Integrated Geophysical and GIS-Based Analysis of Facies Control on Groundwater Flow Direction in Uromi, Edo State, Nigeria

Abstract

This study evaluates the influence of lithofacies variation on groundwater flow direction in Uromi, Edo State, Nigeria, using integrated electrical resistivity surveying and GIS-based spatial analysis. Three Vertical Electrical Sounding (VES) stations were occupied using the Schlumberger electrode configuration, with current electrode spacing extended to 700 m. Resistance measurements obtained with a terrameter were converted to apparent resistivity and interpreted through curve matching and inversion to infer geoelectric layers and lithological variation. GPS coordinates of survey locations were processed in ArcGIS 9.2 for spatial interpolation, facies delineation and groundwater flow interpretation. The apparent resistivity values ranged from 6.20 to 8957.4 Ωm, indicating a heterogeneous subsurface composed mainly of alternating clay, sandstone and lignite-bearing units. The interpreted geoelectric sections show aquiferous intervals associated principally with medium- to coarse-grained sandstone units at greater depths, reflecting comparatively favourable groundwater storage and transmission conditions. The resistivity curves generally display A-type characteristics, suggesting increasing resistivity with depth. GIS-based interpretation indicates a lateral facies transition from kaolin-rich sandstone to lateritic sandstone across the study area. This variation is inferred to influence groundwater movement by creating contrasts in permeability, with the predominant flow direction interpreted as southwest to northeast. Inferred hydraulic conductivity values range from approximately 10⁻⁹ to 10⁻³ m/s, with seepage velocities of about 10⁻⁹ to 10⁻⁴ m/s, increasing from kaolin-rich units to lateritic and coarse sandstone facies. The findings indicate that lithofacies heterogeneity exerts an important control on groundwater flow behaviour in the study area and demonstrate the value of combining geophysical data with GIS-based spatial analysis for preliminary groundwater assessment in sedimentary terrains.