EFFECTS OF PLANT EVAPOTRANSPIRATION ON GROUNDWATER CARBONATE ACCUMULATION IN RECREATED TREE ISLANDS OF THE EVERGLADES

Tree islands in the Everglades have declined substantially in number and area in the past 65 years because of human alteration to the hydrology of South Florida. Preserving the remaining tree islands and restoring lost ones is necessary to re-establish mechanisms of nutrient redistribution and phosphorous sinks across the interior Everglades landscape, which is composed of tree islands, ridges, and sloughs. All of these features are present at the Loxahatchee Impoundment Landscape Assessment (LILA) facility, located at the Arthur R. Loxahatchee National Wildlife Refuge, in Boynton Beach, Florida. This research facility has provided us with a unique opportunity for conducting a long-term monitoring of the hydrogeochemical and hydrogeological conditions across a recreated Everglades landscape, which contains islands of different lithological characteristics and has the capacity of managing the hydrologic conditions.

Preserving and restoring tree islands in the Everglades requires understanding their mechanisms of formation and maintenance. Recently, a 50 to 70 cm thick carbonate horizon was found within the soils of 20 large Everglades tree islands. The original environmental conditions and the depositional and cementation processes under which these sediments formed have not been fully determined. Cemented layers are common features in arid environments, but have not been well documented in sub-humid locations such as South Florida. Long-term monitoring of hydrogeological and hydrogeochemical conditions at LILA shows that seasonal evapotranspiration lowers the water table and increases the concentrations of major dissolved ions and phosphorus in the groundwater beneath the center of the tree islands, with respect to their surrounding landscape. Furthermore, the decreasingly lower groundwater saturation indices with respect to calcite on each subsequent dry season suggests the potential for mineral precipitation. In addition, preliminary results of elemental analyses on tree island soil samples showed a horizon with an increased carbonate content. More detailed and extensive soil analyses and hydrogeochemical modeling are ongoing in order to discern a potential evapotranspiration-driven mechanism of carbonate mineral precipitation within the tree islands at LILA.