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COMPARISONS OF SUB-SLAB SOIL GAS MEASUREMENTS TO MODELED EMISSIONS FROM SUBSURFACE CONTAMINATION

COMPARISONS OF SUB-SLAB SOIL GAS MEASUREMENTS TO MODELED EMISSIONS FROM SUBSURFACE CONTAMINATION by John A. Menatti and Robin V. Davis Utah Department of Environmental Quality for The 16 th Annual AEHS Conference on Soils, Sediments and Water March 13-16, 2006 San Diego, California.

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COMPARISONS OF SUB-SLAB SOIL GAS MEASUREMENTS TO MODELED EMISSIONS FROM SUBSURFACE CONTAMINATION

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  1. COMPARISONS OF SUB-SLAB SOIL GAS MEASUREMENTS TO MODELED EMISSIONS FROM SUBSURFACE CONTAMINATION by John A. Menatti and Robin V. Davis Utah Department of Environmental Quality for The 16th Annual AEHS Conference on Soils, Sediments and Water March 13-16, 2006 San Diego, California

  2. Utah Vapor Intrusion Project • Project evaluated 6 Utah LUST sites. • All 6 sites failed the Utah RBCA vapor intrusion-to-indoor air screening evaluation. • Johnson & Ettinger Model (J&E Model). • Input parameters for the J&E Model are site-specific and include soil type, moisture content, air-filled porosity, capillary fringe thickness, depth to contamination, etc. • Sub-slab soil gas samples were collected at 5 sites. • Nested vapor wells were installed and sampled at 1 site.

  3. Utah Vapor Intrusion Project LUST Sites 1. Price Rental, Price Aug. 8, 2003 2. Hal’s Chevron, Green River Nov. 20, 2003 3. Logan Lube, Logan March 9, 2004 4. Bountiful Bicycle, Woods Cross Aug. 26, 2004 5. Wheel-In Market, Salt Lake City Nov. 22, 2004 6. Gold Cross Ambulance, Salt Lake City May 10, 2005

  4. RESULTS • The Utah RBCA vapor intrusion screening evaluation (J&E Model) at the 6 LUST sites indicated that the vapor intrusion pathway may be complete and may pose an unacceptable risk to building inhabitants. • Sub-slab soil gas sampling and nested vapor well sampling indicated that the vapor intrusion pathway was not complete or was not significant at these 6 LUST sites.

  5. Utah RBCA Vapor Intrusion Screening Levels Groundwater-to-Indoor Air • 1. Benzene concentrations in groundwater (ug/L) that pass a 1x10E-6 risk vapor intrusion assessment for residential (Res.) and commercial (Com.) exposure scenarios. • 2. J&E Model used to back-calculate these benzene concentrations in groundwater.

  6. Price Rental, Price, Utah August 8, 2003 • Utah RBCA (J&E Model) was used to back-calculate site-specific soil and groundwater screening levels for the vapor intrusion-to-indoor air exposure pathway. • Risk Level = 1 in 1,000,000 (1x10E-6) • Exposure Scenario = Commercial Building • Soil Screening Level = 0.4 mg/kg benzene • Groundwater Screening Level = 2,334 ug/L benzene • A paper describing the work at this site was presented at the Air & Waste Management Association’s 97th Annual Conference in Indianapolis, Indiana in June 2004.

  7. PRICE RENTAL Sub-Slab Soil Gas = 0.85 ug/m3 benzene (Acceptable Sub-Slab Soil Gas Concentration = 5-50 ug/m3 benzene) 20 Feet Clayey Silt Soil Contamination @ 20 feet bg = 20 mg/kg benzene (Soil Screening Level = 0.4 mg/kg benzene) Groundwater Contamination = 11,800 ug/L benzene (Groundwater Screening Level = 2,334 ug/L benzene)

  8. Logan Lube, Logan, Utah March 9, 2004 • Utah RBCA (J&E Model) was used to back-calculate site-specific soil and groundwater screening levels for the vapor intrusion-to-indoor air exposure pathway. • Risk Level = 1 in 1,000,000 (1x10E-6) • Exposure Scenario = Commercial Building • Groundwater Screening Level = 1,000 ug/L benzene

  9. LOGAN LUBE Sub-Slab Soil Gas = 10 ug/m3 benzene (Acceptable Sub-Slab Soil Gas Concentration = 5-50 ug/m3 benzene) 13 Feet Silty Clay Groundwater Contamination = 1,500 – 3,000 ug/L benzene (Groundwater Screening Level = 1,000 ug/L benzene)

  10. Slab Attenuation Factors • EPA OSWER Subsurface Vapor Intrusion Guidance (2002) allows the use of a 10-fold (0.1) slab attenuation factor. • San Diego SAM Manual (2004) allows the use of a 10-fold (0.1) slab attenuation factor for an “old slab” and a 100-fold (0.01) attenuation factor for a “new or improved slab.” • Sager, et.al. (1997) discussed a study on slab infiltration in 218 U.S. homes. Based on the study, he recommended that a 1,000-fold (0.001) slab attenuation factor be used. Reference: Sager, S.L., Braddy, L.D., and Day, C.H., 1997, The Infiltration Ratio in Vapor Intrusion Calculations, Proceedings of the Society for Risk Analysis Annual Meeting, Washington, D.C., December 9, 1997.

  11. Bountiful Bicycle, Woods Cross, Utah

  12. Bountiful Bicycle, Woods Cross, Utah August 26, 2004 • Soil contamination remaining under building after vapor extraction remediation was completed = 2,300 mg/kg TPH-G and 900 mg/kg xylenes at 17 feet below the building. • Utah LUST Program Cleanup Level for TPH-G = 1,500 mg/kg and xylenes = 23 mg/kg in soil. • Two sub-slab soil gas (SS) samples and one indoor air sample collected. All concentrations in ug/m3. ContaminantSS #1SS#2Indoor Air Benzene <0.58 <0.58 7.7 TPH <4.1 <4.1 175 Xylenes <1.3 <1.3 36 • The indoor air sample is higher than the sub-slab soil gas samples. • ASTM RBCA (1995) - National Ambient Indoor Air Concentrations: Benzene = 3 - 22 ug/m3 Xylenes = 5 - 48 ug/m3

  13. Bountiful Bicycle Ambient Indoor Air Sampling

  14. Wheel-In Market, Salt Lake City, Utah November 22, 2004 • Utah RBCA (J&E Model) was used to back-calculate soil and groundwater screening levels for the vapor intrusion-to-indoor air exposure pathway. • Risk Level = 1 in 1,000,000 • Exposure Scenario = Commercial Building • Groundwater Screening Level = 3,522 ug/L benzene (5 feet of clay) or 940 ug/L benzene (5 feet of silt). • Soil Screening Level = 1 mg/kg benzene (5-10 feet of clay).

  15. Wheel-In Market Sub-Slab Soil Gas = 4.2 ug/m3 benzene and <1.1 ug/m3 MTBE (Acceptable Sub-Slab Soil Gas Concentration = 5-50 ug/m3 benzene) 3 Feet Fine Sand with Silt 4 Feet Lean Clay with Silt Soil Contamination = 3 mg/kg benzene @ 7 feet bg. (Soil Screening Level = 1 mg/kg benzene). Groundwater Contamination = 3,260 ug/L benzene and 12,300 ug/L MTBE. (Groundwater Screening Level = 3,522 ug/L benzene [5 feet of clay] or 940 ug/L benzene [5 feet of silt])

  16. Gold Cross Ambulance, Salt Lake City, Utah May 10, 2005 • Utah RBCA (J&E Model) was used to back-calculate soil and groundwater screening levels for the vapor intrusion-to-indoor air exposure pathway. • Risk Level = 1 in 1,000,000 • Exposure Scenario = Commercial Building • Groundwater Screening Level = 720 ug/L benzene • Soil Screening Levels = <1 mg/kg benzene 1,500 mg/kg TPH.

  17. Gold Cross Ambulance Collected 5 Sub-Slab Soil Gas Samples under 5 Buildings 6 Feet Silty Sand Soil Contamination = 1.5 mg/kg benzene and 2,200 mg/kg TPH @ 6 feet bg. (Soil Screening Levels = <1 mg/kg benzene and 1,500 mg/kg TPH) Groundwater Contamination = 88 ug/L benzene. (Groundwater Screening Level = 720 ug/L benzene)

  18. Gold Cross Ambulance Sub-Slab Soil Gas Results (ug/m3) * Top Number: Indoor Air, 10E-6 Risk, Residential Exposure Scenario Bottom Number: Sub-Slab Soil Gas

  19. Hal’s Chevron, Green River, Utah November 20, 2003 • Free product (gasoline) plume underlying the Oasis Motel & Café at about 9 feet below the bottom of basement storerooms. • Free product (gasoline) plume underlies parking lots and a road at about 14.5 to 17 feet below grade. • 11 nested (multi-depth) vapor wells were installed in the basements, parking lots, and street.

  20. Hal’s Chevron

  21. Hal’s Chevron

  22. Hal’s Chevron

  23. Hal’s Chevron V W - 5 Oasis Cafe Basement 8 Feet Benzene TPH (ug/m3)(mg/m3) 11 Feet 3 <8 Silt and Clayey Silt 34,000 20,000 15 Feet 16 Feet Free Product - Gasoline

  24. Hal’s Chevron Oasis Motel V W - 4 Basement 8 Feet Benzene TPH (ug/m3) (mg/m3) 11 Feet 980 340 Silt and Clayey Silt 3,400 1,000 15 Feet 17 Feet Free Product - Gasoline

  25. Hal’s Chevron V W - 7 Parking Lot Benzene (ug/m3)TPH (mg/m3) 3 Feet 160 <8 7 Feet 280 <12 Silt and Clayey Silt 11 Feet 53 <8 15 Feet 50,000 5,300 16 Feet Free Product - Gasoline

  26. Hal’s Chevron V W - 10 Road Benzene (ug/m3)TPH (mg/m3) 3 Feet 19 <8 7 Feet 450 27 Silt and Clayey Silt 110,000 30,000 11 Feet 470,000 35,000 15 Feet 17 Feet Free Product - Gasoline

  27. Hal’s Chevron V W - 11 Road Benzene (ug/m3)TPH (mg/m3) 4 Feet 2200 110 Silt and Clayey Silt 8 Feet 1,100,000 110,000 12 Feet 810,000 120,000 14.5 Feet Free Product - Gasoline

  28. EPA-State Regulator Petroleum Vapor Intrusion Workgroup • The November 2002, EPA OSWER Draft Subsurface Vapor Intrusion Guidance (VI Guidance) presented a process to evaluate the vapor intrusion exposure pathway at contaminated sites. • Basically, the VI Guidance recommended that sub-slab soil gas sampling be conducted if a site fails a J&E Modeling analysis. • The EPA acknowledged that the VI Guidance may be too conservative for use at petroleum hydrocarbon sites. • In late 2003, the EPA formed an EPA-State Regulator Petroleum Vapor Intrusion Workgroup headed up by Joe Vescio (EPA Headquarters). • Robin Davis and I represented Utah on the workgroup.

  29. Biodegradation of Petroleum Hydrocarbon Vapors • The workgroup had many conference calls and compiled an extensive repository of literature on petroleum vapor migration and biodegradation. • Using the literature sources from the workgroup’s reference repository, Robin Davis tabulated and evaluated multi-depth soil gas data from 16 petroleum hydrocarbon sites in the U.S. and Canada. • Robin published her findings in LUSTLine, Bulletin 49, March 2005 and concluded the following: • Biodegradation of petroleum hydrocarbon vapors is a probable mechanism of attenuation in subsurface soils. • The vapor intrusion pathway may not be complete at sites where at least 2 feet of uncontaminated soil overlies the contaminant source and sufficient oxygen is present to facilitate biodegradation.

  30. Biodegradation of Petroleum Hydrocarbon Vapors (cont.) • Robin has continued to scour the scientific literature for additional sites to add to her database. • As of February 2006, she had amassed data from 31 more sites (a total of 47 sites). • Her conclusions are pretty much the same: • 2 to 5 feet of clean vadose zone soil overlying the contamination significantly attenuates petroleum hydrocarbon vapors. • The data supports the use of a 100 to 1000 biodegradation factor on the J&E Model. • Robin’s work will be published in an upcoming LUSTLine article and she will be presenting her research at the 18th Annual National Tanks Conference in Memphis, TN next week.

  31. Guidance Documents Incorporating Biodegradation Factors: Health Canada October 7, 2004 Soil Vapour Intrusion Guidance for Health Canada Screening Level Risk Assessment • Allows the use of a 10-fold biodegradation factor with the J&E Model for petroleum hydrocarbons if: 1. The contaminant source is greater than 13 feet below the building. 2. There is no significant capping that would prevent oxygen migration to below the building, i.e., paved surfaces around the building cover less than 80% of the surface area surrounding the building.

  32. New Jersey Department of Environmental Protection Vapor Intrusion Guidance October 2005 • Allows the use of a 10-fold biodegradation factor with the J&E Model for BTEX vapors in the vadose zone (minimum 2 foot thick vadose zone soil and 4% oxygen). Nebraska Department of Environmental Quality Environmental Guidance Document February 2002 • States that the vapor intrusion pathway for petroleum hydrocarbon contaminated soil and groundwater to indoor air is generally considered incomplete if the contaminated soil or groundwater is greater than 15 feet below the structure.

  33. Wisconsin Division of Public Health Chemical Vapor Intrusion and Residential Indoor Air February 13, 2003 • States that petroleum hydrocarbons biodegrade relatively well in unsaturated soils. Therefore, petroleum-related VOCs generally have to be in “free product” state or groundwater very near, if not in contact with, the building foundation to result in vapor intrusion.

  34. Conclusions & Recommendations • Biodegradation of petroleum hydrocarbon vapors in vadose zone soils is well documented. • Clean vadose zone soils (soil, not gravel) are excellent bioreactors for petroleum vapors. • A biodegradation factor of 100 to 1000 should be used to modify the J&E Model for petroleum hydrocarbon vapors. • There should be a minimum of 5 feet of clean vadose zone soil separating the subsurface contamination from the bottom of the building. • Sites with free product or preferential pathways should be sampled, not modeled.

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