QUALITY OF GROUNDWATER IN THE COASTAL-PLAIN SANDS AQUIFER OF THE AKWA-IBOM STATE, NIGERIA

The Coastal Plain Sands Formation is exploited by most of the populati on of the Akwa Ibom State in southeastern Nigeria. The aquifer is most ly coarse-grained, pebbly and poorly sorted sands with minor clay inte rcalations. It is up to 1500 m thick near the coast, but only a few me tres thick along the northeastern boundary. Groundwater occurs princip ally under unconfined conditions. Boreholes penetrating less than 130 m yield over 300 m(3) hr(-1). The main groundwater flow direction is s eaward from north to south. The probable location of the fresh water-s ea water interface is seaward. Forty-two groundwater and two surface w ater samples were analysed. Groundwater quality meets the WHO standard s for potability and is dominated by bicarbonates of Na, Ca and Mg. Th e Sodium Adsorption Ratio lies between 0.2 and 2.0, indicating that th e water is suitable for irrigation. The area has very high annual rain fall exceeding 2000 mm annually. Groundwater recharge should be high, although it is recommended that groundwater levels and quality near th e coast be monitored regularly, especially in urban areas with high gr oundwater abstraction, to detect the onset of sea water intrusion whic h remains a potential hazard in this area. (C) 1998 Elsevier Science L imited.