Geoelectrical Subsurface Imaging for the Mapping of Leachate and Contaminant Plume around Septic Tanks Within the Staff Quarters in University of Benin, Benin City, Nigeria

Abstract

2-Dimensional imaging was used to investigate the presence of leachate and contaminant plume around septic tank within the Staff Quarters in University of Benin, Benin City, Nigeria. This conventional 2-D electrical resistivity survey method was carried out by engaging the Wenner Alpha array configuration using PASI 16 GL Terrameter. Four traverses with a spread of 160 m each were covered around the septic system using DIPRO and RES2D Inversion software for interpretation so as to delineate the plume and probable trend of migration. To test for the presence of heavy metals, Physiochemical analyses were done on the water collected through the borehole that was very close to the septic system. The interpretation of the inverted 2-D electrical resistivity data revealed five subsurface layer: topsoil, clayey sand and sandy layers, clayey sand and sand zones constitute the major aquifer units in the study area. The 2 D Inversion delineated contamination plumes as low resistivity zones. As revealed by the inversion model, there was a strong evidence of vertical and horizontal plume migration westwards in third layer of traverse 1 and 2. Also, traverse 3 displayed a low resistivity anomaly ranging from 60 Ωm to 109 Ωm (15m below the surface) in third layer. It was only traverse 4 that showed a strong evidence of horizontal plume in top soil with a low resistivity <100 Ωm at a depth of 0 to 4 m with a lateral distance of 10 to 160 m. Also, there was strong evidence of topsoil contamination in traverses 2 and 4 (resistivity < 100 Ωm) and this constitutes the contaminated shallow water zone. This low resistivity zone, occasioned by infiltration of leachate from the septic tank. For physiochemical analysis on heavy metals, and with exception of pH, other parameters analyzed compared favourably with WHO, and ASTMD standard for drinking quality water. The pH values of 5.84 obtained from water samples were below 6.5-8.5 as recommended by WHO and this shows that the site was acidic, and a clear evidence of physical pollution exist. However, sodium carbonate should be used as a purifier for acidic groundwater also, future borehole should be constructed beyond far away from the pollution source (septic system).