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Modeling decadal timescale interactions between surface water and ground water in the central Everglades, Florida, USA

6. Summary

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1. The depth in the shallow, interactive layer of ground water near the top of the Surficial aquifer that exchanges water with surface water on a decadal timescale was estimated in WCA-2A using observations of the depth-distribution of tritium in ground water. That value (3.1 m, after correction by porosity to represent actual water storage) was used as input to a coupled model of transport and decay of tritium in surface water and ground water of Water Conservation Area 2A in the central Everglades. Average ground-water residence time in the model was adjusted to match the average tritium concentration in interactive ground water. Those results were then used to calculate time-averaged recharge and discharge fluxes across the Everglades ground surface, which were on the order of 0.01 cm d-1. A check on modeled residence times of interactive ground water was possible by comparison with available measurements of 3H/3He isotopic ratios. That comparison demonstrated that vertical mixing of young ground water was occurring with very old (tritium-dead) ground water, which caused both the residence time and depth of interactive ground water to be overestimated. We believe that our resulting estimates long-term average rate of recharge and discharge fluxes are not very sensitive to vertical mixing with tritium-dead water, because of the compensating effect that overestimating both the residence time and depth of shallow interactive ground water has if vertical mixing occurs. As a result, we believe our long-term average estimate of recharge and discharge of 0.1 cm d-1 is reliable. For future studies a greater number of 3H/3He measurements would be preferable of the desirable effect that resulting residence times are relatively insensitive to vertical mixing. Higher analysis costs and frequent failures of 3H/3He sampling and analysis are not always tolerable, and the present study shows that low-level (0.4 TU Minimum Detection Limit) tritium measurements still have value to investigations of interactions between surface water and ground water.

2. The depth distribution of tritium in ground water of WCA-2A in the central Everglades indicated that relatively recent recharge water had penetrated to a depth of 8 m in the aquifer. This is only approximately 30% of the penetration depth measured by Price et al. (2003) in the southern Everglades within Everglades national Park, which suggests that (since 3H/3He estimated residence times were similar between the two studies), that recharge and discharge might be as much as three times larger in the southern Everglades compared with the central Everglades. This hypothesis is consistent with what is known about the hydraulic properties of the Surficial aquifer in these two areas of the Everglades (i.e. higher hydraulic conductivity in the Surficial aquifer in many areas of Everglades National Park, particularly on its eastern side).

3. There have been relatively few attempts to develop long-term simulations of contaminant fate and transport in the Everglades that consider ground water-surface water interactions, in part due to a lack of reliable estimates of recharge and discharge. The decadal estimates of recharge and discharge resulting from the present work are complementary to previous short-term estimates of recharge and discharge in WCA-2A. For example, decadal estimates of recharge and discharge are indicative of interactions between surface water and the sand and limestone aquifer, and not just the interactions with peat porewater that are characterized by the short-term estimates in Krest and Harvey (2003); Harvey et al. (2004). Results of this study, when combined with previous investigations, will therefore be useful for guiding future simulations of Everglades water quality.

4. Recharge and discharge fluxes in WCA-2A on the order of 0.01 cm d-1 are significant relative to other water balance fluxes such as precipitation evapotranspiration, and surface flow (SFWMD, 1999). One of the most significant side effects of recharge and discharge are its potential influence on solute fate and transport processes in the wetlands. In particular there is concern about the storage of surface-water contaminants, such as phosphorus, in peat and ground water due to recharge and discharge. For example, the effects in the future of recharge of surface-water contaminants today could be evident decades from now, due to the potential for stored contaminants to be returned to the wetland over a period of decades after they were first recharged. More work is needed to understand the potential for such processes to produce historic legacies of contamination in the Everglades long after surface-water inflows from agricultural drainage have been cleaned up.

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