Application of Electrical Resistivity and Hydrochemistry Methods for Mapping Groundwater Contamination around Okun Ilashe Island Area, Lagos State, Southwestern Nigeria

An application of Electrical Resistivity and hydrochemistry investigation involving Vertical Electrical Sounding (VES) and water quality analysis was conducted around Okun Ilashe Island area of Lagos state, southwestern Nigeria where there have been reported cases of groundwater contamination. The hydrochemical analysis was performed on five water samples in the area; two from boreholes and others from hand dug wells. Present study investigation showed that the groundwater had been contaminated by hydrocarbon arising from pipeline leakages in the studied area. The hydrocarbon contaminated plumes are specially characterized by relatively high resistivity values (> 800 Ω-m) and were delineated to a depth of about 12 m. Hydrochemical results showed that three of the sampled water have higher total dissolved solids (TDS) (>400 ppm) in compared to the remaining water samples. The major ions identified includes Na+, K+, Cl, Mg2+, Ca2+ and NO3-. Total Dissolved Solid (TDS) and Electrical Conductivity (EC) showed values not in agreement with WHO standards values. Ten (10) Vertical Electrical Sounding (VES) stations were occupied along four traverse lines trending E-W direction. The vertical electrical sounding results indicate maximum of four subsurface layers; Top soil, hydrocarbon contaminated sand, clay/clayey sand and sand/sandy clay. The contaminant plume has migrated to a significant depth of 12 m thus posing an inherent danger to the inhabitant of the area.