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Key Concepts Module 3: Criteria

This module provides an introduction to water quality criteria and provides answers to the following:

  • What is the purpose of water quality criteria? What is the basis?
  • How are the criteria expressed?
  • What are the different types of criteria?
  • What do the criteria protect and how?

At the end of the module is a brief quiz intended to touch on some Core Modules regarding water quality criteria that are further examined in the classroom session of this module.

This module's main pages and brief quiz at the end take about 15 minutes to complete.

On this page:

Protecting Designated Uses of a Water Body

Water quality criteria are limits on particular chemicals or conditions in a water body. As elements of State/Tribal water quality standards, the criteria protect particular designated uses, such as propagation of fish and wildlife, recreation, and public water supply. The criteria can be expressed as acceptable levels (constituent concentrations) or as narrative statements. 

How Are Criteria Developed?

EPA develops recommendations for many water quality criteria that individual States or Tribe's adopt into their water quality standards. However, States/Tribes develop some criteria that reflect local or site-specific conditions themselves.

EPA develops water quality criteria under the authority of the Clean Water Act §304(a). Consistent with EPA's water quality standards regulation (40 CFR 131.11), criteria that States/Tribes adopt into their standards must meet the following requirements:

  • Be based on a sound scientific rationale.
  • Include parameters (e.g., acceptable concentrations) that are sufficient to support protection of the particular water body's designated uses, including the most sensitive use.

Key Point. EPA's recommended criteria are not rules, nor do they automatically become part of the State or Tribe's water quality standards. EPA develops criteria based on the best available science, extensive scientific literature review, established procedures for risk assessment and management, EPA policy, external scientific peer review, and public input on potentially useful scientific information.

Key Point. A State or an authorized Tribe can apply water quality criteria to a specific water body or a selection of water bodies or apply the criteria to all water bodies throughout the State/Tribe.

Water quality criteria have no force of law under the CWA until they have been adopted into the particular State or Tribe's water quality standards, EPA has approved them, and they are implemented in a regulatory tool (e.g., a permit issued under the National Pollutant Discharge Elimination System).

Establish a Risk Threshold for the Water Body

A water quality criterion establishes a threshold for a pollutant or condition, above or below which the designated uses for a water body may be threatened. If the criterion is exceeded, for instance, the water quality may pose a potential human health or ecological risk. In such cases, protective or remedial action may be needed.

Scientific and Expert Judgment

All limits on pollutants or conditions adopted as water quality criteria should be based on the latest scientific data and expert judgment. Criteria should be revised from time to time as new scientific data or methodologies are developed.

Key Point. Factors such as technological feasibility, social and economic costs, and the benefits of achieving criteria levels are considered in a preceding step in which a water body's designated uses are established. Thus, these factors are NOT directly involved in the process of developing water quality criteria.

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Numeric and Narrative Criteria

Most water quality criteria are expressed as numeric—or quantitative—parameters. This is generally expected for toxic pollutants under most circumstances. Criteria expressed in this way specify the precise, measurable levels of particular chemicals or conditions allowable in a water body. When pollutants cannot be precisely measured, narrative criteria are used to express a parameter in a qualitative form.

Key Point. EPA believes that comprehensive State/Tribal water quality  standards should include both numeric and narrative criteria.

Question. Why do you think EPA advocates that both numeric and narrative criteria are useful in water quality standards?

Answer. Because the use of both helps protect a water body from the effects of specific chemicals as well as from the effects of pollutants that are not easily measured, such as chemical mixtures and floatable debris.

Here are some examples showing the utility of the different forms of expression for water quality criteria:

Numeric Criteria

For the protection of human health against the ingestion of contaminated water and contaminated aquatic organisms: The ambient water quality criterion for cadmium is recommended to be identical to the existing drinking water standard, which is 10 µg/L (micrograms per liter)

Narrative Criteria

For protection from various oils in water bodies: Surface waters shall be virtually free from floating non-petroleum oils of vegetable or animal origin, as well as petroleum-derived oils

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The Basis for Each Form

There are specific expectations when developing narrative or numeric criteria.

Numeric Criteria

Should be adopted based on:
  • EPA recommendations provided under Section 304(a) of the Clean Water Act, or
  • The Section 304(a) recommendations, but modified to reflect site-specific conditions, or
  • Other scientifically defensible methods.

Narrative Criteria

Should be adopted based on: Biological monitoring and assessment methods to supplement numerical criteria.

Specific but Complementary

While each type of water quality criterion has a specific focus, the different criteria that a State or Tribe adopts into its water quality standards should operate together to support overall protection of the water body.

Key Point. In making water quality management decisions, a State or Tribe should independently apply each criterion that has been adopted into its water quality standards. If a water body has multiple designated uses with different criteria for the same pollutant, States/Tribes should use the criterion protective of the most sensitive use.

Resources. For a definition of eutrophication and related terms, visit EPA's Glossary page.

Learn More. Details about the six criteria types. Proceed to the Learn More Topic.

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Addressing Priority Pollutants

The Clean Water Act requires states/Tribes to adopt criteria for the so-called priority pollutants identified by an EPA task force (Section 303(c)(2)(b)).

For most of the priority pollutants, EPA has developed water quality criteria to protect human health and aquatic life.

Key Point. States/Tribes are required to adopt criteria for any priority pollutants that are a concern for a particular water body if EPA has developed criteria for those pollutants. However, States/Tribes are not required to adopt EPA's criteria, which are intended to provide guidance for States/Tribes.

Key Point. If States/Tribes use narrative criteria to address priority pollutants, they must identify how they intend to regulate point sources of the pollutants.

Learn More. Refer to a list of these priority pollutants (PDF), (1 pg, 41 K, About PDF) as presented in Section 307(a).


  • Water quality criteria protect the designated uses of a water body.

  • Criteria establish acceptable levels of pollutants or conditions in a water body based on scientific data and expert judgment.

  • Each State/Tribe is expected to adopt criteria as part of its water quality standards. States/Tribes typically adopt criteria recommended by EPA, except when site-specific criteria are necessary.


  • A comprehensive set of criteria includes numeric criteria to establish specific threshold values for known entities and narrative criteria to help protect against what can't be precisely measured or where specific risk levels are best determined on a case-by-case basis.

  • Many types of criteria operate together to guide protection of a water body.

  • In their criteria, States/Tribes must address the "priority pollutants" as identified by EPA.

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To complete your review of the topic in this module, please take the self-assesement quiz by reviewing each question and considering the possible responses.

A note about the quiz:

Your answers will NOT be scored or recorded. However, selecting the Submit button for each question will provide you with the correct answers on screen.

At the end of the quiz is a Get Password button. Select this after you have completed the quiz to obtain one of the six passwords you will need to obtain your Certificate of Completion at the end of the course.

Answer each of the questions

1. Statement: A State/Tribe can establish one set of water quality criteria for all of its water bodies.

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2. How often is a State/Tribe expected to revise its criteria?


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3. Statement: Where any of the “priority pollutants” identified by EPA are a concern for a particular water body, the State/Tribe must adopt EPA’s criteria for that pollutant.


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4. Which of the six criteria types always takes precedence over the others?

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5. Statement: EPA’s water quality criteria carry the weight of law and are enforceable.


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6. The principal regulatory test of whether a criterion's value can be approved in a State or Tribe's water quality standards is:

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For informational purposes only–Not official statements of EPA policy.