The spatial location and the geochemical conditions of the oligohaline ecotone are a long-term result of the balance between upstream inflows and marine influences. This balance is mediated on seasonal and annual time scales by climatological forcing (local rainfall and evapotranspiration) as well as regional water management, which together regulate surface water geochemical conditions and the underlying brackish groundwater mixing zone. Increased freshwater inflows to SRS (the "Grand Experiment") and in the long run to both transects (associated with Everglades Restoration) are likely to impact surface and groundwater hydrodynamics in the oligohaline ecotone. Seawater intrusion into the Biscayne Aquifer ranges from 10-30 km inland from the coast in the TS/Ph and SRS drainages (Fig. 1). High levels of phosphorus occur in this seawater intrusion zone as a result of geochemical reactions such as ion-exchange and carbonate mineral dissolution (Fig. 2). FCE investigators expect that increased inflows from the "Grand Experiment" will: 1) shift the location of salinity mixing in the oligohaline ecotone towards the coast; 2) suppress brackish groundwater discharge to the oligohaline ecotone, thus changing the geochemical conditions in this area; and 3) reduce water residence times in the SRS oligohaline ecotone, but these changes will not occur in the TS/Ph ecotone. The position of the seawater mixing zone will be monitored using groundwater wells located along the Shark and Taylor Slough Transects. (Fig. 1). Groundwater discharge and surface water residence times will be quantified with a combination of geophysical and geochemical methods including : 1) mass-balance mixing models of major cations and anions run monthly and seasonally to determine the proportions of brackish groundwater discharge and surface seawater in the surface water environments, and 2) subsurface heat flux measurement from thermocouple sensor arrays that will be installed at various depths in the soil combined with heat flux modeling. Surface water residence times will be determined using combination of geochemical methods including 1) synoptic surveys of 222Rn and streaming resistivity, and 2) SF-6 tracer students. Finally, the groundwater discharge rates will be combined with surface water flow, rainfall and ET measurement to provide a water mass balance estimate for Shark and Taylor Slough at varying time scales.
Fig. 1. Seawater intrusion as determined by salinity contours in shallow groundwater (<28 m) beneath ENP. Monitoring stations indicate current and proposed groundwater monitoring sites.
Fig. 2. Seawater intrudes into the coastal surficial aquifer system beneath ENP and mixing with the fresh groundwater to produce a brackish groundwater mixing zone. This brackish groundwater contains elevated concentrations of phosphorus due to geochemical reactions such as ion exchange and carbonate mineral dissolution. The brackish groundwater discharges to the surface water of the Everglades.