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Water: Water Quality Standards Academy

Introduction: Two Criteria Specifying Concentration-Related Thresholds

Aquatic Life Criteria

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Basic Course: Supplemental Topics

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Aquatic life criteria establish numeric thresholds for allowable concentrations of particular chemicals (or other substances) in water bodies above which—studies indicate—aquatic life may be harmed. The criteria are intended to address unacceptable adverse effects from both short-term (acute) and long-term (chronic) exposure, with the objective of protecting aquatic life from lethal as well as sub-lethal effects (e.g., immobility, slower growth, reduced reproduction).
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Key Point. For a given chemical, aquatic life criteria specify limits for the magnitude of a concentration, the duration (period of time) over which the concentration is averaged, and the frequency of exposure to the concentration.

Acute and chronic criteria for aquatic life addressing magnitude, duration, and frequency are expressed with two terms:

  • Criterion Maximum Concentration (CMC). An estimate of the highest concentration of a material in ambient water to which an aquatic community can be exposed briefly without resulting in an unacceptable adverse effect. This is the acute criterion.
  • Criterion Continuous Concentration (CCC). An estimate of the highest concentration of a material in ambient water to which an aquatic community can be exposed indefinitely without resulting in an unacceptable adverse effect. This is the chronic criterion.
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Key Point. CMCs and CCCs are usually derived for both freshwater and saltwater organisms. Thus, EPA guidelines for a particular chemical can include four criteria.

Nationally recommended criteria are expected to protect aquatic organisms from unacceptable effects assuming the following default exposures:

  • Acute = Exposure to a 1-hour average concentration of the chemical does not exceed the criterion more than once every 3 years on average.
  • Chronic = Exposure to a 4-day average concentration of the chemical does not exceed the criterion more than once every 3 years on average.
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Learn More. As an example, the published freshwater and saltwater aquatic life criteria for diazinon. Proceed to the Learn More Topic. » (Note: This link launches a pop-up window.)

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

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