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Water: Module 3

Basic Course: Key Concepts (Module 3.e)

Forms of Expression: Numeric and Narrative Criteria

Module 3. Criteria Development

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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

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

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