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Journal of Applied Sciences & Environmental Management, Vol. 9, No. 1, 2005, pp. 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 Code Number: ja05029 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 55-236Ωm 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 411Ωm 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 168Ωm while the thickness varies form 15 to 35m. This layer is composed of fine to medium-grained sand. The soft layer has resistivity ranging from 326-2308Ωm. This layer is composed of medium coarse-grained 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
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, Alama-Ossai, 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
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 thicknesses obtained from the partial curve matching results were used
for computer iteration using the resist software Vander, (1988).
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 55-236Ωm 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 37-73ΩΩm 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 367-627Ω>m 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 411Ωm 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 168Ωm 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 326-2308Ωm. 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). Conclusion: The results of the interpreted data and the lithologic logs from boreholes indicate five geo-electric 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. REFERENCES
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