ESTIMATION OF AQUIFER PARAMETERS FROM SURFACE GEOELECTRICAL MEASUREMENTS AROUND OKITIPUPA, SOUTHWESTERN NIGERIA

Abstract

The failure of a good number of groundwater projects may be largely attributed to inadequate evaluation of the aquifer. An attempt has been made in this study to estimate the hydraulic parameters of the coastal plain sands aquifer around Okitipupa, southwestern Nigeria using the Dar Zarrouk parameters. The work is also geared towards establishing the relationship between aquifer parameters and resistivity measurements. Fifty vertical electrical sounding (VES) data using the Schlumberger electrode configuration with electrode spacing (AB) range of 2m to 1000m were acquired. Five of the VES data were obtained around existing boreholes (Parametric sounding). The sounding data were first interpreted quantitatively using manual partial curve matching and further iterated to refine the parameters. The interpretation of the data enabled the delineation of the aquifer. The hydraulic conductivity (K) measured in some reference boreholes have been combined with the electrical conductivity (σ) parameters obtained from parametric sounding data. The resulting relation of the product of K and σ i.e. (Kσ) was interpreted with Dar Zarrouk parameters to infer the trasmissivity and hydraulic conductivity variations at other VES locations where K values are unknown. The results of the analysis of water samples from available boreholes and wells show similarities in the chemistry of the groundwater. pH ranges from 4.45 to 7.34. The Total dissolved solids (TDS) vary from 6.7 to 1005. Alkalinity ranges from 12- 20 in four samples. The work reveals that the transmissivity in the delineated aquifer unit varies from 1.72m2/day to 543m2 /day, and K values ranging between 0.4m/day and 11.60m/day across the study area. These values are close to the values (0.11m/day to 25.5m/day) obtained via the convectional pumping test data of boreholes around the area. From the hydraulic conductivity (K) and transmissivity (Tr) maps, K values ranging between 4m/day and 12m/day characterize Igbobini, Irele, Lekun Camp and Lamudifa areas. Similarly, Tr values ranging between 60m2/day and 540m2/day of (T) are found within the central portion of the map. This research work has shown that apart from employing electrical resistivity method in ground water exploration, it can also serve as a valuable tool for the evaluation of the water bearing medium (aquifer) before exploitation stage (drilling). It may be inferred from the foregoing that there exist a direct linear relationship between aquifer hydraulic parameter and electrical resistivity measurement. It is, therefore, recommended that this type of study is integrated in groundwater prospect evaluation in sedimentary terrains.