Mercury contamination is a concern in the Florida Everglades because mercury in the form of methyl mercury (MeHg) can biomagnify and become concentrated to toxic levels in species near the top of the food chain. Although the principal source of mercury is atmospheric deposition, processes occurring in oxygen-deficient (anoxic) wetlands such as the Everglades can enhance the toxicity of mercury through production of the methyl mercury species. One process is sulfur cycling, which is inferred to be linked with the transport, accumulation, and cycling of mercury through oxidation-reduction processes mediated by microbes. Therefore, the amount and rate of sulfate reduction and sulfide oxidation may correlate with mercury species concentrations in the Everglades.

We are studying trends in sulfur speciation and isotopic composition in sediments and surface waters which may be related to location, season, nutrient loading, and rates of sedimentary deposition. We are analyzing these trends to interpret how they relate to mercury cycling in the Florida Everglades. Our studies have focused mainly on determining trends in sulfate contents and isotopic composition in surface waters in the Water Conservations Areas (WCAs) and their bordering canals. These data have been correlated with mercury contents determined by other researchers (D. Krabbenhoft, USGS, WRD, Madison, Wisconsin).

Results suggest that both sulfur and mercury cycling display trends which are related to sampling localities along transects in WCA 2A from the Hillsboro Canal out into the interior of the WCA. Sulfate concentration trends and sulfate-oxygen isotopic ratios reveal that, during periods of low rainfall, sulfide oxidation to sulfate appears to be greater at sampling sites in the interior of the WCA far from the Hillsboro Canal than at sites close to the canal. Isotopic ratios in reduced sulfur species indicate that sulfate reduction rates are higher at nutrient impacted areas near the canal. We hypothesize that MeHg formation is related to sulfate reduction rates. Both total mercury and MeHg fixation are greater at sites far from the Hillsboro Canal where we believe sulfate reduction rates are lower. The sulfate reduction rates may be stimulated by high organic productivity in nutrient enriched areas near the Hillsboro Canal. If this is the case, high rates of sulfate reduction appear to suppress rather than stimulate MeHg production. Possibly, MeHg production may be optimized at lower sulfate reduction rates. A buildup of hydrogen sulfide in sediments with high sulfate reduction rates may actually poison the MeHg production process, or mercuric sulfide precipitation may be favored over MeHg production at high sulfate reduction rates.

Currently, our efforts are focused on determining the sources of sulfate in the WCA in the Everglades. Our data at this time are limited; however, results appear to indicate that sulfate concentrations in surface water from the Hillsboro and New North River Canals in the Everglades Agricultural Areas (EAAs) are slightly higher than in the canals bordering the WCA. Sulfate-sulfur isotopic ratios in the surface water in the EAA canals (³⁴S ~ 17 per mil) are close to that of sulfur fertilizer (³⁴S ~ 15 per mil) and sulfate in the EAA soil (³⁴S ~ 15 per mil). These isotopic ratios are lower than those found in the canals bordering the WCAs (³⁴S from 18 to 21 per mil). A single ground-water sample has been analyzed to date. The sulfate concentration (1.04 meq/L) is higher than in most surface water samples in the WCAs. The sulfate-sulfur isotopic ratio in ground water (³⁴S ~ 29 per mil) is higher than in sulfate from most surface water samples (³⁴S from 20 to 26 per mil). Understanding the source of sulfate to the wetlands of South Florida may be key to understanding why mercury methylation rates are high in certain areas of the Everglades.

Back to Project Homepage