GILL, IVAN, Department of Geology, Tulane University, New Orleans, LA

ABSTRACT: Strontium and Other Isotopic Tracers in Insular Groundwater Systems, St. Croix, US Virgin Islands

St. Croix, US Virgin Islands, contains an aquifer composed primarily of hemi-pelagic limestones of Miocene age. Groundwater in the aquifer system ranges from 250 to 6500 ppm total dissolved solids and often exceeds Federal limits for dissolved solids and choride. Despite this, the aquifer is expected to play an increasing role in island water supply. The purpose of this paper is to illustrate the utility of isotopic techniques in understanding groundwater relationships and water-rock reactions.

Explanations for the high groundwater TDS content on St. Croix have ranged from contamination by "connate" water, to the evaporative concentration of rainwater. Recent groundwater modelling work by the US Geological Survey supports the latter explanation, showing that the low-permeability characteristics of the aquifer system would support the concentration of rainwater-derived solutes, and could produce the concentrations of solutes found the aquifer system.

However, the {87}Sr/{86}Sr ratio of St. Croix groundwater ranges from 0.7076 to 0.7085, well below the ratio of modern seawater, as well as the majority of the carbonate rocks that make up the aquifer system. The Sr content of St. Croix groundwater indicates that interaction with aquifer rocks, in particular volcaniclastic and intrusive rocks in the recharge areas, may also add to the geochemical signature of the groundwaters. The stable isotopic composition of the groundwater suggests preferential recharge during storm events, while geographic solute patterns indicate that seawater contamination may occur in porous aquifer zones close to the coastline. These contributions are relevant to island groundwater models.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.