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Water: Water Quality Standards

Water Quality Criteria

States and authorized Tribes adopt water quality criteria with sufficient coverage of parameters and of adequate stringency to protect designated uses. In adopting criteria, States and Tribes may:

  • adopt the criteria that EPA publishes under §304(a) of the Clean Water Act;
  • modify the §304(a) criteria to reflect site-specific conditions; or
  • adopt criteria based on other scientifically-defensible methods.

States and Tribes typically adopt both numeric and narrative criteria. Numeric criteria are important where the cause of toxicity is known or for protection against pollutants with potential human health effects. Narrative criteria are also important—narrative "free from" toxicity criteria typically serve as the basis for limiting the toxicity of waste discharges to aquatic species (based on whole effluent toxicity testing).

Section 303(c)(2)(B) of the Clean Water Act requires States and authorized Tribes to adopt numeric criteria for §307(a) priority toxic pollutants for which the Agency has published §304(a) criteria, if the discharge or presence of the pollutant can reasonably be expected to interfere with designated uses. The §307(a) list contains 65 compounds and families of compounds, which the Agency has interpreted to include 126 priority toxic pollutants.

In addition to narrative and numeric (chemical-specific) criteria, other types of water quality criteria include:

Biological criteria: a description of the desired aquatic community, for example, based on the numbers and kinds of organisms expected to be present in a water body.


Nutrient criteria: a means to protect against nutrient over-enrichment and cultural eutrophication.


Sediment criteria: a description of conditions that will avoid adverse effects of contaminated and uncontaminated sediments.


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