Hydrogeochemistry of Grand Cayman, British West Indies: implications for carbonate diagenetic studies

Abstract

Groundwater in the dolostone aquifers of the Bluff Group (Oligocene-Miocene) on Grand Cayman is divided into fresh, lightly and highly brackish, and saline (Type I and II) zones according to chemical characteristics that were determined during a 3 year (1985–1988) monitoring program. Brackish and Type I saline waters display the greatest variation in chemical properties whereas the Type II saline water has the most stable chemical characteristics. Most groundwaters from these dolostone aquifers are thermodynamically capable of precipitating calcite and/or dolomite. The saturation indices for these minerals, however, vary through time and space even in the context of small water lens. Simple mixing of fresh and sea water cannot explain the chemistry of the water found in the joint and karst controlled dolostone aquifers of Grand Cayman. Deviation from a simple mixing model is due to variations caused by tidal fluctuation, the rate of rain water recharge, influx of Ca-rich groundwater from the surrounding limestone aquifers, influx Of CO 2-rich surface water from sinkholes and swamps, and water-rock interactions (dissolution and precipitation of calcite and dolomite). Sustained groundwater abstraction from a lens can significantly alter the hydrochemistry of the water lens. This suggests that hydrochemical characterization of small fresh water lenses, like those on Grand Cayman, cannot be based on spot or short-term sampling. Interpretation of such fluids in terms of calcite-dolomite precipitation and/or dissolution must be treated with caution if the data base has not been derived from long-term monitoring.