GEOCHEMICAL EVIDENCE OF BRACKISH GROUNDWATER DISCHARGE TO COASTAL FRESHWATER WETLANDS

Coastal wetlands are often regions of groundwater discharge. In the southern Everglades, seawater intrudes into the coastal aquifer forcing brackish groundwater to discharge to the overlying freshwater wetlands. The brackish groundwater reflects reducing conditions with elevated concentrations of calcium and total phosphorus (TP). Brackish groundwater discharge is identified in the surface water during the dry season by enhanced chloride, sodium, and calcium concentrations. Geochemical mixing models identify even higher concentrations of calcium in surface water then is predicted by conservative mixing. The elevated calcium concentrations may be the result of calcite dissolution driven by a drop in pH upon mixing of the two waters. The concentrations of TP observed in the brackish groundwaters (1 to 2.3 µM) exceed the expected values predicted by conservative mixing of local fresh groundwater and intruding seawater, which both have TP < 1 µM. The additional source of TP is most likely from water-rock interactions (such as carbonate mineral dissolution and ion exchange reactions) induced by mixing fresh groundwater with intruding seawater. Despite the high concentrations of TP in the brackish groundwaters, only low TP values (<0.5 µM) are detected in the overlying surface waters. These results suggest that the phosphorus transported with the brackish groundwater is either removed from the surface water column quickly by biotic processes or is retained in the Everglades soils by sorption. The high concentrations of TP in the groundwater along with any sorbed to the soils is then available for plant root uptake, and therefore, may be an additional source of phosphorus (a limiting nutrient) to the coastal wetlands of the southern Everglades.