Integrated Geophysical and Geochemical investigations of saline water intrusion in a coastal alluvial terrain, Southwestern Nigeria.

Deterioration of water quality in the coastal areas of Lagos due to saltwater infiltration into the freshwater aquifer has become a major concern. Geophysical techniques involving 2D electrical resistivity tomography (ERT) using Wenner and dipole-dipole arrays were incorporated with geochemical evaluation using ICP-OES and 1C analytic methods to study the extent of saline water intrusion and its environmental impacts. The resistivity data were acquired with minimum electrode spacing of 5m (Traverses 1, 2, and 3) and 6 m (Traverse 4). Data level of 8 (maximum electrode spacing of 40.0 m) was achieved in each of the profiles; dipole length with n-factor of 6 was employed for the dipole-dipole array. The depth of inverse models from the geoelectrical resistivity data obtained in the area revealed significant impact of the saline water on delineated aquifers with very low resistivity values uniquely below 4.0 . ERT results also show the lateral invasion and up-coning of saline water within the aquifer systems. Results of the physiochemical analysis indicate that the water is alkaline with pH values range 8.6-8.9. The salinity is also high with the total dissolved solids TDS values and chloride concentration ranges 10405 – 12005 (mg/L) and 432 – 724 (mg/L) respectively. The water has very high electrical conductivity (EC) values range 2005-3013 μS/cm and very Hard with 121 to 180 mg/L range of total hardness thereby falling below the minimum permissible standard of the Indian standard institution (ISI), European union (EU) and world health organization (WHO) for portable water consumption