Delineating Saline Water Zones in an Inland Brine Area Using Electrical Resistivity Indicators (Published)
This paper presents results of saline groundwater zone delineation using electrical resistivity sounding. The geologic setting of the study area, Awe Town is that of the Biddle Benue Trough, a structure filled with Cretaceous sediments that include Awe Formation. The town is a predominantly rural area plagued with saline groundwater resulting from brines associated with Awe Formation. As a result of this challenge, water supply scheme for the area was situated about 6km away from the main settlement making access to potable water difficult. The present study was prompted by the need to delineate the extent of groundwater contamination by the brines and in so doing identify aquifers close to the settlement for exploitation. Electrical resistivity method has been applied extensively in delineation of saline groundwater zones. Electrical resistivity surveys were done around the town with the sole aim of delineating the saline groundwater and fresh groundwater regions around the settlement. A total of 20 vertical electrical soundings were carried out using the Schlumberger configuration. Geophysical indicators (longitudinal conductance and transverse resistance (S and T respectively) as well as transverse resistivity (ρt) and longitudinal resistivity (ρl)) were used to assess saline water distribution in the aquifers of the study area. Of the four indicators, Longitudinal Unit Conductance and Transverse Unit Resistance clearly demarcated saline groundwater zones. Contour maps showing the distribution of these indicators aided in the identification of fresh groundwater, 1.2km from the main town. Analysis of borehole logs in the fresh groundwater region revealed fresh groundwater aquifers within Younger rocks i.e. Ezeaku and Keana Formations, which overlie the Awe Formation. Geologic setting of the study area is such that Awe Formation is exposed in the Old Town due to uplift and weathering, while younger rocks of Ezeaku and Keana Formations overlie Awe Formation in the New Town area. Aquifers in these formations have resistivity in the range of 150 to 300Ωm, with thicknesses ranging between 20m and 35m; these zones were interpreted to have high yield potential in terms of quality.
Previous researches shows that resistivity values are not sufficient in adequately and correctly delineating aquiferous zone or areas of high groundwater yield in crystalline basement terrain, a complementary has been recommended, the aim of this study is to delineate fractured zones and area of groundwater potential from reflection coefficient and overburden thickness above the bedrock. Geoelectric measurements using the vertical electrical sounding (VES) method were carried out in Alakia, Ibadan Oyo State, Nigeria. Eight profiles were carried out using the Schlumberger array configuration. The data was interpreted using the method of curve matching and computer iteration (WinResist Software). Results showed that four to six geologic layers were identified in the study area and they include the topsoil, clayey, clayed sand, sandy soil, lateritic soil, weathered/highly fractured basement. The values of the reflection coefficient ranges from 0.16 to 0.78, the low values imply the study area is well fractured while the overburden thickness varies between 8.1 and 59.4m. 88% of the study area has overburden thickness that is greater than 30m, while all the VES points have reflection coefficient that is less than 0.8. The result revealed that the area is well fractured and the groundwater prospect within the area is very high.