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Ecological Risk Assessment: Exposure Assessment
- Identifying Receptors of Concern
- Selecting Contaminants of Potential Concern (COPCs)
- Exposure Point Concentrations
- Dealing with Biased Data
- Exposure Factors
- Probabilistic Risk Assessment
In the exposure assessment step, the risk assessor identifies which types of ecological receptors are likely to be exposed at a site (e.g., fish, birds, mammals, plants), the pathways by which exposure may occur (e.g., ingestion of contaminated water, ingestion of contaminated soil or food, direct contact with contaminated water or soil), and the degree (magnitude and frequency) of exposure. In most cases, exposure assessment is performed using single values (point estimates) for each exposure parameter, but exposure may also be assessed using probabilistic risk assessment (PRA) methods. In addition, exposure may be assessed by biomonitoring (collection of samples of environmental receptors and measuring chemical levels in their tissues).
There may be a large number of different ecological receptors present at a site, and it is generally not feasible or desirable to attempt to perform a quantitative risk evaluation for each individual species. Rather, the usual approach is to identify several groups of related species that are likely to be similar to each other with regard to the pathways and degree of exposure. For example, a risk assessment might choose to evaluate one or more of the following groups, depending on which types of receptors are likely to be present at the site:
- Aquatic receptors
- Benthic invertebrates
- Aquatic plants
- Semi-aquatic receptors
- Piscivorous birds
- Piscivorous mammals
- Terrestrial receptors
- Small mammals
- Large mammals
- Passerine birds
- Soil organisms
- Soil invertebrates
- Soil microbes
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One way to identify which types of ecological receptors are likely to be present at a site is to perform a site survey. Another way is to review available information for species that are known to occur in certain areas and certain habitat locations, as provided in the following resources:
Region 8 state conservation sites: The following links exit the site Exit
- Colorado Natural Heritage Program
- Montana Natural Heritage Program
- North Dakota Wildlife and Conservation
- South Dakota Wildlife Diversity Program
- Utah Conservation Data Center
- Wyoming Natural Diversity Database
Additional Region 8 state sites: The following links exit the site Exit
- Colorado Division of Wildlife
- Colorado Department of Natural Resources
- Montana Fish, Wildlife & Parks
- Montana Department of Natural Resources and Conservation
- North Dakota Game and Fish
- South Dakota Department of Environment and Natural Resources
- South Dakota Department of Game, Fish & Parks
- Utah Division of Wildlife Resources
- Utah Department of Natural Resources
- Wyoming Game and Fish
The process of selecting contaminants of potential concern (COPCs) to ecological receptors is generally similar to the process used for human receptors. Factors that are often used to help select COPCs for an ecological assessment include detection frequency, comparison to background, and a comparison of maximum detected levels to established toxicity reference values (TRVs). This last step is complicated by the fact that different types of receptors may have differing sensitivities to different chemicals. Thus, either the COPC selection procedure must be conducted independently for each type of receptor that is present, or it must use a screening-level TRV that is likely to be protective for even the most sensitive types of receptors.
Several EPA guidance documents that are helpful in the ecological COPC selection process are listed below.
The Role of Screening-Level Risk Assessments and Refining Contaminants of Concern in Baseline Ecological Risk Assessments (PDF) (Publication 9345.0-14, June 2001 ECO Update) (8 pp, 650 K)
Guidance for Comparing Background and Chemical Concentrations in Soil for CERCLA Sites (PDF) (EPA 540-R-01-003, OSWER 9285.7-41, September 2002) (89 pg, 1.3 MB)
An exposure point (also called an exposure area or exposure unit) is a location within which an exposed receptor may reasonably be assumed to move at random and where contact with an environmental medium (e.g., soil) is equally likely at all sub-locations. For ecological receptors, the exposure point is often approximately the same as the home range. An exposure point concentration (EPC) is an estimate of the true arithmetic mean concentration of a chemical in a medium within some specified exposure point. However, because environmental concentrations of a chemical contaminant may vary in both time and space, it is important that EPC values used to estimate exposures of various ecological receptors are based on sound rationale. The strategy that is selected usually depends both on the nature of the receptor and on the nature of the chemical.
Variability Over Space
In most cases, the ideal exposure point concentration for an ecological receptor is the average (arithmetic mean) concentration over the home range of that receptor. For receptors with large home ranges (e.g., a deer or a raptor), it is usually acceptable to compute average concentration values over all or large areas of a site. However, for receptors that have small home ranges (e.g., a mouse, some passerine birds, benthic invertebrates, some fish), it is generally not appropriate to average concentration values over an area larger than the home range. In this case, it is best to calculate the risks for each home range and present data on how many of the individual home ranges exceed a level of concern, and by how much.
Variability Over Time
For chemicals that cause unacceptable effects only after repeated exposures, the EPC should usually be based on an average over time. However, for chemicals that can cause unacceptable effects following only a brief exposure, averaging over time is usually not appropriate. In these situations, attention should be placed on the peak exposures that may occur. For example, consider a population of fish in a steam exposed to copper that is released from a mine site. Concentration values may be below a level of concern for most days of the year, but could exceed lethal levels for fish during storm or springtime snowmelt events. If averaging of values over time were used, the average could be below a level of concern, even though a peak exposure might be lethal.
Uncertainty in Exposure Point Concentrations
Whenever an EPC is computed as an average of multiple data points (over time, space, or both), there is uncertainty in the resulting estimate of the mean. Therefore, it is customary to use the 95 percent upper confidence limit (UCL) of the sample mean as a conservative estimate of the true EPC. The equation used to compute the 95 percent UCL of a data set depends on the distribution (normal, lognormal, other) of the values.
In the past, it was common practice to test each environmental data set for normality and, if it did not pass, to assume that the data set was lognormal. While this is mathematically convenient, the approach is inherently limited because no environmental data set can ever truly be lognormal, and this approach can substantially overestimate the true UCL. To address this problem, EPA has recently developed software (ProUCL) that computes the UCL for a given data set by a variety of alternative statistical approaches (including several approaches that do not require the assumption of normality or lognormality) and then recommends specific UCL values as being the most appropriate for that particular data set.
Region 8 recommends the use of ProUCL as the default approach for computing exposure point concentrations in most cases. The software and User's Guide for ProUCL are available from the following site:
If the ProUCL software is not selected for use at a site, Region 8 recommends following the guidance for computing UCLs:
Calculating Upper Confidence Limits for Exposure Point Concentrations at Hazardous Waste Sites (PDF) (32 pp, 949 K) (OSWER 9285.6-10, December 2002)
The mathematical approaches described above for computing 95 percent UCL values are all based on the presumption that the data set being evaluated was collected from the exposure point either using a random or systematic sampling strategy. However, in some cases, environmental samples are collected using a biased strategy, where more samples are collected from areas of contamination than from areas without contamination. In this situation, it is usually appropriate to compensate for the bias in the sampling strategy when computing the mean and the 95 percent UCL of the mean.
The simplest way to do this is to divide the exposure area into a series of smaller units (all of the same size), and to calculate the average value for all data points that fall in each smaller unit. The best estimate of the true mean is then the average of the averages across all smaller units, and the UCL of the mean can be estimated from the variability between the means of the different units. While simple, this approach is only a rough approximation, and more reliable estimates require use of geostatistical modeling techniques. This type of analysis usually requires an expert geostatistician. Guidance documents and software that describe and implement this type of analysis include the following:
Exposure of aquatic receptors and soil organisms is generally evaluated based on the concentration of a contaminant of potential concern (COPC) in the water or soil, so exposure factors are usually not needed for these groups. However, exposures of mammals are birds are generally characterized in terms of the amount of chemical ingested from contaminated environmental media, so exposure factors are usually needed for these groups. Depending on which environmental media are contaminated, values may be required for the following:
- Ingestion rate of water
- Incidental ingestion rate of soil
- Inhalation rate for air
- Ingestion rate for various food web items (plants, birds, insects, mammals, etc.)
EPA has compiled exposure data for a number of common birds and mammals, and the results are presented in the Wildlife Exposure Factors Handbook:
Wildlife Exposure Factors Handbook (EPA/600/R-93/187, December 1993)
This handbook is an excellent source of data for conducting exposure assessments for wildlife species exposed to toxic chemicals in their environment. If a species of group is of potential concern at a site but relevant exposure data are not available for that species or group in the Wildlife Exposure Factors Handbook, a search of published literature reports should be conducted to seek the required data.
Equations for computation of exposure of ecological receptors contain a number of terms that are inherently variable. For example, not all receptors in a group have the same dietary fractions, the same body weights, the same intake rates, or the same exposure frequencies or durations. Rather, there is a distribution for each of these terms across different individuals in a population of receptors. If data are available to describe the distribution of each of these terms, then a mathematical method is needed to combine the distributions.
While there are a number of different methods available, the most common and convenient is Monte Carlo simulation. In this approach, each term in the exposure model is described by a distribution rather than a single value. The computer draws a value at random from each distribution, computes the exposure, and saves the value. This process is repeated many times, resulting in a distribution of exposure values. This distribution usually provides a more complete description of exposure than the point estimate approach and helps ensure that the range of exposure levels across the members of the population are characterized. In addition, Probabilistic Risk Assessment (PRA) may also be used to evaluate variability between individuals in the toxicity term (TRV). Key guidance documents dealing with PRA include the following:
RAGS III Part A: Process for Conducting Probabilistic Risk Assessment (OSWER 9285.7-45, December 2001)
Note: In particular, see Chapter 4 - Probabilistic Analysis in Ecological Risk Assessment (PDF) (49 pp, 2.1 MB).
Guiding Principles for Monte Carlo Analysis (EPA/630/R-97/001, March 1997)
At sites where it is acceptable to collect tissue samples from ecological receptors at the site (e.g. fish, field mice, bird eggs, tissue from game animals taken by hunters), analysis of the tissues provides a direct measure of the total exposure of the organisms. This approach has the advantage that is a direct measure of exposure, and does not depend upon obtaining accurate measures of concentration and intake for each medium. Note that simply detecting the presence of a chemical in a tissue does not necessarily imply that an unacceptable exposure is occurring. Proper interpretation of tissue concentration data requires appropriate tissue-based TRVs, often referred to as Maximum Allowable Tissue Concentrations (MATCs). In addition, in cases where exposure occurs by contact with more than one medium, it may not be possible to know with certainty which of the media is/are most important sources of exposure and risk.
Bioaccumulation Testing and Interpretation for the Purpose of Sediment Quality Assessment: Status and Needs (EPA-823/R-00-001, February 2000)
Methods for Measuring the Toxicity and Bioaccumulation of Sediment-associated Contaminants with Freshwater Invertebrates - Second Edition (PDF) (212 pp, 3.7 MB) (EPA 600/R-99/064, March 2000)
Standard Guide for Determination of the Bioaccumulation of Sediment-Associated Contaminants by Benthic Invertebrates (American Society for Testing and Materials (ASTM) Book of Standards 11.05, Standard E1688-00a) Exit
Standard Guide for Conducting Laboratory Soil Toxicity or Bioaccumulation Tests with the Lumbricid Earthworm Eisenia Fetida and the Enchytraeid Potworm Enchytraeus albidus (ASTM Book of Standards 11.05, Standard E1676-04) Exit
Beyer, W.N., G.H. Heinz, and A. W. Redmon-Norwood. 1996. Environmental Contaminants in Wildlife: Interpreting Tissue Concentrations. CRC Press (SETAC Special Publications Series).
Fish Field and Laboratory Methods for Evaluating the Biological Integrity of Surface Waters (PDF) (325 pp, 4.66 MB) (EPA/600/R-92/111, March 1993)
National Study of Chemical Residues in Fish, Volume I (PDF) (325 pp, 15.2 MB) (EPA 823-R-92-008a, September 1992)
National Study of Chemical Residues in Fish, Volume II (EPA 823-R-92-008b, September 1992)
Guidance for Assessing Chemical Contaminant Data for Use In Fish Advisories: Volume 1: Fish Sampling and Analysis - Third Edition (EPA 823-B-00-007, November 2000)
Field Sampling Plan for the National Study of Chemical Residues in Lake Fish Tissue (PDF) (40 pp, 761 K) (EPA 823-R-92-004, September 2002)
A Survey of Fish Contamination in Small Wadeable Streams in the Mid-Atlantic Region (PDF) (110 pp, 2.5 MB) (EPA/600/R-01/107, February 2001)
Interim Report on Data and Methods for Assessment of 2,3,7,8-Tetrachlorodibenzo-p-dioxin Risks to Aquatic Life and Associated Wildlife (PDF) (159 pp, 1.3 MB) (EPA/600/R-93/055, March 1993)