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J. Appl. Sci. Environ. Mgt. 2005 Vol. 9 (1) 157 - 160

Geoelectric investigation of the aquifer characteristics and groundwater potential in Kwale, Delta state, Nigeria OSEJI J O; ATAKPO E A; *OKOLIE E C

Department of Physics, Delta State University Abraka ABSTRACT: Subsurface geo-electrical survey using the electrical resistivity (VES) method and drillers logs were carried out in Kwale, Ndokwa West Local Government Area of Delta State in order to investigate the aquifer characteristics and ground water potential of the subsurface formations. Nineteen vertical electrical soundings were carried out within the town and its environment using the Schlumberger array configuration. The data was interpreted using the conventional curve matching and computer iteration method. The results reveal a five layer HKH, KQH and AKH type curves. The true resistivity of the top soil varies from 55-236m while the thickness varies from 0.7-1.0m. The second layer has resistivity ranging from 37-627 and thickness ranging from 1.3-3.6m. This layer is composed of clay to sand. The third layer, which constitutes an aquifer, has resisitivity varying from 2.7 to 411m while the thickness varies from 4-17m. It is composed of fine-grained sand. The fourth geoelectric layer has resistivity values ranging from 114 to 168m while the thickness varies form 15 to 35m. This layer is composed of fine to mediumgrained sand. The soft layer has resistivity ranging from 326-2308m. This layer is composed of medium coarsegrained soil and it constitutes an aquifer of very good quality groundwater. The average depth of this aquifer is between 30-40m. These results were correlated with lithological logs from boreholes drilled in the study areas and were found to be consistent. @JASEM In the past, residence of Kwale depended on the slow-running River Niger for their domestic water needs but today increased activities from companies have drastically polluted the river and rendered it unfit for use. Unfortunately, this is the only available source despite increased demand for potable water in Kwale due to increase in population within the last few years. However, with recent technological development groundwater is the choice for domestic and industrial use. A better knowledge of the aquifer formation and characteristics (confined or unconfined) in this area is therefore important so as to ascertain whether the aquifer is prone to contamination or not. To achieve this, hydrogeological and geophysical studies were necessary in the study area. Hence the study was carried out to establish a baseline geophysical data and hydrological characteristics using the vertical electrical sounding method and lithological logs of the area. The vertical electrical method was chosen for this study because the instrumentation is simple, field logistics are easy and straight forward while the analysis of data is less tedious and economical Zhody et al , (1974), Ekine and Osobonye, (1996), and Ako and Olorunfemi (1989). The resisitivity technique was successfully used in investigating groundwater potential in different geological settings. Emenike (2000) also used this method to explore for groundwater in a sedimentary environment. The geophysical data obtained and the analyses were correlated with groundwater lithological logs from ten boreholes in the study area. *Corresponding author Geology Of The Study Area:The study area, Kwale is located in Ndokwa West L.G.A of Delta State. It is underlain by the continental sands of the Benin formation. The geology of the Niger Delta has been extensively described by several authors including, Akpokodje and, Etu-Efetobor (1987), Short and Stauble (1965), Asseez (1976). The subsurface sedimentary sequence has been subdivided into three stratigraphic units.-the Benin, Agbada and Akata formations (Kogbe and Asseez (1979). The Benin formation consists of sand, gravely sand, sandy clay and clay intercalations. The formation is known for its high aquifer potential. The lithological units of this area are generally composed of sands and clayey sand. The area has a flat topography and is situated by the bank of river Niger. It is thickly populated and of high economic importance.

MATERIALS AND METHODS

The electrical resisitivity method was used for the investigation. A total of nineteen VES stations were surveyed in the study area. Four VES stations were established and surveyed in Ishiukpo quarter while three VES stations were established and surveyed in each of the following quarters; Ogbe-Uku, AlamaOssai, Ogbe-Akala, Ogbe- Utim and Ogbe-Ovili, The resisitivities of the layers were measured using the ABEM SAS 300 terrameter and SAS 2000 Booster. The schlumberger electrode configuration having a maximum current electrode spread of 500m was used. The apparent resisitivity values obtained from the measurement were plotted against half the current electrode spacing on a log-log graph in order to determine the apparent resisitivities and thicknesses

Geoelectric investigation... of the layers. The curves were interpreted quantitatively by matching small segments of the field curves using two-layer model curves and the corresponding auxiliary curves. The resistivities and

158

thicknesses obtained from the partial curve matching results were used for computer iteration using the resist software Vander, (1988).

Table 1: Resistivities, Layer Thickness and Curve Types of Selected Ves Stations. VES STATION LAYER 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 RESISTIVITIES m 55 37 337 123 1029 217 165 411 114 2308 96 73 387 166 625 283 627 313 156 1138 236 367 217 164 326 36 40 301 168 1142 THICKNESS (m) 0.7 1.3 6.6 16.3 0.7 1.4 3.6 141 0.8 1.7 17.3 38.0 0.5 1.6 5.9 30.9 0.9 3.0 7.6 17.1 0.8 3.6 12.9 32.9 CURVE TYPE

1

> < > < HKH 1

2

3

4

5

3

> < HKH

> < 1 2 3 4 5

> HKH

< > < 1 2 3 4 5

6

< KQH

> > < 1 2 3 4 5

9

< KQH

> > < 1 2 3 4 5

12

17

< < > < AKH

1

2

3

4

5

RESULTS AND DISCUSSION

The resistivities, thickness and curve types from the interpreted sounding curves of randomly selected VES stations from each of the Quarters studied are as shown in table 1. From the table, VES 1, 3 and 6 are five layer HKH type curves, while VES 9 and 12 are five layer KQH type curves. VES 17 is a five layer AKA type curve. The interpreted curves are presented as geo-electric section (Fig. 1). The first geoelectric layer corresponds to the top soil with resistivity ranging from 55-236m reflecting the various compositions and moisture constant of the top soil. It is composed of clay, fine sand and decomposed organic materials. The thickness varies from 0.7-1.0m. The second geoelectric layer of VES 1, 2, 6 and 17 has resistivity ranging form 37-73m and thickness ranging from 1.3-3.6m. The resistivity is diagnostic of clayey layer. This may act as a confining layer however because of the small clay thickness, it may be susceptible to pollution. The resistivity of the second layer of VES 9 and 12 ranges from 367-627m and the thickness ranges from 1.6-3.0m. This layer is composed of fine sand and does not act as a confining layer. The third geoelectric layer has resistivity ranging from 217 to 411m and thickness ranging from 4-17m. It is composed of fine-grained sand. This layer constitutes the first aquifer. The fourth geoelectric layer has resistivity value ranging from 114 to 168m and thicknesses from 15 to 35m. This layer is composed of fine to medium grain sand with little clay and iron content. The fifth geoelectric layer has resistivity ranging from 326-2308m. The thickness of this layer is not defined since it is the last layer. This layer consists of medium to coarser grained sand which constitutes an aquifer of very good quality groundwater. The average depth of this aquifer is between 30-40m. The VES interpretations correlate well with information from borehole lithologic logs obtained in the study area (Fig.2).

Oseji et al

Geoelectric investigation...

159

VES 17 Bore hole 2 40 m 301 m 10

Bore hole 1

VES 1 37 m

VES 3

VES 6

VES 9 627 m 313 m

VES 12 367 m 217 m

165 m 73 m 411 m 387 m

10 20

337 m

123 m

114 m

956 m

20 164 m 168 m 30 30 20

166 m 30

326 m 2308 m 40 1029 m 1138 m sand 1142 m 625 m FIG 1 : GEOELECTRIC SECTION OF STUDY

LEGEND Topsoil Fine Sand Fine-medium Clay Silty Coarse

50 60

Depth [m]

LATERITIC SOIL

Depth [m]

LITHOLOGY LATERITIC SOIL FINE GRAINED SAND

3 3 6

MEDIUM GRAINED MEDIUM GRAINED

FINE GRAINED SAND

6

9 12 15 18 21 24 27 30 33

FINE GRAINED SAND MEDIUM GRAINED SAND FINE GRAINED SAND FINE GRAINED SAND MEDIUM GRAINED MEDIUM GRAINED

9 12 15 18 21 24 27 30

VERY FINE MEDIUM GRAINED SAND

FINE GRAINED SAND CLAYEY SAND FINE GRAINED SAND FINE GRAINED SAND FINE GRAINED SAND COARSE GRAINED SAND

FINE GRAINED SAND

FIG.2b: BOREHOLE LITHOLOGY KWALE VES 2

FIG.2a: BOREHOLE LITHOLOGY KWALE VES 1

Oseji et al

Geoelectric investigation... Conclusion: The results of the interpreted data and the lithologic logs from boreholes indicate five geoelectric layers, which are not confined. The second layer, which may act as a confining bed, is not thick enough so in the event of pollution the ground water may occur. From the study, it is recommended that boreholes are drilled to 30-34m to harness potable water within the 2nd aquifer.

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Etu- Efeotor, J O; Akpokodje, E G (1990): Aquifer systems of the Niger Delta. Nigerian Journal of Mining Geology Vol 26 (2) pp 279-284. Kogbe, L A (1981): Geology of Nigeria. Rock view (Nig) Ltd. Jos pp 235-335. Short, K C; Stauble, A J (1967): Outline of Geology of Niger Delta. American Association of petroleum Geologist Vol. 51 No 5 pp. 761779. Vander, V (1988): RESIST ITL MBL Research Project Zhody, A R; Eaton, G I; Masay, D R (1974): Application of Subsurface Geophysics to Groundwater investigation. Technical Water Resources Investigation, Washington Geological Survey No 2401-2543.

REFERENCES

Ako, A O; Olorunfemi, M O (1989): Geoelectric survey for groundwater in the Newer Basalts of Vom Plateau State. Nig Journal of Mining and Geology Vol 25 (1& 2) pp 247-450. Akpokodje, E U; Etu-Efetobor, J O (1967): The occurrence and economic potential of clean sand deposits of the Niger Delta. Journal of African Earth Sciences Vol 1 pp 61-65 Ekine, A S; Osobonye, (1996): Surface Geo-electric sounding for the determination of Aquifer characteristics in parts of Bonny Local Government Area of River State. Nigeria Journal of Physics Vol. 85, pp 93 ­ 97.

Oseji et al

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