Region 8

Eco Toxicity Assessment

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Ecological Toxicity Reference Values (TRVs) are species-specific and chemical-specific estimates of an exposure level that is not likely to cause unacceptable adverse effects on growth, reproduction, or survival. There are three different types of TRVs:

  • Dose-based (expressed in units of mg/kg-day): This type of TRV is usually used when evaluating risks to wildlife via ingestion pathways.
  • Concentration-based (expressed in units of mg/unit medium, e.g., mg/L water, mg/kg soil): This type of TRV is usually used when evaluating risks to receptors that have direct contact with the contaminated medium (e.g., fish in water, plants in soil).
  • Tissue-based (expressed in units of mg/kg tissue in the exposed receptor): This type of TRV can be used to assess almost any type of receptor (fish, benthic invertebrates, birds, mammals).

Most ecological TRVs are based on studies of laboratory animals exposed to varying levels of chemical for varying durations of exposure. These studies are designed to establish the quantitative relationship between exposure level/duration and the occurrence of adverse effects. The results are usually referred to as "dose-response curves."

Laboratory-based dose-response curves have the advantage that most variables can be well-controlled, and the resulting dose-response data are usually easy to analyze and interpret. However, one limitation of this approach is that dose-response data from laboratory animals exposed under laboratory conditions may not be a good surrogate for populations of wild receptors exposed under field conditions. Therefore, whenever it is feasible, it is usually desirable to seek field-based methods to confirm predictions based on laboratory toxicity data (see weight of evidence discussion in the Risk Characterization section).

Species-Specific vs. Group-Specific TRVs

As discussed in the section on Exposure Assessment, most ecological risk assessments do not focus on risks to individual species, but seek to evaluate risks to groups of related receptors (e.g., raptors, passerine birds, small omnivorous mammals, large herbivorous mammals, piscivorous birds, fish, benthic invertebrates). However, there may be important differences in sensitivity between the individual species present within a group and the selection of a TRV to represent the group must take this potential variability in sensitivity between species into account. Conceptually, if reliable TRVs were available for all of the species in the group, these could be arrayed in rank order to form a species sensitivity distribution (SSD), and the TRV selected to represent the group would be from the low end (e.g., 5th percentile) of the SSD. A good example of this approach is the method used by EPA to derive national Ambient Water Quality Criteria (AWQC) values for aquatic receptors (see Sources for TRVs section below).

When this approach is used, if the HQ is less than 1, it may be concluded that most or all of the species in the group are protected. If the HQ exceeds 1, it is not correct to assume that all species in the group are at risk. Rather, it should be concluded that the most sensitive species of the group may be at risk, and it is unknown whether other (less sensitive) species of the group are or are not at risk. In cases where the HQ for the group is only moderately above 1, some (perhaps many) of the individual species may not be at risk, with the threat being restricted only to the most sensitive species. As the HQ for the group becomes larger, it is expected that more and more species in the group would be at risk.

Unfortunately, in real life, toxicity data are generally not available to define a reliable SSD for all the species of concern at a site, or even for all of the species in any particular group. In these cases, the toxicity data are often evaluated on the basis of much broader taxonomic groups (e.g., all birds, all mammals). The basic strategy used to select the TRV for the broad group is the same as described above: the TRV is intended to be at the low end of the SSD for the broad group.

It is important to recognize that this approach adds a second assumption, namely that inter-species variability in TRVs within a small group (e.g., raptors) is similar to the inter-specific variability within the broad group (e.g., all birds). Conceptually, it is possible that this assumption is not valid. For example, it might be true that all waterfowl tend to have TRVs near the high end of the all-bird SSD, while all passerines have TRVs near the low end of the all-bird SSD. In most cases, it will not be known whether this hypothetical situation exists (if the data were good enough to answer the question, then it would be possible to develop small group-specific SSDs and TRVs). The usual approach is to assume that the SSD for the broad group is a good surrogate for the SSD for each small group, and to employ a single TRV (based on the data for the broad group) to estimate the TRV for the most sensitive members of each small group.

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Sources of Ecological Toxicity Reference Values (TRVs)

As noted above, most TRVs are based on dose-response data published in the toxicity literature. Numerous groups have derived TRVs for various groups of receptors, as summarized below. In addition, the University of Tennessee has assembled a database of ecological benchmarks called SADA (Spatial Analysis and Decision Assistance). Exit The SADA website contains links to many useful source documents, organized by medium: surface water, sediment, and soil.

Surface Water TRVs for Aquatic Receptors

Toxicity values for the protection of aquatic life from contaminants in surface water are available from several sources. EPA's Current National Recommended Water Quality Criteria, along with any state-specific water criteria, should be utilized as the primary source for surface water TRVs. The Current National Recommended Water Quality Criteria table lists aquatic life, human health, and nutrient criteria for 158 pollutants as guidance for states and tribes in adopting water quality standards for surface waters. Details on the basis of these values (chemical-specific criteria documents from the 1980s) may be found in the corresponding EPA Ambient Water Quality Criteria (AWQC) Reference Documents. The Aquatic Life criteria list chemical concentration goals to protect surface water for aquatic life use.

Other documents that present data and aquatic TRVs include the following:

Toxicological Benchmarks for Screening Potential Contaminants of Concern for Effects on Aquatic Biota: 1996 Revision (PDF) (Oak Ridge National Laboratory ES/ER/TM-96/R2) (151 pp, 414 K) Exit

Canadian Water Quality Guidelines for the Protection of Aquatic Life Exit


Bulk Sediment TRVs for Benthic Invertebrates

Toxicity values for the protection of benthic invertebrates from contaminants in sediment are available from several sources. These TRVs include a variety of different types, depending on the nature of the data (e.g., no effect levels, threshold effects levels, probable effects levels) used to derive the TRVs. Currently, EPA does not provide national recommendations for which set of TRVs are most appropriate for use in ecological risk assessment, but the consensus-based values derived by MacDonald et al. (2000) are usually preferred:

MacDonald, DD, CG Ingersoll, and TA Berger. 2000. Development and Evaluation of Consensus-Based Sediment Quality Guidelines for Freshwater Ecosystems. Archives of Environmental Contamination and Toxicology 39:20-31.

Alternative sediment-based TRVs that may be used when values are absent from MacDonald et al. (2000) include the following:

Ingersoll, CG, PS Haverland, EL Brunson, TJ Canfield, FJ Dwyer, CE Henke, and NE Kemble. 1996. Calculation and Evaluation of Sediment Effect Concentrations for the Amphipod Hyalella azteca and the Midge Chironomus riparius. National Biological Service Final Report for the EPA Great Lakes Nation Program Office (GLNPO) Assessment and Remediation of Contaminated Sediment (ARCS) Project. EPA/905/R96/008.

Long, ER, and LG Morgan. 1990. The Potential for Biological Effects of Sediment-Sorbed Contaminants Tested in the National Status and Trends Program. National Oceanic and Atmospheric Administration (NOAA) Publication. Technical Memorandum NOS OMA 52. March 1990.

Long, ER, DD MacDonald, SL Smith, and FD Calder. 1995. Incidence of Adverse Biological Effects Within Ranges of Chemical Concentrations in Marine and Estuarine Sediments. Environmental Management 19(1):81-97.

Canadian Water Quality Guidelines for the Protection of Aquatic Life Exit

Guidelines for the Protection and Management of Aquatic Sediment Quality in Ontario (PDF) (ISBN 0-7729-9248-7, August 1993) (32 pp, 1.8 MB) Exit

Toxicological Benchmarks for Screening Contaminants of Potential Concern for Effects on Sediment-Dwelling Biota: 1997 Revision (PDF) (Oak Ridge National Laboratory ES/ER/TM-95/R4) (48 pp, 182 K) Exit

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Soil TRVs for Terrestrial Plants and Soil Organisms

Toxicity values for the protection terrestrial plants and soil organisms from contaminants in soil are available from several sources. The Ecological Soil Screening Level (Eco-SSL) for plants should be utilized as the primary source for terrestrial plant and soil organism TRVs:

Ecological Soil Screening Level (Eco-SSL) Guidance and Documents

Other reports which contain TRVs for plants and/or soil organisms include the following:

Toxicological Benchmarks for Screening Contaminants of Potential Concern for Effects on Terrestrial Plants: 1997 Revision (PDF) (Oak Ridge National Laboratory ES/ER/TM-85/R3) (123 pp, 410 K) Exit

Toxicological Benchmarks for Contaminants of Potential Concern for Effects on Soil and Litter Invertebrates and Heterotrophic Processes: 1997 Revision (PDF) (Oak Ridge National Laboratory ES/ER/TM-126/R2) (151 pp, 487 K) Exit

Dutch Target and Intervention Values:

  • Swartjes, FA. 1999. Risk-Based Assessment of Soil and Groundwater Quality in the Netherlands: Standards and Remediation Urgency. Risk Analysis 19(6):1235-1249.
  • Crommentuijin, GH, EJ Van der Plassche and JH Canton. 1994. Guidance Document on the Derivation of Ecotoxicological Criteria for Serious Soil Contamination in View of the Intervention Value for Soil Clean-up. RIVM report 950011003. RIVM, Bilthoven, The Netherlands.

Canadian Soil Quality Guidelines for the Protection of Environmental and Human Health Exit

Wildlife TRVs for Ingestion of Contaminants

There are three primary sources of dose-based wildlife TRVs (listed in order of preference below):

Ecological Soil Screening Level (Eco-SSL) Guidance and Documents

Engineering Field Activity West. 1998. Development of Toxicity Reference Values for Conducting Ecological Risk Assessment at Naval Facilities in California, Interim Final. EFA West, Naval Facilities Engineering Command. United States Navy. San Bruno, CA. September 1998.

Toxicological Benchmarks for Wildlife: 1996 Revision (PDF) (Oak Ridge National Laboratory ES/ER/TM-86/R3) (217 pp, 1.3 MB) Exit

What to Do if No TRV Can Be Located

If a TRV for a particular chemical for a particular receptor species or group is not available in one of these sources, it may be possible to develop a TRV based on a review of published toxicity studies. This task should not be undertaken without the help of an expert toxicologist. The basic methods for performing this type of assessment are detailed in Ecological Soil Screening Level (Eco-SSL) Guidance and Documents.

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Chemical-Specific Resources and Guidance

Analyses of Laboratory and Field Studies of Reproductive Toxicity in Birds Exposed to Dioxin-like Compounds for Use in Ecological Risk Assessment (EPA/600/R-03/114F, April 2003)

Dioxin Analyses for Ecological Risk Assessment: Scientific Approach and Practical Considerations (PDF) (SETAC, 1998)


The consequences of exposure to a chemical depend on many things, including the degree to which the chemical is absorbed into the body from the environmental medium. This is referred to as bioavailability. A detailed discussion of bioavailability and how it may be measured at a site is provided in the human health bioavailability section of this website. Nearly all of the basic concepts and approaches discussed in this section are also applicable to evaluation of exposures of wildlife receptors by the oral route. Further information is provided in:

Viewpoints on Bioavailability for Wildlife (PDF) (Region 8/Ecological Risk Assessment Forum (ERAF), June 1998)

Some ecological receptors may also be exposed by direct contact with contaminated media (e.g., fish exposed to water, benthic organisms exposed to sediment, plants exposed to soil), and bioavailability may also be important in these situations. However, rather than measuring bioavailability and adjusting risk estimates for these types of receptors, it is usually most efficient to perform toxicity tests using site-specific media (e.g., expose fish to site water, grow plants in site soil) (see section on Site-Specific Toxicity Testing). One of the main advantages of this approach is that if bioavailability of a chemical in a site medium is low, this will be reflected in lower-than-expected toxicity in the test, and these results can be used to improve the accuracy of the risk assessment.

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