INTEGRATION OF GEOLOGIC AND GEO-ELECTRICALLY DERIVED PARAMETERS FOR AQUIFER VULNERABILITY ASSESSMENT IN AKURE METROPOLIS, SOUTHWESTERN NIGERIA

Abstract

The assessment of groundwater vulnerability was carried out in north-western part of Akure, Southwestern Nigeria where presence of automobile workshops is the major source of pollution. One hundred and eighty-seven (187) Vertical Electrical Sounding data were acquired using Schlumberger array technique with (AB/2) ranging from 65 - 150 m. Fifteen (15) water samples were obtained from wells for the chemical analysis across the area. The VES results delineated 3 to 5 geoelectric layers across the area which corresponds to five subsurface layers; topsoil, weathered layer, partially weathered basement, partially fractured basement and the presumed fresh basement. The subsurface layer resistivities vary respectively as 4.7 - 790.1 Ω-m, 3.4 - 3409.9 Ω- m, 51.1 - 2597.8 Ω-m, 144.2 - 509.3 Ω-m and 695.5 - ∞ Ω-m in the topsoil, weathered layer, partially weathered basement, partially fractured basement and the fresh basement. The thickness of the layers varies respectively as 0.4 - 4.1 m, 0.9 - 18.6 m, 7.7 - 13 m and 1.2 - 58.2 m. The overburden thickness across the area varies from 6.7 - 61.4 m across the study area. Six parameters of hydrogeological importance were used to develop groundwater vulnerability model for the study area. The parameters consist of surface elevation, lithology, aquifer overlying layer resistivity, aquifer overlying layer thickness, coefficient of anisotropy and hydraulic conductivity. These factors were subjected to the Fuzzy Analytical Hierarchy Process (FAHP) method of weightage determination to assign weights to each criterion of the groundwater vulnerability conditioning factors. Groundwater vulnerability map (GWVM) was produced using the determined groundwater vulnerability index. The GWVM of the area classified the area into four (4) vulnerability zones; very low, low, moderate and high. Two parameters were used to validate the developed GWVM; namely physiochemical parameters determined from water samples collected across the area (Lead, Pb) and longitudinal conductance derived from the geo-electric parameters. The validations were done by superimposing the values of the validating parameters on the GWVM. The validation using Lead (Pb) and longitudinal conductance values gave accuracy of 73% and 74% respectively and they both proved the efficacy of the model.