Hydrochemical Analysis of Ground‐Water Flow and Saline Incursion in the Clarendon Basin, Jamaica

Abstract

AbstractThe Clarendon Basin of south‐central Jamaica has been recognized for its productive sugar cane industry since the late 1800s. Much of its success is attributed to rich alluvial soils, a year‐round tropical climate and copious supplies of good quality irrigation water from a karstic limestone aquifer of Tertiary age. This aquifer extends throughout the northern part of the basin, but ends abruptly at the South Coast Fault, an east west feature that forms the northern boundary of a deep graben filled with alluvial sediments. These alluvial sediments are the 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‐water source. By the early 1970s nearly 200 wells supplied irrigation water for over 20,000 hectares of land. As development increased, the salinity of the ground water increased. Consequently, many wells were closed down and several sugar plantations were abandoned. In the study presented here, major ion (Ca, Mg, Na, K, HCO3, Cl, SO4, and NO3), minor ion (F, Br, and I), and environmental isotope (δ18O,δD) hydrochemistry is used to resolve the hydrodynamics of ground‐water flow 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, recharge originates almost exclusively as rainfall in the cooler elevated parts of the basin above 750 m (asl). Subsurface conduit flow brings this water to the limestone well fields, and any excess water is able to 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 attributed to natural upward vertical leakage in the northwest of the Clarendon Plain, the remainder can be attributed to the seepage of irrigation water drawn, at least in part, from wells developed in the underlying 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 beneath the thick alluvial aquifer of the Vere Plain is an obvious candidate for the source of salinity observed in wells from this aquifer. Closer examination of the chemical data reveals, however, that wells in the limestone and alluvial aquifers to the north of the South Coast Fault do not derive their salinity from this source location, and instead draw sea water from the east and west along the relatively permeable South Coast Fault zone. This water can move to limestone wells directly; wells in the alluvium of the Clarendon Plain, however, receive most of the saline water indirectly via leakage of irrigation water pumped initially from the limestone. Sea water drawn along the fault may also be the primary source of salinity in the aquifer of the Vere Plain but this cannot be confirmed on the basis of hydrochemical evidence alone.