Modeling of hydrogeological parameters and aquifer vulnerability assessment for groundwater resource potentiality prediction at Ita Ogbolu, Southwestern Nigeria

Groundwater prospects and vulnerability mapping to vertical contamination, utilizing multi-criteria evaluation techniques and analytical hierarchy process were carried out within five geologic units in Ita-Ogbolu, southwestern Nigeria, comprising older granite (OGu), migmatite (M), charnockite (Ch), medium to coarse grained biotite granite (OGe), and coarse to porphyritic biotite hornblende granite (OGp). Seventy one electrical soundings involving Schlumberger array with 65 m electrode spread; in-situ (hydraulic) pumping tests; and hydrogeological parameter measurements from fifty one wells were used to acquire the hydrogeological data. The vulnerability evaluation was done using geoelectrical parameters obtained from longitudinal conductance (LC); and the aquifer vulnerability index (AVI), while the groundwater potential index value modeling (GWPIV) was used develop the groundwater potential map. All parameters measured were weighted, rated and assessed according to their significance to groundwater accumulation. The major aquifer unit was the weathered layer, having a 301 ohm m average resistivity, thickness of 14.1 m, 0.67 m/d hydraulic conductivity (K), and 9.82 m(2)/d transmissivity. The average depth of the overburden is 19.6 m, which is a moderate depth for groundwater accumulation. In addition, weak positive coefficients model between K and formation factor were found for all rock units. The relationship or correlation model of hydraulic conductivity (K) and formation factor (Fm) gives OGp (0.6005e(-0.018x)), M (0.4601e(0.0695x)), Ch (0.0838e(-0.0673x)), OGu (1.1623e(-0.091x)), and OGe (0.5937e(0.0578x)). The relationship shows weak positive (< 0.5) for all the rock units. The obtained GWPIV varied from 2.80 to 7.73, with an average of 5.04 suggesting moderate potential; with the potential decreasing in the following order: OGu-OGe-M-OGp-Ch. The calculated AVI model values range from 0.23 to 1.74, with an average of 1.22 indicating extremely vulnerable aquifer to vertical contamination; likewise the vadose zone thickness (6.41 m avg.), and LC (0.1470 mhos avg.) all points to lack of protective capability.