WATER-ROCK INTERACTIONS AND SEASONAL HYDROLOGIC PROCESSES IN CONSTRUCTED EVERGLADES TREE ISLANDS

The decline of tree islands in the freshwater-Everglades wetland has compromised valuable ecosystem services because of hydrologic manipulation, including water drainage by canals and compartmentalization in water conservation areas. Although the role of tree islands in maintaining freshwater quality stems largely from evapotranspiration processes, fundamental questions remain about the effects of different geologic materials on their hydrogeochemical functioning, under seasonal hydrologic conditions. To address this issue, the lithological composition of a set of man-made tree islands composed of different geologic substrates was investigated coupled with long-term hydrologic and hydrochemical data, obtained from the Loxahatchee Impoundment Landscape Assessment (LILA) facility. The LILA facility simulates a large-scale physical model of the freshwater-Everglades wetland, including the tree island-slough-ridge landscape, which provides an advantageous setting to study the hydrogeochemical conditions of developing tree islands.

Groundwater-flow directions, calculated from observed water levels across LILA, coupled with the use of Cl^- concentrations and d¹⁸O values of water samples as hydrochemical tracers, and hydrogeochemical modeling, indicated that both limestone and peat substrates with elevated proportions of sand facilitated surface water-groundwater interactions and mineral dissolution. However, limestone-based tree islands were more effective in supporting evapotranspiration processes to lower the water table and concentrate solutes during low surface water stages, compared to peat-based tree islands with elevated sand content. In addition, the peat substrate of an island with low sand content favored the thermodynamic conditions for calcite accumulation in the phreatic zone, despite seasonally variable surface-water stages. Furthermore, phosphorus concentrations in groundwater were associated with organic-matter breakdown in the peat-based tree islands. Future efforts for restoring Everglades tree islands should focus on characterizing the mineralogy of their deeper soils to better constrain hydrogeochemical models, providing more robust information about the long-term stability of constructed tree islands under managed hydrologic conditions.