A SUMMARY OF WATER-ROCK INTERACTIONS IN CARBONATE COASTAL AQUIFERS AFFECTED BY SALTWATER INTRUSION

Coastal aquifers affected by saltwater intrusion often experience changes in groundwater composition that are not representative of simple mixing of fresh groundwater with coastal seawater. Instead, the changes in groundwater chemistry along a salinity mixing zone are often non-linear. This presentation will discuss groundwater chemistry observed in coastal carbonate aquifers affected by saltwater intrusion in three regions: 1) Mallorca, Spain; 2) Everglades, Florida; and 3) the Florida Keys. Each of these carbonate aquifers is shallow and unconfined. Excess calcium and supersaturated conditions with respect to calcite were observed in each of these aquifers. Excess phosphorus was observed in the coastal Everglades aquifer. Geochemical processes found to be important in these carbonate aquifers include dissolution of carbonate minerals in the vadose zone with rainwater and infiltration of calcium ions to the shallow groundwater. The excess calcium combined with a loss of carbon dioxide from the shallow groundwater to the atmosphere can explain the supersaturated conditions with respect to the carbonate minerals. Carbonates typically adsorb phosphate dissolved in fresh waters. Although calcium carbonate dissolution is not expected in the mixing zone groundwater, phosphorus can be contributed to the mixing zone water via dissolution of the limestone in the vadose zone. Phosphorus is also released into the groundwater via desorption from the limestone upon contact with the intruding seawater. The excess phosphorus in the mixing zone groundwater can be transported to nearby coastal waters with groundwater discharge contributing to biological processes.