This article was downloaded by: [University of Delaware]On: 10 August 2014, At: 06:52Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Human and Ecological Risk Assessment: AnInternational JournalPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/bher20

An Approach for Incorporating Information on ChemicalAvailability in Soils into Risk Assessment and Risk-Based Decision Making, Prepared by: The New EnglandEnvironmentally Acceptable Endpoints WorkgroupCharles Menzie a , Anne Marie Burke b , Domenic Grasso c , Marion Harnois d , Brian Magee e

, Dave McDonald f , Cheryl Montgomery g , Amy Nichols h , Joseph Pignatello i , Barbara Pricej , Richard Price j , Jane Rose d , Jo Anne Shatkin k , Barth Smets c , James Smith l & SusanSvirsky ba Chairman, Menzie Cura &Associates, Chelmsford, MAb U.S. Environmental Protection Agency, Boston, MAc University of Connecticut, Storrs, CTd Massachusetts Department of Environmental Protection, Boston, MAe Ogden Environmental, Inc., Westford, MAf U.S. Environmental Protection Agency, Lexington, MAg C.R. Montgomery &Associates, Inc., Arlington, MAh University of Massachusetts, Amherst, MAi The Connecticut Agricultural Experiment Station, New Haven, CTj Applied Science and Analysis, Inc., Portland MEk Menzie Cura &Associates, Inc. Chelmsford, MAl Oak Creek, Inc. Gorham, MEPublished online: 03 Jun 2010.

To cite this article: Charles Menzie , Anne Marie Burke , Domenic Grasso , Marion Harnois , Brian Magee , Dave McDonald ,Cheryl Montgomery , Amy Nichols , Joseph Pignatello , Barbara Price , Richard Price , Jane Rose , Jo Anne Shatkin , BarthSmets , James Smith & Susan Svirsky (2000) An Approach for Incorporating Information on Chemical Availability in Soils intoRisk Assessment and Risk-Based Decision Making, Prepared by: The New England Environmentally Acceptable EndpointsWorkgroup, Human and Ecological Risk Assessment: An International Journal, 6:3, 479-510

To link to this article: http://dx.doi.org/10.1080/10807030091124581

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Human and Ecological Risk Assessment: Vol. 6, No. 3, pp. 479-510 (2000)

1080-7039/00/$.50© 2000 by ASP

An Approach for Incorporating Information onChemical Availability in Soils into Risk Assessmentand Risk-Based Decision Making*

Prepared by:

The New England Environmentally Acceptable Endpoints Workgroup

Charles Menzie1, Chairman, Menzie-Cura & Associates, Chelmsford, MA

Anne Marie Burke, U.S. Environmental Protection Agency, Boston, MA

Domenic Grasso, University of Connecticut, Storrs, CT

Marion Harnois, Massachusetts Department of Environmental Protection,Boston, MA

Brian Magee, Ogden Environmental, Inc., Westford, MA

Dave McDonald, U.S. Environmental Protection Agency, Lexington, MA

Cheryl Montgomery, C.R. Montgomery & Associates, Inc., Arlington, MA

Amy Nichols, University of Massachusetts, Amherst, MA

Joseph Pignatello, The Connecticut Agricultural Experiment Station, NewHaven, CT

Barbara Price, Applied Science and Analysis, Inc., Portland ME

Richard Price, Applied Science and Analysis, Inc. Portland, ME

1 Menzie-Cura & Associates, 1 Courthouse Lane, Suite 2, Chelmsford, MA 01824; Tel:(978) 970-2620; Fax: (978) 970-2791; e-mail: camenzie@menziecura.com

* The support of the various institutions and agencies made this project possible. The GasResearch Institute (GRI) also supported administrative activities.

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Jane Rose, Massachusetts Department of Environmental Protection, Boston, MA

Jo Anne Shatkin, Menzie-Cura & Associates, Inc. Chelmsford, MA

Barth Smets, University of Connecticut, Storrs, CT

James Smith, Oak Creek, Inc. Gorham, ME

Susan Svirsky, U.S. Environmental Protection Agency, Boston, MA

ABSTRACT

A regional workgroup comprised of individuals from regulatory agencies, uni-versities, and consulting companies was formed to develop an approach for incor-porating information on chemical availability in soils into risk assessment and risk-based decision making. The approach consists of the following decision frameworkfor including information on chemical availability: (1) Determine the usefulnessof incorporating information on bioavailability; (2) Identify information needsfrom a conceptual model of exposure for the site and from exposure pathwaysjudged critical to the assessment; (3) Identify soil factors that affect bioavailability;(4) Determine the type or form of information (measures and/or models) thatcan be used within the risk assessment and risk management process; (5) Selectmethods (measures and/or models) based on the “weight of evidence” or strengthof the bioavailability information they will provide and how that information willbe used for risk assessment and risk-based decision making; (6) Incorporateinformation into the risk assessment and risk-based decision making. These fac-tors can be integrated into existing risk-based approaches for site managementsuch as Superfund, state approaches, and the ASTM Risk Based Corrective ActionProcess (RBCA). Consistent with risk assessment guidance, an assessment ofchemical availability in soils must consider current as well as reasonably foresee-able conditions. The approach recognizes that information on chemical availabil-ity is contextual and depends on the receptor and pathway. Further, the value ofinformation depends on how well it is accepted and/or validated for use inregulatory decision making. The workgroup identified four principles for select-ing methods (measures and/or models) for obtaining information on chemicalavailability and for evaluating information on chemical availability for use in riskassessments: (1) soil-chemical relevance, (2) pathway relevance, (3) receptorrelevance, and (4) acceptance of the method.

Key Words: soil, bioavailability, human health, ecological risk, organics, metals.

INTRODUCTION

A regional workgroup — known as the New England Environmentally Acceptable/Protective Endpoints Group — was convened in 1996 to identify technical issues relatedto how to incorporate information on chemical availability in soils into risk assessmentsand risk-based decision making. The workgroup is comprised of individuals from U.S. EPA

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New England, the Massachusetts Department of Environmental Protection, local univer-sities, and consulting companies. The workgroup included toxicologists, soil scientists,ecologists, biologists, and both human health and environmental risk assessors.

A growing body of information indicates that the availability of chemicals insoil matrices is often reduced relative to the chemicals’ availability in othermedia (Ruby et al., 1992, 1993, 1996; U.S. DOE, 1994; Smith et al., 1996; Mageeet al., 1996, Linz and Nakles, 1996). Accounting for chemical availability insoils can lead to more accurate estimates of exposure and guide the selectionof appropriate and protective remedial measures. For example, it may bepossible to reduce exposure by reducing the fraction of chemical that isavailable in the soil (Alexander, 1996; Loehr and Webster, 1996). More accu-rate information on exposure can lead to reduced physical disturbances at asite because remedies can be focused on smaller areas. Consequently, associ-ated health and ecological effects that stem from site disturbance would besmaller.

Contaminant availability in soils is recognized as having a role in the assessmentof exposure. Yet, there is little guidance concerning how to include this informa-tion in risk assessments and risk-based decision making for contaminated sites(Menzie et al., 1996). In this paper we identify an approach for utilizing informa-tion on chemical availability in assessing exposure and risks. We also describe howto integrate the information into existing risk-based approaches for site manage-ment.

STEPS TO INCORPORATING BIOAVAILABILITY

Six steps for incorporating bioavailability information into existing risk assess-ment and risk-based decision making procedures include:

1. Determine the usefulness of incorporating information on bioavailability;

2. Identify information needs from a conceptual model of exposure for the siteand from exposure pathways judged critical to the assessment;

3. Identify soil factors that affect bioavailability;

4. Determine the type or form of information (measures and/or models) thatcan be used within the risk assessment and risk management process;

5. Select methods (measures and/or models) based on the “weight of evidence”or strength that the information will provide as well as on how that informa-tion will be used for risk-based decision making;

6. Incorporate information into the risk assessment and risk-based decisionmaking.

These steps can be integrated into tiered (or iterative) assessment strategies andtailored to specific federal or state risk assessment frameworks such as those used forSuperfund (USEPA, 1997), many states, and the ASTM Risk Based Corrective Action

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process (ASTM, 1998). Within the generic risk assessment framework, informationon chemical availability can be included as part of the exposure or effects assessment(Figure 1). The relationship among the six factors is illustrated in Figure 2. Depend-ing on the particular existing risk-based approach (e.g., Superfund, particular states,or ASTM RBCA), the six factors can be integrated into various stages of theassessment.

Figure 1. Generic framework for risk assessment.

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Figure 2. Framework for including information on chemical availability in soils.

ASSESSING THE USEFULNESS OF INCORPORATING BIOAVAILABILITYINFORMATION

When judging the value of including information on bioavailability of chemicals insoils, an assessor or manager should ask: (1) Do the governing site-management regula-tions or policies allow for risk-based approaches that can incorporate bioavailabilityinformation? (2) Within these risk-based approaches, when is it appropriate to considerinformation on bioavailability? (3) Will the information be valuable for decision making?

Do the Governing Site-Management Regulations or Policies Allow for Risk-BasedApproaches That Can Incorporate Bioavailability Information?

In the United States, approaches for evaluating contaminants in soils vary amongregulatory programs at the state and federal levels. Some rely upon specific soiltarget levels or waste-specific clean-up procedures as a default approach to thedetermination of site-specific cleanup levels. In such cases, it may not be possible,appropriate, or cost effective to consider information on the site-specific availability

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of the chemicals. In other cases, information on chemical bioavailability must beconsidered in setting risk-based cleanup levels. Other approaches may stipulatewhat information should be obtained for assessing availability. For example, statesand the U.S. Environmental Protection Agency (USEPA) frequently identify theToxicity Characteristic Leaching Procedure (TCLP) and the Synthetic PrecipitationLeaching Procedure (SPLP, USEPA, 1994) as methods for evaluating the leachabil-ity of chemicals in soils.

Some approaches identify bioavailability as a component of a risk-based approach,but do not stipulate how such information should be obtained. For example, theASTM Standard Guide for Risk-Based Corrective Action (ASTM, 1998) provides adefinition for bioavailability — a measure of the fraction of the chemical(s) of concern inenvironmental media that is accessible to an organism for absorption — and indicates thatsuch information may be utilized to assess risks within a Tier 3 evaluation. Additionalinsight into the manner in which soils may affect the bioavailability of chemicals isgiven in Appendix 2 of the ASTM RBCA standard. However, the standard does notidentify specific methods by which bioavailability should be determined or integratedinto the assessment. In general, many state and federal regulatory programs arebound by law or policy to consider the best available science and technology, includ-ing bioavailability, for setting cleanup levels at hazardous waste sites.

Within These Risk-Based Approaches, When Is It Appropriate to ConsiderInformation on Bioavailability?

Many risk-based approaches are tiered, beginning with simple but conservativeassessments and proceeding, as needed, to more site-specific assessments. Thedevelopment and use of soil screening levels (SSLs and RBSLs) by U.S. EPA, DOE,the states, and as described in the ASTM RBCA standards are all examples of simple,conservative approaches that employ generic information on exposure and effects.To the degree that “default” values for bioavailability exist, these may be used todevelop SSLs and RBSLs used for screening-level risk assessments. Uses of defaultvalues for bioavailability are currently agency specific. Some examples are providedin Table 1.

Site-specific information on bioavailability is most likely to be useful in later tiersof risk-based approaches when, for example, soil concentrations exceed SSLs orRBSLs, and there is reason to believe that chemical availability is reduced. Menzieet al. (1996) proposed a tiered approach within which site-specific information onbioavailability is considered in later tiers (Figure 3).

Will the Information be Valuable for Decision Making?

Considering the bioavailability of a chemical within a risk assessment will enablethe analyst to provide better estimates of exposure. However, gathering informationon bioavailability and incorporating it into a risk-based approach that is acceptableto regulatory agencies can be a resource- and time-intensive process. Therefore, itis useful to consider the added value that this information will have for decisionmaking regarding site soils. This judgement requires experience and a good under-standing of options for site management. Discussions among risk assessors, sitemanagers, and regulatory agencies are especially important for making decisions

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Table 1. Examples of default values used to adujust exposures to account forreduced contaminant availability.

about whether and how to include bioavailability information. Ideally, all partici-pants in the process (including stakeholders) should have a common understand-ing concerning the value, application, and use of chemical availability approachesand information early in the process of site evaluation.

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Figure 3. Conceptual decision-making flowchart for testing.

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The following questions can help the manager and assessor judge the value ofincluding bioavailability information:

1. Is the chemical in a form that is considered — for all practical purposes — tobe physically and chemically unavailable as supported by a regulatory determi-nation, previous studies, or simple tests accepted by the regulatory agency?This question presumes that there are some materials that can be classified asbiologically unavailable. For example, the U.S. EPA (1996) has not derived achronic oral RfD for elemental mercury since it is “only slowly absorbed by thegastrointestinal tract (~0.01 %), and because of this is thought to be of notoxicological consequence” when ingested. In these cases, the analyst woulddocument that the chemicals are associated with these materials and that thereis little or no exposure. If this can be established early in the process, there maybe no need to evaluate exposure further.

2. Are risks associated with site soils being driven by chemicals for which theacquisition of bioavailability information is being considered? If the answer tothis question is no, then there may be limited value to gathering such infor-mation.

3. Are other factors associated with the presence of the chemicals (e.g., thepresence of a free product or visibly stained soils) likely to determine sitemanagement options? Regulatory agencies rely upon various criteria for man-aging contaminated soils including the presence and thickness of free prod-uct, presence of hot spots, and aesthetic criteria. In situations where these willdrive clean-up decisions, the value of refining risk estimates to includebioavailability information for incorporation into remedial decisions may belimited. In such cases, refined estimates of risk may still be helpful in theselection of appropriate remedial technologies and in evaluating conditionsfollowing remediation.

4. Will incorporating bioavailability information change the risk estimate suffi-ciently to affect how the site will be managed? The value of bioavailabilityinformation at any particular site may be assessed by weighing the likelihoodthat changes in risk estimates will affect the areas or volumes of soil that mayneed to be removed, contained, or treated. The value of the bioavailabilityinformation can be judged by comparing the costs associated with obtainingand incorporating the information (including achieving regulatory accep-tance) to the corresponding reduction in site remedial costs.

USING A CONCEPTUAL SITE MODEL

Conceptual site models are important starting points for both human and eco-logical risk assessments (USEPA, 1989; USEPA, 1997; ASTM, 1998). The purpose ofa conceptual model is to illustrate or define the relationships among sources ofcontamination, potential pathways of exposure, and potential receptors by usingpicture diagrams, flow charts, narratives, or tables. Because reduced bioavailability

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of chemicals in soils affects exposure, a conceptual model of exposure pathwaysprovides a useful framework for determining where information on bioavailabilitymay be important in estimating exposure. Examples of conceptual models forhuman and ecological receptors are shown in Figures 4 and 5.

Because exposure to chemicals in soils is pathway and receptor dependent,conceptual models can help the risk assessor identify the types of measurements ormodels that may be needed for quantifying bioavailability. Tests or exposure modelsthat may be appropriate for a certain pathway and receptor may not be applicableto another. The conceptual model helps insure that the acquisition of bioavailabilityinformation is appropriately matched with exposures.

Identifying Potential Receptors

The availability of chemicals in soils is considered with respect to three categoriesof receptors: ground water, ecological receptors (plants and animals), and humans.We considered ground water as a receptor because contamination of this mediumis often the basis of decision making at sites. The importance of these receptors willvary from site to site depending upon local conditions, land use, and proximity toecological habitats. Receptors are typically selected as part of the specific regulatoryprocess and the analyst should be aware of guidance governing selection of recep-tors. The principal pathways for receptors are summarized in Table 2.

It may be useful to consider the relative importance of major exposure pathwaysto receptors. An example of a qualitative scheme for considering the relativeimportance of pathways is provided in Table 3.

Table 2. Major exposure pathways by which receptors could be exposed tocontaminants in soils.

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IDENTIFYING SOIL FACTORS THAT AFFECT BIOAVAILABILITY

The availability of chemicals depends on specific physical and geochemicalbinding mechanisms. These vary among chemicals and are controlled by a numberof soil factors. The strength of these binding mechanisms can be affected by physicaland chemical alterations. For this reason, it is important for the analyst to under-stand the role these factors have under current and potential future conditions. Werecommend that the major factors be identified early in the evaluation and quali-tatively assessed for how they may affect bioavailability. This will help the analystidentify appropriate measures or models of bioavailability and evaluate howbioavailability may change in the future.

Chemicals interact with soil primarily through physical interactions of adsorption(interactions with the surface of particulate material in soils), absorption (pen-etrates through the surface and becomes encased or sequestered in the material),and through chemical reactions (sometimes called chemisorption). In order for achemical to be bioavailable, it generally must be freed from its interaction with thesoil.

The chemical and physical nature of soil can be modified upon contact withbiological membranes. For example, incidental or intentional ingestion of soils canchange its physical structure, pH, and expose organic soil matter to a variety ofdigestive enzymes and biological membranes (Menzie et al., 1996). Such physicaland chemical processing of the soil can alter chemical availability within the soilmatrix (Ruby et al., 1992, 1993, 1996). Knowledge of these processes leads to agreater understanding of the overall chemical availability from soil.

Chemical availability in soils can also change over time. For example, the avail-ability of many organic chemicals in soil declines over months or years (Alexander,1996; Loehr and Webster, 1996). This aging or weathering process may involvechanges in the physical relationship between the chemical and soil particle includ-ing the movement of chemicals into soil micropores (Alexander, 1996). Changes insoil properties could also result in an increased availability of chemicals. For ex-ample, as soils become increasingly acidified, metal availability could increase.

Soil Factors Affecting the Availability of Organic Chemicals

Substantial research is underway to identify, characterize, and quantify factorsthat affect the availability of organic chemicals in soils. Table 4 summarizes howvarious soil factors could affect availability under equilibrium and kinetic condi-tions. Behavior of chemicals at equilibrium provides insight into the role of differentfactors over long time frames and therefore is useful for exposure situations wheretime is sufficient for equilibrium conditions to develop. Many exposure situations,however, are dominated by discrete events often of short duration (e.g., dermalcontact, incidental ingestion of soil). Under these circumstances, the kinetics ofchemical release from soils (i.e., the amount released per unit time) into anothermedium is most important because time is often not sufficient for these processesto approach equilibrium. As indicated in Table 4, relationships between soil prop-erties and availability are not always straightforward and in some cases are poorlyunderstood. Therefore, caution should be exercised when generalizing phenom-ena.

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Table 4. Effects of various soil parameters on bioavailabilty of hydrophobicorganic compounds.

The bioavailability of organic compounds in soil is attenuated by their sorptionto soil particles. Sorption encompasses any process that leads to association of thecompound with the solid phase. It includes adsorption on surfaces, absorption bysoil organic matter (SOM), precipitation in solid form, condensation in liquid form,and dissolution in other condensed phases such as non-aqueous phase liquids(NAPLs; e.g., solvents and tars). The driving force for sorption is the combinedeffect of weak intermolecular (non-covalent) forces between contaminant mol-ecules and particle surfaces or NAPL and hydrophobic expulsion from water due topoor solubility. When sorption occurs from the vapor phase the magnitude dependson the compound’s volatility, as reflected by its vapor pressure.

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The sorption equilibrium may be affected by the following soil properties: SOMcontent, quality of SOM, surface area, porosity and pore size distribution, presenceof expanding smectite clays, and presence of other substances such as NAPLs. Insurficial soils containing negligible levels of NAPLs, the SOM content measured assoil organic carbon is often the most important soil property governing the equilib-rium distribution of contaminants. The equilibrium distribution coefficient refer-enced to soil organic carbon content (Koc) may be obtained experimentally orestimated from linear free-energy relationships based on octanol-water partitioncoefficient (Kow) or water solubility (Schwarzenbach et al. 1993). There is alsogrowing evidence that sorption depends on the quality of SOM. The properties ofSOM that may influence sorption include biomass source, degree of humification,polarity, aromatic content, density, nanoporosity, and geologic age. Furthermore,particles of ‘soot’ carbon resulting from incomplete combustion through natural oranthropogenic processes may be widespread in the environment. Such particles mayhave a higher affinity for hydrophobic organic compounds than ordinary SOM.

Attainment of sorption equilibrium is generally limited by molecular diffusion ofmolecules through intraparticle fixed pores and/or through SOM or NAPL phases(Xing and Pignatello, 1996; Luthy et al., 1997). Diffusion through pores is hinderedby the tortuousity of the pore network, adsorption on pore walls, and by sterichindrance. Diffusion in SOM is thought to occur as though it were taking place ina three-dimensional liquid-like phase rather than an extended pore network. Thus,diffusion of molecules is hindered by the extreme viscosity of SOM. Recent evidencesuggests that some forms of SOM may contain fixed internal sub-nanometer sizepores (Xing and Pignatello, 1997). Moreover, ancient organic matter and sootparticles may contain extended micropore networks.

Bioavailability of an organic chemical in soil depends on mass transfer of con-taminant molecules from the external surface of the particle to the organism andthe ability of the organism to absorb them. The issue of whether bacteria cells arecapable of extracting molecules directly from the sorbed state has not been conclu-sively settled. However, this capability is irrelevant since the vast majority of sorbedmolecules at any moment are located remotely to cells. This is because SOM, theprincipal sorbent material, is impenetrable by cells and because the preponderanceof particle surface area exists in mesopores and micropores (<50 nm), which are toosmall to fit even the smallest bacterial cells. Hence, bioavailability is dependent onthe rates of molecular diffusion through intraparticle pores or SOM to the externalsurfaces which are in contact with the organism.

At equilibrium, the sorption of neutral organic chemicals is proportional to thefraction of organic carbon and aqueous solubility of the organic chemical. Thisrelationship provides the best estimate of sorption for neutral water insoluble(hydrophobic) chemicals. Chemical sorption is often estimated by using either anorganic carbon (Koc) or octanol water (Kow) partition coefficient. This empiricalapproach ignores the contributions of factors such as the ionic capacity of the soil,the structure of pores in the soil, clay content, and others, and is often usedinappropriately. For example, partition coefficients not useful for evaluating chemi-cals with appreciable water solubility, such as TCE, or for chemicals that are ex-tremely insoluble in water (i.e., log(Kow) > 5.5), such as highly chlorinated biphenylsand dioxins.

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Chemical Availabilty in Soils

Soil Properties Affecting the Availability of Metals

The interaction of metals with soils is more complex than organic chemicals withsoils. Metals are a major component of soil and are often bound in mineral struc-tures. Dissolved or free metals can interact with those associated with mineralstructures, both on the surface and within the structures. Metals can occur in soilsin different forms that can vary across a site as a result of historical release, thepassage of time, and chemical transformations in soil. Recent studies have demon-strated that the form, or speciation, of a metal in soil controls its environmentalmobility or solubility, oral bioavailability, toxicity, and response to specific remedialactions. Therefore, understanding the form of a metal in soil is essential in evalu-ating its relative availability, exposure, toxicity, and risk. Because of the extraordi-nary variability in metal-soil interactions, some generalities and a few examples arepresented that highlight important features related to understanding metalbioavailability from soils.

Both soil and metal characteristics are important. Soil properties can often bemeasured from site-specific samples in the field and laboratory. Many of the impor-tant chemical properties of the metals have been measured in the laboratory andthese values can be used in determining important interactions.

Important Soil Properties for Metals

Soil organic matter, clay minerals, metal oxides, particle size and surface area allstrongly influence the bioavailability of metals. Kabata-Pendias (1992) describes theinfluence on metals of the three main components of soils organic matter, clayminerals, and oxides (Table 5).

Organic matter influences the availability of metals in three major ways:

1. By offering a nonionic surface on which hydrophobic chemicals can absorb.This can affect metal bioavailability if the metals react with organic com-pounds that are relatively insoluble in water and are poorly absorbed in thegut.

2. By reacting with some metals to form water-soluble chelates that decrease orincrease metal availability. In this regard, microbial processes responsible forconverting inorganic metals into organo-metalic complexes, such as methylmercury, are also important.

3. By changing the reduction and oxidation, or redox potential, (i.e., soil EH) andthe microbial environment of soil, usually by increasing its reducing potential

Table 5. Degrees to which soil characteristics affect binding of selected metals.

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and possibly changing the oxidation state of the metal. This latter effect is themost important because the chemical behavior of metals is dependent on theiroxidation state.

Iron, manganese, and aluminum form hydrous oxides (and colloids) in soils and arethe predominant metals in most rock minerals and soils. The presence of these formsinfluences the behavior and bioavailability of other metals present in soils. Because theseoxides form colloids, and colloids govern cation exchange and ion adsorption, the watercontent of the soils can dramatically affect the availability of some of the metals. Metalsthat are sorbed to colloids on the soil surface are often more stable and therefore lessavailable because the soil dries out and the oxides crystallize.

The cation exchange capacity (CEC) of soils ranges from 1 to 100 meq/g of soiland is a measure of the ability of a soil to adsorb and release cations in soils. Soilsthat have high CECs (most soil CECs are about 30 meq/g) also have high surfaceareas and high adsorption capacities. Heavy metals will often replace sodium,potassium, and magnesium ions in high CEC soils and, consequently, are often lessbioavailable.

Mineral size and shape determines surface area and thus the chemical andphysical extent of surface interactions with metals. The CEC of the soil reflects someof these surface properties. The clays with higher CECs bind metals more tightly andthe CECs for the five common clay minerals decrease in the following order:smectites (montmorillites)> vermiculite > illite, chlorite > kaolinite > halloysite. Allthe mineral clays have relatively high CECs and react with soil organic compounds.Thus, metals in a soil that is high in both organic matter and clay are less bioavailablethan in a soil with little organic matter and high in sand.

Minerals, especially those of carbonates and phosphates, may bind metals.Once incorporated, these metals may be affected by any of the processes thataffect carbonate and phosphate salt stability, including soil and water pH. How-ever, carbonate- and phosphate-metal complexes have varying degrees of solubil-ity and reactivity depending on the metal, its oxidation state, and the ligand towhich it is bound. For example, lead phosphate is practically insoluble in water atnormal soil and water pH, remaining so until a high acid pH is reached, acondition that is unlikely to occur in the environment under normal conditions.

Important Metal Properties

Some of the important features that are predictive of metal availability includeoxidation state and mineral form, redox potentials, kinetics of redox reactions,coordination by organic chemicals (including humic portions of soils), transforma-tion by microbial action, and solubility of metal compounds. Generally, there isgood information regarding the solubility of metals and their salts under laboratoryconditions. However, there is little information regarding their solubility in differ-ent types of soils. Many features of common soils may augment or hinder metalavailability. Therefore site-specific study is often required to estimate the availabilityof metals in soils.

The availability of metal from soil is most easily estimated for metals with only oneoxidation state (e.g., magnesium). In those cases, the metal will occur either in ametallic or un-oxidized state (very uncommon) or the oxidized state (very com-

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mon). The most important properties of metals affecting their availability forabsorption is the oxidation state, which mineral forms are present, and what inter-actions govern their solubility in soil. Three metals — chromium, mercury, andarsenic — are used as used as examples to illustrate the importance of different soilor metal characteristics on the metals’ bioavailability.

Example for Chromium

The availability of chromium for absorption from soil illustrates the importance ofoxidation state, mineral form, solubility, and rate of formation. Chromium existsprimarily in two different oxidation states, Cr (III) and Cr (VI). Cr (III) is thedominant chromium oxidation state found in soil. Much of the Cr (III) may bepresent as chromic oxide (Cr2O3), a very insoluble form of chromium. Very little Cr(VI) is found naturally in the environment. However, some soil conditions may existthat favor Cr (VI). The kinetics of oxidizing Cr (III) to Cr (VI) are very slow, in partlimited by the solubility of the Cr2O3 and the availability of manganese dioxide(MnO2) and oxygen. Thus, very little Cr (III) in the environment is converted to Cr(VI). The dominant chromium species present in soil is controlled by pH. Undernormal aerobic conditions (pH between 5 and 10) chromic oxide is the dominantform of chromium present. In contrast, at hazardous waste sites with extremely acidsoils (pH <3) a measurable fraction of the chromium present may be Cr (VI). Cr (VI)can form strong complexes with both organic and inorganic compounds and can bepresent as a cation (e.g., Cr (NH3)6

3+) or an anion (e.g., CrO42-), but is predominantly

found linked to oxygen as either the chromate (Cr2O42-) or the dichromate (Cr2O7

2-

). Recently, Cr(VI) has been shown to bind with soil organic matter, adsorb onto ironoxide coatings, and to form an insoluble precipitate with iron (Olazabal, 1997).

Example for Mercury

The chemical form of mercury in a soil will control its mobility in the soil, itsbioavailability when ingested, and its response to specific remedial actions. Mercuryusually exists as an inorganic species in soil, either as Hg (I) or Hg (II). Under somereducing conditions, mercury may be reduced to methylmercury by microbialaction. This can occur in the sediments of eutrophic lakes and ponds and in soilscontaining methylating bacteria. Organic forms of mercury are nearly completelyabsorbed through the gastrointestinal system and are also available to an organismthrough dermal contact and inhalation exposure pathways. Mercury may occur insoils as liquid elemental mercury, as organic mercury compounds, mercuric chlo-ride, or one of several different mineral species, including mercury oxides, carbon-ates, and sulfides. In general, organic mercury and mercuric chloride are verysoluble and bioavailable, mercury oxides and carbonates are less soluble, andelemental mercury and mercury sulfides are generally insoluble and not availablefor absorption (USEPA, 1996a). Liquid elemental mercury is highly volatile and isnearly completely absorbed when inhaled, but poorly soluble if ingested.

Example for Arsenic

Arsenic exists in four different oxidation states (As3-, As0, As3+, and As5+) and asinorganic and organic compounds. The predominant forms of arsenic in the

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environment are the arsenates (AsO43-) and arsenites (AsO2

5-). Arsenic availabilityfrom soil is governed by its solubility in water, oxidation/reduction potential (EH),ligand exchange, and biotransformation. The pentavalent form of arsenic (As5+) isthe favored form of arsenic in water under conditions with high levels of dissolvedoxygen, basic pH, high oxidation/reduction potential, and reduced organic carboncontent. Methylated forms of arsenic predominate at warmer temperatures and insoils with biotic transformation, while at colder temperatures pentavalent arsenicpredominates. Arsenates readily sorb onto colloidal humic material under condi-tions of high organic carbon content, acidic pH, low phosphate, and low mineralcontent. Arsenates can also adsorb to or co-precipitate with hydrous iron oxides andform insoluble precipitates with calcium, sulfur, aluminum, and barium compounds(US DOI, 1988). These arsenic compounds are not generally considered availablefor absorption in the gastrointestinal tract. However, phosphate can displace arsenicfrom arsenic compounds making it more available for dissolution and absorption.Recently, arsenic has also been shown to form methylarsenites in the environment.Methyl arsines and arsenates are typically well absorbed through skin, lungs and thegastrointestinal tract, but are seldom either the predominate form of arsenic in theenvironment or are at levels so low as to be inconsequential to human and environ-mental health. There is very little absorption of arsenic through dermal or inhala-tion exposure pathways (ATSDR, 1993).

Changes in Soil and Chemical Conditions Influencing Bioavailability

At many sites, risk-based approaches consider potential “reasonably foreseeable”future as well as current risks. Therefore, factors that may affect the availability ofthe chemicals in soil in the future should be considered and addressed eitherqualitatively or quantitatively. Changes in soil conditions can affect both the kineticsand equilibrium of bioavailability. Conditions that may affect the future availabilityof chemicals in soils are described in Table 6.

TYPES OF BIOAVAILABILITY INFORMATION THAT CAN BE USED

Bioavailability information for risk assessment purposes tends to fall into thefollowing categories: (1) values or factors that are used in deterministic or equilib-rium exposure equations; (2) kinetic models that take into account the time ofexposure and other factors; and (3) direct measures of chemical concentrations orbiological responses to exposure. These are not mutually exclusive categories.

Equilibrium Models and Algebraic Exposure Equations

For evaluating leaching to ground water and for estimating exposure to ecologi-cal receptors and humans, equilibrium models and simple algebraic equations arecommonly used. These approaches are usually simpler to use than kinetic models.They may be used in lieu of or may be supplemented by measurement methods andgenerally involve the incorporation of factors to account for chemical availability.This is illustrated below with examples for ground water, ecological risk assessment,and human health risk assessment.

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Ground Water Example

The following example is taken from a review by Linz (1998). The expression forrelating the total chemical i in a soil sample with the concentration of the chemicalin the aqueous phase (pore water) for organic contaminants is given by USEPA(1996b) based on Feenstra et al. (1991) as:

where qiT = total chemical i in soil including pore spaces, mg/kg

Ciw = the concentration of i in the water, mg/L

Kip = foc * Ki

oc (fraction of organic carbon, chemical’s organic carbon partition co-efficient)Θw, Θa = water-filled and air-filled porosity, L/LHi = dimensionless Henry’s constantρb = soil bulk density, kg/L

Table 6. Factors that may affect the availability of soil contaminants over time.

q = Ci iT W K Hi w a

i

b

p + +

Θ Θρ

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This equation, used in the USEPA SSL and ASTM RBCA procedure, takes intoaccount the total concentration of chemical i measured in a field soil sample toestimate bioavailability to ground water as a receptor. It includes chemical, which isin a sorbed phase, dissolved in the water in the water-filled void space and in thevapor phase in the air-filled void spaces of the soil.

Ecological Example

Simple equations are commonly used to estimate non-human exposure to soilcontaminants. One example is the earthworm bioaccumulation model developed byMarkwell et al. (1989) and introduced by Menzie et al. (1992) for use in ecological riskassessment. The simple form of the model used for non-polar organic chemicals is

Bioaccumulation factor in worms = YL/(0.66 FOC)

Where: YL = lipid content of the worms, andFOC = fraction organic carbon content of the soil

Contaminant concentration in the worm is the concentration in soil times thecalculated bioaccumulation factor. This simple equilibrium model only considersone soil characteristic, organic carbon, to evaluate the availability of non-polarorganic chemicals, but it could be expanded further to consider other soil charac-teristics that may be found to be important in estimating bioaccumulation.

Human Health Example

Simple algebraic equations are commonly used to estimate exposures of peopleto soils. They are usually implemented in a deterministic fashion but may also beused in probabilistic risk assessments as well. An example equation is given below forexposure via incidental ingestion of soil:

Exp = CS x IR x F x D x RAFBW x AVG x 365 dys/yr x 106mg/kg

Where:Exp = average daily exposure dose for the ingestion pathway (mg/kg/dy)CS = contaminant concentration in soil (mg/kg)IR = soil ingestion rate (mg/dy)F = exposure frequency (dys/yr)D = exposure duration (yrs)RAF = relative absorption factorBW = body weight (kg)AVG = averaging period (yrs)

The relative absorption factor (RAF) is the variable used to incorporate informationon bioavailability. It is used to adjust the absorption of chemical from soil to that for theexposure medium used to derive the toxicity data that will be used in the risk assessment.When selecting an appropriate metric for the RAF, care must be taken to insure that theresultant exposure estimate is consistent with the dose-response toxicity informationthat will be used in assessing risks. Usually, the RAF is expressed as a simple ratio:

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RAF = absorption of the chemical form in soilabsorption of the chemical form in the study medium

The RAF may be an estimated or measured factor. The dose-response or toxicityinformation can be based on the administered dose, the absorbed dose, or theinternal dose, so it is critical to know which applies when incorporating informationon bioavailability. In most cases, the information is based on an administered dose inanimal studies. Usually, the chemical is given orally dissolved in drinking water oradded to the diet. Typically, these studies use the form of the chemical that is mostbioavailable from the test medium. This provides the greatest sensitivity for theobservation of toxic effects. For example: in studies of the toxicity of organic chemi-cals, the chemical is dissolved in oil and administered by gavage (forced feeding).Similarly, in studies of metal toxicity, a very soluble salt of the metal is administeredin food or water. In animals, as in humans, the administered dose of a compound isnot necessarily completely absorbed. Moreover, differences in absorption may existbetween laboratory animals and humans, as well as between different matrices androutes of exposure. A RAF should take these differences into consideration.

RAFs can be less than one or greater than one. If the absorption from soil is foundto be the same as absorption in the laboratory study where animals were administeredthe chemical in some other medium, then the RAF is 1.0. An RAF of 1.0 does notindicate that absorption is 100%, but that absorption is known or estimated to be thesame as that of the chemical used in the toxicology study from which the toxicitycriterion or dose-response value was derived.

Kinetic Models

Equilibrium models and simple relationships are satisfactory for representingexposure when conditions permit equilibrium relationships to develop. These toolsare generally well accepted for risk assessment purposes. However, because theavailability of chemicals in soils is typically related to the rate of release, kinetic modelsare useful for estimating exposure when the contact with soil is not continuous. Insuch cases, the exposure times may be much shorter than those needed to achieveequilibrium. Examples of the use of kinetic models in risk assessment are given below.

Ground Water Example

The following example is taken from Linz (1998) and takes into account slow andfast desorption phenomena for non-polar organic chemicals in soils. Taking intoaccount these phenomena can be important in cases in which the available fractionof chemical has been biodegraded (naturally or by active management), and theremainder of the chemicals are stabilized and only released very slowly. In thedynamic situation, the mass of chemical associated with the fast release fraction(qi

1 ) is assumed to be in equilibrium with the pore water at all times, but the massassociated with the slow release fraction (qi

2 ) is not.A differential material balance of a volume of soil, assuming linear first order

mass transfer rate coefficients for the sorbed to dissolved phase transfer, yields anexpression that relates the bulk pore water concentration to the total soil concen-

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tration, the infiltration rate of water, the biodegradation rate, the partition coeffi-cient, and the slow release rate of chemical from soil to pore water:

whereCi

w = the concentration of i in the pore water, mg/Lqi

T = total chemical i in soil including pore spaces (= qi1 + qi

2 ), mg/kgI = water infiltration rate, meter/yearH = thickness of soil bed, meterλ = Biodegradation rate constant in pore water, year-1

ρb = soil bulk density, kg/LΘw = water filled porosity, L/Lki

2 = first-order mass transfer constants for the slow compartment, (year)-1

KiT = Partition coefficient for chemical i, L/kg

Note that if the slow release rate ki2 is relatively large (fast) compared with the

infiltration rate and/or the biodegradation rate, then this equation reduces to thefamiliar equilibrium expression Ki

T = qiT / Ci

w . However, when the slow release rate isslow enough, then the pore water is diluted by infiltration water and/or the biodegra-dation in the pore water reduces the bulk pore water concentration. This can besignificant in defining soil concentration limits based on transport to groundwaterbecause soil concentrations greater than the limits calculated by the equilibrium expres-sion can still lead to acceptable groundwater concentrations, provided the release ratefrom the soil is low enough.

Ecological Example

A kinetic model that was developed to evaluate the accumulation of polynucleararomatic hydrocarbons in an amphipod (Landrum, 1989) is described below. Themodel could be applied similarly for soils.

dCa/dt = (ks*Cs*e-rt) / (ke*Ca)Where:Ca = concentration in the animal (nmol/g wet weight)ks = conditional uptake clearance rate (g dry weight sediment cleared of chemical/

g wet weight tissue/d)Cs = initial sediment concentration (nmol/g dry weight)r = is the rate constant (d-1) for the reduction in the bioavailable fraction of

chemical in sedimentke = is the rate constant for the elimination rate (d-1)t = time (d).

Human Health Example

Examples of kinetic models that have been developed to evaluate exposure tohumans include the skin uptake model (Shatkin et al., submitted) and physiologi-cally based pharmacokinetic (PBPK) models.

CiW q

I H

kKT

i w

bi T

i= + +

−

/ λρ

Θ2

1

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To predict the effect of various system parameters, related either to exposure or soilcharacteristics, on dermal absorption, the concept of fugacity can be applied todetermine the amount of contaminant in a soil matrix that would be absorbed by theskin (McKone, 1990). Shatkin et al. (submitted) developed a modified version of theMcKone and Howd (1992) fugacity model for bioaccumulation into the skin. Themodified model for uptake of organic chemicals from soil into the skin takes intoaccount time of exposure, soil layer thickness on skin, soil organic carbon character-istics, soil moisture content and chemical-specific kinetic rate constants. To incorpo-rate the two-phase (fast and slow) kinetics into the fugacity model, the uptake fractionis modified to account for a soil with both slow and fast desorbing sites.

Other kinetic approaches include physiologically based pharmacokinetic (PBPK) models.PBPK models have been used to estimate the oral and dermal absorption of PAHs,dioxins, and other persistent chemicals in humans. Typically, direct and/or indirectmeasures of chemical levels in tissues and excreta are used as input into the PBPK model.Rate constants and assumptions relating to the mass balance of the chemical in theorganism are used to determine a kinetic rate of absorption for the different tissues underconsideration (i.e., stomach, skin, and lung). Typically, PBPK models must be validatedthrough experimental evaluation of the different kinetic parameters in several test species.

Direct Measures

Measurements of chemical availability have also been used within risk-basedapproaches. These are often incorporated into the classes of equations and modelsdescribed above. Examples of methods that have been developed and accepted orwhich are in the developmental stage are given in Table 7.

There are a number of measures of chemical availability in soils that can be usedin risk-based approaches. However, many of the methods that are currently acceptedare time consuming and expensive. As a result, there is interest in developing fasterand less expensive methods that can provide estimates of chemical availability that canbe used for specific categories of receptors and pathways. The use of simulated systems(e.g., skin or the digestive tract), biomimics, and extraction media that are correlatedwith chemical availability are examples of efforts to meet these objectives.

SELECTING METHODS (MEASURES AND/OR MODELS)

A wide range of measures has emerged for measuring or estimating bioavailability(Menzie et al., 1996). As a result, there is often confusion over what represents agood measure or estimate. Within a regulatory context, this often translates into acautious approach toward incorporating bioavailability information into risk esti-mates. In general, regulatory agencies want to be sure that the information hassufficient technical merit or strength to warrant its use for risk-based decisionmaking that is protective of health and the environment.

An example of such concerns is in the Agency for Toxic Substances and DiseaseRegistry’s (ATSDR) expert panel report on the bioavailability of mercury in soils(Canady et al., 1997). The following quotes are illustrative of the bioavailability issueand the kinds of information and tests that would be useful and acceptable to theagency:

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Tab

le 7

. E

xam

ples

of

met

hods

for

mea

suri

ng b

ioav

aila

bilit

y in

soi

l fo

r us

e in

ris

k-ba

sed

deci

sion

mak

ing.

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Tab

le 7

. (c

onti

nued

)

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The commonly used default value of 100% relative bioavailability appears to beexcessively conservative for mercury in soil. Additional research is needed toestimate a value (i.e., a default value in lieu of site-specific information) that ismore realistic but still protective of public health.

It seems likely that site-specific in vitro assays could be developed that couldpractically and economically reduce the uncertainty of bioavailability estimation.However, the assays need to be validated against the human digestive system, andperhaps against the digestive system of the animal used in the toxicity studiesfrom which dose-response relationships are derived.

We developed a weight-of-evidence approach that will help analysts select meth-ods and evaluate the technical strength of bioavailability information. This ap-proach does not minimize the value of engaging regulatory authorities in frequentand open discussions at every step in the process of evaluating, developing, andusing information on chemical availability from soil.

The approach involves evaluating or weighing information against four principles:

1. Soil-chemical relevance

2. Receptor relevance

3. Pathway relevance

4. Acceptance or validity of method.

More weight is assigned to information that accounts for these principles.Soil-chemical relevance refers to the extent to which methods and resultant informa-

tion take into account the soil factors that govern the bioavailability of the chemical.The approach outlined here involves an explicit consideration of these factors. Thisshould enable the analyst to determine the soil-chemical relevance of bioavailabilityinformation. For example, the bioavailability of non-polar organic chemicals is gov-erned, in part, by soil organic carbon. This information can be used early in the siteevaluation process to direct additional sampling efforts. These efforts would bedesigned to obtain site-specific soil organic carbon content at locations relevant to theevaluation of exposure pathways for receptors identified in the conceptual model forthe site. Later, in an analysis of chemical availability, methods that consider chemicalavailability are given higher weight than those that have not.

Receptor-relevance refers to the extent to which methods and resultant information arespecific to the receptors that are the subject of the risk assessment. These are identifiedin the development of the conceptual model. For example, if the receptor of concernis a mammal, methods that provide information on this or related mammals would begiven higher weight than those developed for other receptors such as bacteria or worms.

Pathway relevance refers to the extent to which methods and information are specificto the exposure pathway. The critical pathway(s) are also identified in the conceptualmodel. For example, if the relevant pathway is incidental ingestion, information derivedfrom feeding studies or physiological models of ingestion would be given greater weightthan those developed for other pathways such as aqueous leaching or dermal contact.

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This will vary on a case to case basis based on the level of acceptable risk specificallyassociated with the site and/or the risk management group requirements.

Acceptance or validity of method refers to the degree to which the method has beenaccepted by the scientific and risk management communities as a tool for evaluatingbioavailability. Validation of a method refers to its reliability and repeatability. Amethod that is well accepted and validated is typically given greater weight than onethat is new or experimental.

The technical strength of bioavailability information increases with the degree to whichthe four factors are satisfied (Figure 6). For example, information that takes into accountone or two factors may provide insight into qualitative aspects of the chemical’s availabilityfrom soil but may not be judged adequate for making quantitative estimates of bioavailability.

The four principles presented above should help people identify methods that willbe most useful for meeting the needs of the assessment. The relevance and specificityof information in relation to chemicals, pathways, and receptors all add value to theweight of evidence, the combination of information on each of these types increasesconfidence. The information needed for decision making will vary case by case.

INCORPORATING INFORMATION INTO THE RISK ASSESSMENT

An approach for considering whether and how to use information on bioavailabilityis illustrated in Figure 2 and is designed so that it may be integrated into existing risk-based approaches for site management (e.g., Superfund, state approaches, and theASTM RBCA process.). The approach begins with an initial set of considerations relatedto the value of information. If information on bioavailability is judged useful for decisionmaking, the process advances further and the approach identifies considerations foracquiring information. Finally, the approach shows how such information might beused in decision making. Information can be used in different ways depending on theweight of evidence. If information is judged adequate for use in quantitative riskassessment, then exposure and risk estimates are made that incorporate bioavailability.If information is not considered adequate for quantitative risk estimates, it may still have

Figure 6. Factors that affect confidence in measures of chemical availability in soils.

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value at the risk management stage. For example, if the information suggests that thebioavailability of the chemicals be reduced, then it may be reasonable to conclude thatthe exposure and risk estimates are likely to be conservative. The additional informationon bioavailability may help support a less stringent management decision, especiallywhen risk estimates are slightly above target risk levels.

Risk estimates typically need to consider current and foreseeable future condi-tions. Therefore, risk analyses and management decisions that utilize bioavailabilityinformation should consider how bioavailability of the chemicals might be affectedby reasonably foreseeable future conditions. Some of these factors are describedabove. Site-specific risk assessments should include a discussion of how these orother factors might be important at a site.

REFERENCESAlexander M. 1996. Chapter 1. Sequestration and Bioavailability of Organic Compounds in

Soil. In: Environmentally Acceptable Endpoints in Soil. Risk-Based Approach to Contaminated SiteManagement Based on Availability of Chemicals in Soil. pp. 44–136. (Linz, D. G. and Nakles,D.V., Eds.). American Academy of Environmental Engineers.

ASTM (American Society for Testing and Materials). 1998. Standard Provisional Guide for Risk-Based Corrective Action. ASTM PS 104–98. 101 p.

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