Elevated mercury concentrations were identified previously in freshwater fish in the Everglades, Savannas State Reserve, and receiving waters of the Okefenokee Swamp. The goals of this research were to: 1) determine historic baseline concentrations of mercury in wetland soils of the Florida Everglades, Savannas Marsh, and Okefenokee Swamp; 2) determine post-development changes in sedimentary mercury accumulation; and 3) identify the spatial distribution of mercury throughout the Florida Everglades. Sixty sediment cores were retrieved between January 1992 and February 1993, and were analyzed for total mercury, percent solids, and bulk density. Selected cores were analyzed for carbon (total and organic), and additional metals (Cd, Cr, Cu, Fe, Ni, Pb, and Zn), and were chronologically analyzed after radionuclide analysis for 210Pb and 137Cs.

The average mercury concentration in surface sediment (0 - 4 cm) of 121 ng g^-1 (n=51, 17-411 ng g^-1) was 2.5 times (0.2-10.6, n=51) higher than corresponding deep sediment (11-17 cm) concentrations. The largest increases were measured in Water Conservation Areas 1 and 2 (WCA1 and WCA2)(3.7 times higher for both) of the Florida Everglades, while Okefenokee Swamp sediment showed the smallest relative increase (1.4). Because concentration data are vulnerable to temporal variations in bulk sediment accumulation rate, the interpretive problem of co-variance was avoided by determining mercury accumulation rates after radionuclide dating. Post-1985 mercury accumulation rates averaged 53 ug m^-2 y^-1 (23-141 ug m^-2 y^-1) corresponding to a 6.4 (1.6-19.1, n=18) times rate increase since the year 1900. The largest rate increases occurred in WCA1 and WCA2 cores (7.8 and 8.7 times higher, respectively), while the Savannas State Reserve cores showed the smallest rate increase (3.4). Mercury accumulation rates increase starting about 1940, due perhaps to mid-century alteration of the hydrologic structure of the Everglades, and to increased agricultural and urban development to the north and east. There is presently insufficient information regarding regional inputs to quantify any direct causal relationship between mercury accumulation rate increases and regional human activities. However, apparent nonuniform accumulation of mercury in the Everglades hydrologic basins, coupled with increased accumulation rates of other trace metals (Cd, Cr, Cu, Pb, Ni, and Zn), indicate some atmospheric contribution of mercury from regional anthropogenic activities. The findings are similar to trends reported for lakes in Minnesota, Wisconsin, and Sweden. This agreement is significant, perhaps indicating a global process that leads to similar accumulation rates over widely varying geographic regions. This research provides the first data on mercury accumulation in subtropical wetland systems and demonstrates the feasibility of radiochemical dating of wetland cores.