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Impaired Waters and TMDLs

Overview of Listing Impaired Waters under CWA Section 303(d)

What is a Clean Water Act Section 303(d) list of impaired water?

The term "303(d) list" or “list” is short for a state’s list of impaired and threatened waters (e.g. stream/river segments, lakes). States are required to submit their list for EPA approval every two years. For each water on the list, the state identifies the pollutant causing the impairment, when known. In addition, the state assigns a priority for development of Total Maximum Daily Loads (TMDL) based on the severity of the pollution and the sensitivity of the uses to be made of the waters, among other factors (40 C.F.R. §130.7(b)(4)).

In general, once a water body has been added to a state’s list of impaired waters it stays there until the state developes a TMDL and EPA approves it. EPA reporting guidance provides a way to keep track of a state’s water bodies, from listing as impaired to meeting water quality standards. This tracking system contains a running account of all of the state’s water bodies and categorizes each based on the attainment status. For example, once a TMDL is developed, a water body is no longer on the 303(d) list, but it is still tracked until the water is fully restored.

How do states identify impaired waters?

States may use any number of ways to determine whether or not a water body meets the water quality standard. However, federal regulations say states must evaluate "all existing and readily available information" in developing their 303(d) lists (40 C.F.R. §130.7(b) (5)). This means that states cannot select what data/information they use and purposely disregard other. EPA's regulations contain a nonexclusive list of information that must be considered.

What do states need to submit to EPA?

In addition to section 303(d) lists of impaired waters states are required to submit section 305(b) water quality reports to EPA. Section 305(b) reports provide information on the water quality status of all waters in the state, whereas section 303(d) lists are a subset of these waters – those that are impaired by a pollutant and in need of a TMDL. Given that both the 305(b) report and the 303(d) lists are due at the same time (April 1 of every even numbered year), EPA recommends that states combine them into a single “Integrated Report.” EPA approves or disapproves the state’s 303(d) list of impaired waters needing TMDLs. If EPA disapproves a state’s list, EPA is required to identify any additional impaired or threatened waters for the state. In most of these circumstances, EPA partially approves and partially disapproves a list because some waters have been omitted and adds these waters to the state's list.

Impaired Waters Restoration Process: Listing