Observed Versus USEPA “Limited Site-Specific” Soil Gas-to-Indoor Air Attenuation Factors for a S...
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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

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

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Objectives
Objectives Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate

  • 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.


Usepa 2002 tiered approach
USEPA (2002) Tiered Approach Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate

  • 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.


The site and surrounding area
The Site and Surrounding Area Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate

  • 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


Subsurface geology hydrogeology
Subsurface Geology/Hydrogeology Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate

  • 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)


Tce groundwater concentrations
TCE Groundwater Concentrations Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate


Pce groundwater concentrations
PCE Groundwater Concentrations Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate


Tier 1 conclusions
Tier 1 Conclusions Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate

  • 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


Tier 2 generic usepa attenuation factors
Tier 2 “Generic” USEPA Attenuation Factors Gas-to-Indoor Air Attenuation Factors for a Site in a Semi-Arid Climate

  • 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


Observed versus usepa limited site specific soil gas to indoor air attenuation factors for a site in a semi arid clima
Range of USEPA, CalEPA, and Site-Specific “Default” Assumptions Used to Model Tier 2 Attenuation Factors


Tier 2 subsurface to indoor air attenuation factors
Tier 2 Subsurface-to-Indoor Air Attenuation Factors Assumptions Used to Model Tier 2 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).


Property boundary subsurface samples

Residential Property-Boundary Groundwater and Soil-Gas Monitoring

Property-Boundary Subsurface Samples


Property boundary soil gas results
Property Boundary Soil-Gas Results Monitoring

  • 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


Conceptual site model
Conceptual Site Model Monitoring

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


Tier 3 sampling program
Tier 3 Sampling Program Monitoring

  • Data Collected:

  • Subslab soil gas

  • Subcrawlspace soil gas

  • Crawlspace air

  • Indoor air

  • Outdoor air

  • Meteorological

  • Indoor products survey


Indoor air crawlspace air and subslab subcrawlspace soil gas concentrations g m 3
INDOOR AIR, CRAWLSPACE AIR, AND SUBSLAB/SUBCRAWLSPACE MonitoringSOIL-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


Tier 3 conclusions
Tier 3 Conclusions Monitoring

  • 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


Observed versus usepa limited site specific soil gas to indoor air attenuation factors for a site in a semi arid clima

[Indoor Air] Monitoring

[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.


Empirical subsurface to indoor air attenuation factors conclusions
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