Geoelectric delineation of hydrocarbon spill in Abesan Lagos, Nigeria

This study was undertaken at Abesan Estate in Lagos with the aim of determining the spatial and depth extent of hydrocarbon contamination occasioned by vandalization of petroleum pipeline crossing the area. The area is underlain by the Benin Formation. The electrical resistivity method was adopted. Sixteen Vertical Electrical Sounding (VES) stations were occupied with two horizontal profiles using Schlumberger and Wenner electrode arrays respectively. Three to four geoelectric layers were delineated from the VES curves interpretation, with resistivity varying from 46 - 249 Ωm for the topsoil, 2.5 - 166 Ωm for the second geoelectric unit, 387 - 17,192 Ωm for third geoelectric unit; and thicknesses of 0.8 - 2.8 m, and 1.3 - 7.1 m for the topsoil and second layer respectively. Iso-resistivity depth slices generated from 1 to 6 m enabled the establishment of lateral and vertical distribution of the hydrocarbon contamination plume and identification of the contaminant migratory pathway. The unconfined hydrogeologic setting of the subsurface sequence has likewise enabled the downward migration of the hydrocarbon spill to depth of 5 - 6 m as observed in relatively high resistivity values characterizing the layers. The resistivity distribution pattern indicates migration towards the northern flank while the contaminant source is on the southern edge. The aquifer in the area is overlain by materials of weak protective capacity (0.03 - 1.12 mhos) with fairly high anisotropy (1.03 ≤ l ≥ 4.22). Thus, shallow aquifers in Abesan Estate and the adjoining areas are under severe threat of hydrocarbon contamination.