Groundwater-yielding capacity, water–rock interaction, and vulnerability assessment of typical gneissic hydrogeologic units using geoelectrohydraulic method.

Geohydraulic parameters, namely hydraulic conductivity (K), transmissivity (T), effective porosity (ϕ ), permeability ( k p ), anisotropy coefficient (λ), and longitudinal conductance (S), of aquifer units in Etioro-Akoko, southwestern Nigeria, were evaluated using the Schlumberger vertical electrical sounding (VES) technique. This study aimed to understand the hydrodynamics and water–rock interaction of the near-surface crustal architecture to determine the groundwater yield and vulnerability of the aquifer units in the study area. A total of 7 model curve types were generated for fifty-two geoelectrical surveyed points, with percentage distributions in the order of HA > AA > H > KH > A > HK > AK. The VES curve models constrained the subsurface layers into topsoil, weathered units, weathered/fractured bedrock units, and fresh bedrock. The weathered and fractured aquifer zones occurred at the depths of 8 m and > 16 m (with depths exceeding 26.5 m for some sections). The K and T values for the aquifer units varied from 0.1901 to 0.6188 m/day and 0.7111 to 6.3525 m2/day, respectively. These parameters coupled with the aquifer ϕ (18.03–23.35%) and k p (0.028–0.089 µm2) classified the delineated aquifer units as low to moderate groundwater-yielding capacity aquifers, with recorded resistivity values between 85.1 Ω-m and < 613.0 Ω-m. The observed positive correlations and R2 values with > 32–100% prediction rates affirmed the dependence of K on T, ϕ , and k p for effective water–rock interactions and groundwater transmissibility. The recorded S values (0.0146–0.162 mhos) and low logarithm hydraulic resistance, Log C (0.89–1.75 years), suggested poor to weak aquifer protective capacity ratings, resulting in high aquifer vulnerability index delineated across the study area. As a result, deep-weathered/fractured aquifers should be exploited for sustainable potable groundwater supplies. However, intended wells/boreholes in the study area must be developed properly for long-term groundwater abstraction to alleviate potable groundwater deficit and optimize future operational drilling costs. [ABSTRACT FROM AUTHOR]