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The Everglades TMDL Pilot Final Report and Recent Findings: November 2003


A report on the results of the Everglades Mercury Total Maximum Daily Load (TMD) Pilot Study has just been released. Exit.

Through a grant to the Florida Department of Environmental Protection, EPA began the TMDL pilot portion of the project in 1999 to evaluate technical approaches (such as computer models) for developing TMDLs in which much of the mercury is from atmospheric sources. Although the TMDL pilot study had been completed in 2002, the report has been updated to include data showing declines in mercury emissions in south Florida, as well as declines in mercury levels in fish and wildlife in the Everglades. In addition to the TMDL pilot project, a comprehensive research effort on the Everglades mercury issue is being conducted under the South Florida Mercury Science Program, a consortium of federal, state, and electric utility interests.

Technical Approach

The Florida pilot involved the evaluation of computer modeling techniques for their potential use in a TMDL analysis. A key objective of the project was to "test" an approach that combines both atmospheric and aquatic models. Another objective was to conduct an analysis using already available data and tools, rather than collecting new data or developing new models. The project was able to rely on the wealth of data from the ongoing mercury studies in the Everglades.

Scientists at the University of Michigan Air Quality Laboratory developed the atmospheric modeling approach. Using a combination of tools, the modelers predicted the amount of mercury deposition to the study area from local sources in southern Florida. Aquatic modelers then used results from the atmospheric model and other data to examine how mercury levels in fish might respond to reductions in deposition. Project scientists applied the Everglades Mercury Cycling Model (developed by Tetra Tech for the Electric Power Research Institute), which simulates mercury cycling in the Everglades and its accumulation within the food chain.
The pilot project examined two key questions: first, what is the relationship between atmospheric mercury deposition and fish mercury concentrations, and can the models predict how much atmospheric loadings of mercury need to be reduced to reach the desired levels of mercury in fish; and second, how fast would mercury concentrations in fish decrease when atmospheric mercury loadings are reduced.

Key Findings

The Everglades TMDL pilot study was successful in demonstrating that, through close coordination between air and water scientists and managers, it is possible to link the results of air and water computer models. The project illustrates one approach for examining how levels of mercury in fish might respond to changes in mercury deposition. The key modeling results from the pilot are as follows:

  • There is a linear relationship between mercury deposition and levels of mercury in fish: when atmospheric deposition of mercury is reduced, the levels of mercury in fish (largemouth bass) show a corresponding decline. The relationship between fish mercury levels and deposition is almost 1:1.
  • Fish mercury levels may respond relatively quickly to changes in deposition. When deposition is reduced, mercury levels in fish went down by 50% within about 10 years, and by 90% within 25-30 years.
  • If deposition were eliminated, fish may have small amounts of mercury because of historic mercury in sediments, and this remaining mercury may persist for decades.

The TMDL modeling study also highlights a number of scientific uncertainties and other technical issues which need to be taken into account when examining the project results. Areas of uncertainty include emissions inventories (i.e., data on the amount and form of mercury emitted by various types of sources), meteorological data, atmospheric chemistry, the effect of sulfate reduction on aquatic mercury cycling, and others.

In addition to the TMDL study, the November 2003 report describes reductions in mercury emissions levels in south Florida, along with declines in mercury levels in fish and wildlife in the Everglades. The data are based on work conducted by other investigators separately from the TMDL modeling study. The declines in mercury appear to be consistent with the modeling results from the TMDL pilot project.

  • Local emission rates of mercury in south Florida appear to have declined by over 90% since the late 1980s to early 1990s. Reductions appear to be related to reductions in emissions from medical waste incinerators and municipal waste combustors.
  • Statistically significant declines in mercury declines in largemouth bass and great egret chicks have been observed for a number of sites in the Everglades. Declines over the past decade for both species appear to be on the order of 80%.

The results from the Everglades may not be transferrable to other sites because of the unique properties of the Everglades. For example, the Everglades is a shallow marsh and may respond to deposition differently from other aquatic systems. However, the TMDL pilot study demonstrates the types of modeling tools that could be applied elsewhere, such as other versions of the aquatic model. In addition, although the product of the pilot project was not a TMDL, the effort illustrates some of issues to be considered when developing a TMDL involving air deposition. [A similar pilot project is being conducted in Wisconsin and may give some indication of the similarities and differences in results from site to site.] The pilot results were peer-reviewed, and the final report summarizes peer reviewers' comments and responses to comments.

Background on TMDLs and the Everglades Mercury Issue

Under the Clean Water Act, states must prepare lists of their "impaired waters" and develop Total Maximum Daily Loads (TMDLs) for those waters. A TMDL identifies pollutant reductions needed in a waterbody to meet state water quality standards, and divides the reductions among pollutant sources. Air deposition can be a significant source of toxic chemicals and nitrogen compounds to the nation's waters; however, few TMDLs have included a detailed analysis of pollutant loadings from air deposition. As a result, EPA launched pilot projects to examine technical tools for analyzing air deposition and its impacts on water quality, with a focus on mercury. Mercury is a toxic pollutant that bioaccumulates in fish, and, in many waterbodies, mercury loadings are largely the result of air deposition. Nationwide, 44 states have fish consumption advisories due to high levels of mercury in fish.

The Florida pilot project was conducted on a portion of the Everglades called Water Conservation Area 3A-15. The study area was selected because it is on Florida's list of "impaired" waterbodies and has a fish consumption advisory due to high levels of mercury in fish. Virtually all of the mercury load to the Everglades is from atmospheric sources. The mercury problem in the Everglades is the subject of extensive study by state and federal agencies and universities under the South Florida Mercury Science Program.