Bioavailability of Contaminants in Soils and Sediments Processes, Tools, and Applications (2003) / Chapter Skim
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2. Current Use of Bioavailability in the Management of Contaminated Soil and Sediment
2. Current Use of Bioavailability in the Management of Contaminated Soil and Sediment
Pages 52-118
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From page 52... ...
Risk management decisions for soils or sediments focus on identifying relevant pathways of exposure that pose a risk to human health or the environment and then developing appropriate remedial measures that could include treating or removing sources or cutting off pathways or both. Many of the exposure pathways discussed above are affected by the bioavailability processes shown in Figure 1-1.
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From page 53... ...
This information can be used to both refine a risk assessment calculation and help develop models of bioavailability processes that can be used at other sites. This chapter first describes human health risk assessment to illustrate how bioavailability processes are considered in that arena, followed by an overview of the use of bioavailability processes in ecological risk assessment.
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From page 54... ...
. Although bioavailability processes can be considered explicitly in both human health and ecological risk assessments, there are some important differences.
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From page 55... ...
, while ecological risk assessment frequently considers bioaccumulation of contaminants in animal tissues (bottom photo)
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From page 56... ...
Therefore, there are many exposure pathways and a larger number of bioavailability processes that may require simultaneous evaluation during ecological risk assessment as compared to human health risk assessment, where it is more feasible to evaluate one pathway at a time. A manifestation of this difference is that human health risk assessment often involves distinct exposure equations for the direct pathways of ingestion, dermal contact, and inhalation, within which a variable is included to account for absolute or relative bioavailability.
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From page 57... ...
Exposure variables: Numerical adjustments to account for Bioavailability processes related to entry of soil or sediment contaminants into the body are typically included among the exposure variables. This is the usual means by which Bioavailability adjustments" are made in human health risk calculations.
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From page 58... ...
Incidental Ingestion Incidental ingestion is often an important exposure route for contaminated soils in human health risk assessments. In its basic form, the intake equation for incidental ingestion of soils is: Intake = ~ Cs x IR x RAF )
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From page 59... ...
This simply means that the relative bioavailability is assumed to be 1.0. Under some circumstances, the oral toxicity value might be expressed as an internal dose.
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From page 60... ...
If this approach is used, the ABS term has a different meaning. Instead of representing the absolute bioavailability of the chemical through the skin, ABS is instead a relative bioavailability term, in this case quantifying the expected difference in absorption from the dermal route versus the absorption implicit in the toxicity value.
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From page 61... ...
Unlike ingestion, however, both the doses and the toxicity values are often expressed in terms of concentration in air, rather than an amount of chemical per unit body weight. For example, a toxicity value for non-cancer health effects by inhalation exposure may be simply a safe concentration limit for the chemical in air.
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From page 62... ...
Instead, the implicit assumption is that the relative bioavailability associated with environmental exposure is 100 percent that is, the pulmonary absorption of the chemical under environmental exposure conditions is equivalent to the pulmonary absorption that existed in the critical study used to derive the inhalation toxicity value. Leaching to Groundwater Leaching from soil to groundwater is another common pathway by which humans can be exposed to contaminants (see Figure 2-2~.
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From page 63... ...
Also, information on absorption implicit in the toxicity values used in the calculations is required for determining absolute bioavailability. Unfortunately, the extent of absorption of a chemical that occurred as part of a critical toxicity study is almost never measured.
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CURRENT USE IN RISK ASSESSMENT 65 continues
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From page 66... ...
These limitations are overcome to a large extent by conducting relative bioavailability studies at specific sites instead of attempting to determine absolute bioavailability. Nonetheless, the paucity of absorption data, and the expense and difficulty associated with doing site-specific studies of relative bioavailability (see Chapter 4)
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From page 67... ...
In this situation, an assumption of 100 percent relative bioavailability will underpredict the toxic potential of the exposure. As discussed above, there are many situations in which information on absolute bioavailability is needed.
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From page 68... ...
For dermal absorption of chemicals from soils, when chemical-specific data are not available, a default absolute bioavailability of 1 percent for organics and 0.1 percent for inorganics is recommended (EPA Region 4, 2000~. Table 2-1 lists default absolute and relative bioavailability values for dermal and oral routes, respectively, used by EPA and the states.
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From page 69... ...
1.0 [1] Volatile organic compounds 0.1 [5]
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From page 70... ...
bRAF equals the relative bioavailability of the compound (i.e., in soil vs. in the medium used in the toxicity study)
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From page 72... ...
. The dashed line represents the 60 percent relative bioavailability used to set the national default value for absolute bioavailability of lead in soil used by EPA.
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From page 73... ...
Table 2-2 considers where explicit bioavailability information has been typically used for four exposure pathways. To illustrate further how specific bioavailability processes are currently considered in ecological risk assessment and risk management, the following section focuses on direct contact of invertebrates with soils or sedi
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From page 74... ...
play an important role in this exposure pathway and are considered during ecological risk assessment in a variety of ways. One relatively simple technique has been to develop models that predict the partitioning of metals and organics between different phases of which there are many levels of detailand then incorporate these into exposure assessment.
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From page 75... ...
Estimates of the available fraction of a contaminant pool from the exposure assessment are used in ecological risk assessments directly by comparing the
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From page 76... ...
(2000) Journal of Human and Ecological Risk Assessment.
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From page 77... ...
For example, vertical redox gradients and sulfide were found to control concentrations of cadmium in lake pore water in a field setting in Quebec (Hare et al., 1994) and for cadmium, zinc, and nickel in an experimental setting (Lee et al., 2000a)
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From page 78... ...
These methods assume that organic compounds are associated with organic matter in soils and sediments, that pore water concentrations vary depending on the octanolwater partition coefficient for the compound and the amount of organic matter present, and that the pore water concentrations of these contaminants determine bioavailability to invertebrates. BSAF Values.
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From page 79... ...
As discussed later, BSAF values are also used as input to intake equations for wildlife exposure. Because BSAF values are dependent on the chemical-physical properties of both the organic compound and solid as well as on the lipid content of the organism, they are site- and species-specific (Lake et al., 1990~.
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From page 80... ...
The average TOC-normalized total PCB concentration in sediments was 1.7 mg PCB/g TOC with a range of more than 34-fold between the least and greatest values, resulting in a range of as much as 35-fold for BSAF values calculated in this manner. Although the BSAF method is empirical, it could be more mechanistically based (e.g., on fugacity theory see Clark et al., 1988; Mackay and Paterson, 1991; Ling et al., 1993)
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From page 81... ...
from one location to another (Neely and Mackay, 1982; Velleux and Endicott, 1994~. For example, for total PCBs in sediments, a global average BSAF value of 1.7 has been suggested for use in risk assessments for infaunal invertebrates where BSAF values have not been determined for a particular site (Landrum and Poore, 1988~.
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From page 82... ...
(1994) have estimated the amounts of soil and sediment ingested by various species, and these data are frequently used in ecological risk assessments for wildlife.
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From page 83... ...
The more sophisticated wildlife exposure models take into account the foraging behavior of individual animals in the population, food and habitat quality, and the spatial distribution of habitat and contamination (Hope, 2001~. Although it is recognized that wildlife may also be exposed via incidentally ingested soils or sediments (the second term in the equation)
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From page 84... ...
Other than this assumption, there are few if any default values related to bioavailability that are commonly used in ecological risk assessment unlike with human health risk assessment. Because of a lack of information, and because it is thought to be less significant than the food and soil ingestion pathways, dermal contact is rarely considered when estimating exposures of wildlife species, and therefore no default values for dermal absorption have been suggested.
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From page 85... ...
Bioavailability processes have also commonly been included in ecological risk assessments, although they have not been labeled as "bioavailability assessments or adjustments" per se. Nonetheless, there are certain pathways (e.g., sediments to invertebrates)
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From page 86... ...
CAlthough studies generally determine the relative bioavailability of lead, the absolute bioavailability of lead in soil is used in the IEUBK model. The default value in this model is 30 percent absolute bioavailability.
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From page 87... ...
C 33% 25% 10% 30% 6% <0.5% 29% 29% EPA Region 7 400 mg/kg 650 mg/kg 230 mg/kg 100 mg/kg 925 mg/kg 100 mg/kg 60 mg/kg 68 mg/kg 300-500 mg/kge 150 mg/kg No cleanup requiredf EPA Region 4 EPA Region 3 EPA Region 10 EPA Region 8 Oklahoma DEQ Michigan DEQ California EPA New Jersey DEP California EPA DISC RBA used; reduced area of EPA Region 5 remediation No cleanup levels calculatedh Massachusetts DEP No cleanup levels calculatedh Ohio EPA and EPA Region 5 dThere are two numbers for each because more than one soil was analyzed. Both values were used in the risk assessment modeling.
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From page 92... ...
Baird & McGuire, Holbrook, MA, and Oak Ridge National Laboratory, Oak Ridge, TN. Sediments to Lead A number of risk Chesapeake Bay, MD, and Waterfowl assessments have considered Couer d'Alene River Basin, the relative bioavailability of ID.
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From page 93... ...
Assuming that adequate information is obtainable, an explicit consideration of bioavailability processes should lead to more scientifically accurate and cost-effective remediation, with no greater actual risk to human health or ecological receptors than under the traditional generic approach to cleanup. The following discussion considers the current use of bioavailability as a concept in laws and regulation governing hazardous and solid waste cleanup, and it considers whether the law allows for the implicit assumptions currently made about bioavailability to become more explicit via site-specific risk assessment.
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From page 94... ...
For example, if EPA, a regional EPA office, or a state environmental agency issues a guidance document on the use of bioavailability in risk assessment, the risk assessor generally assumes that any departure from that guidance will be closely scrutinized and questioned. Alternatively, lack of clear authorization or guidance on bioavailability would lead the risk assessor to conclude the approach is not favored or even prohibited.
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From page 95... ...
CURRENT USE IN RISK ASSESSMENT 95 mental regulation. The only statutory reference is a brief mention of the bioavailability of restricted metals in the Clean Water Act (CWA)
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From page 96... ...
There is no agency-wide guidance on the data necessary to substantiate such an adjustment, however, leaving that critical determination to EPA regional offices, state environmental agencies, or the judgment of the risk assessors, risk assessment reviewers, remedial project managers, and risk managers to whom RAGS is addressed. The fact that the term "bioavailability" does not appear in the laws and regulations, but does appear in the informal guidance of regulatory comments and guidance documents necessarily leads to confusion and even conflict over the acceptability of the concept in risk assessment between regulators and risk assessors.
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From page 97... ...
The hazardous waste sites in the Superfund program are on the NPL. Over the last 15 years, EPA has produced numerous guidance documents both on risk assessment conducted under CERCLA and on remediation strategies for meeting cleanup goals.
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From page 98... ...
Brownfields are abandoned, idled, or under-used industrial and commercial sites where expansion or redevelopment is complicated by real or perceived environmental contamination (EPA Region 5, 1996~. The goal of state and EPA Brownfields programs is the restoration of Brownfields so they can once again be used as a fruitful resource.
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From page 99... ...
In addition, questions were directed toward human health risk assessment managers rather than ecological risk assessment managers. Table 2-8 summarizes responses to the questionnaire received from various EPA regional personnel.
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From page 100... ...
Region 8 Region 8 has no guidance other than the national default values for lead and the two national dermal values. However, the region has spearheaded many basic, site-specific studies of absolute and/or relative bioavailability primarily at large metal contaminated sites in the Rocky Mountain west.
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From page 101... ...
Hesitancy to explicitly consider bioavailability processes during site-specific risk assessments, especially for human health, may reflect agency concern with costs (which can be very large for an in vivo bioavailability study) , anxiety about public and community acceptance of the concept and the methods (see Chapter 5)
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From page 102... ...
states responded to the survey, although three major states California, Massachusetts, and Texas did not. In general, the state environmental agencies do not have guidelines currently in place for the use of site-specific absolute or relative bioavailability adjustments in human health risk assessments and rely nearly exclusively on EPA risk assessment (RAGS)
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From page 103... ...
In summary, although there is no legal recognition of the term "bioavailability" in soil remediation statutes, bioavailability processes can be encompassed by both the human health and ecological risk assessment paradigms used under CERCLA and RCRA. Several states and some EPA regions have specified default values (other than 100 percent)
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From page 104... ...
104 BIOAVAILABIL[l Y OF CONTAMINANTS IN SOILS AND SEDIMENTS biosolids, the bioavailable fraction rather than the total concentration of the compounds of concern formed the basis of the rule (Chancy et al., 1982; O'Conner et al., 1990~. The body of research used was more extensive for certain elements
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From page 105... ...
The new proposed molybdenum limit is based on a soilto-plant uptake coefficient derived from 29 field studies. Multiple exposure pathways, and hence bioavailability processes, were used to formulate the regulatory requirements.
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From page 106... ...
The risk assessment calculation included the following steps. First, relative metal uptake coefficients (similar to absolute bioavailability factors)
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From page 107... ...
Aside from some conceptual similarities, however, the different agencies' approaches for defining sediment quality and the links to bioavailability and Contaminated sediments at the Wingate Road Incinerator Superfund Site.
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From page 108... ...
incorporates bioaccumulation factors into the derivation of sediment quality criteria and values to protect human health and wildlife. EPA Approach EPA has formulated sediment quality criteria to be consistent with previously established standards for water quality using an approach referred to as equilibrium partitioning.
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NOAA Approach In contrast to the ESGs used by EPA, numerical sediment quality guidelines (SQGs) have been suggested by researchers at NOAA (Long et al., 1995, 1998~.
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From page 110... ...
* The methodology and approaches used for sediment quality criteria differ among the three agencies in fundamental ways, not the least in how certain bioavailability processes are assessed and taken into consideration.
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From page 111... ...
For example, EPA has focused on the absorption aspect of bioavailability (through the use of default values for dermal and oral relative bioavailability and BSAF values) while many of the other processes have been less explicitly examined.
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From page 112... ...
More formal recognition of "bioavailability" in state and federal regulatory contexts would eliminate at least some of the hesitancy and confusion on the part of risk assessors and managers. The lack of clear authorization or guidance on using bioavailability in site-specific risk assessments from EPA has generally led to the perception that the approach is not favored.
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From page 113... ...
1991. Technical basis for establishing sediment quality criteria for nonionic organic chemicals using equilibrium partitioning.
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From page 114... ...
1992a. Ecological risk assessment guidance for Superfund: process for designing and conducting ecological risk assessments.
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From page 115... ...
1992. Relative bioavailability of lead from mining waste soil in rats.
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1994. Toxicity of metal contaminated sediments from the Upper Clark Fork River, Montana, to aquatic invertebrates in laboratory exposures.
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1992. Assessment of methods for estimating ecological risk in the terrestrial component: a case study at the Baird & McGurie Superfund site in Holbrook, Massachusetts.
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From page 118... ...
1999. Advances in evaluating the oral bioavailability of inorganics in soil for use in human health risk assessment.
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