Groundwater geochemistry fluctuations along a fresh-saltwater gradient on the carbonate islands of the lower Florida Keys

Abstract

Climate change will have long-lasting effects on the availability of fresh water on small, carbonate islands that have isolated fresh groundwater lenses, particularly as sea level rises and rainfall regimes shift. The carbonate islands of the Florida Keys provide an ideal location to study the effect of variable rainfall on the aqueous geochemistry of the islands' groundwater. In a rainfall-driven carbonate system, the expectation is that limestone dissolution will occur within the vadose zone resulting in increased ions in the groundwater. However, geochemical processes are also affected by the salinity of groundwater and the extent of the mixing zone between fresh and salt water. We chose two islands to conduct the study of the shallow groundwater: the largest island in the lower Florida Keys, Big Pine Key (BPK), and a smaller island, Upper Sugarloaf Key (SLK). From May 2011 through April 2012, monthly groundwater samples were collected from 24 shallow (1 m deep) wells located along a fresh to saline gradient on both islands. Groundwater chemistry was compared with rainfall amounts from a weather station on BPK. Saturation indices for aragonite and calcite, generated with geochemical modeling in PHREEQC, were compared to conservative mixing between Gulf of Mexico water and freshwater. Equilibrium to supersaturated conditions with respect to carbonate minerals dominated in all of the groundwater samples. Saturation indices varied with rainfall with the most supersaturated samples observed after a large rain event and samples approaching equilibrium after the longest period without rainfall. Calcium in excess of what would be expected from conservative mixing of fresh water and seawater was observed in all groundwater samples and was elevated at near-shore locations, especially on BPK. Contrary to expectations, dissolution resulting from mixing of freshwater and seawater was not supported in the shallow groundwater. Instead, dissolution within the narrow vadose zone from rain events likely resulted in the excess calcium in the groundwater. Seasonal fluctuations in groundwater composition were primarily observed on the smaller island and were related to the fresh water balance, changing rapidly after a heavy rain event, and suggest that a size threshold has been surpassed for a stable lens. Rising seas will further decrease lens extent and vadose zone depth, reducing the potential for future limestone dissolution.