Observed Versus USEPA

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
1 parsons
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
Parameter Range of Values Used in Model
Depth to Bottom of Floor (cm) 15 - 200
Depth to Water Table (cm) 260 - 300
Soil-Gas Sampling Depth (cm) 15 - 210
Soil Temperature (C) 10 - 17
Thickness of Soil Stratum (cm) 15 - 210 (soil gas model) 100 - 260 (groundwater model)
SCS Soil Type sand, loam, or clay
Soil Dry Bulk Density (g/cm3) 1.3 - 1.5
Soil Total Porosity (unitless) 0.38 - 0.43
Soil Water-Filled Porosity (cm3/cm3) 0.054 - 0.3
Enclosed Space Floor Thickness (cm) 10 - 25
Pressure Differential (g/cm-s2) 40
Floor Length (cm) 960 - 1,700
Floor Width (cm) 960 - 1,300
Enclosed Space Height (cm) 270 - 370
Crack Width (cm) 0.1
Indoor Air Exchange Rate (1/h) 0.25 - 1
Vapor Flow Rate into Building (L/min) Modeled or 5
11
Tier 2 Subsurface-to-Indoor Air Attenuation
Factors
USEPA Generic USEPA Limited Site-Specific CalEPA Area-Specific Slab-on-Grade Site-Specific
Groundwater 1E-03 (1/1,000) 1E-03 (1/1,000) 8.6E-06 (1/120,000) 2.2E-05 (1/45,000)
Soil Gas 1E-01 (1/10) 2E-03 (1/500) 1E-03 (1/1,000) 2.5E-04 (1/4,000)

• Tier 2 attenuation factors differ by gt2-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
Property-Boundary Subsurface Samples
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Property Boundary Soil-Gas Results
Maximum Soil-Gas Level (µg/m3) Tier 2 Screening Criteria (µg/m3) Tier 2 Screening Criteria (µg/m3) Tier 2 Screening Criteria (µg/m3)
Chemical Maximum Soil-Gas Level (µg/m3) USEPA Generic USEPA Limited Site-Specific CalEPA Area-Specific
TCE 39,000 0.22 11 1,200
PCE 3,800 8.1 410 410
1,1-DCA 3,300 1,000 50,000 1,500
cis-1,2-DCE 150,000 70 3,600 7,300
Vinyl Chloride 3,000 2.8 140 31
Benzene 150 3.1 160 84

• 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
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Tier 3 Sampling Program

• Data Collected
• Subslab soil gas
• Subcrawlspace soil gas
• Crawlspace air
• Indoor air
• Outdoor air
• Meteorological
• Indoor products survey

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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

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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
Site-Specific Empirical Soil Gas-to-Indoor Air
Attenuation Factors Without Considering
Background Sources
Residence Empirical Empirical Empirical USEPA Default a/ 0.1 (Generic) 0.002 (Limited) Site-Specific b/ 0.00025
Residence TCE PCE Benzene USEPA Default a/ 0.1 (Generic) 0.002 (Limited) Site-Specific b/ 0.00025
A1 0.09 0.37 2.9 USEPA Default a/ 0.1 (Generic) 0.002 (Limited) Site-Specific b/ 0.00025
A2 1.5 0.75 0.87 USEPA Default a/ 0.1 (Generic) 0.002 (Limited) Site-Specific b/ 0.00025
A5 0.01 0.37 0.52 USEPA Default a/ 0.1 (Generic) 0.002 (Limited) Site-Specific b/ 0.00025
A7 0.12 1.1 1.4 USEPA Default a/ 0.1 (Generic) 0.002 (Limited) Site-Specific b/ 0.00025
A9 0.07 0.25 0.05 USEPA Default a/ 0.1 (Generic) 0.002 (Limited) Site-Specific b/ 0.00025
C1 0.02 0.19 1.9 USEPA Default a/ 0.1 (Generic) 0.002 (Limited) Site-Specific b/ 0.00025
a/ USEPA (2002) Generic empirical-based
Limited Site-Specific from Figure 3a b/
Site-Specific modeled using site-specific
input parameters.
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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