Environmental Geophysics Investigation of Tar Sand Exploitation Impacts on Groundwater in Ondo State, South West Nigeria

The Ondo State tar sand belt represents Nigeria's largest known bitumen deposit, and ongoing exploitation activities have raised concerns about the impact of open-cast mining and crude bitumen extraction operations on shallow groundwater chemistry and the physical integrity of near-surface aquifer systems. This study investigates the geophysical and hydrochemical impacts of tar sand exploitation on groundwater in the Agbabu and Loda areas of Okitipupa Local Government Area using time-lapse 2D ERT, EM profiling, and systematic groundwater sampling. Baseline geophysical surveys were acquired prior to mining area expansion, with repeat surveys conducted 12 and 24 months after the commencement of a new mining cell. Differential ERT sections comparing pre-mining and post-mining resistivity distributions reveal a progressive zone of elevated resistivity emanating from the mine pit boundaries, interpreted as desaturation of the previously saturated sand horizon above the bitumen layer. A new low-resistivity zone develops downgradient of the processing facility between months 12 and 24, consistent with infiltration of hydrocarbon-contaminated process water. Groundwater samples from 16 monitoring boreholes show significant increases in dissolved hydrocarbons, particularly total aromatic hydrocarbons and benzene, at the four boreholes within the ERT-identified contamination zone. The study concludes that current operational practices at the surveyed site are causing measurable groundwater quality degradation and proposes geophysically monitored containment measures as part of an environmental management plan. Keywords: tar sand, ERT, groundwater impact, Ondo State, bitumen extraction