Basic Course: Supplemental Topics: The 303(d) List: Identifying Impaired/Threatened Waters

As a fundamental step for addressing impaired or threatened waters, Section 303(d) and EPA regulations require a State/Tribe to do the following:

• List the impaired/threatened waters within its boundaries that require development of TMDLs. Then for each water body, identify the pollutants causing or expected to cause the impairment, which will be the focus of the TMDLs.

In this first step, as established by the regulation, each State/Tribe must identify "water quality-limited segments still requiring TMDLs within its boundaries" for which (a) technology-based effluent limitations, (b) more stringent effluent limitations, and (c) other pollution-control requirements "are not stringent enough to implement any water quality standards (WQS) applicable to such waters." (40 CFR 130.7)

The assessment of impaired waters has traditionally been a process defined by the State/Tribe, and thus can vary in approach and complexity. Regardless, the process must consider "all existing and readily available water quality-related data and information" (e.g., information included in watershed plans and other types of water quality management plans) (40 CFR 130.7(b)(5)).

Key Point. The 303(d) list submission to EPA must include a description of the methodology that the State/Tribe used in developing the list, a description of the data and information considered, and a rationale for any data or information left out of its assessment (see 40 CFR 130.7(b)(6)).

Key Point. While approaches vary for identifying impaired/threatened waters and the pollutants at issue, States/Tribes typically use both existing information and new data collected from ongoing monitoring programs to assess whether water quality standards are being met and to detect water quality trends.

Resource. For more information on impaired waters lists, see the Overview of Impaired Waters and Total Maximum Daily Loads Program.