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by Robin V. Davis, P.G. Project Manager Utah Department of Environmental Quality

Understand causes, mechanisms, and distances of vapor bioattenuation in petroleum vapor intrusion; apply screening criteria and avoid unnecessary investigation.

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by Robin V. Davis, P.G. Project Manager Utah Department of Environmental Quality

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  1. Methods for Developing and Applying Screening Criteria for the Petroleum Vapor Intrusion Pathway Workshop 7 Tuesday March 24, 2015 6:30 pm – 9:30 pm Association for Environmental Health & Sciences (AEHS) 25th Annual International Conference on Soil, Sediment, Water & Energy San Diego, California by Robin V. Davis, P.G. Project Manager Utah Department of Environmental Quality Leaking Underground Storage Tanks rvdavis@utah.gov 801-536-4177

  2. OBJECTIVES • Understand why petroleum vapor intrusion (PVI) is very rare despite so many petroleum LUST sites • Understand causes of PVI • Show mechanisms, characteristics, degree of vapor bioattenuation • Show distances of vapor attenuation, apply as Screening Criteria, screen out low-risk sites • Avoid unnecessary additional investigation, soil gas/air sampling

  3. SCOPE • Field studies published by work groups, individuals • Data compiled to an empirical database: EPA Petroleum Vapor Database Jan. 2013 • Source strength: LNAPL in soil and GW, dissolved-phase • Associated soil gas measurements from 1000s of sample points at 100s of sites • Extensive peer review and quality control checks • Guidance Documents Issued: • Some US States • Australia 2012 • EPA draft PVI April 2013 • ITRC October 2014 • EPA ORD Issue Paper 2014

  4. Petroleum Vapor Database of Empirical Studies EPA OUST Jan. 2013 Canada 4/13 United States 70/816 Australian sites evaluated separately MAP KEY REFERENCES Australia 70 # geographic locations evaluated Davis, R.V., 2009-2011 McHugh et al, 2010 Peargin and Kolhatkar, 2011 Wright, J., 2011, 2012, Australian data Lahvis et al, 2013 EPA Jan 2013, 510-R-13-001 124/>1000 # paired concurrent measurements of subsurface benzene soil vapor & source strength 816 Perth Sydney Tasmania

  5. Conceptual Characteristics of Petroleum Vapor Transport and Biodegradation KEY POINTS • Aerobic biodegradation of vapors is rapid, occurs over short distances • LNAPL sources have high mass flux, vapors attenuate in longer distances than dissolved sources • Sufficient oxygen supply relative to its demand, function of source strength 0 1 O2/Hydrocarbon Vapor Profile 0 1 O2/Hydrocarbon Vapor Profile After Lahvis et al 2013 GWMR

  6. Subsurface Petroleum Vapor Bioattenuation Study Results • >100 years of research proves rapid vapor biodegradation by 1000s of indigenous microbes • Studies show vapors biodegrade and attenuate within a few feet of sources • No cases of PVI from low-strength sources • Causes of PVI are well-known

  7. 2 4 1 3 Groundwater-Bearing Unit Causes of Petroleum Vapor Intrusion Preferential pathway: bad connections of utility lines; natural fractured and karstic rocks BUILDING High-strength source in direct contact with building (LNAPL, high dissolved, adsorbed) Unsaturated Soil LNAPL LNAPL High-strength source in close proximity to building, within GW fluctuation zone Affected GW Preferential pathway: sumps, elevator shafts LNAPL Drawing after Todd Ririe, 2009 High-Strength Sources • Direct contact or close proximity to buildings • Preferential pathways: engineered & natural

  8. Collect Basic Data, Characterize Site, Construct Conceptual Site Model • Define extent & degree of contamination • Apply Screening Criteria Building Soil Boring/MW Soil Boring/MW Utility line LNAPL in soil Clean Soil UST system High vapor concentrations, high mass flux from LNAPL & soil sources Low vapor concentrations, low mass flux from dissolved sources LNAPL in soil & GW Dissolved contamination

  9. Signature Characteristics of Aerobic Biodegradation of Subsurface Petroleum Vapors LNAPL • Vapors aerobically biodegraded by oxygen-consuming microbes, waste product carbon dioxide • Vapors attenuate in short distances

  10. Vapor Bioattenuation Limited by Contaminated Soil LNAPL in Soil (sand, silty sand)

  11. Importance of Shallow Vapor Completion Points Example of apparent non-attenuation until shallow vapor point installed in non-contaminated soil VW-11 Hal’s, Green River, Utah 6/27/07 8/26/06 Shallow completion too deep Shallower point confirms attenuation above contaminated soil zone No attenuation within contaminated soil zone

  12. EPA OUST Jan. 2013 Results of Empirical Studies http://www.epa.gov/oust/cat/pvi/PVI_Database_Report.pdf • Thickness of clean soil required to attenuate vapors associated with LNAPL and dissolved sources • Screening Criteria

  13. Dissolved Sources 4.94 feet Benzene in GW 12,000 ug/L 4 feet Benzene in GW 3180/ ug/L Thickness of clean soil needed to attenuate vapors = Distance between top of source and deepest clean vapor point

  14. LNAPL Sources 8 feet Thickness of clean soil needed to attenuate vapors = Distance between top of LNAPL and deepest clean vapor point

  15. Screening Distances Dissolved Sources Benzene Vapors vs. Distance of Attenuation LNAPL Sources (small sites) Benzene Vapors vs. Distance of Attenuation 5 ft 15 ft 95%-100% Confidence

  16. Lahvis et al 2013 Results of Vapor Attenuation from LNAPL Sources • Different analysis, similar results • 13 ft vertical separation attenuates LNAPL source vapors 13 feet, 95% Confidence

  17. LNAPL Indicators (after EPA 2013; Lahvis et al 2013)

  18. Results of Empirical Studies for Developing Screening Criteria • Various methods of data analysis yield similar results • Dissolved Sources require 5 feet separation distance: • Benzene <5 mg/L • TPH <30mg/L • LNAPL Sources require 15 feet separation distance: • Benzene >5 mg/L, >10 mg/kg • TPH >30mg/L, >250-500 mg/kg • 18 feet separation required for large industrial sites • Soil within separation distance: • LNAPL-free soil contains sufficient oxygen to bioattenuate vapors • “Clean” (non-source), biologically active, sufficient oxygen and moisture • EPA: <100 mg/kg TPH “clean” soil

  19. Measuring Magnitude of Subsurface Vapor Attenuation Subsurface Attenuation Factor (AF) = Ratio of shallow to deep vapor concentration Shallow SV Benzene, ug/m3 AF = Deep SV Benzene, ug/m3 Field Example: ~1 ug/m3 AF = 7E-06 = 145,000 ug/m3 ~7,000,000x contaminant reduction

  20. Distribution of Magnitude of Subsurface Petroleum Vapor Attenuation Factors 3 Reasons for Insignificant AFs 10x-100x Most events exhibit Significant AFs >10,000x Screen these out Reasonable Screening AF 100x-1000x

  21. EPA Modeling Studies Vertical and Lateral Attenuation Distances http://www.epa.gov/oswer/vaporintrusion/documents/vi-cms-v11final-2-24-2012.pdf

  22. Building with Basement LNAPL Vapor Source 200,000,000 ug/m3 8 m deep (26 ft) 0 Model: 20 ft vertical 16 ft lateral Field Data: 15 ft vertical 8 ft lateral Conclusions: - Models under-predict attenuation - Vapors attenuate in shorter distances laterally than vertically 2 Vertical Attenuation 6m (20 ft) 4 Lateral Attenuation 5m (16 ft) 6 8 10 20 30 40 50 60 70 80 90 Vertical Distance Below Grade, meters Oxygen 0 2 4 6 8 Lateral Distance, meters

  23. Screening Criteria U.S. and Australia

  24. THANK YOU

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