Effect of Ground-water and Surface-water Interactions on Mercury Transport in the North-central Everglades

A major part of research on aquatic cycling of mercury concerns chemical reactions in the subsurface (Gilmour and others, 1998). For example, the conversion of inorganic forms of mercury to methylmercury is a critical step that makes mercury more bioavailable in aquatic ecosystems. This reaction is thought to occur predominantly in the subsurface; for example, in porewater of the wetland peat (Gilmour and others, 1991). In order to understand the biogeochemical controls on mercury methylation, it also is necessary to understand physical transport processes that affect movement of mercury between surface water and peat porewater. Diffusion often has been investigated as an important mechanism that releases methylmercury from porewater into surface water, whereas advective transport (with discharging or recharging water) less often has been considered (Krabbenhoft and Babiarz, 1992).

Mercury mobility with recharging and discharging ground water in the Everglades is evaluated in this section. Discharge is the upward flow of ground water through peat into surface water, whereas recharge is the downward flow of surface water through peat into the aquifer. In addition to potentially transporting mercury, discharge and recharge also may affect mercury cycling by transporting other constituents that could affect chemical transformations of mercury. An example is transport of sulfate, which potentially affects mercury cycling through the effect on sulfate-reducing bacteria and sulfide levels, and their effects on methylation rates. The specific objectives of this part of the study are listed below:

Characterize the distribution of dissolved total mercury and methylmercury (Hg_T and MeHg) in the Everglades Nutrient Removal (ENR) project and Water Conservation Area 2A (WCA-2A) in surface water, peat porewater, and ground water.

Relate patterns in mercury distribution to factors such as aquifer lithology, ancillary geochemistry, and source of recharge water.

Quantify ground-water exchange fluxes of mercury to improve the overall mercury mass budget at the ENR.

ENR is a relatively large constructed wetland (3,815 ac) in the north-central Everglades built as a prototype for testing the effectiveness of a larger set of constructed wetlands called Stormwater Treatment Areas (STAs) designed to remove excess nutrients from agricultural runoff. A concern developed among water-resources managers that STAs might release large amounts of inorganic mercury to dissolved phases that subsequently would be exported in surface-water outflow. Much attention was focused on the ENR project because it represented an area of the Everglades where that concern could be tested by developing a reliable mercury balance. Previous hydrological work in ENR (for example, Choi and Harvey, 2000) demonstrated clearly that recharge and discharge of ground water had to be considered, in order to develop a reliable budget for mercury (or any chemical constituent that differs in its average concentration in surface water). The general approach used here was to combine recent estimates of recharge and discharge in ENR (Choi and Harvey, 2000) with mercury data from ground water and surface water to compute wetland to ground-water fluxes.

Although ENR is relatively large for a constructed wetland, it is small in comparison with hundreds of thousands of acres that make up the larger basins in the north-central Everglades. A disadvantage of working primarily at ENR is that it does not represent spatial variability across diverse Everglades landscapes. For that reason, the mercury distribution in WCA-2A also was investigated. Data from WCA-2A are not as comprehensive as from ENR, but, nonetheless, serve to broaden the relevance and applicability of conclusions given here.