ERA has several considerations that HHRA lacks. One of the most important factors affecting the exposure assessment is the spatial and temporal scale of the assessment. Spatially, exposure estimates must take into account the home range of, and the availability of, suitable habitat for the receptor species, relative to the areal extent of contamination. Temporal considerations include whether the receptor species is a resident or migrant species, and whether contaminant concentrations vary over the course of the year due to seasonal changes.
Another concept that is not often addressed in HHRA is the different level of protection afforded to different species. HHRAs are designed to protect individuals. In ERA, only threatened and endangered species, or other species of special legal (e.g. migratory birds) or public concern are evaluated for impacts at the individual level. For other species, protection is primarily afforded at the population level. For example, it is important to protect a population of deer at a site; individual deer will not be protected. Practically, this means that impacts on measures relevant to the population as a whole, such as survival and reproduction, are evaluated. Individual quality of life is not considered.
As in HHRA, for an exposure pathway to be complete, there must be a contaminated medium, a transport medium, receptor species, and an exposure route that enables the contaminant to enter the organism. However, ERA has unique exposure routes, such as respiration of water by fish. In the exposure assessment, contaminant concentrations at an exposure point are determined, or intake rates calculated. In the risk characterization, these concentrations are related to toxicological benchmarks; which are contaminant concentrations that are assumed not to be hazardous to the receptor species.
The exposure scenario in an ERA may not be the same scenario as the HHRA. ERA does not have a default “residential scenario,” or “industrial scenario.” However, hazardous waste sites often are industrial in nature. Scenarios are developed which are appropriate to the current land use. Like the human health assessment, the ERA may make assumptions regarding future land use. This future scenario may assume the site is abandoned and undergoes natural succession. Therefore, it is unreasonable to assume that the same wildlife species will be present in the current and future scenarios, especially if the habitat changes. All assumptions regarding exposure scenarios must be documented early in the ERA process.
During characterization of the exposure environment, the relationship between the receptor species and the environment is detailed. Ecosystem characteristics can modify the nature and extent of contaminants. Chemicals may be transformed by microbial communities or through physical processes such as hydrolysis and photolysis. The environment also may affect bioavailability of contaminants. Physical stressors such as stream siltation and water temperature fluctuations may have considerable impact on ecological risks, and, therefore, must be described.
As part of the characterization of the exposure environment, it is also important to consider both the habitat requirements of receptor species and the amount of suitable habitat available at the site. Availability of habitat will determine the amount of use that a site receives. Because exposure cannot occur if receptor species are not present and receptor species will not be present if suitable habitat is not available, it is important to identify habitat requirements and availability early in the exposure assessment. Selecting exposure routes depends on the endpoints to be evaluated. Several endpoint categories and exposure routes are discussed in Sections 5.3.2.1–5.3.2.5.
Fish Community
Fish are exposed to contaminants in surface water through respiration and dermal absorption. They also may be exposed through the consumption of contaminated sediment or food. There are two important considerations for the fish community. The first is that for inorganic contaminants, it is the dissolved fraction of the contaminant in the surface water that the fish are exposed to by inhalation (i.e. gill uptake). Practically speaking, this involves filtering the water sample through a nanometer filter prior to analysis. HHRA calculates exposures using the total inorganic concentration in water. However, the particulate‐bound fraction is not available to fish at the gill. Secondly, dermal absorption as a separate exposure route is not evaluated, because existing toxicity data for fish were generated either by feeding contaminated food to fish or exposing fish to contaminants in the water, without attempting separate evaluations of the various uptake routes.
Benthic Macroinvertebrate Community
Benthic macroinvertebrates live in or on contaminated sediments. They may be exposed through ingestion of the sediment or contaminated food. Also, benthic organisms may respire overlaying water or the sediment pore‐water. Special considerations for this endpoint include the need for bulk sediment contaminant concentrations and pore‐water analyses, in order to compare these concentrations to benchmark concentrations. For nonionic/nonpolar organic contaminants, bulk sediment concentrations are used. The organic carbon content of the sediment is also required. For ionic/polar organic contaminants, the sediment pore water must be analyzed. For inorganic contaminants, either analysis is adequate.
Soil Invertebrate Species
Soil invertebrates, such as earthworms, are in direct contact with contaminated soil. Also, the earthworm ingests large amounts of soil during feeding. Contaminants are in contact with and may be absorbed by the gut of the worm.
Terrestrial Plants
Plants may take up contaminants from the soil at the root. Also, contaminants in shallow groundwater may be taken up by the plant roots. Airborne contaminants (e.g. ozone, acid gases) also may enter the plant through the leaf stomata and cause damage.
Terrestrial Wildlife
As terrestrial wildlife move through the environment, they may be exposed to contamination via three pathways: oral, dermal, or inhalation. Oral exposure occurs through the ingestion of contaminated food, water, or soil. Dermal exposure occurs when contaminants are absorbed directly through the skin. Inhalation exposure occurs when volatile compounds or fine particulates are respired into the lungs. While methods are available to assess dermal and inhalation exposure to humans, data necessary to estimate dermal and inhalation exposure are generally not available for wildlife. However, these routes are generally considered to be negligible relative to other routes.
Ideally, seasonal data would provide the most complete evaluation of contaminants present in the environment. Wherever possible, site‐specific data should be used, rather than modeled data. Where EPCs must be modeled, the same methods and considerations are applicable to ERA as in HHRA. EPCs are developed differently according to endpoint. For the fish community, the concentration of contaminant in water or sediment is used as the EPC. No exposure models are required. The upper 95% confidence limit on the mean water concentration may be used instead of the mean or maximum detected concentration. This is because chronic exposures of the maximally exposed aquatic organisms would be to spatially and temporally varying contaminant concentrations. For the benthic, soil invertebrate and plant communities, the concentration in the sediment or soil at each sample location is used as the EPC. Again, no exposure models are required. However, in each of these cases, the maximum concentration in the sediment or soil should be used as the EPC because these organisms are not particularly mobile. The entire community could be exposed to the maximum concentration present in the medium.
For wildlife species, contaminant concentrations in food, water, and soil are used in exposure models to estimate dose. Because wildlife are mobile, use various portions of a site, and are exposed through multiple media, the upper 95% confidence limit on the mean best represents the spatial and temporal integration of contaminant exposure wildlife will experience. Exposure estimates for wildlife are usually expressed in terms of a body weight‐normalized daily dose or mg contaminant per kilogram body weight per day (mg/kg/day). Exposure estimates expressed in this manner may then be compared to toxicological benchmarks for wildlife, or to doses reported in the toxicological literature. A very few wildlife consume diets that consist exclusively of one food type.
To account for varying contaminant concentrations in different food types, exposure estimates should be weighted by the relative proportion of daily food consumption attributable to each food type, and the contaminant concentration in each food type. Each parameter in a wildlife contaminant intake equation must be obtained from the literature because few site‐specific values are likely to be available. The EPA Wildlife Exposure Factors Handbook (USEPA 1993) contains a compilation of values for parameters such as diet composition, food intake rate, body weight, and home range for 15 birds, 11 mammals, and 8 reptiles and amphibians. The primary and secondary literature must be consulted for any parameter values not contained in this document or if the values provided are not appropriate for the site or become outdated.
One advantage that ERA has over HHRA is the ability to sample the receptor species itself. Rather than introducing modeling uncertainties, fish, benthic macro‐invertebrates, soil invertebrates, plants, and some wildlife species (e.g. small mammals) can be sampled directly to give an indication of the bioavailability of environmental contaminants. Of course, it is not acceptable to destructively sample many species, such as rare, threatened, and endangered species, or those with high societal value or low abundance. However, when possible the additional sampling and analytical costs will be worth the added certainty in the exposure assessment and risk characterization.
Ideally, contaminant analysis of whole fish are used when conducting an exposure assessment on piscivorous species. However, fish body burdens may be estimated using bioaccumulation factors. Professional judgment is required when selecting a parameter value for the exposure model. Full rationale for the selection of any parameter value must be provided in the exposure assessment. Exposure assessments will use a variety of data with varying degrees of uncertainty associated with them. Each assumption made will be a result of professional judgment, but it will still have some uncertainty. It is important that the exposure assessment document and characterize each source of uncertainty, including those associated with analytical data, exposure model variables, contaminant distribution and bioavailability, receptor species presence and sensitivity, and other incomplete exposure information.
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