Overview of Impaired Waters



Section 303(d) of the Clean Water Act

The goal of the Clean Water Act (CWA) is “to restore and maintain the chemical, physical, and biological integrity of the Nation’s waters” (33 U.S.C §1251(a)). Under section 303(d) of the CWA, states, territories, and authorized tribes, collectively referred to in the act as “states,” are required to develop lists of impaired waters. These are waters for which technology-based regulations and other required controls are not stringent enough to meet the water quality standardsset by states. The law requires that states establish priority rankings for waters on the lists and develop Total Maximum Daily Loads (TMDLs), for these waters. A TMDL is a calculation of the maximum amount of a pollutant that a water body can receive and still safely meet water quality standards.

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How does the 303(d) Impaired Waters and TMDL Program fit into the Clean Water Act?

The CWA includes two basic approaches for protecting and restoring the nation’s waters. One is a technology-based, end-of-pipe approach, whereby EPA promulgates effluent guidelines that rely on technologies available to remove pollutants from waste streams. These guidelines are used to derive individual, technology-based National Pollutant Discharge Elimination System (NPDES) permit limits. The other approach is water-quality based and is designed to achieve the desired uses of a water. This approach may ultimately result in more stringent NPDES permit limits. The 303(d) program is at the core of the water-quality based approach and serves to link the water quality goals to the NPDES permit limits.

Water Quality-Based Approach of the Clean Water Act

Flow chart depicting several Clean Water Act activities and how they relate to the 303(d) Program to provide a water quality-based approach. Graphic begins with adopting water quality standards, then monitoring and assessing waters, listing impaired and threatened waters, developing TMDLs for those waters, and the implementation of control sources for managing nonpoint and point source pollution.

Water quality standards are the foundation of the water-quality based control program mandated by the Clean Water Act. Water quality standards define the goals for a waterbody by designating its uses, setting criteria to protect those uses, and establishing provisions to protect water quality from pollutants. A water quality standard consists of four basic elements:

  1. designated uses of the waterbody (e.g. recreation, water supply, aquatic life, agriculture),
  2. water quality criteria to protect designated uses (numeric pollutant concentrations and narrative requirements),
  3. an antidegradation policy to maintain and protect existing uses and high quality waters, and
  4. general policies addressing implementation issues (e.g., low flows, variances, mixing zones).

By adopting water quality standards, states are able to determine which healthy waters need protection, which waters must be restored and how much pollutant reductions are needed. Consequently, these water quality standards set a goal for restoring and protecting a watershed over the long term.

Water quality monitoring provides the data to characterize waters and identify changes or trends in water quality over time. The collection of monitoring data enables states to identify existing or emerging water quality problems and determine whether current pollution control mechanisms are effective in complying with the regulations. The CWA requires that each state monitor and assess the health of all their waters and report their findings every two years to EPA. This list of data and findings is called the 305(b) report or “biennial water quality report.”

Under section 303(d), monitoring data as well as other information, must be used by the states to develop a list of “water-quality limited segments,” i.e., waters that will not meet water quality standards for a particular pollutant even after a technology-based permit is in place. States must develop TMDLs, or Total Maximum Daily Loads, for every water body/pollutant combination on the 303(d) list.

The TMDL calculates the maximum amount of a pollutant allowed to enter a waterbody, also known as the loading capacity, so that the waterbody will meet and continue to meet water quality standards for that particular pollutant. The TMDL allocates that load to point sources, (Wasteload Allocation or WLA), and nonpoint sources (Load Allocation or LA) which include both anthropogenic and natural background sources of the pollutant.

In many cases, the TMDL analysis is the trigger for determining the source(s) of pollutants. A TMDL may contain WLAs only, LAs only, or a combination of both. Under the CWA TMDLs are not self-implementing, meaning EPA cannot enforce implementation of a TMDL once the analysis is complete. Although, if the TMDL WLA requires more stringent permit limits for point sources these must be implemented in the appropriate NPDES permits at the time of their renewal. If the TMDL identifies nonpoint sources of pollutants as a major cause of impairment, states can apply for EPA funded grants, called section 319 grants. These grants can be used to fund state programs for nonpoint source assessment and control as well as individual projects.

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