USING TEMPERATURE AND WATER LEVELS TO IDENTIFY GROUNDWATER-SURFACE WATER INTERACTIONS IN PEAT TREE ISLANDS

Tree islands in the Everglades act as a nutrient sink with total soil phosphorus concentrations three to six-fold higher than the surrounding oligotrophic surface water system. The elevated concentrations of phosphorus in the tree islands may be attributed to the interaction of high nutrient groundwater and low nutrient surface water in a system that is inundated half of the year. To gain a finer understanding of groundwater-surface water interactions in tree islands, research was conducted on man-made tree islands at Loxahatchee Impound Landscape Assessment (LILA), a large physical model of the Everglades located in Boynton Beach, Florida. LILA contains 8 tree islands, 4 made entirely of peat, and 4 have limestone cores with a thin peat cover. Nine groundwater monitoring wells were installed on each of the 8 islands. Both temperature and water level were used to identify an exchange of surface water and groundwater. During the summer wet-season when the surface water temperature averaged 30C, the groundwater temperature varied spatially across the tree island from 28C along the perimeter to 26.5C at the center. In the winter dry-season when the average surface water temperature dropped below the groundwater temperature a diurnal temperature change of 1-2C was detected in the groundwater. This diurnal signal in temperature suggests a density dependent movement of water between the colder surface water and the warmer groundwater. A second type of diurnal signal occurred when surface water levels were more the 0.4m below that the top of the tree islands. This second diurnal signal consisted of a groundwater level drawdown of 1-3.5cm most likely in response to evapotranspiration. While these two diurnal signals were evident in tree islands with peat cores, the signals were severely dampened or undetectable in the limestone core tree islands. These results suggest there are two potential mechanisms for surface water-groundwater interactions in peat tree islands, and these mechanisms may contribute to nutrient dynamics within tree islands.