Investigating the Effects of Leachate on Groundwater Repositories in Major Dumpsites of Akwa Ibom State, Nigeria, Using Integrated Approach

Abstract

Integrated study involving electrical resistivity method and physicochemical analysis of water samples was carried out to investigate the effect of dumpsite leachate on groundwater repositories around the dumpsites in Ikot Ekpene, Uyo and Eket Local Government Areas of Akwa Ibom State. A total of thirty three (33) vertical electrical sounding (VES) employing Schlumberger electrode configuration and five profiles of electrical resistivity tomography (ERT) were carried out around the dumpsites. The field data were interpreted using computer software (Winresist for VES and RES2DINV for ERT). The field data gives a resolution of three to five geoelectric layers and the observed curve types. From the measured aquifer resistivity and thickness, the Dar-Zarrouk parameters (longitudinal conductance and transverse resistance) and aquifer conductivity were determined. The resistivity of the aquifer varies from 65.0 to 1867.7Ωm, 8.1 to 1125.8Ωm and 78.4 to 1669.8Ωm in Ikot Ekpene, Uyo and Eket respectively. Aquifer thickness varies from 17.4 to 94.5m in Ikot Ekpene, 7.4 to 56.3m in Uyo and 9.2 to 155.0m in Eket. The longitudinal conductance and transverse resistance ranged from: 0.03 to 1.28Ω-1 and 5401.50 to 138291.30Ωm2 in Ikot Ekpene; 0.0196 to 1.8272Ω-1 and 119.88 to 34449.48Ωm2 in Uyo; 0.0055 to 1.0893Ω-1 and 6695.36 to 34921.50Ωm2 in Eket. The low resistivity observed is a reflection of leachate infiltration into the subsurface aquifer. The high resistivity zone depicts areas with less or non-conductive materials. The spread of the leachate/groundwater based on the elevation contour maps show a northwest and southwest trend in Ikot Ekpene, northeast - southwest trend in Uyo and southwest - northeast trend in Eket. The low protective capacity observed in parts of the study area suggests high permeability of the overlying geoelectric layers, hence, showing the possibility of ingress of leachate into the subsurface aquifer. The water samples were subjected to physicochemical analysis and the result compared with the World Health Organization (WHO) permissible standard values for drinking water. The results show the ranges of concentrations of the physical and chemical parameters. In Ikot Ekpene, SO42-, Cl-, Na2+, Mg2+,Ca2+,Mn2+ and Fe2+ have values below WHO standard in all boreholes tested while F-, K+, and Cu2+ have values above WHO standard in some boreholes while the pH range was within the WHO standard. In Uyo, the pH ranged from 4.45 to 6.92 and it was observed that two of the boreholes have values below WHO standards while the chemical parameters tested fall below the WHO limit. In Eket, the pH range was within WHO standard, F-, K+, Fe2+ and Mn2+ have values that exceeded the WHO standard in some boreholes while SO2-, Cl-, Na2+, Ca2+, Mg2+, and Cu2+ have values below WHO standard in all the boreholes. The hydrochemical parameters with concentrations below WHO standards indicate that the loaded leachate does not give rise to increase in these minerals. The realization of the possibility of ingress of leachate into the subsurface aquifer through the estimated geoelectric and physicochemical parameters show the goodness of fit and the effectiveness of integrating the indirect geophysical method (electrical resistivity) and the direct physicochemical (laboratory) method.