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Bioavailability of Contaminants in Soil: Considerations for Human Health Risk Assessment

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1 Introduction
1 Introduction Overview
1.1 Using Bioavailability Information
1.2 Background
1.3 Definition of Terms
2 Regulatory Background
2 Regulatory Background Overview
2.1 Current Practices: Survey of State Regulators
3 Technical Background
3 Technical Background Overview
3.1 Soil Mineral Phases
3.2 Soil pH, Organic Matter, and Reactive Clay Minerals
3.3 Soil Particle Size
4 Decision Process
4 Decision Process Overview
4.1 Decision Process Flowchart
4.2 Is there a Method Available?
4.3 Could Bioavailability Assessment Affect the Remedial Decisions?
4.4 Do the Benefits of Bioavailability Assessment Justify the Costs?
4.5 Further Considerations
5 Methodology
5 Methodology for Evaluating Contaminant Oral Bioavailability Overview
5.1 In Vivo Approach
5.2 In Vitro Approach
6 Lead
6 Lead Overview
6.1 Fate and Transport
6.2 Toxicology and Exposure
6.3 Methodology for Quantifying RBA of Lead in Soil
6.4 When Does a Bioavailability Study Make Sense?
6.5 Case Studies
6.6 Using Bioavailability Methods to Evaluate Remedies (Bioavailability-Based Remediation)
7 Arsenic
7 Arsenic Overview
7.1 Fate and Transport
7.2 Toxicology and Exposure
7.3 Methodology for Evaluating Arsenic Bioavailability
7.4 When Does It Make Sense to Use Bioavailability?
7.5 Case Studies
7.6 Using Bioavailability Methods to Evaluate Remedies (Bioavailability Based Remediation)
8 PAHs
8 Polycyclic Aromatic Hydrocarbons (PAHs) Overview
8.1 PAH Sources and Exposure
8.2 General Toxicity of PAHs
8.3 Influences of Soil on Bioavailability of PAH
8.4 Methodology for Evaluating PAH Bioavailability
8.5 Dermal Absorption
8.6 Amendment Strategies and Permanence of Bioavailability
8.7 Case Study
9 Risk Assessment
9 Using Bioavailability Information in Risk Assessment Overview
9.1 Risk Calculations
9.2 Other Considerations and Limitations
10 Stakeholder Perspectives
10 Stakeholder Perspectives Overview
10.1 Stakeholder Concerns
10.2 Specific Tribal Stakeholder Concerns
10.3 Stakeholder Engagement
11 Case Studies
11 Case Studies Overview
11.1 Arsenic, Mining, CA
11.2 Arsenic, Pesticide, AR
11.3 Arsenic, Naturally occurring, UT
11.4 Arsenic, Smelter, AZ
11.5 Arsenic-contaminated tailings, OR
11.6 Lead, Industrial, Midwest US
11.7 PAH, Skeet targets, TX
11.8 Arsenic, Copper precipitation, UT
11.9 Arsenic, CCA wood preservative, CA
11.10 Arsenic, MGP coal ash, MI
11.11 Lead, Mining MT
11.12 Lead, Mining, MT
11.13 Lead, Smelter, UT
Additional Information
Review Checklist
Appendix A: Detailed Survey Responses
Appendix B: Chemical Reactions of Metals
Acronyms
Glossary
Acknowledgments
Team Contacts
Document Feedback

 

Bioavailability of Contaminants in Soil
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7.2 Toxicology and Exposure

Arsenic can exist in both organic and inorganic compounds. Exposures to organic arsenic occur primarily in the food supply. Exposures through the consumption of fish (usually in the form of arsenobetaine and arsenocholine) have a relatively low level of associated toxicity (Sabbioni et al. 1991). Inorganic arsenic, however, is highly toxic and associated with risks to human health. Inorganic arsenic is ranked as the top chemical of concern on the Priority List of Hazardous Substances (ATSDR 2015). This list ranks substances that present the greatest risk to public health based on prevalence, toxicity, and potential for human exposure. Human exposure to inorganic arsenic can occur through dietary intake, which is primarily from rice consumption (Rahman and Hasegawa 2011), as well as ingestion of contaminated water or soils. Chronic exposure to arsenic has been associated with a variety of cancers (skin, lung, bladder, liver, and kidney), cardiovascular disease, and neurological impairments in exposed populations (ATSDR 2007a; Mitchell 2014).

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The toxicity criteria for arsenic that are applied in human health risk assessments are based on exposure to arsenic solubilized in groundwater (USEPA 2015b). Absorption is highly influenced by solubility (Marafante and Vahter 1987); an estimated 95% of arsenic is absorbed following consumption of contaminated groundwater in healthy individuals (Zheng et al. 2002). The dose-response relationship for toxicity of a substance is directly related to how readily it is absorbed by the body (its bioavailability). The relative oral bioavailability of arsenic in soil is greatly influenced by the source of the arsenic and by mineralogical associations within the soil matrix and has a reported range from 3 to 100% (Freeman et al. 1995; Rodriguez et al. 1999; Juhasz et al. 2007; Bradham et al. 2011; Mitchell 2014; Meunier et al. 2010; USEPA 2012a).

Arsenic in soils is primarily inorganic arsenate, As(V) or arsenite, As(III). In most soils, aerobic conditions result in predominantly As(V). Trivalent As, As(III), has been reported to have higher toxicity than As(V) (Thomas, Styblo, and Lin 2001; Hughes 2002; Singh, Goel, and Kaur 2011). While valence state has been reported to affect toxicity, arsenic valence is not typically considered in human health risk assessments due to the reduction of As(V) to As(III) early in the biotransformation pathway (ATSDR 2007a). The arsenic valence information for a specific site could be addressed qualitatively in the uncertainty section of a human health risk assessment, although this is not common practice.

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