GEOPHYSICAL AND HYDROCHEMICAL INVESTIGATIONS OF SALINE WATER INTRUSION INTO AQUIFERS IN PARTS OF EASTERN DAHOMEY BASIN, NIGERIA

Abstract

The study is aimed at mapping saline water intrusion, subsurface extent of saline water incursion and fresh-saline water boundary within aquifer settings in the easternmost Dahomey basin, covering six Local Government areas (Odigbo, Irele, Okitipupa, Ese-Odo and Ilaje in Ondo State and Ogun Waterside in Ogun State) in Southwestern Nigeria. The study adopted an integrated method comprising hydrochemical analysis, vertical electrical sounding, induced polarization sounding and borehole logs analysis. Sixty one (61) water samples were collected across the study area and analyzed for 17 relevant parameters such as conductivity, pH value, total hardness, calcium hardness, magnesium hardness, total dissolved solids, total suspended solid, alkalinity concentration, and concentration of the following anions and cations; chloride, calcium, sulphate, bicarbonate, calcium, magnesium, sodium, manganese and nitrate. Equivalent salinity was also derived from the hydrochemical analysis results. A total of one hundred and eight (108) Vertical Electrical Soundings (VES), using Schlumberger array were carried out across the area with maximum current electrode separation (AB/2) of 750 m. Induced polarization soundings (IPS) was also carried out in some selected locations where saline water intrusion was suspected in the area. The hydrochemical analysis results shows possible saline water intrusion in the coastal area and Agbabu in the north central area as evident from high concentration values of chloride (372 - 1500 mg/l), alkalinity (105 - 330 mg/l), equivalent salinity (135 - 2808 mg/l), total dissolved solid (181 - 1005 mg/l) and conductivity (541 - 1500 μs/cm). The VES results delineated three aquifer units; first, second and third aquifer units. Low resistivities in these aquifer units suggest possible saline water intrusion in the coastal areas and in parts of the mainland (Ilubirin, Agbabu, Idepe, Igodan Lisa, Legbogbo, Irele, Omi and Ajagba). However, IPS results show high chargeability values (above 50 msec) in all the low resistivity layers delineated in northern parts of the study area including Agbabu. This implies that the low resistivity values recorded in northern parts of area are due to presence of sandy clay/clayey sand. In the coastal areas, low chargeability values were delineated at Obinehin and Zion Pepe (12 msec and below). Although it is difficult to correlate results of hydrochemical analysis with surface geophysical method results, because 85 percent of the water samples were obtained from hand dug wells, surface water (3 - 6m) and shallow boreholes, however the results confirmed presence of saline water in Agbabu area. The Igboegbugurin borehole log does not show presence of saline water. The Igbokoda borehole log shows alternation of fresh and brackish water layers. Asisa and Rewoye borehole logs show the presence of saline/brackish water at depths of 30 - 350 m and 28 - 320 m respectively. Only Obenla borehole log intercepted fresh water sand between depth of 240 and 270 m. The hydrochemical analysis, VES, IPS and borehole logs analysis show presence of saline water intrusion in Agbabu, coastal areas and parts of the mainland adjoining the coastal area. Depth to saline water at any given location within saline water intruded area strongly correlate (R² = 0.8564) with distance from salinity source. The correlation curve can therefore serve as a predictive model for determining depth to saline water at any location within the saline water zone in the area.