Integration of borehole logs and 2D electrical resistivity imaging in the investigation of saltwater intrusion in Lagos Island, Southwestern Nigeria

Abstract

The study area is bounded by Lagoon in the North and Atlantic Ocean in the South both being source of Saltwater pollution. Detailed investigation of saline intrusion into freshwater aquifers have been carried out by integrating Borehole logs and 2D Electrical Resistivity Imaging. The electrical resistivity methods was acquired with Wenner, Dipole-Dipole and Pole-Dipole with spread range of 415 to 810m for deeper investigation with the aid of multi-electrode system. Gamma ray log was used for lithology identification while resistivity logs where used for freshwater/saltwater delineation based on trend analysis. Six aquifers have been delineated on the borehole logs based on their separation by clay layers. Shallow fresh aquifer that straddles above the saltwater intruded zone. This is underlain by saltwater zone with to a depth range of 150 to 168m. Two aquifers delineated at deeper subsurface region are with fresh water within the depth range of 153 to 214m and are overlain by thin and thick layer of clay. This suggests that the fifth aquifer is prone to pollution because the thin overlying clay may not impede further intrusion into it unlike the sixth aquifer which is overlain by thick clay. The 2D Electrical Resistivity on the traverses shows similar result with borehole log interpretation and have provided lateral variation of the subsurface information. Investigation from the 2D Electrical resistivity surveys reveals that shallow freshwater zone ranges between depth of 8 to 22m and electrical resistivity range of 33 to 2036?m. Saltwater/Clay/Clayey Sand region is characterised by electrical resistivity range of 0.11 to 64?m within depth range of about 110 to 180m. Traverses close to the ocean are with high interface of saltwater and shallow freshwater are barely present. This study have shown the versatility of integrating Borehole logs with 2D Electrical Resistivity in delineating regional interplay between saltwater intrusion sources and freshwater in coastal environment. The 2D Electrical Resistivity Tomograms have provided invaluable information on the vertical and lateral variation of subsurface aquifer.