Abstract Introduction Study Area Coring & Pore Water Extraction Analytical Methods Results & Discussion Summary Acknowledgments References PDF Version

The results of chemical analysis of surface and pore waters from 22 selected sites throughout south Florida showed that distinct regional patterns exist in the distribution of chemical species in this wetland ecosystem. Most chemical species in both surface and pore water showed enhanced concentrations in WCA 2A, with generally decreasing concentrations to the south and west into WCA 3A, ENP, and Big Cypress National Preserve. The largest enhancements observed were for sulfide and phosphate, with concentrations in pore waters at sites near the Hillsboro Canal in WCA 2A up to 500 times higher than "pristine" sites in WCA 1A, WCA 3A, and ENP. Other chemical species (DOC, alkalinity, chloride, fluoride, calcium, magnesium, sodium, potassium, and strontium) showed much lower enhancements of 2 to 10 times at WCA 2A near-canal sites compared to "pristine" areas. Ammonium concentrations in pore waters were quite variable, and were frequently very high (> 1,000 µg/l) at pristine sites with concentrations comparable to contaminated marsh sites near the Hillsboro Canal in WCA 2A.

Within WCA 2A, sites near the Hillsboro Canal had higher pore water concentrations of several dissolved chemical species compared to a site near the center of WCA 2A (2A-U3). This trend is most pronounced for phosphate, consistent with previous studies of the sediments which show a distinct pattern of sedimentary phosphorus enrichment proceeding toward the Hillsboro Canal (Koch and Reddy, 1992; Craft and Richardson, 1993). The phosphorus enrichment near the Hillsboro Canal apparently originates from discharge of canal water that contains phosphate derived from the Everglades Agricultural Area. This phosphate is rapidly assimilated by aquatic macrophytes (notably cattails which have recently colonized areas near the Hillsboro Canal in WCA's 1A and 2A) and ultimately deposited in the sediments. Diagenetic processes in the sediments recycle the sedimentary phosphorus, producing high pore water phosphate concentrations at the contaminated sites. Titration alkalinity and DOC concentrations are also somewhat higher at sites near the canal compared to the central marsh site in WCA 2A. This likely reflects the active decomposition of organic matter derived from macrophytes (notably cattails) that flourish in the nutrient-rich environment adjacent to canals. Other chemical species in pore water, such as chloride, fluoride, calcium, magnesium, sodium, and strontium were only nominally higher at one of the canal sites (WCA 2A-F1) compared to the center marsh site (WCA 2A-U3). Sulfate showed somewhat unusual behavior, with similar surface water sulfate concentrations and pore water sulfate profiles at both the canal and center marsh sites, but > 10x higher sulfide concentrations in pore water at the center marsh site. Rooted aquatic macrophytes (cattails at eutrophied sites and sawgrass at non-eutrophied sites) may play a key role in transporting dissolved gases such as sulfide between the sediments and the atmosphere.

In addition to the observed regional trends, the pore water data provides information on the nature of diagenetic processes occurring in the sediments. In most cores, maxima for phosphate and ammonium in the pore water occur in the upper 20 cm of sediment. This suggests that recycling of organic P and N occurs principally in the near-surface sediments where microbial activity is highest. High concentrations of phosphate and ammonium in the pore waters establish concentration gradients between the pore water and surface water, and diffusional fluxes of pore water nutrients may represent a significant source of nutrients to marsh surface waters. Sulfate reduction is evident at most sites, but high levels of sulfide (> 100 µg/l) were only observed in pore water from sites in WCA 2A, a canal site in WCA 1A, and a brackish water site in ENP. Sulfate reduction is thought to be the principal mechanism for the microbial methylation of mercury (Gilmour et al., 1992), an important environmental issue in south Florida. When abundant sulfate (e.g. @ 5-10 mg/l or greater) is present, however, the production of high levels of sulfide in the pore water may inhibit mercury methylation by immobilizing the mercury as insoluble mercuric sulfide. Thus the high levels of sulfate and sulfide in WCA 2A may retard mercury methylation here. At sites further south (e.g. WCA 3A-15) measurable but much lower levels of sulfate and sulfide may favor mercury methylation. Preliminary results (Gilmour et al., 1997) suggest that rates of mercury methylation are in fact greater at site WCA 3A-15 compared to sites in WCA 2A. Further work is underway to better evaluate the relation between mercury methylation and sulfate reduction in various wetland areas of the south Florida.