Delineation of Saline Water Intrusion Lateral Extent Using Hydro-chemically and Geo-electrically Derived CRS - Model Parameters - A Case Study of Eastern Dahomey Basin, Nigeria

Abstract

This study was another attempt to map the subsurface lateral extent of saline water intrusions into aquifers at the eastern part of Dahomey basin, Nigeria. The study comprised geo-electric sounding method consisting of 108 vertical electrical soundings (VES) data acquired using Schlumberger array technique and hydro-chemical analysis of 61 water samples. The geoelectric survey results were presented as average longitudinal Conductance and average longitudinal Resistivity maps. The longitudinal conductance values vary from moderate to high (20 - 180 mhos) in the coastal areas, while lower values (lesser than 10 mhos) were recorded in the mainlands. Conversely the average longitudinal resistivity values were low to moderate (0.26 - 41.7 ohm-m) in the coastal areas and in few places such as Agbabu, Legbogbo and Sabomi in the mainland, while higher values (more than 60 ohm-m) were obtained from the mainlands. The hydro-chemical analysis results was presented as equivalent Salinity map; its values range from 25.8 mg/l (Ode-Irele) to 2808 mg/l (Asisa), high values were obtained from Ugbo, Ugbonla, Odonla, Idogun and Ayetoro area in the southeastern part of the study area and likewise at Agbabu in the north central part of the study area. The results were synthesized using CRS-model. The final CRS-model map showed that the saline water intrusion extended to 35 % of the total area which covers; most part of the coastal area and Agbabu in the mainland.

Keywords: Saline water intrusion, saline-freshwater boundary, vertical electrical sounding (VES), hydrochemical analysis, synthesis and CRS-model.