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Observed Versus USEPA “Limited Site-Specific” Soil Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Clima

Observed Versus USEPA “Limited Site-Specific” Soil Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate. Modeling Vapor Attenuation Workshop The Annual International Conference on Soils, Sediments and Water Amherst, MA. October 2004 Loren Lund, Ph.D. 1

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Observed Versus USEPA “Limited Site-Specific” Soil Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Clima

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  1. Observed Versus USEPA “Limited Site-Specific” Soil Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate Modeling Vapor Attenuation Workshop The Annual International Conference on Soils, Sediments and Water Amherst, MA. October 2004 Loren Lund, Ph.D.1 Senior Toxicologist/Risk Assessor Terry Feng, Ph.D.1, Joy Su, M.S.1, and Benny DeHghi, B.S.2 2 1parsons

  2. Objectives • Present case study of USEPA (2002) Vapor Intrusion guidance tiered approach applied to data collected at the Site. • Discuss different methods and assumptions used by USEPA and CalEPA to derive Tier 2 subsurface-to-indoor air attenuation factors. • Present Site-specific empirical soil gas-to-indoor air attenuation factors without accounting for background levels. • Discuss factors that should be considered when deriving empirical or modeled soil gas-to-indoor air attenuation factors.

  3. USEPA (2002) Tiered Approach • Tier 1: Primary Screening • Chemicals sufficiently volatile/toxic? • Inhabited buildings present (or expected) above/near subsurface contamination? • Current conditions warrant immediate action? • Tier 2: Secondary Screening • Compare measured concentrations with generic or limited site-specific numerical criteria derived using attenuation factors. • Tier 3: Site-Specific Pathway Assessment • Generally involves direct measurement of subslab, subcrawlspace, crawlspace, indoor, and/or outdoor air concentrations and a survey of potential indoor sources.

  4. The Site and Surrounding Area • Former chemical storage and solvent recovery facility • Mixed industrial/residential • Residences • East of Site: Two-story, slab-on-grade, 45x60 ft footprint, attached garages, and forced-air heating/cooling (built in 2000-01) • North of Site: One-story, 1½ - 2 ft crawlspace (no basement), with or without forced-air heating/cooling and attached garages (built in 1950-60s) Residential ` ` FormerIndustrial Site

  5. Subsurface Geology/Hydrogeology • Subsurface stratigraphy • Surface to 2-4 ft bgs: Dry fill (primarily silt) • 2-4 to 11-12 ft bgs: Moist-to-wet silty clay • 11-12 to 18-30 ft bgs: Saturated (primarily silty sand) • 18-30 to 43-49 ft bgs: Saturated clay/silty clay • Static groundwater levels (semi-confined) • 4.5 to 6 ft bgs beneath the Site and northern homes • 8.5 to 10 ft bgs beneath eastern homes because ~4 ft fill used during construction in 2000-2001 • General groundwater flow is from east-to-west • Primary Site-related chemicals of potential concern (COPCs) in groundwater • Trichloroethylene (TCE), tetrachloroethene (PCE), and1,1-dichloroethene (1,1-DCE) Shallow Groundwater Zone (SGZ)

  6. TCE Groundwater Concentrations

  7. PCE Groundwater Concentrations

  8. Tier 1 Conclusions • Tier 2 (Secondary Screening) warranted • Inhabited buildings above/near groundwater impacted by Site-related chemicals • Site-related chemicals in groundwater sufficiently volatile and toxic (per Table 1 of USEPA, 2002) • Immediate action not warranted • No evidence of existing odors or adverse physiological effects • No short-term safety concerns

  9. Tier 2 “Generic” USEPA Attenuation Factors • USEPA (2002) “Generic” Subsurface-to-Indoor Air Attenuation Factors • Based on a review of limited empirical paired subsurface/indoor air data • 0.1 to 0.0000007 (groundwater-to-indoor air factors) • 0.9 to 0.0002 (soil gas-to-indoor air factors) • Unclear how or if background outdoor/indoor air sources addressed when deriving factors Default of 0.001 (1/1000) selected Default of 0.1 (1/10) selected

  10. Range of USEPA, CalEPA, and Site-Specific “Default” Assumptions Used to Model Tier 2 Attenuation Factors

  11. Tier 2 Subsurface-to-Indoor Air Attenuation Factors • Tier 2 attenuation factors differ by >2-orders of magnitude due to variability in empirical data or sensitivity of model and uncertainties in assigning “default” inputs. • Therefore, practical application of Tier 2 factors is limited, particularly when compounded by uncertainties in toxicity (e.g., TCE).

  12. Residential Property-Boundary Groundwater and Soil-Gas Monitoring Property-Boundary Subsurface Samples

  13. Property Boundary Soil-Gas Results • TCE, PCE, 1,1-DCA, cis-1,2-DCE, vinyl chloride, and benzene concentrations in shallow soil-gas samples exceeded Tier 2 screening criteria in vicinity of residences • Therefore, Tier 3 assessment warranted

  14. Conceptual Site Model Infiltration of Regional Outdoor Air Potential Inhalation of Chemicals in Indoor Air Potential Sources in Home Potential Soil Gas Diffusion Potentially Impacted Soil Gas Volatilization Volatilization Potentially Impacted Soil Potentially Impacted Groundwater

  15. Tier 3 Sampling Program • Data Collected: • Subslab soil gas • Subcrawlspace soil gas • Crawlspace air • Indoor air • Outdoor air • Meteorological • Indoor products survey

  16. INDOOR AIR, CRAWLSPACE AIR, AND SUBSLAB/SUBCRAWLSPACESOIL-GAS CONCENTRATIONS (µg/m3) • 7 of 18 chemicals were detected in at least one 24-hour indoor air sample above CalEPA Tier 2 screening level • However, concentrations generally were consistent with background outdoor air concentrations

  17. Tier 3 Conclusions • Chemicals (except PCE) above Tier 2 levels were not (or only sporadically) detected in nearby groundwater • Indoor air levels generally consistent with background outdoor air • Chemicals present in consumer products (e.g., glues, paints, cleaners) • Subslab soil-gas levels below Tier 2 screening criteria • PCE indoor air levels exceeded Tier 2 criterion in 10 homes • Indoor levels (except one sample) within the range of background outdoor air • Crawlspace and subsurface air concentrations lower than indoor levels for the single sample with PCE above outdoor air levels • Indoor levels similar to background levels in 450 California homes • Present in consumer products (e.g., dry-cleaned clothes) Therefore, Site-related COPCs in subsurface are not significant contributors to indoor air concentrations

  18. [Indoor Air] [Subslab Air] Empirical Attenuation Factor = (per USEPA, 2002 Appendix F) Site-Specific Empirical Soil Gas-to-Indoor Air Attenuation Factors Without Considering Background Sources a/ USEPA (2002) “Generic” = empirical-based; “Limited Site-Specific” = from Figure 3a b/ “Site-Specific” = modeled using site-specific input parameters.

  19. Empirical Subsurface-to-Indoor Air Attenuation Factors: Conclusions • Site-specific factors (without considering background sources) suggested significant vapor intrusion • However, an evaluation of vertical concentration profiles and outdoor/indoor air data indicated subsurface chemicals were not significantly impacting indoor air • Should not be used to derive default screening (e.g., USEPA “generic”) attenuation factors without accounting for outdoor/indoor background sources • Should not be used to validate modeled screening (e.g., USEPA “limited site-specific”) attenuation factors without considering vertical concentration profiles and outdoor/indoor air sources • Likely not feasible at this site given the high background outdoor air levels and potential indoor air sources

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