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Water: Water Quality Standards Academy

Basic Course: Supplemental Topics - Introduction: The Importance of the 303(d) List and TMDLs Development

Listing Impaired Waters and Developing TMDLs

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Under Section 303(d), states and authorized tribes must identify waters for which a water quality standard has not been met, even after required minimum levels of pollution control technology have been adopted. Such waters are considered "water quality-limited" or impaired waters. In addition, EPA guidance provides for listing threatened waters for which readily available data and information show that the water body is currently meeting standards, but that there is a trend toward impairment within the next listing cycle (2 years).

Consistent with the Clean Water Act's (CWA) overall objective to "restore and maintain the chemical, physical and biological integrity of the Nation's waters" (Section 101(a)), states and tribes are required under Section 303(d) to identify, prioritize, and establish TMDLs for such waters — impaired waters. EPA construes this requirement to apply to waters with water quality that is threatened with impairment as well.

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Key Point. After a State/Tribe adopts its water quality standards, it should regularly monitor the condition of water bodies within its boundaries. Regular monitoring provides information to help states/tribes identify and prioritize impaired and threatened waters.

For waters indentified in their Section 303(d) lists, states and tribes must calculate the maximum amount of a pollutant allowed to enter an impaired water body, known as the loading capacity, so that the water body will meet and continue to meet the State/Tribe's water quality standards for that particular pollutant. These pollutant loadings are calculated in total maximum daily loads (TMDLs), and their development is essential to a State/Tribe's long-term planning for achieving the objectives established by their water quality standards.

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Key Point. A TMDL is a "tool" for conducting a waterbody-specific analysis that uses current water quality conditions to guide pollution control actions focused on meeting water quality standards. The TMDL analysis characterizes the relationship between pollution sources and waterbody conditions to develop a pollutant load that — when allocated among the various pollutant sources — would allow the water body to meet water quality standards.
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Resource. For a range of information on TMDLs, including laws and regulations, technical resources, and guidance, visit EPA's Web site on Impaired Waters and TMDLs.

Disclaimer:
For informational purposes only–Not official statements of EPA policy.

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