Use of tritium and helium to define groundwater flow conditions in Everglades National Park

Abstract

The concentrations of tritium (3H) and helium isotopes (3He and 4He) were used as tracers of groundwater flow in the surficial aquifer system (SAS) beneath Everglades National Park (ENP), south Florida. From ages determined by 3H/3He dating techniques, groundwater within the upper 28 m originated within the last 30 years. Below 28 m, waters originated prior to 30 years before present with evidence of mixing at the interface. Interannual variation of the 3H/3He ages within the upper 28 m was significant throughout the 3 year investigation, corresponding with varying hydrologic conditions. In the region of Taylor Slough Bridge, younger groundwater was consistently detected below older groundwater in the Biscayne Aquifer, suggesting preferential flow to the lower part of the aquifer. An increase in 4He with depth in the SAS indicated that radiogenic 4He produced in the underlying Hawthorn Group migrates into the SAS by diffusion. Higher Δ4He values in brackish groundwaters compared to fresh waters from similar depths suggested a possible enhanced vertical transport of 4He in the seawater mixing zone. Groundwater salinity measurements indicated the presence of a wide (6–28 km) seawater mixing zone. Comparison of groundwater levels with surface water levels in this zone indicated the potential for brackish groundwater discharge to the overlying Everglades surface water.

Groundwater’s significance to changing hydrology, water chemistry, and biological communities of a floodplain ecosystem, Everglades, South Florida, USA

“…4). Similar values of water stable isotope ratios close to precipitation values also have been observed in shallow groundwater beneath short hydroperiod wetlands of the Rocky Glades in the southern Everglades by Price and Swart (2006). The more negative isotopic composition of those waters is consistent with recharge of precipitation occurring quickly and without time for significant evaporation to occur Price and Swart 2006).…”

“…Similar values of water stable isotope ratios close to precipitation values also have been observed in shallow groundwater beneath short hydroperiod wetlands of the Rocky Glades in the southern Everglades by Price and Swart (2006). The more negative isotopic composition of those waters is consistent with recharge of precipitation occurring quickly and without time for significant evaporation to occur Price and Swart 2006). In STA-1W those values reflect the history of the area as part of the EAA, where drainage and farming in the middle part of the century promoted direct infiltration of precipitation into soils until the mid 1990s when the site was re-flooded for operation as a stormwater treatment area.…”

“…Surfacewater specific conductance in the most mineral-rich freshwater areas of Everglades National Park rarely exceeds 400-600 μS/cm (Joffre Castro, Everglades National Park, personal communication, 2007). The relatively high transmissivity of the Biscayne aquifer beneath the southern Everglades (Fish and Stewart 1991;Renken et al 2005) and the thinner peats in the southern Everglades promote significant flow through marginal wetlands into the sloughs of the southern Everglades (Price et al 2003;Harvey et al 2006). The chemistry of shallow groundwaters in short hydroperiod wetlands (Price and Swart 2006) is similar to the subset of eastern canal waters in the northern Everglades that have lower specific conductance, i.e.…”

“…It is instructive to look at the southern Everglades where there are large areas of short hydroperiod wetlands on the margins of the major sloughs in Everglades National Park. The relatively short hydroperiods in those areas promote infiltration of precipitation into the peat and bedrock (Price and Swart 2006). The wetlands also have thinner peat accumulations (Gleason et al 1974) which offers less resistance to subsurface vertical flow.…”

“…Deeper groundwater in the western part of the Biscayne aquifer beneath WCA-2A has a specific conductance often above 10,000 μS/cm and as high as 26,000 μS/cm and is a sodium chloride water type, reflecting its origin as relict seawater trapped in the aquifer during higher sea level stands (Howie 1987;Harvey et al 2002;Price and Swart 2006). The origin of the shallow groundwater is very different.…”