Risks of groundwater pollution in the coastal areas of Lagos, southwestern Nigeria

Abstract Saltwater intrusion is a common problem associated with every coastal region all over the globe, and Lagos state is not an exception. This study was carried out in the coastal belt of Lagos, Southwestern Nigeria with the main objective of assessing the risk of shallow groundwater and its susceptibility to contamination. In this research, geophysical and geochemical techniques coupled with the concepts of Dar- Zarrouk (D-Z) parameters were employed. Resistivity measurements were taken at twelve vertical electrical sounding (VES) stations and eight 2-D resistivity profiles using the Schlumberger, dipole-dipole and pole-dipole arrays were analyzed and interpreted respectively. The D-Z parameters such as longitudinal conductance (SL), transverse resistance (Tr), longitudinal resistivity (??L) and transverse resistivity (??T) data were computed from the qualitative interpretation of VES data to reveal the aquifer hydraulic characteristics. The 2-D sections display the lateral invasion of saline water within the aquifer systems and variations in resistivity of the subsurface being the geo-electric property that differentiates lithologies. The geo-electric model interpretation supported with available geologic information revealed four/five geo-electric layers namely; top sandy layer, clayey sand/sandy clay, clay and sand. Two aquifers (Upper and Lower) were delineated within the depth of investigation hosted mainly by sand and sandy clay/clayey sand. Furthermore, the longitudinal conductance data was used to predict the protective capacity of the coastal aquifer. Generally, the water table aquifer falls within the poor/weak protective capacity, while the lower semi-confined to confined aquifer is within the zone of good to excellent protective capacity. The lower aquifer is essentially impacted with saline intrusion while the phreatic aquifer contaminations are both from saline and anthropogenic sources. These assertions corroborate the geochemical results and previous findings in the study area. Hence, geophysical method with the use of D-Z parameters proved to be useful tool in the pollution assessment of groundwater.