ULTRA-TRACE LEVEL MERCURY IN THE EVERGLADES ECOSYSTEM, A MULTIMEDIA CANAL PILOT-STUDY

The Florida Everglades ecosystem is composed of the largest deposit (9600 km(2)) of near neutral peat in the world extending south of Lake Okeechobee to Florida Bay. The federal Central and South Florida Flood Control Project (C-and-SF) has sectioned the historic Everglades with a system of canals and levees to control water for urban and agricultural development, resulting in pronounced hydrologic modifications to the natural system. As a part of a comprehensive ecological risk assessment of mercury (Hg) contamination in the Everglades ecosystem, a pilot study of canals was initiated in September 1993 to determine the extent and magnitude of total (HgT) and methylmercury (MeHg) in water, sediment and fish. A probability-based random sampling grid was used to obtain consistent estimates of Hg contamination over this large geographic area. Two hundred canal sampling locations were selected as probability samples by associating grid points on the sampling frame with specific canal sections for independent sampling cycles. Of this number 50 locations were randomly selected for sampling in this pilot study. The selected canal points were sampled from north to south during a six day period. Cumulative distributions with 95% confidence intervals were calculated and used to determine a canal system median concentration for selected water, sediment and fish constituents. The percent exceedance of each median, by hydrologic subarea, was determined to demonstrate the existence and direction of spatial gradients in the system. North to south (high to low) gradients were apparent for total phosphorus (TP), sulfate (SO4), dissolved organic carbon (DOC), conductance, HgT and MeHg in water. However, the gradients were reversed from south to north for HgT in sediments and fish (Gambusia sp.) The greatest Hg concentrations in Gambusia sp. occurred in the same canals where largemouth bass had previously been found to be most contaminated.