ABSTRACT

The methods adopted for this research work were the combined geophysical and geochemical investigations along the coastal parts of Akwa Ibom state located within latitude 430 and 500'N of the equator and between longitude 730! and 820'E of the Greenwich meridian in southern Nigeria. This is to show the level of saline water intrusion into the fresh groundwater aquifer. The geophysical method adopted was the 2-D Electrical Resistivity Tomography (ERT) using resistivity meter and other accessories to obtain the field data. Wenner electrode configuration with electrode spacing ranging from 5 to 200 m was used for field measurement and a total of twenty (20) ERT profiles evenly distributed were covered. The modeled field data revealed the resistivities and depth to bottom layers distribution across the study area with predominant lithologies being coastal plain sands. The depth of inverse models from the geoelectrical resistivity data obtained in the area revealed significant impact of the saline water on delineated aquifer with very low resistivity value below 0.466 nm. ERT results also show the lateral invasion and upconing of saline water within the aquifer system. The interpretation showed upper resistivity limit of 1,197 2m for fine to medium grained sand;6,378 2m for lateritic sands; 18,665 Om for gravelly sands and 800 nm for sandy/clayed and clay soils. Results of the physicochemical parameters of groundwater samples displaced in the contour diagram showed that the water is slightly acidic with pondiwn Hydronium (pH) values ranging from 4.99 to 7.45, Electrical conductivity (EC) from 5.41 to 89.90 mho, Total dissolved solid (TDS) from 2.77 to 45.90 mg/L, Total Alkalinity (TA) from 0.99 to 16.10 mg/Land Total Hardness (TH) from 2.50 to 25.50 mg/L. Also from the investigation, the ionic concentrations of groundwater samples for cations values ranging from 0.18 to 12.50 mg/L for Ca, 0.49 to 6.00 mg/L for Mg, 0.24 to 2.50 mg/L for K, 0.02 to 0.21 mg/L for Fe? and 2.15 to 22.32 mg/L for Na while for anions values ranging from 3.09 to 31.95 mg/L for CI and 115.00 to 333.00 mg/L for SO respectively were below the minimum permissible standards of the India standard institution (ISI, 1995); European Union (EU, 1998) and World Health Organization (WHO, 2006) for recommended water consumption. Both natural (e.g rock-water interaction) and anthropogenic (e.g Over pumping) factors at a distance of 5km are responsible for saline intrusion into the interland from the coast. The study therefore recommended effective remediation measures including introduction of artificial recharge of freshwater and saltwater extraction techniques be adopted.