Contamination Investigation Using Geoelectrical Resistivity and Hydrogeological Data: A Case Study of Regolith Groundwater: Around Agwan-Nepa Dumpsite, Keffi, North­-Central Nigeria

Abstract

Groundwater contained in regolith aquifer within Agwan-Nepa dumpsite and environs in Keffi (north-central Nigeria) was investigated to ascertain its contamination condition. Groundwater flow directions, were established from static water table elevation measurements. Transverse unit resistance (T) and longitudinal unit conductance derived from geoelectric sections were employed in conjunction with spacing of water table elevation contours to capture variation pattern in hydraulic conductivity and permeability. Water filtrate obtained from a colloidal solution of soil sample taken at the dumpsite, and water samples from twenty different hand­dug wells were analysed for acidity level (pH), electrical conductance (EC) and total dissolved solids (TDS) at the Acme Laboratory in Vancouver BC, Canada. Topsoil and weathered basement are respectively very thin (0.4-0.8m) and slightly thin (4-7 m) at the dumpsite and its immediate surrounding area. This creates high potential for groundwater contamination from dissolved solids migrating vertically downwards from dumpsite's surface load. Transverse unit resistance values for topsoil at dumpsite and its vicinity (100-160 Ωm2) indicate vertical hydraulic connective between the soil and the regolith below. The values also validate tbe existence of high potential for groundwater contamination at the dumpsite. The transverse unit resistance value for weathered basement increases outward from the dumpsite in a radial pattern. Its longitudinal unit conductance similarly decreases outwards. This reflects hydraulic connectivity between the dumpsite and its iil10lediate surrounding area, and constitutes a potential for contaminant dissolved solids to migrate outwards from dumpsite to its vicinity. Values of pH, TDS and EC decrease outward from the dumpsite, along the southerly direction of groundwater flow. This ascertains that a contamination plume exists at the dumpsite and that the contaminant dissolved solids are dispersed southerly from the dumpsite. Values of pH, EC and TDS are respectively 7.18, 352 µs/cm and 197.6 mg/L at groundwater convergence zone in southwest of the dumpsite. This indicates that water at the convergence zone is uncontaminated and fit for drinking.