Comparative Assessment of Aquifer Vulnerability Near Major Dumpsites around Karu-Abuja and Keffi in Nasarawa State, Nigeria, Using Integrated Geophysical Methods

This study investigates aquifer vulnerability to leachate infiltration near major dumpsites in Karu-Abuja and Keffi, Nasarawa State, Nigeria, using integrated geophysical methods – Vertical Electrical Sounding (VES), 2-D Electrical Resistivity Tomography (ERT), Self-Potential (SP), and Very Low Frequency Electromagnetic (VLF-EM). The study area spans basement complex and sedimentary formations. Data were collected from nine VES points, four ERT profiles, ten SP profiles and sixteen VLF transverses using Ohmega Allied resistivity meter and a Gem VLF receiver. The measurements identified groundwater saturation zones and contamination pathways such as fractures and faults. Data interpretation employed tools such as WINRESIST, RES2DINV, GRAPHER, SURFER and KHFFILT. Results delineated five to six geoelectric layers, including, topsoil, clayey sand, weathered/fractured, and fresh bedrocks. Keffi's topsoil resistivity values (47.1-224.2 Ω.m, in depths ≥ 2.1 m) indicate thicker overburden and better aquifer protective layers compared to Karu-Abuja's resistivity values (16.5-294.0 Ω.m, in depths ≥ 0.5 m). Leachate infiltration is observed in both areas, with materials of low resistivity values ranging from (7.2 to 9.9 Ω.m, in depths ≥ 7.7 m) and (2.8 to 9.6 Ω.m, in depths ≥ 6.37 m) in Karu-Abuja and Keffi study areas respectively, interpreted as contaminated zones. Negative SP anomalies ranging from (– 339.9 to -1.1 mV) and (-135 to -1.65 mV) attributed to electro-kinetic reactions, and high positive VLF current-density ranging from (5 to 10 %, in depths ≥ 14 m), further corroborated contamination pathways. The study evaluated the Aquifer Protective Capacities (APC) of Keffi and Karu-Abuja, revealing that Keffi showed better protection compared to Karu-Abuja, with Keffi showing poor rating of 66%; along VES 1 (0.043 S), VES 4 (0.05 S), VES 5 (0.01 S) and VES 6 (0.02 S) and a moderate rating of 33.33 %, along VES 3 (0.29 S) and VES 4 (0.33 S). Only VES 2 (0.89), representing 16.6%, had a good rating. The Karu-Abuja study area showed poor APC rating of 66.6%, along VES 1 (0.0063 S) and VES 3 (0.002 S), and a weak rating of 33.3% along VES 2 (0.1 S). The findings emphasise the need for regular Environmental Impact Assessments (EIAs) and the installation of geo-synthetic clay liners at dumpsite bases to safeguard groundwater resources.