Abstract

St. Croix is a small island in the northeastern Caribbean with a strong traditional dependence on rainwater for domestic and agricultural use. In recent years, some of this dependence on rainwater has been supplanted by increasing exploitation of limited groundwater resources as well as desalinized seawater. The high dissolved solids content of the groundwater is a potential barrier for its use as a drinking water source and has been the subject of several water-supply publications. Suggested sources for the dissolved solids in St. Croix groundwater range from seawater mixing, concentration of aerosols and rainwater solutes, contamination from “connate” waters, and rock-water interactions within the aquifer system.

Although all of the above sources could reasonably be expected to contribute to the groundwater chemistry of a small island, not all are consistent with the chemical characteristics of the groundwater in this case. Except in areas of heavy groundwater withdrawal close to the coastline, it can be shown that seawater contamination is not a likely source of groundwater solutes. Similarly, the stable and strontium isotopic signature of the groundwater is not consistent with either an aerosol source or a formation water (so-called connate water) source of the solutes. The stable oxygen and hydrogen isotopic characteristics of the groundwater require that meteoric water, not seawater or buried marine water, be the primary source of aquifer fluids. Furthermore, the stable isotopic signature of the groundwater shows no sign of evaporation. The strontium isotopic composition of the groundwater is too depleted in ⁸⁷Sr to be derived from the dissolution of the Kingshill Limestone aquifer and instead must be derived from older materials in the recharge zones or noncarbonates within the Kingshill Aquifer system.

Interaction with siliciclastic materials, although more complex than simple dissolution of carbonate materials, is consistent with the chemistry of St. Croix groundwater. It may be that siliciclastic contributions in dominantly carbonate aquifers are more common in this type of environment than would be expected; these relationships are made clearer through the use of several isotopic systems as tracers of geochemical reactions. Strontium isotopic ratios in particular are not affected by evaporation and precipitation reactions and thus are a powerful complementary technique to the use of stable isotopes of hydrogen and oxygen. The latter are already well-established tools in groundwater investigations. Strontium isotopes may become very useful in groundwater research in determining the sources of recharge for lithologically complex aquifer systems, and for calibrating groundwater flow models.