Determining how many data are needed to address the ERA goals is part of the process of meeting a project’s Data Quality Objective (DQO). All risk assessment stakeholders (e.g. the US EPA, the State, the Fish and Wildlife Service, etc.) should be involved in this process. The DQO is ascertained at the beginning of an assessment, to define both the amount and quality of data required to complete the assessment. Scheduling time to complete DQOs at the beginning of the ERA may save the project time and money in the end. Once the goals and DQOs have been determined, the remainder of the problem formulation may be conducted. The ultimate goal of problem formulation is the SCM.
A wide range of ecosystem characteristics may be considered during problem formulation. These include abiotic factors (e.g. climate, geology, soil/sediment properties) and ecosystem structure (e.g. abundance of species at different trophic levels, habitat size, and fragmentation). The environmental description may be documented using recent photographs and maps. Plant and animal species lists should be compiled.
The scale of the assessment is especially important if a large, complex site has been subdivided into several smaller sites. It also is not uncommon for Superfund sites to be located adjacent to each other. Hence, the areal extent of the assessment must be defined. For example, if an off‐site area is included in the assessment, the distance off‐site must be specified. The development of the SCM and the selection of assessment endpoints will be directly related to the spatial scale. For example, due to their large home ranges, effects of soil contamination on deer would not be assessed if the site encompasses only two acres; assessment of endpoint species with smaller home ranges, such as small mammals, would be more appropriate.
It is necessary to decide if the assessment must consider temporal changes. All historical information should be evaluated. Then, it may be determined how much new information is needed to adequately evaluate impacts and risks. Certain parts of the year may need to be included in the sampling season for the assessment. For example, environmental exposures may change over the course of a year, or over several years, due to various seasonal influences in either chemical form or organism behavior (e.g. salmon returning to a contaminated river to spawn; migrating birds making temporary use of a site).
Site Conceptual Models
The SCM describes a series of working hypotheses regarding how contaminants or other stressors may affect ecological receptors (ASTM 2003). An SCM clearly illustrates the contaminated media, exposure routes, and receptors for the risk assessment. In addition to a written description, a diagrammatic SCM is easy to understand and is useful for ensuring that no relevant component is omitted from the assessment. The model ensure that all exposure scenarios are considered and allows for full documentation of the rationale behind selection and omission of pathways and receptors. The idea behind the SCM is that although many hypotheses may be developed during problem formulation, only those that are expected to contribute significantly to risks at the site are carried through the remainder of the ERA process.
During SCM development, all contaminant sources are identified (e.g. landfills, burial grounds, lagoons, air stacks, effluent pipes), and all contaminated media are represented (e.g. soil, water, sediment, air, biota). Groundwater usually is not considered an exposure medium until it becomes surface water, but contaminants in groundwater can migrate from soil to surface water and biota. An exception is shallow groundwater or seeps where plants may be exposed via their roots. All exposure pathways are represented, unless adequate rationale can be provided to exclude a pathway from the assessment. For example, an effluent pipe releasing metals into a stream would not need an air exposure pathway, and the only soils that would need to be considered are those of the floodplain. Thus, terrestrial receptors would be exposed by direct contact with or drinking from the stream, living in floodplain soils, or obtaining contaminated food from the stream and floodplain. An appropriate food web must be presented. A food web going from contaminated soil to earthworm to shrew may be appropriate for a one acre site, but a significantly larger site may require the food web to continue up to larger predators which have larger home ranges (see Figure 5.5).
For nonchemical stressors such as water level or temperature changes, or habitat disturbances, the SCM describes which ecological receptors are exposed to the physical disturbance, and the temporal and spatial scales of the alterations.
ERAs may have more than one SCM. In predictive ERAs, impacts on different components of the ecosystem from various activities may require several SCMS. In retrospective ERAs, a hypothetical future scenario often requires assessment. For example, an industrial area that now provides little habitat for wildlife (hence little exposure and little risk) may in future become covered in vegetation. It then would be more attractive as wildlife habitat, and hence the risk of exposure to contaminants would become greater. Similarly, a plume of contaminated groundwater that does not now pollute a pond may become a water source in several years. The future risk must be evaluated.
Identifying Endpoints
Before the SCM can be completed, the assessment endpoints of the ERA must be defined and rationale given for their selection. An assessment endpoint is the actual environmental value that is to be protected (Suter 1993; Suter et al. 2000). An example of an assessment endpoint would be “no less than a 20% decrease in the survival, growth, or reproduction in the largemouth bass population in the creek.” Desirable characteristics for assessment endpoint species include the following (Suter 1993; Suter et al. 2000):
- An assessment endpoint must be relevant to decision making.
- The structure and function of components of the ecosystem must be understood in order to determine the ecological relevance or importance of the endpoint. Species that control the abundance and distribution of other species and those that are involved in nutrient cycling and energy flow are generally considered to be ecologically relevant.
- Selection of endpoints may be influenced by societal involvement and concern.
- Only species that are present, or likely to be present at the site, should be used to evaluate risks, regardless of the value or importance of the species.
- Since only some species at a site can be evaluated, endpoint species must be selected which are sensitive to the contaminants at the site and are likely to receive high exposures. In this way, other species that may be less sensitive or receive lower exposures will also be protected. Other information necessary for each receptor species includes diet composition; habitat preference/needs; home range size; intake rates of food, water, sediment, air, and soil; and body weight.
- Finally, an assessment endpoint must be measured or modeled. If there is no method available to measure or model effects on an endpoint, evaluation of risk cannot be completed.
Selecting Measurement Endpoints
Because there are so many species and other ecosystem characteristics from which to choose assessment endpoints, stakeholders must agree on the appropriate assessment endpoints early in the ERA process. The remainder of the assessment cannot be completed until these have been chosen. After assessment endpoints have been selected, ecological risk assessors can select appropriate measurement endpoints for each assessment endpoint. Measures of exposure and effect are measurable environmental characteristics related to the valued characteristic chosen as an assessment endpoint (Suter 1993; Suter et al. 2000). There are three categories of measures (USEPA 1989). “Measures of effect” are measurable changes in an attribute of an assessment endpoint in response to a stressor to which it has been exposed (formerly referred to as “measurement endpoints”). “Measures of exposure” are measures of stressor existence and movement in the environment and this contact or co‐occurrence with the assessment endpoint. “Measures of ecosystem and receptor characteristics” are measures of ecosystem characteristics that influence the behavior and location of assessment endpoints, the distribution of a stressor, and life history characteristics of the assessment endpoint that may affect exposure or response to the stressor. These three difference measures are especially important when completing a complex ERA.
ERAs that involve Superfund remedial actions must meet federal and state standards or criteria called ARARs, for “applicable or relevant and appropriate requirements” (USEPA 1989). ARARs which may need to be considered at a site include those specified in the Clean Water Act, Clean Air Act, Endangered Species Act, Fish and Wildlife Conservation Act, Wild and Scenic Rivers Act, Migratory Bird Treaty Act, and many others. If numerical ARARs exist, modeled or measured chemical concentrations in site media cannot exceed these values.
During problem formulation, historical data and/or site investigation data are used to prepare a preliminary list of contaminants of potential ecological concern (COPEC). In order to obtain a meaningful ERA, selection of COPECs must ensure that all contaminants that may contribute significantly to risk are included. Reasoning must be provided for exclusion of chemicals from the COPEC list. In this initial screening of contaminants, valid reasons may include (but not be limited to) contaminant concentrations at or below background levels; concentrations below ARARS, other regulatory concentrations, or toxicity benchmarks; or chemicals infrequently detected. Exclusion of COPECs because the HHRA excluded them is not a valid reason. This is because protection of human health does not guarantee protection of nonhuman biota. Table 5.4 presents several aspects of this apparent paradox.
Table 5.4 Differences between human health and ecological risk assessments.
| Component | Human health risk assessment (HHRA) | Ecological risk assessment (ERA) |
| Institutional controls | Institutional controls may be considered when selecting exposure parameters | Nonhuman organisms are not excluded from waste sites by controls, such as fences or signs |
| Standard exposure factors | The USEPA provides standard exposure parameters and toxicological benchmarks for humans | Risk assessors must generate their own exposure parameters and toxicity data |
| Receptor species | Humans only | Nonhuman organisms (flora and fauna) and ecosystem properties (e.g. nutrient flow) |
| Exposure routes | Ingestion of food and water, incidental ingestion of soil, inhalation of contaminants from air, dermal contact, ingestion of fish fillets | As well as the exposure routes common to HHRA, other routes exist, such as fish respiring water, benthic organisms consuming sediments, small mammals burrowing in soil leading to enhanced exposure, fish‐eating wildlife consume the entire fish and chemicals accumulate to a different degree in different organs |
| Chemical form | Total metals in water are assumed to be available to humans | Dissolved metals are available to aquatic biota for gill uptake |
| Spatial scale | Often assumes a residential scenario at the site, regardless of appropriateness | Scale is important, since a small site (e.g. a few acres) cannot support a population of larger organisms (e.g. deer, hawks), but it could support small animal populations (e.g. shrews) |
| Temporal scale | Often only considered when seasonality may change chemical concentrations | Seasonality is more important in ERA, often because of habitat changes or changes in organism behavior |
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