SALINE WATER INTRUSION IN THE COASTAL AREAS OF ONDO STATE, NIGERIA

¹Adeyemi, J. F., ²Arubayi, J. B., ³Oteri, A. U. and ⁴Ako, B. D.

²Department of Physics, Federal University of Petroleum Resources, Effurun

¹Department of Physics, Obafemi Awolowo University lIe-Ife

⁴Department of Geology, Obafemi Awolowo University, IIe-Ife

ABSTRACT

Saline water intrusion occurs in virtually all coastal aquifers when they are in hydraulic continuity with seawater. In continuation of  efforts to supply the communities of the coastal areas of Ondo State in Nigeria with reliable freshwater, the Ondo State government through the Ondo State Oil Producing Area Development Commission, decided to drill water boreholes in Asisa, Zion-Pepe and Ajapa in Ilaje and Ese-Odo local government areas of the State. Zion Pepe rests on a sand beach ridge; Ajapa and Asisa are on clays. Saline water intrusion is known to occur in surrounding communities. This study is meant to delineate the fresh water aquifers in the three coastal communities. VES 1 in Asisa was located on the coast within the brackish/saline surface water zone, and VES 2 at Asisa­-Zion is on an area with fresh water vegetation. The very low resistivity value of the surficial clay at both VES stations at Asisa and Asisa-Zion masked the sands below. Using the S-rule method of interpretation, fresh water sand is likely at depths greater than 154 m. It was recommended that a borehole be drilled to a depth of 300 m at Asisa Zion. In Zion Pepe, a deep fresh water aquifer mapped at a depth of 152 m was recommended. The borehole should be logged and screened at the fresh water interval identified from the borehole geophysical logs. In Ajapa, the space available for geophysics coupled with the screening effect of the low resistivity upper layers did not allow mapping of the deeper layers. However the data obtained indicate the occurrence of a fresh water-bearing aquifer underlying the clays and saline water sands. Ajapa is quite close to Bolowo Zion and from the results obtained at Bolowo the drilling of a borehole at VES 1 was recommended, to a depth of 200m. Nigeria has a long coastline which borders eights States, greater attention should be given to pre-drilling geophysical investigation for fresh water aquifer.

Key words: Saline water intrusion; Potable water; geoelectric layer.