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Supplemental Module: Aquatic Life Criteria

Aquatic life criteria are estimates of concentrations of pollutants in ambient water that—if not exceeded—are expected to protect fish, invertebrates, and other aquatic life from adverse effects associated with exposure. This module introduces methods used by EPA to derive ambient water quality criteria for aquatic life (ALC). (Ambient refers to open waters such as rivers, lakes, and streams, in contrast to closed water systems used for distributing treated water or wastewater.)   

In particular, this module provides answers to the following:
  • What is EPA’s role under the Clean Water Act regarding the development of aquatic life criteria?
  • What data are needed for developing criteria?
  • What steps and types of calculations are involved in establishing criteria?

States and authorized Tribes need to address aquatic life criteria in their water quality standards as well as human health, nutrient, and biological criteria. A module in the Key Concepts section of this online training provides an introduction to criteria development, and a Supplemental Topics module focuses on deriving human health criteria. Online modules to address derivation of other criteria are planned for the future.

At the end of the module is a brief quiz intended to touch on aspects of ALC derivation. The topic is further examined in the classroom session of this module.

This module's main pages and brief quiz at the end take about 30 minutes to complete.

On this page:

EPA's Role

Section 304(a)(1) of the Clean Water Act (CWA) requires EPA to develop and publish—and to periodically review and revise as necessary—nationally recommended criteria for water quality that accurately reflect the latest scientific knowledge. The criteria are based solely on data and scientific determinations about the relationship between environmental concentrations of a pollutant and its effects on, for instance, aquatic life. Thus, EPA criteria development does not involve consideration of social and economic impacts or the technological feasibility of not exceeding the chemical concentration values in ambient water.

Key Point. EPA’s nationally recommended water quality criteria provide guidance to States and Tribes for developing numerical and narrative criteria for their water quality standards in support of the CWA. They also provide guidance for the Agency itself when promulgating Federal regulations under the CWA.

The majority of EPA’s currently recommended aquatic life criteria were derived using the methods outlined in EPA’s ALC guidelines published in 1985.

Resource. Links to both finalized aquatic life criteria and those that are under development (along with other ALC resources) are provided on EPA’s Aquatic Life Criteria webpage.

Since publication of the ALC derivation method, important scientific advancements have been made in aquatic toxicology and biology; fate, transport, and effects modeling; and ecological risk assessment. In addition, water quality impairment issues have become increasingly complex. Given these developments, EPA has been leading an effort to update the ALC derivation guidelines.

Learn More. A somewhat recent area of interest related to modifying the 1985 guidelines focuses on so-called contaminants of emerging concern (CECs). Proceed to the Learn More Topic.

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Two Concentration-Related Criteria

hands measuring a substance

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).

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.

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.

Learn More. As an example, the published freshwater and saltwater aquatic life criteria for diazinon. Proceed to the Learn More Topic.


Steps in Deriving the Criteria

EPA’s process for deriving estimates for ALC threshold concentrations involves the steps listed, which are summarized in the following pages of the module.

Getting Started

  • Select the chemical of concern for criteria development (or the chemical in need of reevaluation).

Deriving the Criterion Maximum Concentration

  • Collect and assess effects data (i.e., on median lethal and median effective concentrations).
  • Calculate the Final Acute Value.
  • Assess any effects from water characteristics (e.g., hardness) on the chemical’s toxicity.
  • Derive the CMC.

Deriving the Criterion Continuous Concentration

  • Collect and assess effects data (i.e., on species growth, reproduction, survival).
  • Calculate the Acute/Chronic Ratio, when—as typically is the case—sufficient chronic toxicity test data are not available to meet the minimum requirement.
  • Calculate the Final Chronic Value/CCC.
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Illustration. View graphic showing overview of ALC derivation. Proceed to the Illustration.

Once EPA has developed draft versions of the recommended criteria for a given chemical, the criteria undergo internal and external review before they are released as final.

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Prioritizing Chemicals

hand holding a chemical glass

EPA follows an established, risk-based process for determining which chemicals should be given priority for developing aquatic life criteria. The process involves identifying candidate chemicals, assessing the availability of data, and then ranking the chemicals based on risk. Essentially the same process is used for selecting chemicals with criteria that are due for reevaluation.

To identify candidate chemicals, the Agency solicits and reviews suggestions from EPA Regions, States, Tribes, and stakeholders. EPA then compiles a list of chemicals that have been proposed by two or more of these submitters.

Next, EPA narrows down the list based on the availability of toxicity data. At a minimum, the 1985 Guidelines call for acute data on eight families and chronic data on three species (i.e., for chronic, at least need acute-chronic studies from which the chronic criterion can be derived as a ratio of the final acute value).

The chemicals with the necessary data are then scored and ranked according to risk based on:

  • Occurrence. Frequency at which a chemical is found in ambient water and/or fish tissue.
  • Toxicity. Relative potential to have an adverse effect on aquatic life.

Collecting Effects Data

In the course of selecting a chemical for developing aquatic life criteria, EPA reviews the availability of needed data. Then to start the actual criteria development process, the data are collected and assessed for quality and completeness.

The derivation of criteria relies on two types of values from toxicity tests:

  • Median lethal concentration (LC50). A specific concentration of a chemical that has been found to be lethal to 50 percent of individuals in a group of aquatic organisms exposed for 48 to 96 hours.
  • Median effective concentration (EC50). A specific concentration of a chemical that has been found to cause a particular effect in 50 percent of individuals in a group of aquatic organisms exposed over a given time period (e.g., weeks or years).

For calculating the CMC, the 1985 Guidelines require that acceptable acute values be available for at least eight families with a specified taxonomic diversity in order to address a wide variety of the organisms constituting an aquatic animal community. These minimum requirements include:

Data on three vertebrates:

  • Salmonid fish (e.g., trout, salmon),
  • Fish from a family other than salmonidae (e.g., bass, fathead minnow),
  • Species from a third chordate family (e.g., salamander, frog), and

Data on five invertebrates:

  • Planktonic crustacean (e.g., daphnia),
  • Benthic crustacean (e.g., crayfish),
  • Insect (e.g., stonefly, mayfly),
  • Species from a phylum other than Chordata or Arthropoda (i.e., rotifer, annelid [worm], or mollusk [e.g., mussel, snail]), and
  • Species from another order of insect or a fourth phylum (e.g., an insect or mollusk not already represented above).

Key Point. Where toxicity data are available for multiple life stages of the same species (e.g., eggs, juveniles, adults), the 1985 Guidelines require that the data from the most sensitive life stage be used. This ensures that an individual of that species can survive an exposure at any time during its life and thus have the opportunity to produce a succeeding generation.

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Assessing Acute Effects Data

Assessing the quality and completeness of collected data includes screening the data for validity based on such criteria as:

  • Use of a control.
  • Use of a single species and compound for each test.
  • Use of species from North America.
  • Use of flow-through tests if the compound is volatile or easily degrades.
  • Reporting of water characteristics (e.g., hardness or pH) when relevant to toxicity.

All valid toxicity data are used in ALC development, since confidence in a criterion usually increases along with the amount of acceptable data. A three-step calculation process results in a Final Acute Value, which is then used to derive the Criterion Maximum Concentration. These FAV calculation steps are listed and described on the next several pages of the module.

  1. Calculate the Genus Mean Acute Values.
  2. Rank the Genus Mean Acute Values.
  3. Calculate the Final Acute Value.
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Illustration. View graphic showing overview of ALC derivation. Proceed to the Illustration.


Calculate the Genus Mean Acute Values

Often, available verified data include multiple toxicity tests for a species. In such cases, the species data are pooled and averaged to provide a geometric mean of the species toxic concentration, or Species Mean Acute Value (SMAV).

Similarly, if data from several species in a single genus are available, the SMAVs are pooled to calculate a geometric mean of the genus toxic concentration, or Genus Mean Acute Value (GMAV).

Key Point. If data exist for only one species, the SMAV for that species becomes the GMAV.

Sample calculations of a SMAV (using short-term toxicity data) and GMAV are presented.

Species Mean Acute Value (SMAV)
Daphnia magna EC50 25 µg/L
Daphnia magna EC50 30 µg/L
Daphnia magna EC50 35 µg/L
Daphnia magna EC50 28 µg/L
SMAV = 29 µg/L
Genus Mean Acute Value (GMAV)
Daphnia magna 29 µg/L
Daphnia pulex 38 µg/L
Daphnia ambigua 42 µg/L
GMAV = 36 µg/L

Note: Totals for both are rounded down.

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Rank the Genus Mean Acute Values

The GMAVs from the minimum eight families of aquatic organisms are then ranked based on sensitivity, from high (most resistant to the chemical) to low (most sensitive to the chemical). From this ranking, the lowest four GMAVs are used in deriving the CMC, as described in the next step.

A sample ranking of the lowest four GMAVs is presented.

Rank GMAV SMAV
µg/L Species µg/L
4 100 Rainbow Trout, Oncorhynchus mykiss 100
3 36 Cladoceran, Daphnia ambigua 42
Cladoceran, Daphnia pulex 38
Cladoceran, Daphnia magna 29
2 25 Amphipod, Gammarus pseudolimnaeus 25
1 19 Amphipod, Hyalella azteca 19

Calculate the Final Acute Value

calculator

The lowest four values are then used in regression to estimate the concentration that would cause the threshold effect (i.e., LC50) for the fifth percentile most sensitive species. For acute toxicity tests, this fifth percentile of the effect concentrations is considered the FAV.

Key Point.In most cases, there are not enough data to interpolate the fifth percentile concentration. Thus, typically the four lowest GMAVs are selected for extrapolating.

If a SMAV for a commercially or recreationally important species (e.g., rainbow trout) falls below the FAV, the 1985 Guidelines say that this SMAV can be substituted for the fifth percentile of the GMAVs to protect that important species.

Key Point. If acceptable data are available for a large number of appropriate taxa from an appropriate variety of taxonomical and functional groups, a reasonable level of protection will likely be provided if all except a small fraction of the taxa are covered. To be practical, EPA selected the fifth percentile as this small fraction.

The ALC derivation graph highlights the data and approach used in calculating the FAV.

graph of supporting data for criteria

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From FAV to CMC

After the FAV is calculated, it is divided by two (a safety factor to avoid lethality) to arrive at the recommended acute criterion—that is, the Criterion Maximum Concentration. The CMC is set equal to half of the FAV to represent a low level of effect for the fifth percentile genus, rather than a 50 percent effect.

Key Point. An acute criterion is applied as a limit on the short-term average concentration in the environment. Both the acute and chronic criteria are values that are not to be exceeded more than once in 3 years. In other words, the criteria specify a magnitude, duration, and frequency to be met in order to provide protection of aquatic life.


Factoring in Water Characteristics

gray water

Water hardness (the amount of calcium and magnesium in the water) is known to moderate the toxicity of some metals. In particular, the toxicity of cadmium (Cd), chromium III (Cr III), copper (Cu), lead (Pb), nickel (Ni), silver (Ag), and zinc (Zn) decreases as water hardness increases.

For these materials, the relationship of toxicity to hardness is calculated. Then, the CMC is adjusted accordingly. The same approach is applied for characteristics associated with other materials as appropriate.

EPA has been working cooperatively with various organizations to develop a model called the biotic ligand model, or BLM. The BLM predicts acute toxicity based on site-specific water quality parameters like pH, hardness, and dissolved organic carbon (DOC). This model accounts for the major variables that effect toxicity rather than only accounting for hardness.

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CCC Approach Based on Available Data

For deriving the Criterion Continuous Concentration (or Chronic Criterion), the 1985 Guidelines call for using the same procedure as the one described above to derive the CMC (or acute criterion) when sufficient data are available. That is, if chronic values are available for at least eight families with the required taxonomic diversity, the CCC is set to the fifth percentile of the Genus Mean Chronic Values (GMCVs) in the same way the FAV for the CMC is calculated from GMAVs.

Often, however, available chronic toxicity test data are insufficient to meet the guidelines’ minimum requirements—because chronic toxicity tests take longer to carry out and thus are more expensive than acute effects testing. For such situations, the 1985 Guidelines provide an alternate way to derive a chronic criterion by using ratios derived from studies in which both acute and chronic tests have been conducted simultaneously for the same species.


Assessing Chronic Effects Data

Chronic tests are performed at sub-lethal toxin concentrations and measure effects on, for instance, growth of individuals or their reproductive capability. Assessing the quality and completeness of collected data includes screening for validity based on the same criteria used on data compiled for deriving the CMC. Regarding chronic values, for example, concentrations of the test chemical should have been measured at appropriate times during the test to verify consistency of exposure.

Two effect levels are identified in chronic effects testing:

  • No observed adverse effect concentration (NOAEC). The concentration at which no observable effect occurs (e.g., no statistically significant reduction in growth).
  • Lowest observed adverse effect concentration (LOAEC). The lowest concentration at which effects occur (e.g., statistically significant reduction in growth).

In general, the chronic criterion is derived from the geometric mean of these two effect levels. In some cases (e.g., for ammonia), the EC20 from chronic tests—based on concentration-effect regression analyses—is used.

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Illustration. View graphic showing overview of ALC derivation. Proceed to the Illustration.


CCC Calculation of the FCV

A three-step calculation process results in a Final Chronic Value (FCV), which is then used to derive the Criterion Continuous Concentration. These FCV calculation steps are described briefly.

Step 1. Calculate Acute-Chronic Ratios (ACRs).

ACRs are calculated for each set of parallel acute and chronic tests by dividing the acute value by the chronic value. That is, ACR = AV ÷ CV.  At least three species with a specified taxonomic diversity must be addressed by studies with parallel testing to calculate a valid final ratio.

Step 2. Develop a Final Acute-Chronic Ratio (FACR).

The geometric mean of the ACRs is then determined. That is, the ACRs are averaged to arrive at the FACR.

Step 3. Calculate the Final Chronic Value (FCV).

The FCV is then calculated from the Final Acute Value (FAV)—which was calculated in deriving the CMC—by dividing it by the FACR. That is, FCV = FAV ÷ FACR.

Thus, if the side-by-side tests show that, on average, chronic effects occur at one-half the concentration of acute effects, the chronic criterion will be one-half of the FAV, which is the same as the acute criterion.

Key Point. A chronic criterion is applied as a limit on the 4-day average concentration in the environment. Both the acute and chronic criteria are values that are not to be exceeded more than once in 3 years. In other words, the criteria specify a magnitude, duration, and frequency to be met in order to provide protection of aquatic life.

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FCV to CCC

The CCC can then be set as equal to the FCV, unless an adjustment is made to factor in other available data for deriving the CCC. For instance, the final chronic value can be adjusted downward to reflect available data from plant studies that yield a Final Plant Value (FPV) or from studies on commercially or recreationally important animal species yielding a Final Residue Value (FRV).


Criteria Review Process

Once EPA has developed draft versions of national recommended aquatic life criteria for a given chemical, the estimates are peer reviewed by both EPA and external experts and then subjected to a public comment period. States/Tribes also have an opportunity to review and comment on the draft criteria before individual jurisdictions consider adopting them into their water quality standards.

More specifically, aspects of the review process include the following:

  • EPA review:
    • Internal by expert EPA peer reviewers.
    • External by expert reviewers.
  • Public comment period:
    • Publication in Federal Register.
  • State/Tribal review:
    • Opportunity for public and scientific comment.
  • Release of final criteria:
    • Publication in Federal Register.

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Site-Specific Criteria

no fishing sign

In some locations, the nationally recommended aquatic life criteria may be considered under- or overprotective if the species at a site have different sensitivities than those included in the national criteria data set. For instance, physical and/or chemical characteristics at a site can alter the biological availability and/or toxicity of a material. For this reason, site-specific criteria may be developed to address such conditions.

Site-specific procedures consist of:

  • Defining the site boundaries.
  • Determining the effect of biological, physical, or chemical characteristics on sensitivity or bioavailability and toxicity.
  • Calculating numerical criteria by applying the recalculation procedure, the water-effect ratio procedure, or the resident species procedure.

Key Point. EPA regulations allow States and Tribes to develop site-specific aquatic life criteria. The Agency then considers the adequacy of the criteria during its review of the State/Tribe’s water quality standards. In general, EPA approves site-specific criteria if they are supportive of the designated uses established in the State/Tribe’s water quality standards and the criteria are based on sound scientific rationales.


Summary

  • Aquatic life criteria are estimates of concentrations of pollutants in ambient water that—if not exceeded—are expected to protect fish, invertebrates, and other aquatic life from unacceptable adverse effects associated with short-term (acute) or long-term (chronic) exposure (e.g., mortality, reduced reproduction).
  • EPA develops nationally recommended aquatic life criteria based on the latest scientific knowledge to provide guidance to States and Tribes for developing numerical and narrative criteria for their water quality standards.
  • An acute aquatic life criterion (termed the Criterion Maximum Concentration) is applied as a limit on the 1-hour average concentration of a particular chemical (or other material) in the environment. The chronic aquatic life criterion (termed the Criterion Continuous Concentration) is applied as a limit on the chemical’s 4-day average concentration in the environment. Aquatic life exposure to the chemical should not exceed either the CMC or the CCC more than once every 3 years on average.
  • To develop a CMC for a particular chemical, EPA guidelines require that acceptable acute values be available for at least eight families with a specified taxonomic diversity. For the CCC, if available chronic toxicity test data are insufficient to meet minimum requirements for the standard approach, an alternative is to calculate ratios from studies in which both acute and chronic tests have been conducted simultaneously for the same species.
  • In some locations, the nationally recommended aquatic life criteria may be considered under- or overprotective if the species at a site have different sensitivities than those included in the national criteria data set. In general, EPA approves site-specific criteria if they are supportive of the designated uses established in the State/Tribe’s water quality standards and the criteria are based on sound scientific rationales.

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Quiz

To complete your review of the topic in this module, please take the following brief quiz.

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.

Answer each of the questions

Quiz generates here...

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Disclaimer: This online course presentation and any associated links have been prepared by EPA staff for informational purposes only. Their sole purpose is to make available training online from recent Water Quality Standards Academy classroom courses. As such, this online course and any associated links are not binding on EPA or the public and have no legal effect. They do not constitute an EPA statute, regulation or other requirement and do not substitute for such authorities. In addition, the course and any associated links have not been reviewed or endorsed by EPA management. Thus, they are not intended or written as official statements of EPA's scientific views, policies, guidance, or requirements and cannot be used or cited as evidence of EPA's position on any matter.