How Water Quality is Evaluated

In 1972, Congress passed the Clean Water Act (CWA) to restore and maintain the “chemical, physical and biological integrity” of U.S. waters. CWA establishes a framework for measuring and evaluating water quality throughout the country which is implemented by states.

States are required to establish water quality standards for all surface waters. Water quality standards have three components: designated uses, water quality criteria and antidegradation policies. Designated uses are the state’s goals for the waterbody—for example, is the river used for drinking water? for agriculture? for fishing and swimming? (CWA requires that all waters must be swimmable and fishable). Water quality criteria are the chemical, physical and biological conditions that must be met in order for a waterbody to support its designated use. Those which meet criteria are considered “supporting” or “attaining” (Category 1 on the map below). Those which are monitored for all uses and do not meet criteria are considered “impaired” (Categories 4A, 4B and 5 below). States are not able to monitor all their waters, so some are listed as unassessed (Category 3 below). Other waters meet criteria for a portion of their uses, but are not monitored for others (Category 2 below).

States use a water quality restoration planning approach called Total Maximum Daily Load, or TMDL, to improve impaired waters. Antidegradation policies, which apply to all waters, are intended to prevent further degredation to ensure clean water goals are maintained.

States are responsible for monitoring and evaluating water quality relative to the standards, and reporting the results to EPA and the public. Rhode Island and Massachusetts accomplish this through biennial Integrated Water Quality Monitoring and Assessment Reports. These list all the surface waters of the state—rivers, lakes, streams and estuarine waters—evaluating each according to the five categories described above, and keyed on the map below.

For more on fresh and salt water quality in the Narragansett Bay Region, download the Currents of Change technical report

For Rhode Island’s integrated water quality report, see www.dem.ri.gov

For more on water quality management in Massachusetts, see www.mass.gov/dep

For national water quality reports, compiled by EPA from state reports, see www.epa.gov/305b

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