Use of Soil-Gas Data in Vapor Intrusion Decisions 17 th AEHS West Coast Conference on Soils, Sediments, and Water San Diego, CA March 21-22, 2007 Presented by: Henry Schuver, US EPA OSW See: http://iavi.rti.org
History of Soil Gas being used Qualitatively Determine if tanks had leaked For GW -ingestion exposure-scenarios Define GW plumes (news in 1980 s) e.g., Mapped plume beneath Tucson, AZ w/ GW at 42 meters depth (126 ft) [Marrin & Thompson, GW, vol. 25 (1) 1987]
Now Soil Gas is being used for Quantitative risk-based decisions For VI -inhalation exposure-scenarios: Tier 3 (site-specific) Soil-Gas criteria [depth & soil] USEPA OSWER Draft VIG 2002 can be directly compared to risk-based screening levels or used in predictive models ITRC, 2007, p. D-7 Soil gas sampling is the preferred sampling and investigative tool for most vapor intrusion investigations. ITRC, 2007, p. D-7
OSWER Tier 3 Soil-Gas Exits 2002
Some Questions Re: Soil Gas Samples Representative of concentrations: Vicinity of the probe? Adjacent to building? Concentration able to enter bldg (e.g., sub-slab) Sufficient number (to assess variability)? Area (xy) Depth (z) Time (t)
Some of what we have learned since 2002 3-D Modeling [& Exterior samples] by L. Abreu & P. Johnson, EST, 2005 2002 AF = 0.1 AF = 0.01 AF = 0.001 Soil-gas wells Not representative of the sub-slab Goal of soil-gas sampling is to represent the concentration under the slab. So use SS alpha. Note: SS can be 60% of source conc. (Sandbox Geology) In NY state 5 out of 11 sample pairs shows conc. > under slab than nearby soil-gas implants Bill Wertz of NYS 2/7/06 e-mail w/ Westside soil gas vs subslab.xls [up to 30 x higher under slab]
Evidence indicates high degree of variability in Soil-Gas samples True variability Documented spatial (xyz) Natural features Building influenced Documented temporal (t) (lesser) Sampling variability The quality of [active] soil gas data depends greatly on the collection protocols. ITRC, 2007, p. D-7
How much external data is needed to predict (i.e., model) this reality (SS conc.)? [And this is only SubSlab (add bldg variability to IA)] Variation in Sub-slab conc. within a few hundred feet. Endicott, NY (presentation by B. Wertz)
Endicott, New York Deep SG, Shallow SG, Sub-Slab, Indoor Air I. Hers, IAVI db data hjs Summary: Deep soil vapor samples predictive of sub-slab Only 3/9 shallow soil vapor even close to sub-slab
Slide by Bill Wertz (NYSDEC) Endicott, NY 100000.00 Shallow Soil Gas vs Subslab TCE ug/m3 shallow means same depth as basement floor (8-10 ft) 10000.00 Subslab 1000.00 100.00 10.00 1.00 Higher in Sub-Slab Higher in Soil Gas 0.10 0.01 0 0 1 10 100 1000 10000 100000 Shallow Soil Gas Slide by B. Wertz (NYSDEC) Endicott, NY Showing the range of relationships. (some are 150 ft away)
Decreasing soil-gas concentrations above lenses of finer-grained material w/ higher moisture contents Endicott, NY; Slide from B. Wertz
Sub-Slab Concentrations at House B Raymark site, CT Not a sample of the intruding vapors >44 x 1,1,1-TCA 1,1-DCE TCE 8.1 0.94 30 Soil-Gas Location 89 1,1,1-TCA 1,1-DCE TCE 7.9 3.7 20 MW215 showed no VOCs 1,1,1-TCA 1,1-DCE TCE 120 44 80 1,1,1-TCA 1,1-DCE TCE 7.9 3.7 20 Slide by D. Digiulio (EPA-ORD) w/ mod. HJS Soil-Gas Location 88 IAQ of 11DCE = 0.11 ug/m3 Using sub-slab mean Alpha ss = 0.11/16 = 0.0065 Using sub-slab point Alpha ss = 0.11/0.94 = 0.118
Many advantages of Soil Gas Is the media of inhalation exposure Integrates gw, soil, & gas, sources Multiple sample locations & depths possible/practical due to low costs Near Real-Time analysis possible TRIAD-like benefits However
Before we can understand naturally occurring variability * in soil gas Spatial (xyz), & temporal (t) We need know that we are all sampling what is present in the environment, (at the probe) with: precision & accuracy (or, at least, comparable) results Thus, this workshop on sampling methods *Dominic DiGiulio, MSRAS, Aug. 2006
If time permits
A New Phase of VI Guidance Was Predictive (spreadsheets) Now Observational (spreadsheets) Compare your site to what others have found http://iavi.rti.org database Drs. H. Dawson & I. Hers >2,500 pairs of indoor air and: Groundwater Sub-slab Soil-gas
Draft 3/07 - Vapor Intrusion Assessment Multiple Lines of Evidence Investigative Approach News
We need soil-gas & indoor air paired data submissions >1,100 Groundwater & Indoor Air pairs New understanding and screening tools Want to do the same for Soil Gas But only 186 soil-gas and indoor air pairs Need more Soil Gas data submitted (using methods providing comparable data)
SG Conc > 100 x (90%) bkgd (iavi db - 3/07) Indoor Air Concentration (ug/m3) Indoor Air vs Soil Gas Concentration 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 Data 1.E-01 Alpha = 1.0 Alpha = 0.1 1.E-02 Alpha = 0.05 1.E-02 1.E+00 1.E+02 1.E+04 1.E+06 Alpha = 1.E+08 0.02 Soil Gas Concentration (ug/m3) Alpha = 0.01 Attenuation Factor 1.E+03 IA/SG Attenuation Factor vs Soil Gas Concentration 1.E+02 1.E+01 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1.E-02 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 Soil Gas Concentration (ug/m3) Attenuation Factor 1.E+03 1.E+02 1.E+01 1.E+00 IA/SG Attenuation Factor vs Indoor Air Concentration 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 Indoor Air Concentration (ug/m3)