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Impaired Waters and TMDLs

Impaired Waters and Mercury

Thousands of waterbodies are listed on state Clean Water Act Section 303(d) lists as impaired due to mercury, often due to high mercury levels in fish. Mercury accumulates in fish tissue as methylmercury, the form that presents the greatest risk to human health through consumption of contaminated fish. Many states have issued advisories to limit consumption from certain waters of certain fish due to high levels of mercury.

In many waterbodies, mercury originates largely from air sources, such as coal-fired power plants and incinerators that deposit in waters or adjacent lands that then wash into nearby waters. Contributions may come from a combination of local, regional and international sources. In some cases, the presence of mercury may be a result of past practices that used mercury, such as historic gold mining, or from geologic deposits. Some mercury may be discharged via water point sources, although in many waterbodies, the amounts are usually very small compared to other sources. Products containing mercury may also result in releases to water and air during waste handling and disposal processes.

Given the variety of potential mercury sources, developing and implementing Total Maximum Daily Loads (TMDLs) for mercury-impaired waters may involve coordination among multiple programs - water, air, waste and toxics programs. EPA, states and other stakeholders continue to work on how best to develop mercury TMDLs and reduce sources of mercury. Examples of policy and technical tools to assist in developing mercury TMDLs, and example mercury TMDLs are included below.

Policy and Guidance Documents

Mercury TMDL Examples and Supporting Studies

Mercury TMDL Examples

Examples of TMDLs involving mercury from air deposition and legacy sources.

Multi-state TMDLs addressing mercury predominantly from air deposition sources:

Northeast Regional Mercury TMDL (CT, ME, MA, NY, NH, VT, RI)

Statewide/Regional TMDLs addressing mercury predominantly from air deposition sources:

Minnesota Statewide Mercury TMDL

Atmospheric Mercury Deposition Project for Watershed Planning

The Atmospheric Mercury Deposition Project for Watershed Planning contains the results of a 12-km grid cell modeling analysis for the lower 48 States. In this project, roughly 300 of the top mercury emitters in the United States were "tagged" in order to allow source attribution analysis for individual areas of concern, such as waterbodies, watersheds or catchments.

Examples of Mercury Deposition Modeling Tools

Atmospheric Deposition Models

Atmospheric deposition models can be used to help develop mercury TMDLs by estimating the load falling to a particular waterbody or waterbodies of interest. In addition, they can shed light on the relative contributions from key sources. Because deposition in a given area is typically made up of contributions from local, regional, as well as global air emission sources located in other countries, modeling techniques for TMDL applications need to be able to account for deposition from a broad array of geographically dispersed sources.

Continental-scale Models

Continental-scale models contain detailed emission data from sources in the US, plus Canada and Mexico. Such models incorporate results from global-scale models in order to account for long-range transport of mercury primarily from sources located outside of North America.

Global-scale Models

Global-scale models contain emissions data from sources located around the world. Results from these models tend to be coarser than that from Continental-scale models.

Watershed and Waterbody Models

TMDLs calculate the maximum amount of a pollutant that can enter a waterbody, such that the waterbody will meet water quality standards for that particular pollutant. This "loading capacity," and the specific reductions needed to meet it, is typically calculated using water quality modeling tools. Additional information on water quality models and example of their use in mercury TMDLs can be found below.

Other Mercury Resources