EFFECTS OF TOPOGRAPHY AND PEAT/MARL LAYERS ON GROUNDWATER DISCHARGE AND SALT WATER INTRUSION IN TAYLOR SLOUGH, THE FLORIDA COASTAL EVERGLADES

In the southern Everglades, coastal groundwater discharge is driven by hydraulic head and density differentials among the fresh, brackish and saltwater interfaces. The saturated-unsaturated transport model (SUTRA) is used to simulate variable density groundwater flow and solute transport in the Taylor Slough region of Everglades National Park. A 35km long 2D vertical profile of the slough was established from the L-31W levee in the north to Florida Bay in the south. The width of the model varied from 2km at the upstream to 13 km in Florida Bay. Topography and aquifer characteristics were input to simulate the hydrogeologic conditions of Taylor Slough. Upstream freshwater inputs were derived from water level at monitoring wells along or close to the profiled transect line were included in the model. The model was run from 2002 to 2016 on a monthly time step. Results suggest that in the absence of significant hydraulic head difference, the density variation at the salt water mixing zone is a major factor in coastal groundwater discharge. The mixing zone with in the top 5m depth of the aquifer moves landward and seaward seasonally, with a net inland migration throughout the period of simulation. The lighter fresh groundwater discharges inland of the mixing zone with a seasonal pattern coincident with upstream freshwater delivery. Heavier saline plumes were observed to sink in the mixing zone following on-set of the rainy season. The overall groundwater flow pattern was affected by topographic features such as roads and sloughs as well as the presence and thickness of surficial peat and marl layers. The modeling endeavor will help to better understand the role of small topographic features, peat/marl layers, and sea level rise in coastal groundwater discharge in a region of low hydraulic head.