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projects > aquatic cycling of mercury in the everglades > abstract
Distribution and Transport Mechanisms of Mercury and Methylmercury in Peat of the Everglades Nutrient Removal AreaSusan A. King, Judson W. Harvey, David P. Krabbenhoft, Randy J. Hunt, David E. Armstrong, John F. DeWild, and Mark L. Olson Mercury (Hg) studies in Canada, Wisconsin, and the Florida Everglades have shown that some wetlands are net production areas of methylmercury (MeHg). Concerns over whether net production of methylmercury may also occur in constructed wetlands prompted studies of a constructed wetland in Florida as part of the Aquatic Cycling of Mercury in the Everglades (ACME) project funded by the U.S. Geological Survey and the South Florida Water Management District. The Everglades Nutrient Removal (ENR) project is a 3,800 acre treatment wetland constructed in 1994 to reduce phosphorous levels in agricultural runoff before discharging to the Everglades. The ENR is situated along a ground-water gradient between the Loxahatchee Wildlife Refuge and the Everglades Agricultural Area. Its location on a ground-water gradient provides the opportunity to examine the influence of upward (site P12, on the east side of the ENR) and downward (site P5, on the west side of the ENR) advective ground-water fluxes, and diffusive fluxes (site ENR103, located in the middle of the ENR) on concentrations, partitioning, and transport of mercury species in wetland peat. Ground-water flux may affect several important Hg-cycling processes, including (1) adsorption and desorption of Hg and MeHg; (2) geochemical controls on mercury solubility; and (3) mercury methylation and demethylation by preventing accumulation of reaction byproducts. We hypothesize that observed mercury levels in ENR sediments will differ spatially and temporally between these sites primarily due spatial and temporal differences in advective hydrologic flux, and may be important to the ENR mercury mass balance in low rainfall periods. Ultra-clean sampling techniques were used during all sample collection and laboratory
procedures. Sampling was conducted in January, April, and July 1997, which span the range of
hydrologic conditions generally observed in south Florida - low water levels in January and April, and high water levels in July. Surface water and porewater samples were analyzed for total mercury (HgT), MeHg, dissolved organic carbon (DOC), sulfide and ultra-violet light adsorbence. Surface water and porewater samples were pumped through a Teflon sampling line and filter holder, and filtered with ashed, quartz-fiber filters (nominal pore size 2 mm) to ensure comparability. Sediments were analyzed for HgT, MeHg, organic carbon, and bulk density.
The following is a brief summary of some of our preliminary results from the study. Total
mercury levels in surface waters at all sites ranged between 1.14 and 5.23 ng/L during this study. For all three sampling periods, the diffusion-dominated site (ENR103) had the greatest difference between porewater and surface water Hg Porewater MeHg concentrations were often below the detection limits due to limited sample
volume size and analytical interference by high DOC and sulfide levels. However, we were able to
calculate porewater MeHg concentrations based on partitioning coefficients, sediment concentration profiles, and sediment porosity. We used these values to estimate MeHg mass fluxes from ENR sediments.
Estimated methylmercury levels were highest in the top 5 cm of sediments at all sites, and
concentrations declined rapidly with increasing sediment depth. Highest sediment MeHg
concentrations were observed in January 1997 at site P12 (3.6 ng/g dry weight), coinciding with the higher surface water MeHg levels at this site during the same time period. Sediment MeHg
concentrations at sites ENR103 and P5 were generally lower than site P12, reaching a maximum
concentration of 1.7 ng/g during July 1997 at both sites. Temporal differences in sediment MeHg
levels are hypothesized to be related to spatial differences in ground-water-flux rates. Most of the MeHg values reported here are higher than those reported previously, which may reflect conditions associated with maturation of the ENR wetland or annual variability in water chemistry and methylation rates.
In summary, we observed spatial and temporal variability in the surface waters, porewaters, and sediments of the ENR across a hydrologic gradient. These variations are hypothesized to be controlled by systematic changes in ground-water/surface water interactions along this hydrologic gradient. It is likely that the hydrologic settings are influencing Hg and MeHg sources and sinks in the ENR.
(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)
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