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

[1] The concentrations of tritium (H-3) and helium isotopes (He-3 and He-4) were used as tracers of groundwater flow in the surficial aquifer system (SAS) beneath Everglades National Park (ENP), south Florida. From ages determined by H-3/He-3 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 H-3/He-3 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 He-4 with depth in the SAS indicated that radiogenic He-4 produced in the underlying Hawthorn Group migrates into the SAS by diffusion. Higher Delta(4)He values in brackish groundwaters compared to fresh waters from similar depths suggested a possible enhanced vertical transport of He-4 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.