Poor agreement between 3H/3He ages and CFC-11 and CFC-12 ages suggests that CFCs may not be conservative tracers in the Everglades National Park. 3H/3He ages were used to calculate the expected concentration of CFC-11 and CFC-12 in ground water from wells 2 to 73 m deep. The expected concentration of CFCs were compared to the measured concentration and plots of the % CFC-12 and CFC-11 remaining offered no evidence that significant CFC removal was occurring in the groundwater at depths > 2 m, suggesting that CFC removal occurs at shallower depths. Except where CFC contamination was suspected, CFC-11, CFC-12 and CFC-113 concentrations in fresh surface water were nearly always below solubility with the atmosphere. Measurements of CFC-11, CFC-12, and CFC-113 in pore water indicate a 50 to 90 % decrease in concentration 5 cm below the groundwater-surface water (GW-SW) interface. In the same 5 cm interval CH4 concentrations increased by 300 to 1000 %. This suggested that CFCs were removed at the GW-SW interface, possibly by methane-producing bacteria. CFC derived recharge ages should therefore be viewed with caution when recharging water percolates through anoxic methanogenic sediments.
Journal of Hydrology, Vol. 279, 94-105, 2003.