Saltwater Intrusion from Atlantic Ocean into coastal Aquifers in parts of Nigerian Dahomey Basin

The study of saltwater intrusion was carried out in Dahomey basin of coastal area of southern Nigeria. In the last three decades, aquifers in this area been deteriorated by saltwater intrusion which resulted in the contamination of freshwater along the coast of the Atlantic Ocean. Hydrogeochemical analysis were carried out on thirty-one groundwater samples and twenty-seven dissolved constituents were determined in the laboratory. The results showed that the groundwater samples were highly enriched with magnesium chloride indicating saltwater intrusion and this reduces the storage of freshwater in the aquifers. Magnesium chloride was responsible for the contamination of the freshwater. Ghyben – Herzberg relation has been used to estimate the depth to saltwater based on the thickness of the freshwater zone above the sea level. The depth of the freshwater – saltwater interface in Dahomey basin is directly proportional to the elevation of the water table above the sea level. The density difference and over-pumping of freshwater creates interface causing the mixing of the two fluids. There are three aquifers, the two deeper aquifers were contaminated and most of the boreholes nearer the seashore were abandoned because their water are not drinkable. Cluster analysis shows four groups: group 1, 2, 3 and 4, water from locations 1, 5, 9, 25, 26, 31, 21 and 10 gave rise to the fourth cluster group. Group 4 represents 25.8 % of the groundwater samples. It electrical conductivity average was 227.8 µS/cm, dominant ion was chloride and water here are more polluted than the first three groups.