HYDROCHEMICAL ANALYSIS OF GROUNDWATER-FLOW AND SALINE INCURSION IN THE CLARENDON BASIN, JAMAICA

The Clarendon Basin of south-central Jamaica has been recognized for i ts productive sugar cane industry since the late 1800s. Much of its su ccess is attributed to rich alluvial soils, a year-round tropical clim ate and copious supplies of good quality irrigation water from a karst ic limestone aquifer of Tertiary age. This aquifer extends throughout the northern part of the basin, but ends abruptly at the South Coast F ault, an east west feature that forms the northern boundary of a deep graben filled with alluvial sediments. These alluvial sediments are th e only source of water for the area south of the fault (the Vere Plain ). However, the sediments also lap onto the limestone to the north of the fault (the Clarendon Plain) and provide a supplementary ground-wat er source. By the early 1970s nearly 200 wells supplied irrigation wat er for over 20,000 hectares of land. As development increased, the sal inity of the ground water increased. Consequently, many wells were clo sed down and several sugar plantations were abandoned. In the study pr esented here, major ion (Ca, Mg, Na, K, HCO3, Cl, SO4, and NO3), minor ion (F, Br, and I), and environmental isotope (delta(18)O, delta D) h ydrochemistry is used to resolve the hydrodynamics of ground-water flo w in the basin and identify the source and mode of emplacement of the saline water. Oxygen and hydrogen isotope data confirm that while the major well production areas are located in lowland coastal areas, rech arge originates almost exclusively as rainfall in the cooler elevated parts of the basin above 750 m (asl). Subsurface conduit flow brings t his water to the limestone well fields, and any excess water is able t o cross the South Coast Fault to feed the alluvial aquifer of the Vere Plain. The thin alluvial aquifer of the Clarendon Plain also receives limestone water but this water does not enter entirely by subsurface means. Instead data suggest that while some of the water can be attrib uted to natural upward vertical leakage in the northwest of the Claren don Plain, the remainder can be attributed to the seepage of irrigatio n water drawn, at least in part, from wells developed in the underlyin g limestone. Saline ground waters affect all the aquifers of the basin and several potential sources have been proposed. Major and minor ion data point to a sea-water source, and a sea-water wedge extending ben eath the thick alluvial aquifer of the Vere Plain is an obvious candid ate for the source of salinity observed in wells from this aquifer. Cl oser examination of the chemical data reveals, however, that wells in the limestone and alluvial aquifers to the north of the South Coast Fa ult do not derive their salinity from this source location, and instea d draw sea water from the east and west along the relatively permeable South Coast Fault zone. This water can move tee limestone wells direc tly; wells in the alluvium of the Clarendon Plain, however, receive mo st of the saline water indirectly via leakage of irrigation water pump ed initially from the limestone. Sea water drawn along the fault may a lso be the primary source of salinity in the aquifer of the Vere Plain but this cannot be confirmed on the basis of hydrochemical evidence a lone.