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Region 4 South Florida Project Information

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Text reproduced from the publication, USEPA 1993. Regional Environmental Monitoring and Assessment Program. EPA/625/R-93/012. September 1993. U.S. Environmental Protection Agency, Office of Research and Development, Washington, DC.

Figures and tables not reproduced here. The complete publication may be ordered from the U.S. Environmental Protection Agency, Center for Environmental Research Information, Cincinnati, OH 45268. The publication request desk may be reached by phone at 513-569-7562.


Since 1989, mercury has been found in elevated concentrations in various biota of the Florida Everglades, including fish, the Florida panther, raccoons, wading birds, and alligators. The state of Florida has issued a fish consumption advisory due to mercury contamination, banning or restricting the consumption of largemouth bass and other freshwater fish from 2 million acres encompassing the Everglades and Big Cypress National Preserve (Figure 4-1). Mercury contamination in Florida, although highest in the Everglades, also occurs in largemouth bass in many other lakes and streams across the state. Mercury in its most toxic form, methyl mercury, accumulates in aquatic life, and may pose increased risks to consumers at the top of the food chain (birds, mammals, and humans).

Scientists currently know little about the sources, extent, transport, transformation, and pathways of mercury in South Florida ecosystems. Possible mercury sources in South Florida include natural mineral and peat deposits, atmospheric deposition (global and regional), fossil fuel fired electrical generating plants, municipal waste incinerators, medical laboratories, paint, and agricultural operations. None of these individual sources, however, appears adequate to explain the vast area apparently contaminated.

The proposed Region IV R-EMAP study will identify and coordinate research, monitoring, and regulatory efforts to address this issue, using EPA's ecological risk assessment framework. The study will focus on the Everglades ecosystem, composed of the largest deposit of near-neutral peat in the world, encompassing a region about 40 miles wide by 100 miles long south of Lake Okeechobee to Florida Bay (Figure 4-2). The study area includes the Everglades Agricultural Area (EAA), three Water Conservation Areas (WCAs) including the Loxahatchee National Wildlife Refuge (WCA-1), Big Cypress National Preserve, Everglades National Park (ENP), and other areas drained for urban and agricultural development, resulting in massive hydrologic modifications.

Seven policy-relevant questions have been identified to guide the development of this complex research and monitoring effort:

The proposed Region IV R-EMAP project will focus on the first four questions above and will initiate an ecological risk assessment process. The project will integrate and coordinate the efforts of various state and federal agencies, including EPA's Office of Research and Development and Region IV Environmental Services Division; Florida's Department of Environmental Protection, Freshwater Game and Fish Commission, Department of Health and Rehabilitative Services, and South Florida Water Management District; the U.S. Army Corps of Engineers; the U.S. Geological Survey; and industry representatives. Dr. Ron Jones of the Southeastern Environmental Research Program at Florida International University is cooperating closely with both the Everglades National Park and Region IV on this R-EMAP project.

Significant quantities of mercury cycle through the air, water and solid phases of the global environment. Mercury cycling through the atmosphere is estimated at 6 billion grams per year. Within this global background, certain regional areas may have higher atmospheric background concentrations due to nearby urban or industrial activity. In South Florida, the operation of solid waste incinerators and fossil fuel power plants has increased since 1940. It is possible, therefore, that regional atmospheric mercury might also have increased over this time period. Figure 4-3 depicts atmospheric deposition of mercury from urban sources into the Everglades. Figure 4-4 shows a conceptual model of the biogeochemical cycling of mercury in the Everglades ecosystem.

Important components of the mercury cycle include:

Because the abundant organic matter in wetlands sequesters mercury, Everglades soils contain a substantial mercury pool even without continuing atmospheric deposition. These soils are a suspected source of the mercury contaminating fish in associated waters. The subsidence (loss of surface elevation) of peat and muck in the Everglades Agricultural Area over the years may have resulted in the concentration of mercury at the soil surface, facilitating methylation, transport of mercury downstream, and/or evaporation of mercury.

Inorganic mercury is converted to methyl mercury primarily through the actions of microorganisms. Sulfur-reducing bacteria have been implicated in mercury methylation. Methylation greatly increases the toxicity of mercury, its ability to be bioaccumulated, and its mobility in the environment.

One of the most noticeable changes in the Everglades ecosystem in recent years is eutrophication. Phosphate enrichment in Everglades soils has triggered microbial consumption of organic matter, resulting in anaerobic conditions and a change from oligotrophic to eutrophic ecosystems in some areas. Under eutrophic conditions, inorganic mercury may be converted to methyl mercury and bioaccumulated in the food chain.

This may be an important transport mechanism that moves sediment, phosphorus, and inorganic and organic mercury off the Everglades Agricultural Area via canals to the downstream Water Conservation Areas and toward Everglades National Park. An average of 200 tons of phosphorus flow from the 700,000-acre agricultural basin into downstream habitats each year, resulting in systemic changes in wetland flora and fauna.

Evasion from South Florida wetland habitats, other land uses, and open waters is a component of the mercury cycle that has not yet been quantified.

Critical path analyses for the top terrestrial and aquatic predators (birds, reptiles, and mammals) in several habitat types are an important part of an ecological risk assessment for mercury contamination in the Everglades ecosystem.

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The Region IV R-EMAP study is designed to answer questions that focus on the extent, magnitude, and trends of the mercury problem, as well as to provide information for the initial phase of the ecological risk assessment process. All the activities are part of a larger interagency effort to study mercury contamination in the Everglades. Habitat types that will be sampled include canals, ponds, sloughs, wet prairies, sawgrass marsh, and hammocks/tree islands. Canal sampling is scheduled to begin in September 1993, with other tasks to follow in fall and winter 1993-94.

Water, soil, sediment, and biota will be sampled using the EMAP sampling strategy. Regional air monitoring is being conducted by the Florida Atmospheric Mercury Study, supported by Florida Power and Light, Electric Power Research Institute, Florida Department of Environmental Protection, and EPA Region IV. In addition, the Region IV Environmental Services Division is initiating studies of the sources, fate, and transport of mercury emissions. Data from these studies will be integrated into the Region IV R-EMAP study. Finally, the Region IV R-EMAP study and other projects are jointly developing analytical capabilities to allow researchers to measure mercury at the parts per trillion level in water and air.


The Region IV R-EMAP study will test a number of hypotheses regarding mercury contamination in the Everglades ecosystem. These include the following:

The Region IV R-EMAP results and findings will provide a basis for defining an ecological risk assessment of the impact of mercury on the entire system, as well as on selected rare and endangered species. This assessment will help researchers determine the factors and processes to be incorporated into a mathematical model of the mercury cycle in the Everglades ecosystem.


Region IV R-EMAP scientists will use a random, probability-based sampling strategy, based on the EMAP approach. The strategy will be designed to be integrated with the assessment strategy of the South Florida Geographic Initiative, a Region IV program to address crucial environmental issues in South Florida. The sampling grid is a seven-fold enhancement of the EMAP base grid, resulting in points distributed across the entire 4,000-square-mile study area. The distance between the individual points with the full grid density is about 4 km, with a hexagon area of about 13 km2 around each grid point. Grid points in the Everglades Agricultural Area, Water Conservation Areas, and Everglades National Park have an equal probability of inclusion. The intensity of sampling will be decreased in the areas outside this primary study area.

Table 4-1 summarizes the indicators to be measured during the Region IV study.

Samples will be taken in the following order of priority: Samples based on the enhanced base grid will be taken systematically at 50 sites in a north-to-south sequence over a one-week period. These sites will be accessed by helicopter or boat. This sequence will be carried out twice each year during the wet (May-October) and dry (November-April) seasons. After four cycles, sampling at the initial set of 50 stations will be repeated. This sampling will allow researchers to gain an initial spatial understanding of total mercury and methyl mercury in water, sediment, and biota.

The South Florida Water Management District will carry out this sampling at bi-weekly intervals for one year. Four of the canal structures to be sampled are the main discharge points for water from the Everglades Agricultural Area. The other three canal structures, located at progressive intervals down the canal system toward Everglades National Park, will help the researchers determine whether a spatial gradient exists.

These transects are located across known nutrient gradients. Water, soil, and biota will be sampled to test the eutrophication hypothesis. Depending on the strength of the relationship of total phosphate to methyl mercury, this effort will be used to evaluate the design and practicality of the marsh grid sampling effort.

The initial cycle of the marsh sampling is shown in Figure 4-7a; Figure 4-7b shows the density after four cycles. Water, soil, and biota will be sampled, when available, at each site. Soil will be sampled at depths of 0 to 5 cm (surface), 20 to 25 cm (middle), and 40 to 45 cm (maximum depth). The maximum depth was deposited approximately 100 years ago. Other information, such as the habitat type in which each station is located and the depth of water present, will be recorded as each site is sampled.


To determine the sources and fluxes of mercury in the Everglades ecosystem, the investigators will need to accurately measure mercury at ultra trace levels (parts per trillion) in air, water, sediment, soil, and fish tissue. To accomplish this, researchers will use a technique called automated cold vapor atomic fluorescence spectrometry. The study will employ "clean" sampling protocols for air and water to prevent contamination of the samples during the collection, transport, and storage phases. "Clean" protocols for laboratory analysis of total and methyl mercury in air, water, soil/sediment and tissue are also being developed by related projects.



Jerry Stober, PhD
Environmental Services Division
Region IV
U.S. Environmental Protection Agency
College Station Road
Athens, GA 30613 (706) 355-8705

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