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The Role of Background Soil Levels in Risk Assessment

The Role of Background Soil Levels in Risk Assessment. Teresa S. Bowers Presented at USGS/NRCS Soil Geochemistry Workshop March 4, 2003. Soil’s Contribution to Risk. Human health and ecological systems Cancer and noncancer risks to human health from soil exposure arise from: Ingestion

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The Role of Background Soil Levels in Risk Assessment

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  1. The Role of Background Soil Levels in Risk Assessment Teresa S. Bowers Presented at USGS/NRCS Soil Geochemistry Workshop March 4, 2003

  2. Soil’s Contribution to Risk • Human health and ecological systems • Cancer and noncancer risks to human health from soil exposure arise from: • Ingestion • Dermal contact • Inhalation of suspended soil particles

  3. Example Calculation for Cancer Risks Due to Soil Ingestion Intake = (Concentration)(Ingestion rate)(Exposure frequency)(Exposure duration) (Averaging time)(Body weight) Cancer Risk = (Intake)(Bioavailability)(Cancer slope factor)

  4. Risk Thresholds Drive Remediation Decisions • EPA: • Cancer: 10-6 – 10-4, cleanup preferred to 10-6 for individual contaminants • Noncancer: hazard index = 1 • States: • Cancer: 10-6, some use 10-5, some use different target for residential vs industrial use • Noncancer: hazard index = 0.1, 0.2, 1 UNLESS background presents a higher risk

  5. Risk Thresholds > Background • Most contaminants fall in this category • Background may still be used as a standard to define where remediation stops (not where it starts), or to define “clean fill” • Legal issues: “Environmental trespass” may still rely on a definition of background • Tribal issues: Tribes may prefer background rather than risk threshold

  6. Risk Thresholds < Background • Arsenic, beryllium, dioxin, some PAHs, some radionuclides, many metals for eco risk • How to define background levels if used as a remediation standard • Average vs upper percentile of background • Natural vs anthropogenic background

  7. Comparison of Background and Risk Levels PAH background information from MA DEP As, Be background information from Shacklette and Boerngen

  8. U.S. EPA’s “Role of Background in the CERCLA Cleanup Program” April 26, 2002 • Risk assessment: “Naturally occurring elements that are not CERCLA hazardous substances, pollutants, and contaminants, but exceed risk-based screening levels, should be discussed…” • Risk management: “…the CERCLA program, generally, does not clean up to concentrations below natural or anthropogenic background levels.” • Risk communication: “…knowledge of background risks could help some community members place CERCLA risks in perspective.”

  9. Natural vs Anthropogenic vs Contamination EPA’s definition of Anthropogenic: “Natural and human-made substances present in the environment as a result of human activities (not specifically related to the CERCLA release in question)” Diffuse: Mercury from atmospheric cycling Local: Stack emissions, waste disposal unrelated to site under investigation

  10. Exposure and Depth • Human exposure occurs to surface soils, but how deep? • 1 inch (EPA guidance for lead sites) • 6 inches (Michigan risk guidance) • 1 foot (EPA Region 4 risk guidance) • 3 feet (Massachusetts Contingency Plan)

  11. Background and Depth Natural vs Anthropogenic Background May be Seen at Different Depths Data from site in Western Massachusetts, all units mg/kg

  12. Bioavailability – Which One? • Bioavailability based on geochemical characteristics that limit uptake into circulatory system • Human: • Oral: Solubility at stomach, intestine pH • Dermal: Dissolution in sweat? • Ecological: • Animals: Some animals used as models for humans • Plants

  13. Controls on Bioavailability • Presence in a mineral vs adsorbed to a surface • Other inorganics: Fe, Mn, Al, S • Fraction organic carbon • Clay content • Soil pH

  14. Shacklette and Boerngen Database • Useful as a screen for where background may exceed risk thresholds • Provides guide for when local background study may be useful • Provides indication of when local background may be anthropogenic rather than natural • Useful for narrowing contaminant list for local background study

  15. Things to Think About • Natural vs anthropogenic • Measure both, but distinguish? • Anthropogenic may be more local • Surface vs deep soils • What is demarkation between? • Depth of anthropogenic influence can vary • Should goal be based on defining exposure or separating natural from anthropogenic?

  16. Things to Think About • Additional analytes • Dioxin, PAHs, pesticides (DDT, etc.) • PCB congeners? • Organic carbon • Cr III vs Cr VI

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