Evaluation of the overburden protective capacity of aquifer-aquitard systems in Agbara town, southwestern Nigeria using electrical resistivity techniques

Abstract

This study utilized electrical resistivity techniques to assess the protective capacity of overburden materials within Agbara industrial area in southwestern Nigeria. Forty vertical electrical sounding (VES) stations and six 2- D traverses were occupied using Schlumberger and Wenner configurations, respectively. Both techniques suggest the existence of a multi-layered aquifer-aquitard systems, characterized by a lithological sequence presumably composed of sand and clayey sand, of moderate resistivity, alongside clay of low resistivity and high-resistivity hard clay. At most locations, thin unconfined phreatic sand or clayey sand aquifers were identified at shallow depths less than 5 m. Seasonal variations in groundwater volume and quality usually render phreatic aquifers unsuitable for long-term sustainable groundwater development. Much deeper sand/clayey sand aquifers (5.0 - 68.0 m) were identified at seventy percent of the surveyed locations. At twenty VES stations, impermeable subsurface zones were delineated: clay layers with resistivity in the range 29 - 71 Ωm and thickness range of 2.5 -10.0 m and hard clay medium with resistivity range of 818 - 2557 Ωm and thickness range 1.4 - 52.4 m. Clay materials are common aquitard which act as lithological barriers since they can impede the movement of contaminants into groundwater. In contrast, if the clay is hard, compacted, and consolidated, cracks can quickly allow water and dissolved pollutants to enter aquifer systems. Computed values of overburden protective capacity (longitudinal conductance) portrayed significant variations with twenty-seven (67.5%) locations showing poor ratings (0.0027 - 0.0813 S); nine locations (22.5%) exhibiting weak ratings (0.1044 - 0.1824 S); and only four (10%) indicating moderate ratings (0.348 - 0.472 S). Moderate to very high overburden transverse resistance (filtration coefficient) values (544 - 81105 Ωm²) also support these ratings. The findings indicate that the aquifers in these areas are vulnerable to contamination, which should be a critical concern in groundwater projects in this area.