SALT WATER INTRUSION STUDIES IN THE COASTAL AQUIFERS OF JAMAICA, A CASE STUDY FROM ST CATHERINE, JAMAICA

Jamaica, the third largest island of the Caribbean, has shown remarkable decrease in groundwater quality from salt water intrusion, typically one of the main factors affecting coastal aquifers all over the world. Salt water intrusion in Jamaica has affected many hydrological basins, including both limestone and alluvium aquifers leading to closure of many wells resulting in scarcity of water in several parishes. Previous hydrochemical study on the groundwater wells of Kingston and St Andrew parish has shown high levels of Na (235 ppm), Cl (234 ppm) in the well waters located close to the coast exceeding the ambient Jamaican water quality standards. In the present research attempt has been made to delineate the saline-water fresh water interface in the alluvium and limestone aquifers of the Rio Cobre basin of St Catherine, the next most extensively used hydrological basin after the Kingston basin. Direct Current resistivity studies were conducted along Ferry Springs and Caymanas Estate in St Catherine, Jamaica. Results show presence of a three layer sub-surface lithology with a top 3-12 layer of sand and silty clay of low resistivities (1-3 Ωm) overlying the thicker (>30m) limestone aquifers. The area thus shows the presence of an unconfined alluvium aquifer overlying the limestone aquifer. Static water level data from 1987 showed that the estimated depth to saline interface from furthest inland to the central and finally closest to the coast ranged respectively from 272m, 124m, and 112m below msl. This was considered safe as the wells were pumping and in operation at that time. However, from 1996 to recent the depths of the salt-fresh water interface as estimated from water levels showed range from 276m, 84m, and 13.6m to 0.8 below msl. The major contributor to saline intrusion was high abstraction from the wells, and the reduction of groundwater gradients which permitted saline-water to displace fresh water within the aquifer. The limestone aquifer that is exposed to the ocean also contributes to a frontal intrusion problem that is directly responsible for salinity problems in the Rio Cobre basin