TRACING GROUNDWATER EVOLUTION IN A LIMESTONE AQUIFER USING SR ISOTOPES - EFFECTS OF MULTIPLE SOURCES OF DISSOLVED IONS AND MINERAL-SOLUTIONREACTIONS

Uplifted Pleistocene coral-reef terraces on Barbados, West Indies, con stitute an aquifer that is built on low-permeability Tertiary pelagic rocks that overlie the Barbados accretionary prism. The downdip segmen ts of the aquifer are composed of younger reef limestones that contain more aragonite and have higher Sr-87/Sr-86 and Sr/Ca ratios than the updip parts of the aquifer. Ground waters and host limestones display similar stratigraphic trends in Sr-87/Sr-86 and Sr/Ca. The ground wate rs have lower Sr-87/Sr-86 values, however, indicating that they acquir e a significant fraction of their dissolved Sr through interaction wit h components of Tertiary rocks, which compose the underlying aquitard and parts of overlying soils. Geochemical modeling results indicate th at ground-water evolution is controlled by (1) variations in the age a nd composition of the aquifer and aquitard rocks and (2) the relative roles of calcite dissolution, calcite recrystallization, and the trans formation of aragonite to calcite. Sr isotopes can provide unique info rmation for tracing ground-water evolution, which requires considerati on of the multiple components and processes that make up even relative ly simple limestone aquifer systems.