundwater that discharges to coastal lagoons. Chloride concentrations in waters of the cenotes exceed the MDWS recommendation; the high values suggest salt-water intrusion. Two sources of salt water occur in the Yucatan Peninsula (Lesser and Weidie 1988, Perry et al. 1996, Steinich and Marín 1996, Steinich et al. 1996). The first is the dissolution of evaporite deposits interbedded in the carbonate rocks, and the second is seawater. Over-exploitation of the aquifers, combined with the shallowness of the fresh-water/salt-water interface is modifying the ionic dominance in the shallow fresh-water lens. In the northwestern and central parts of the peninsula, dissolution of evaporites is the main source of salt water. Nitrates are often reported as common groundwater contaminants concurrently with bacteria (Scanlon 1990). Fertilizers, pesticides, and organic wastes increase nitrate concentrations in karstic systems. Nitrate contamination plumes emanating from septic tanks have also been reported (van der Kamp 1995). Natural nitrate sources, such as leguminous and non-leguminous terrestrial nitrogen-fixing plants and plant-litter decomposition, also contribute to nitrate species in groundwater. An additional source of nitrate in groundwater is ammonia, as suggested by Stoessell et al. (1993). Ammonia may be oxidized to nitric acid and may further dissociate into nitrate species in the presence of oxygenated waters. The northeastern part of the Yucatan Peninsula is poorly cultivated, and the population density is still low. Both factors explain the low nitrate concentrations in waters of the five cenotes, all of which are much less than the MDWS (5 mg/L).

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The presence of fecal coliform bacteria suggests the potential presence of pathogenic viruses that are responsible for various diseases, including infectious hepatitis; the low concentrations reported in this study indicate a low risk of infection. Sewage and domestic animals are potential sources of biological contamination. The five cenotes are regularly visited by tourists and, thus, are subject to biological contamination. The Casa cenote has a small restaurant nearby, and the people who run the restaurant live in small huts near the cenote. Although wastes from the restaurant discharge into a septic tank, the inhabitants and their domestic animals (turkeys, dogs, etc.) rely on a less formal means of personal waste disposal. The yards near or adjacent to the living quarters in the Nohoch cenote and to the touristic facilities (dressing rooms and kiosk) in the other cenotes (i.e., the Cristal, Carwash, and Mayan Blue cenotes) are used as latrines. Few domestic animals (dogs) are known near the Cristal, Mayan Blue, and Carwash cenotes, but in the Nohoch cenote, numerous dogs, cats, horses, and cows have been observed. Cristal cenote has the highest count of bacteria (460 MPN/100 mL); the high value is probably associated with the high volume of monthly visitors. In general, aesthetic characteristics are satisfactory for potable water and recreational uses, except during the rainy season, when the possibility increases of the clear, blue water turning into a red, turbid one. Four of the five cenotes (Carwash, Nohoch, Mayan Blue, and Cristal) currently meet the MDWS. The Casa cenote does not meet the MDWS, due to its high TDS content. However, two potential problems exist with respect to the other four cenotes being used as sources of drinking-water supplies: (1) unless a proper groundwater extraction scheme is developed that minimizes saltwater intrusion, the water quality is likely to be degraded quickly; such a scheme should be supplemented by monitoring the chloride content carefully; and (2) the alkalinity, chloride, and TDS concentrations in several of the cenotes exceed the MDWS. The dissolvedoxygen content required by the MDWS is not met; however, this does not pose a major health hazard. Thus, the conclusion is that from a water-quality standpoint the cenotes, except for the Casa cenote, are suitable as drinking-water sources. In summary, low levels of the reduced forms of nitrogen (i.e., nitrites and ammonium), as well as low nitrate values, indicate that waters in the Casa, Carwash, Cristal, Nohoch, and Mayan Blue cenotes are not contaminated. Currently, the main problem is the high TDS content at the Casa cenote, and the potential for substantial increases in TDS at the other cenotes, unless a groundwater withdrawal scheme is conducted that considers the potential for salt-water intrusion. Furthermore, in order for these cenotes to provide potable water for many years to come, proper sanitary facilities should be constructed in the areas surrounding the cenotes.

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301 Acknowledgments This project was financially supported by Dirección General de Asuntos del Personal Académico de la UNAM project IN203894. L.E. Marín acknowledges support from the Consejo Nacional de Ciencia y Tecnología (project 0258PT). The authors thank Dr. D. Valdés and Chem. E. Real de León (CINVESTAV Unidad Mérida) for carrying out nutrient and microbiological analyses. Special thanks are given to the Instituto de Ciencias del Mar y Limnología field station Puerto Morelos for providing lodging and laboratory support, and to Mike Madden and his team (especially Chuck Stevens) of CEDAM Dive Center at Puerto Aventuras for providing cave-diving equipment and logistical support. Biologists V. Urbieta, M.E. García, M. Sánchez, L. Peralta, and L. Oseguera are acknowledged for helping in the collection of biological and water samples, and for conducting chemical analyses. The authors sincerely thank Dr. H. Mooers for suggestions and comments that greatly improved this paper. North America. Geological Society of America, pp 237­241 Margalef R (1983) Limnología [Limnology]: Omega, Barcelona Marín LE (1990) Field investigations and numerical simulation of a karstic aquifer of northwest Yucatan, Mexico. PhD Dissertation, Northern Illinois University, De Kalb, 127 pp Marín LE, Perry EC (1995) The hydrogeology and contamination potential of northwestern Yucatan, Mexico. Geofísica Internacional 33 : 619­623 Moore YH, Stoessell RK, Easley DH (1992) Fresh-water/sea-water relationship within a ground-water system, northeastern coast of the Yucatan Peninsula. Ground Water 30 : 343­350 Pacheco J, Cabrera A (1997) Groundwater contamination by nitrates in the Yucatan Peninsula, Mexico. Hydrogeol J 5 : 47­53 Parsons TR, Maita Y, Lalli CM (1984) A manual of chemical and biological methods of seawater analysis. Pergamon Press, London Pearse AS, Creaser EP, Hall FG (1936) The cenotes of Yucatan. A zoological and hydrographic survey. Carnegie Institution of Washington, Washington DC Perry EC, Marín LE, McClain J, Velazquez G (1996) The ring of cenotes (sinkholes) in northwest Yucatan, Mexico: its hydrogeologic characteristics and possible association with the Chicxulub impact crater. Geology 23 : 17­20 Sánchez M, Alcocer J, Lugo A, Sánchez MR, Escobar E (in press) Variación temporal de las densidades bacterianas pláncticas en cinco cenotes y dos cuevas sumergidas del NE de Quintana Roo, México. In: Mancilla JM, Vilaclara G (comps) Cuadernos de investigación interdisciplinaria en ciencias de la salud, la educación y el ambiente, vol. 1. UNAM, México pp 66­80 Scanlon BR (1990) Relationships between groundwater contamination and major-ion chemistry in a karst aquifer. J Hydrol 119 : 271­291 Steinich B, Marín LE (1996) Hydrogeological investigations in northwestern Yucatan, Mexico, using resistivity surveys. Ground Water 34 : 640­646 Steinich B, Velázquez G, Marín LE, Perry EC (1996) Determination of the ground water divide in the karst aquifer of Yucatan, Mexico, combining geochemical and hydrogeological data. Geofísica Internacional 35 : 153­159 Stirling HP (1985) Chemical and biological methods of water analysis for aquaculturists. Institute of Aquaculture, University of Stirling, Scotland Stoessell RK, Moore YH, Coke JG (1993) The occurrence and effect of sulfate reduction and sulfide oxidation on coastal limestone dissolution in Yucatan cenotes. Ground Water 31 : 566­575 Stoessell RK, Ward WC, Ford BH, Schuffert JD (1989) Water chemistry and CaCO3 dissolution in the saline part of an open-flow mixing zone, coastal Yucatan Peninsula, Mexico. Bull Geol Soc Am 101 : 159­169 Strickland JDH, Parsons TR (1972) A practical handbook of seawater analysis. Fisheries Research Board of Canada Bulletin, vol 167, 310 pp Velázquez G (1995) Estudio geoquímico del anillo de cenotes, Yucatán [Geochemical study of the enotes ring, Yucatan]: MSc thesis, Universidad Nacional Autónoma de México, Mexico, 77 pp van der Kamp G (1995) The hydrogeology of springs in relation to the biodiversity of spring fauna: a review. J Kansas Ent Soc 68 : 4­17 Ward WC, Weidie AE, Back W (1985) Geology and hydrogeology of the Yucatan and Quaternary geology of northeastern Yucatan Peninsula. New Orleans Geological Society, New Orleans, 160 pp

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