Geoelectrical evaluation of overburden protective capacity in an Alluvial deposit microenvironment in the vicinity of an industrial district, southwest Nigeria: Insights on aquifer vulnerability and groundwater contamination

In this study, electrical resistivity techniques were employed in the evaluation of overburden protective capacity at a location designated for industrial activities in southwest Nigeria. 40 VES stations and 6 resistivity imaging traverses were occupied using Schlumberger and Wenner configurations respectively. Results indicate the presence of typical multi-layered aquifer and aquitard system, presumably composed of sand, clayey sand, clay and hard clay. Predominant phreatic (unconfined) sand/clayey sand aquifers mapped at shallow depth (< 5m) are not suitable for long term sustainable groundwater development, since volume and quality of groundwater vary with seasons. 70% of surveyed locations are underlain by much deeper (5.0-68.0 m) sand/clayey sand aquifer. The occurrence of impermeable clay with resistivity range 29-71 Ωm and thickness range 2.5-10.0 m and/or hard clay with resistivity in the range 818-2557 Ωm and thickness range 1.4-52.4 m is depicted across 20 VES locations. Clay materials are lithological barriers to pollutants infiltration into the aquifer zone. However, fractures in hard compacted and consolidated clay can serve as pathways for rapid movement of water and dissolved contaminants into aquifer systems. The overburden protective capacity rating at the locations exhibits abysmally poor rating (0.0027-0.0813 S) in 27 (67.5%) l; weak (0.1044-0.1824 S) in 9 (22.5%) and moderate rating (0.348-0.472 S) in 4 (10%). These values suggest high probability of pollutants migration into groundwater resources. This is also supported by high values of transverse resistance (filtration coefficient).