Geophysical exploration to assess leachate percolation and aquifer protectivity within hydrogeological units at a major open dump in Eket, Nigeria

Abstract

Leachate contamination was investigated with geo-electrical methodologies at a major open dump in Eket, Southern Nigeria. The open dump is heavily polluted, degrades the environment, and has released toxins into the atmosphere. To model leachate percolation, vertical electrical soundings (VES) and electrical resistivity tomography (ERT) surveys, constrained by well lithological information, were undertaken at the site. The field equipment used for the geo-electrical investigations was the ABEM Terrameter SAS 1000 and its accessories. WINRESIST software was employed to process the sounding data and RES2DINV was employed to generate 2D tomograms indicating subsurface heterogeneity. Results obtained from sounding data showed that the subsurface had three lithological units (motley topsoil, fine sand and coarse sand), with coarse sand being the main hydrogeological unit comprising water-bearing formations. The first layer’s resistivity ranged from 84.9 – 625.4 Ωm with a mean of 360.4 Ωm; the second layer’s resistivity ranged from 7.5 – 1008.1 Ωm with a mean of 142.9 Ωm, and the third layer’s resistivity values ranged from 11.6 – 745.6 with a mean of 205.5 Ωm. The 2D resistivity tomograms indicated that leachate had percolated into groundwater resources. This is concerning as groundwater is the major water resource within the area. Measures of the Dar-Zarrouk indices and electrical reflection co-efficient indicated that the highly heterogenous region had moderate aquifer protective capacity and moderate aquifer potentiality. To mitigate contamination at the site, sanitary landfills, phyto-remediation, impermeable liners, or impermeable earth materials should be employed to diminish the rate of leachate percolation.