8 Remedial Goals Diseases desperate grown by desperate appliance are relieved, or not at all . —William Shakespeare, Hamlet The first step of a feasibility study in the RI/FS process is to identify remedial goals (also called remedial action objectives) for protecting human health and the envi- ronment. The remedial goals specify contaminants and media of concern, potential exposure pathways, and cleanup criteria (EPA, 1990a). The criteria are typically concentrations of chemicals in the specified media that are expected to protect human health and the environment adequately, based on risk assessments of the specified routes of exposure. Chemical concentrations are the usual criteria because they are the single line of evidence used in a human health risk assessment. However, ecological risk assessment offers the possibility that remedial goals can be defined more broadly than chemical criteria. These remedial goals are developed iteratively, beginning with the DQO process in the problem formulation and ending with site- specific goals that are set by regulatory agencies, and agreed to by site managers if the site is a U.S. federal facility. The remedial goals are the basis for the selection of candidate remedial alternatives by engineers and site managers. Remedial goals must specify a receptor and exposure route, because the EPA acknowledges that protection can be attained by actions that decrease exposure as well as by decreasing concentrations of chemicals in environmental media (EPA, 1990a). The risk assessor’s primary input to risk management is proposed cleanup criteria, alternatively termed preliminary remediation goals (EPA, 1991d), treatment endpoints (Alexander, 1995), corrective action goals (ASTM, 1999), or remediation objectives (CCME, 1996a). The term remedial goal options (RGOs), used by some EPA Regions, is preferable to the other four, because it emphasizes (1) that reducing risks from contamination to minimal levels is only one of the risk manager’s options when making a remedial decision and (2) that risk assessors may present multiple options for remedial goals, based on different levels of risk, different endpoints, or different definitions of the remedial action objective. Thus, we use the term RGO throughout this chapter. In this chapter, the term preliminary remedial goals (PRGs) is restricted to toxic concentrations of individual chemicals that are generically derived and serve as default RGOs (Section 8.1). Thus, the following definitions apply: preliminary remedial goals are concentrations in media that are starting points for developing cleanup targets; remedial goal options are the assessors’ recommen- dations concerning ways that remediation might achieve protection of the assessment endpoints; and remedial goals are the ultimate cleanup targets set by risk managers that engineers attempt to achieve. The final rule for the National Oil and Hazardous Substances Pollution Contingency Plan states that the EPA sets remedial goals (EPA, 1990a). The EPA and states set these goals at many sites; however, for U.S. federal © 2000 by CRC Press LLC facilities, these goals are negotiated between site managers and the EPA, with a high degree of involvement by risk assessors. 8.1 PRELIMINARY REMEDIAL GOALS PRGs are upper concentration limits for specific chemicals in generic soils, waters, or sediments that are anticipated to protect human health or the environment. The following discussion is focused on the development and use of PRGs for ecological endpoints. PRGs are more generic than RGOs and can be used as a starting point for the development of RGOs for remedial investigations at multiple facilities, sites, or units. There are two important questions to the risk assessor: (1) Which chemical concentrations in environmental media should be used as PRGs? (2) How may PRGs be modified to generate site-specific RGOs? The EPA has published guidance entitled “Risk Assessment Guidance for Super- fund: Volume I—Human Health Evaluation Manual, Part B” (RAGS), which is a useful aid in developing PRGs intended to protect human health (EPA, 1991d). However, no guidance is available in the United States on how to develop PRGs based on ecological risk or even what level of protection is analogous to the 10 -6 risk for human carcinogens. (See Section 9.2.3 for a discussion of balancing health and ecological risks.) PRGs should not be higher than any numerical applicable or relevant and appropriate requirements (ARARs) for the chemical of concern. For ecological endpoints, the only ARARs are National Ambient Water Quality Criteria (NAWQC) that are available for many contaminants in surface waters. Other gov- ernment entities have published guidelines that are recommended for use as PRGs. The Canadian Council of Ministers of the Environment (CCME), for example, has published a protocol for the derivation of soil quality guidelines (CCME, 1996b) and guidelines for 20 chemicals derived using the protocol (CCME, 1997). These guidelines are intended to be used or modified by site managers as remediation criteria (CCME, 1996a, b, 1997); thus, they are PRGs. Risk assessors may use existing PRGs (often national or state guidelines) or derive PRGs that are less generic. In the latter case a risk assessor may, for example, derive soil PRGs using generic toxic doses and site-specific wildlife food uptake factors (LMES, 1997). Many ecological risk assessors do not distinguish between screening benchmarks (Section 4.1.8) and PRGs; however, PRGs are useful if (1) more than one assessment endpoint that is exposed to a single medium (e.g., piscivorous birds and fish in water) requires protection; (2) screening benchmarks are no observed adverse effects levels (NOAELs) and therefore cannot serve as PRGs (PRGs should be minimal effects levels unless the endpoint is a threatened or endangered species); or (3) multiple screening benchmarks for an endpoint exist. In the latter case, the selection of general PRGs serves to assure that particular bench- marks (e.g., among aquatic or sediment toxicity benchmarks) are consistently the starting points for developing final remedial goals. It is acknowledged that screening ecotoxicity benchmarks are biased to avoid eliminating contaminants that are pos- sible contributors to risk; thus, excessive cleanups could result if they were used as final remedial goals (TNRCC, 1996). Sheppard et al. (1992) provide a short history of the development of “generic guidelines” that are applicable to remediation of a © 2000 by CRC Press LLC broad range of sites. Most of these focus on health effects, but environmental effects are incorporated in others. When considering the use of existing PRGs, the risk assessor must be aware of (1) the intended use and (2) how they were derived. For example, ecotoxicity benchmarks are often not appropriate PRGs, particularly if they represent no-effects levels. Also, if PRGs for an arid state in the United States are derived using arid soil data, they are not appropriate PRGs for humid locations. Similarly, PRGs derived from toxicity data for organisms that are not related to endpoint species should not be used. For example, PRGs based on toxicity to osprey should not be used for remediation of a small stream. Toxicity tests on which the PRGs are based vary, depending on needs of the institution or facility that developed them. For Canadian soils, test endpoints include mortality, reproduction, growth, development, behavior, activity, lesions, physiological changes, respiration, nutrient cycling, decomposition, genetic adaptation, and physiological acclimatization (CCME, 1997). Since the PRGs may be used as default remedial goals, the assessor should be aware of the level of conservatism associated with the goals. In general, existing PRGs correspond to small effects on individual organisms, and these chemical concentrations would be expected to cause minimal effects on populations and communities. Far more studies that concern effects on individual organisms are available than those that demonstrate effects at higher levels of organization. PRGs developed in this manner may not be sufficiently protective of individual organisms among threatened and endangered populations if they are also sensitive species. Because these species are protected at the individual level, remedial goals for such species should be developed ad hoc and should be based on NOAELs. PRGs may apply to one of three environmental media: surface water, sediment (including pore water), and soil. At hazardous waste sites where cleanup is contem- plated, it is unlikely that an air source is the major contributor to the ecological risk and even less likely that air will be remediated. Similarly, ecological PRGs for groundwater exposure have not been developed. Groundwater contamination has greater consequences for human health than for nonhuman organisms, data on microscopic and other small biota of groundwater are scarce, and regulatory agencies do not typically advocate their protection. Although contaminants of potential con- cern at a site can be identified based on concentrations in wildlife food or in the assessment endpoint organism’s tissues, ultimately one of the three abiotic media is remediated. Therefore, the media for which PRGs have been developed do not include “foods” and are limited to surface water, sediments, and soil. In addition, indirect effects of contamination (such as avoidance of contaminated food) are not typically considered in the derivation of PRGs (CCME, 1996b). 8.1.1 PRG S FOR S URFACE W ATER In the United States, PRGs for surface waters should be at least as conservative as NAWQC, unless the particular NAWQC are based on effects on humans. Numerous other benchmarks for aquatic toxicity may also serve as PRGs. At Oak Ridge National Laboratory, PRGs for surface waters are chosen by comparing the ORNL benchmarks for screening toxicity of contaminants to aquatic life (chronic NAWQC © 2000 by CRC Press LLC or secondary chronic values; Suter and Tsao, 1996) with those for toxicity to piscivorous wildlife (LOAEL; Sample et al., 1996b). The lower of the two bench- marks is the PRG (LMES, 1997). It should be noted that this PRG may be too conservative in the case of small streams, where avian or mammalian piscivores may not forage. The PRG for a particular chemical cannot be assumed to protect piscivorous wildlife if information on the toxicity is not available. As in the risk assessment for aquatic organisms, the filtered concentration of a chemical in water should be assumed to be more representative of exposure than the total concentration. The only exception would be in the drinking water concentrations used in the calculation of PRGs for piscivorous wildlife. 8.1.2 S EDIMENT Both the concentrations of chemicals in the solid phase of sediments and concen- trations in the pore water are relevant to the exposure of benthic (sediment-inhab- iting) organisms, and PRGs may be developed for both media (LMES, 1997). If PRGs are available for both sediment and pore water, the PRG that is determined by the remedial investigation to be the best estimate of risk to sediment biota should take precedence. At ORNL, PRGs for sediments have been defined as the lowest concentration of six types of sediment toxicity benchmarks (LMES, 1997). Most PRGs for nonionic organic chemicals are based on equilibrium partitioning. In the United States, PRGs for chemicals for which sediment quality criteria have been proposed should be at least as low as those values. Few peer-reviewed publications exist in which PRGs are derived. However, one is worth mentioning. Comber et al. (1996) provide sediment guideline values (which may be used as PRGs) for dioxins and dibenzofurans. They derived values using two independent sets of parameters: (1) an aquatic guideline value corresponding to a toxic level for rainbow trout fry and the K oc and (2) a toxic residue level for benthic organisms and the sediment–organism uptake factor. The values for 2,3,7,8- TCDD were within a factor of 6 of each other. 8.1.3 S OIL Standard benchmarks for soil do not exist in the United States, although they are currently being developed by the EPA. At ORNL, PRGs for soil are protective of avian and mammalian wildlife, plant communities, and soil invertebrate communities (LMES, 1997). Microbial processes are not included. The EPA and state regulatory agencies in the United States rarely make remedial decisions based on the protection of soil invertebrates, and to the authors’ knowledge have never based a decision on protecting microbial processes. Thus, regulatory priorities and precedents should be considered when the assessor is selecting or developing PRGs for use at a particular site. The CCME adds nutrient cycling processes to the ecological receptors of concern for the derivation of its effects-based soil quality guidelines or PRGs (CCME, 1996b). Soil PRGs should be derived under the assumption that plants and soil invertebrates are exposed through direct contact with the chemical in soil and that wildlife are exposed primarily through ingestion. Interestingly, the CCME (1996b) assumes that PRGs developed for soil-dependent organisms should be © 2000 by CRC Press LLC protective of wildlife exposed via ingestion and dermally, except in the case of a few specific chemicals (e.g., molybdenum and selenium). The CCME (1996b) provides a methodology for estimating the threshold effects concentration (TEC) using single-chemical toxicity tests, where the first method is preferred to the second, and the second is preferred to the third: (1) weight-of- evidence analysis (percentile of the combined effects and no-effects distributions of concentrations), (2) extrapolation from the LOEC, or (3) extrapolation from the EC50 or LC50. A microbial effects concentration (nitrogen fixation, nitrification, nitrogen mineralization, respiration, and decomposition) is compared with the TEC. If the microbial concentration is lower than the TEC, the geometric mean between the two is used as the guideline for soil contact. As stated above, guidelines for soil contact are assumed to protect wildlife exposed through ingestion (CCME, 1996b). An exception is ingestion by herbivores in an agricultural scenario (CCME, 1997). A methodology for calculating soil PRGs for terrestrial wildlife has been devel- oped for the Oak Ridge Reservation (LMES, 1997). PRGs were calculated as a concentration in soil that would result in an estimated dose by all oral routes equal to the contaminant-specific and species-specific LOAEL (Figure 8.1). Exposure estimates were iteratively calculated using varying soil concentrations and soil-to- biota uptake models. The soil concentrations were manipulated to produce an expo- sure estimate equivalent to the wildlife endpoint-specific and contaminant-specific LOAEL. Because different diets may dramatically influence exposures, and sensi- tivity to contaminants varies among species, PRGs were developed for six species present on the Oak Ridge Reservation: short-tailed shrew, white-footed mouse, red fox, white-tailed deer, American woodcock, and red-tailed hawk. For each chemical, the PRG for each of the wildlife species was compared, and the lowest soil concen- tration was selected as the final wildlife PRG. Estimates of oral exposure to con- taminants were generated using a generalized exposure model (Section 3.10). Among the 18 chemicals and six wildlife species for which PRGs were derived, the final PRG for protection of wildlife was always based on either the short-tailed shrew or the American woodcock (LMES, 1997). Reports in which PRGs are compiled or derived do not typically recommend a soil depth to which the quantities should apply (e.g., CCME, 1996b). The relevant depth of exposure is left to the assessor to determine. The considerations related to the depth of sampling for ecological risk assessments (e.g., Section 3.4.2) apply here. 8.1.4 M ODIFICATION OF PRG S The specific, numerical PRGs that an assessor uses are less important than how they are modified (below) or how ultimate RGOs are chosen (Section 8.2). PRGs that are not ARARs or based on ARARs may be modified during the remedial investi- gation and feasibility study using site-specific data (EPA, 1991d). These modified PRGs may be recommended by assessors as RGOs for the site. The use of the same remedial goal at sites with varying soils or exposure pathways would result in variable risks (see Labieniec et al., 1996, for an analysis of this risk variability with respect to human health). The Canadian Council of Ministers of the Environment © 2000 by CRC Press LLC (CCME, 1996a) suggests that generic guidelines (PRGs) should be modified if (1) a particular site has high background concentrations of a chemical; (2) contaminants may move from soil to groundwater or air; (3) toxicological data used to derive the guidelines are not relevant to the site (different receptors, complex mixtures of chemicals); or (4) land uses necessitate modification of PRGs. The EPA emphasizes that multiple chemicals and multiple exposure pathways may justify the modification of PRGs (EPA, 1990a). California adds that if multiple media contribute to exposure, the media should be assumed to be in equilibrium for the development of remedial goals (Cal EPA, 1996). In summary, modifications of PRGs may be based on: • Land-use assumptions; • Exposure assumptions and habitat considerations (e.g., fraction of land that is suitable habitat); FIGURE 8.1 Procedure for calculation of PRGs for soil based on toxicity to wildlife (LMES, 1997) © 2000 by CRC Press LLC • Environmental assumptions used to derive PRGs (e.g., water hardness, soil pH, and organic content); • Absence of type of organism (e.g., wide-ranging predator) which the PRG is intended to protect (or exclusion from list of assessment endpoints); • Synergistic, antagonistic, or additive effects of multiple pollutants; • Form of chemical different from assumption in derivation of PRG (e.g., nonaqueous-phase liquids); • Exposure from multiple media; • Impacts of contamination of one medium on another (EPA, 1991d); • Impacts of remediation of one medium (such as sediments) on contami- nation of another medium (such as surface water); • Effects of remediation on organisms and their habitat (Chapter 9); • Desirable level of protection; • Background concentration of element higher than PRG; or • Indirect effects of contamination. 8.1.5 L AND U SE Land-use scenarios play a different role in ecological risk assessments than they do in human health risk assessments. For human health risk assessments, remediation depends on the land-use scenario because land use determines human exposure. Exposure pathways for humans can change, for example, depending on whether the land is industrial, agricultural, or residential. Soil ingestion by children may occur in residential areas and not in industrial areas; plants for human consumption are not grown on industrial sites; and inhalation of soil particles would be more signif- icant in agricultural than in residential areas. Therefore, because humans engage in different activities in different locations, exposure depends on land use, and risk-based remediation should depend on land use. In ecological risk assessments, three issues must be considered with respect to post-remedial land use. 1. What habitat will occur on the site given the proposed land uses? Plants and most animals are more likely than humans to engage in all activities on a particular site. Therefore, land-use scenarios are important primarily because they determine which receptors will find habitat on the site. Land use determines habitat, which determines the presence of end- point populations and communities, which determines exposure and risk. Even for migratory species, the question arises: Will the site provide habitat for the species during the time it might spend on the site? There- fore, for an industrial site, one might develop a soil PRG for a continued industrial use scenario based on leaching and runoff to an off-site stream, because there would be no on-site ecological endpoint receptors. However, if the site is to be converted to a park with a plant community, birds, and small mammals, the soil PRG should be protective of on-site as well as off-site endpoint receptors. In some cases, as in human health risk assess- ment, land use may eliminate habitat for some activities but not all. For example, waterfowl may rest on and drink from an industrial pond during © 2000 by CRC Press LLC migration but would not breed there. Therefore, a PRG might be developed for that limited exposure. 2. Will land use affect the value of ecological endpoint receptors? In some land uses, endpoint populations or communities are not as highly valued as they would be with other uses. For example, earthworms in soil adjacent to a factory are less valued than in agricultural or natural land uses where maintenance of soil texture and fertility is important. These differences in value could result in different degrees of protection. An example of this concept is the identification of “designated uses” for receiving waters under the Clean Water Act. The various designated uses, which vary among states, require different water qualities, which in turn require different levels of pollutant regulation. 3. Will land use affect the sensitivity of endpoint receptors? Land use may modify the species composition of an endpoint community or the life stages of an endpoint population on the site. For example, streams in industrial, agricultural, or residential areas are physically disturbed and tend to have little riparian vegetation and highly variable flows, resulting in low species richness. Similarly, stream reaches with no spawning hab- itat (e.g., suburban channelized reaches) may contain adult fish but very few or no fish eggs or larvae. Such stream reaches may require less protection, because (a) sensitive species or life stages are absent, (b) no amount of waste remediation will result in recovery, and (c) the land use effectively precludes habitat restoration. For any of those reasons, risk managers may decide that different levels of protection should be associated with different land uses, thus modifying the PRG for a medium. However, it should be noted that no particular land-use scenario is assumed for the application of ARARs (e.g., NAWQC) to a site. Protection levels for different land uses may be determined generically for a nation or other political entity. Soil quality guidelines developed by the CCME (1997) and used as PRGs are specific to different land uses: agricultural, residen- tial/parkland, commercial, and industrial. Agricultural lands include agricultural edge habitats. The residential/parkland use assumes that the land serves as a buffer zone between residences but is not broad wilderness. Commercial land includes managed systems, such as cultivated lawns and flowerbeds (CCME, 1996b). How- ever, it is assumed that the normal range of activities on commercial lands does not rely as much on ecological services as agricultural or residential/parkland. Although the same receptors are generally examined in the derivation of the PRGs, the level of protection for commercial and industrial land uses is lower than for the other two (CCME, 1997). 8.2 REMEDIAL GOAL OPTIONS Differing quantitative or qualitative definitions of RGOs are possible because of the multiple lines of evidence that are available in ecological risk assessment. Conven- tionally, an RGO is defined as a concentration of a particular chemical that constitutes © 2000 by CRC Press LLC a threshold for unacceptable risk. The risk assessor modifies the PRG to derive the RGO. Media with chemical concentrations below the RGO are assumed to be acceptable, but those with concentrations above the RGO may be remediated. Indeed, the EPA definition of a remedial goal is a concentration of a chemical in an envi- ronmental medium (EPA, 1990a). Alternatively, RGOs may be defined in terms of media toxicity test results (Office of Emergency and Remedial Response, 1994a). That is, one may specify that areas where a particular test endpoint (e.g., >20% mortality of earthworms) or any one of a set of test endpoints is exceeded are candidates for remediation. For example, an RGO may be defined in terms of toxicity if the risk assessment identifies a medium as toxic without clearly isolating the cause. Potential reasons for the use of a toxicity RGO would be inadequate data on concentrations of all chemicals, poor relationship between bioavailability at the site and in laboratory media, failure of any of the chemicals measured in the medium to be toxic by themselves, or high variance in the relative contributions of individual chemicals to toxicity at different sample locations. In such cases an appropriate RGO could be a direction to remediate all toxic areas. This was the case for a sediment depositional area in Bear Creek in Oak Ridge, where the sediments were unambiguously toxic (i.e., Hyalella azteca survival was reduced by 37% relative to reference and control), but a causative agent could not be determined from the available chemical data. Alternatively, an RGO may be to perform a toxicity identification and evaluation (TIE) procedure and remediate the chemicals that are identified by the TIE to be causing the toxicity (Section 4.2). Finally, one may specify that areas where biological surveys indicate levels of effects in exceedence of some measure of effect (e.g., dead plants or fewer than x earth- worms per square meter) are candidates for remediation. Derivation of chemical-concentration-based RGOs should incorporate site-spe- cific exposure and effects data, to the extent practical. One way to do this is to derive a site-specific no apparent effects level (SSNAEL) for an environmental medium (Jones et al., 1999). The SSNAEL is the highest measured concentration of each chemical at which toxicity was never observed in standard tests of the ambient medium from the contaminated site. If the tests are sufficiently sensitive, the RGO for a particular chemical should not be lower than the concentrations which were shown to be nontoxic in the site medium. This approach can also be used for survey data (e.g., species richness or abundance of benthic invertebrates), provided that the surveys are sufficiently sensitive and the measured chemical concentrations are representative of the exposures associated with the observed effects. An example of the application of this approach to the assessment of risks to benthic invertebrates in Poplar Creek, TN is presented by Jones et al. (1999). The recommended RGO was the higher of the generic probable effects level from the literature (e.g., the effects range–median; see Section 4.1.8) and the SSNAEL. As the word options in the phrase suggests, multiple RGOs may be provided to the risk manager as alternative levels of protection. If the best basis for remediation is unclear, the assessors may provide a set of concentrations or other criteria from which the risk managers could select or derive the final remedial goals. For example, RGOs for water might include (1) the chronic NAWQC for the chemical that is believed to cause significant toxic effects on an endpoint community, (2) a threshold © 2000 by CRC Press LLC for toxicity of that chemical in a toxicity test such as the EPA subchronic Ceri- odaphnia test performed with site water as the diluent, (3) chemical concentrations derived by performing a TIE on the contaminated water (Section 4.2), (4) a concen- tration of a chemical bioaccumulated to toxic levels in fish tissue but not detected in water, and (5) a requirement that toxicity be eliminated, as determined by one or more specified test endpoints. Combinations of these types of RGOs may be used. For example, to confirm that apparent effects are due to contamination, risk managers may require that areas to be remediated show some level of toxicity and have some minimum level of a chemical that is the primary contaminant of concern. In the Netherlands, site-specific risk assessment is not used to determine whether or not remediation is required at a particular site. Under the Dutch Soil Protection Act, remediation is required if “serious soil contamination” is present, i.e., if risk- based intervention values are exceeded (Swartjes, 1997). The intervention value incorporates both human health and ecotoxicological screening criteria; the ecotox- icological criterion is the hazardous concentration 50 (HC50), the concentration at which 50% of species is assumed to be protected. Risk assessment is used to determine the priority for remediation of sites where concentrations exceed the intervention values. The prioritization of sites may be based on the factors above, such as the diversity of ecological receptors, soil char- acteristics, and results of ambient media toxicity tests. The CCME (1996a) provides five alternative procedures for developing RGOs (termed site-specific objectives ). They are: • Adopt a PRG (termed generic guideline) directly as site-specific objec- tive . This alternative provides a conservative level of protection for eco- logical and human receptors under known land uses. • Modify a PRG. PRGs may be modified if receptors or other site conditions are somewhat different from the assumptions used in deriving the PRG (see Section 8.1.4). For example, if specific toxicity data used to derive the generic guidelines are not relevant to the site, PRGs may be recalcu- lated without them. Data used in the calculation of PRGs may be elimi- nated for ecological receptors not present at the site, but the adjusted data set must retain values for plants, vertebrates, and invertebrates from fam- ilies that are or could be represented at the site. Also, properties of the medium such as organic carbon content may be used to modify the PRGs. • Develop RGO using risk assessment. If the site is considerably different from the assumptions used in the derivation of PRGs, risk assessment is recommended. More specifically, risk assessment is recommended: (a) if critical habitats are on or near the site; (b) if a large degree of uncertainty is associated with the fate and transport of contaminants (e.g., periodic flooding); (c) if sensitive populations or endangered species are present; (d) if a large degree of uncertainty is associated with the fate or toxicity of contaminant mixtures or metabolites; or (e) if multiple sources of contamination or exposure pathways exist and were not considered in the derivation of PRGs. © 2000 by CRC Press LLC [...]... above focuses on the most contaminated sites, with the goal of reducing population-level risk to an acceptable level As a consequence, some sites, where point estimates of exposure indicate risks may be present, will not be recommended for remediation While these sites may present a hazard to individuals that use these sites extensively, their remediation is not required to prevent risks at the population... development and use of ecological PRGs, specifically It is advisable to develop remedial goals for ecological endpoints separately from those for human health, and to compare the two when all site-specific considerations (such as those discussed in Section 8. 1.3) have been factored into the final ecological remedial goals (e.g., CCME, 1996b) The ecological risk assessor’s role is to provide the risk manager with... 1992) 8. 3 SPATIAL CONSIDERATIONS Soil concentrations that constitute remedial goals are usually applied on a point-bypoint basis, rather than averaged over the area of exposure (Bowers et al., 1996) Although this criticism of risk assessment by Bowers and colleagues is based on human health risk, the same statistical argument applies to wildlife assessment endpoints Remedial action objectives for wide-ranging... the remediated set until an acceptable level of risk has been achieved Exposures at sites recommended for remediation are set to background © 2000 by CRC Press LLC because inorganic contaminants that present risks are frequently present at low concentrations in uncontaminated background soils which are likely to be used to remediate the contaminated sites (For organic contaminants, such as PCBs, exposure... residual risk can be determined through the use of GIS Thus, if only chemical concentrations in soil were available as the line of evidence, the RGO could be defined in terms of risk reduction An approach for determining remedial goals in a spatial context is presented by Sample (1996) At large contaminated sites such as the Oak Ridge Reservation and component watersheds, population-level risks to wildlife... policy differences, others are based on real differences in bioavailability and toxicity among sites Zhang (1992) shows that the final remedial goals for arsenic and pentachlorophenol in soils in Records of Decisions for U.S Superfund sites have ranged from 1.1 to 300 mg/kg for arsenic and 0.0012 to 83 ,000 mg/kg for pentachlorophenol These goals have typically been based on human health exposure scenarios... is adequate To determine which sites should be remediated to reduce the estimated population-level risk to an acceptable level (e.g., proportion of population experiencing exposures greater than the LOAEL is . play a different role in ecological risk assessments than they do in human health risk assessments. For human health risk assessments, remediation depends on the land-use scenario because land. PRGs based on ecological risk or even what level of protection is analogous to the 10 -6 risk for human carcinogens. (See Section 9.2.3 for a discussion of balancing health and ecological risks.). developed for six species present on the Oak Ridge Reservation: short-tailed shrew, white-footed mouse, red fox, white-tailed deer, American woodcock, and red-tailed hawk. For each chemical, the PRG for