Evaluating the Petroleum Vapor Intrusion Pathway Studies of Natural Attenuation of Subsurface Petroleum Hydrocarbons & Recommended Screening Criteria NEIWPCC Webinar on Petroleum Vapor Intrusion June 26, 2012 11:30 am-2:00 pm MDT by Robin V. Davis, P.G. Project Manager Utah Department of Environmental Quality Leaking Underground Storage Tanks rvdavis@utah.gov 801-536-4177
Slide provided by Tom McHugh, 2012
OBJECTIVES Understand why there are so many petroleum LUST sites yet petroleum vapor intrusion (PVI) is very rare Use Screening Criteria to exclude low-risk sites from PVI pathway SCOPE Build Petroleum Vapor Database from field studies Soil type, depth to GW, LNAPL presence, contaminant source concentrations Show mechanisms & characteristics of petroleum hydrocarbon vapor biodegradation
Petroleum Vapor Database Compilation of concurrent source strength & soil vapor data ~170 Sites ~1000 measurements Canada United States 56/304 2/13 Australia 112/608 Perth Sydney MAP KEY 56 304 # Geographic Locations (sites) Evaluated # Paired concurrent measurements of benzene subsurface soil vapor & source strength Tasmania
Characterize Site Gas Station Building Define full extent & degree of soil & GW contamination Construct Conceptual Site Model UST system High vapor concentrations from LNAPL Clean soil Low vapor concentrations from dissolved source Contaminated soil & LNAPL Dissolved contamination
Case Study 1 Tesoro #40 Salt Lake City, Utah Apartments Contamination from dispensers & USTs Very high concentrations of contaminant source in soil & GW <5 feet below apartment building foundation Vapors are biodegraded & fully attenuated within few feet of clean soil overlying the source PVI pathway is not complete
Case Study 1: Front View of Apartments 35 feet wide x 65 feet long=2275 feet 2 Apartment slab 3 feet bgs
Residential basement apartments A Dissolved Benzene Plume Map Dissolved source Benzene 14,000 ug/l TPH 28,000 ug/l Sub-slab vapors Benzene 9.9 ug/m3 TPH 130 ug/m3 A SG-1 Sub-Slab soil vapor monitoring point Groundwater monitoring well Groundwater flow direction
Long-Term GW Monitoring 7 MW-21 DTW, feet bgs Benzene, ug/l 16000 Contaminated Soil Zone 14000 12000 8 10000 Depth to GW, feet bgs 9 Partial Removal of On-Site Contaminated Soil Zone 8000 6000 4000 2000 Benzene in GW, ug/l 10 0 5/14/10 12/17/10 4/25/11 7/23/11 10/14/11 1/17/12 3/29/12 Date of GW Sampling Event
Boring Log near Apartments Clean Soil Contaminated soil
PVI Investigation
NW Feet Below Grade 0 Apartments Cross-Section A-A 0 20 Scale, feet SE 5 Silty Clay Sub-slab SG-1 9.9 ug/m3 benzene <100 ug/m3 TPH 22% O2 <0.2% CO2 Sub-slab SG-2 Benzene <3.2 ug/m3 TPH 130 ug/m3 O2 22% CO2 <0.2% Soil Benzene 160 ug/kg TPH-gro <30,000 ug/kg Benzene 4000 ug/kg 10 Sand Dissolved Benzene 14,000 ug/l TPH-gro 28,000 ug/l TPH-gro 750,000 ug/kg Benzene 480 ug/kg Silty Clay TPH-gro 130,000 ug/kg Benzene 11 ug/kg 15 TPH gro <3100 ug/kg
Case Study 2 Ogden Mini Mart, Ogden, Utah Gasoline LNAPL directly beneath building slab, PVI reported by building occupants, mitigation implemented immediately LNAPL
Results of Field & Published Studies Clean soil contains sufficient oxygen needed to biodegrade vapors (aerobic) A few feet of clean soil provides a natural barrier to PVI No reported cases of PVI from lowstrength sources Causes of PVI are predictable & wellunderstood
Causes of Petroleum Vapor Intrusion 1 High-strength source (LNAPL, high dissolved/adsorbed) in direct contact with building BUILDING 4 Preferential pathway allows vapors to enter building Unsaturated Soil LNAPL LNAPL 3 Sump draws LNAPL/high dissolved into building Affected GW Groundwater-Bearing Unit LNAPL 2 LNAPL/high dissolved in close proximity to building
The Science of Petroleum Hydrocarbon Biodegradation & Vapor Attenuation
Aerobic Biodegradation and Oxygen Mass Balance Petroleum Hydrocarbon C6H6 + 7.5O2 Aerobic Bacteria 6CO2 + 3H2O Aerobic bacteria use oxygen to degrade the hydrocarbon for the carbon. The waste product is carbon dioxide and water
Conceptual Model of Aerobic Petroleum Vapor Biodegradation Aerobic biodegradation is a robust & rapid process a) LNAPL SOURCE UNSATURATED ZONE high mass flux CAPILLARY ZONE O 2 VOCs sharp reaction front Clean/uncontaminated soil is sufficiently aerobic to biodegrade & attenuate vapors 8 feet for LNAPL 5 feet for dissolved SATURATED ZONE b) DISSOLVED-PHASE SOURCE UNSATURATED ZONE limited mass flux CAPILLARY ZONE SATURATED ZONE constituent distributions O 2 VOCs constituent distributions sharp reaction front Lahvis, Hers, Davis, Wright, DeVaull (2012, in process)
Signature Characteristics of Aerobic Biodegradation Typical O2, CO2, PHC vapor profiles as petroleum vapors are naturally biodegraded & attenuated with sufficient thickness of clean vadose zone soil
Non-Attenuation of Vapors due to Lack of Clean Overlying Soil Conneaut, OH VMP-1 (Roggemans, 1998; Roggemans et al., 2001) AF Subsurface 7E-01 Bio-AF=7E-01 Oxygen Carbon Dioxide Benzene O2 & CO2 (% V/V) 0 0 5 10 15 20 5 Contaminated Soil zone 10 15 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 Benzene (ug/m3)
Importance of Shallow Vapor VW-11 8/26/06 Completion Points Example of apparent non-attenuation due to no shallow soil completion point, attenuation shown in later sample points VW-11 Hal s, Green River, Utah Benzene SV, ug/m3 TPH SV, ug/m3 VW-11 8/26/06 6/27/07 6/27/07 Benzene SV, ug/m3 TPH SV, ug/m3 0 0 2 2 Depth, feet bgs 4 6 8 10 12 Shallow completion too deep. No attenuation within contaminated zone Depth, feet bgs 4 6 8 10 12 Shallow points confirm attenuation above contaminated zone 14 1.00E+00 1.00E+03 1.00E+06 1.00E+09 14 1.00E+00 1.00E+03 1.00E+06 1.00E+09 SV Concentration, ug/m3 SV Concentration, ug/m3
Comparison of Field Data to Models that Account for Biodegradation & Vapor Attenuation - Abreu & Johnson Numerical Model (Abreu & Johnson) - BioVapor Analytical Model (DeVaull & McHugh)
Numerical Model Effect of Oxygen-Driven Biodegradation & Magnitude of Subsurface Attenuation of Benzene Vapors Beneath Buildings Benzene Vapors Oxygen Atmospheric Oxygen diffuses downward Low source strength Medium source strength High source strength Hydrocarbon vapors diffuse upward Depth below grade (meters) Horizontal Distance from Building Center (meters)
Comparison of Field-Measured Soil Gas Data to Numerical Model (LNAPL example) Chatterton Research Site, British Columbia, Canada (Hers et al 2000) 0 Slab-on-Grade Building Benzene, Field-Measured, Chatterton Benzene, Numerical Model-predicted Depth, feet bls 5 Sand, Gravel Soil Deep SV Sample Depth to GW reported 10 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 1.E+10 Benzene, ug.m3
Comparison of Field-Measured Soil Gas Data to BioVapor Analytical Model Beaufort, NJ-VW-2 (Lahvis et al, 1999) AF=0.1, O2=1%, foc=0.5% Find it at: api.org
BioVapor Model Compared to Dissolved Site, Beaufort, South Carolina (Lahvis et al 1999) - Soil vapors associated with Dissolved Benzene 16,000 ug/l, TPH-g 67,100 ug/l Beaufort, SC (Lahvis et al, 1999) Soil Vapor Field Data Compared to BioVapor Model from dissolved source - BioVapor Model under-predicts subsurface attenuation by 100x to 10,000x Beaufort, SC (Lahvis et al, 1999) Soil Vapor Field Data Compared to BioVapor Model from Dissolved Source Benzene Field-Measured, ug/m3 Benzene BioVapor Prediction, ug/m3, AF=0.1, O2=1%, foc=0.5%, Bare Earth Benzene BioVapor Prediction, ug/m3, AF=0.1, O2=1%, foc=0.5%, Pavement TPH-gro Field-Measured, ug/m3 TPH-gro Bio Vapor Prediction, ug/m3, AF=0.1, O2=1%, foc=0.5%, Bare Earth TPH-gro Bio Vapor Prediction, ug/m3, AF=0.1, O2=1%, foc=0.5%, Pavement TPH-gro Bio Vapor Prediction, ug/m3, AF=0.1, O2=1%, foc=0.5%, Aerobic Depth Spe Benzene BioVapor Prediction, ug/m3, AF=0.1, O2=1%, foc=0.5%, Aerobic Depth=3.4 0 0 3 3 Depth, feet bls 6 Depth, feet bls 6 9 Benzene in GW 16,000 ug/l 9 TPH in GW 67,100 ug/l 12 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 Benzene, ug/m3 12 1.0E-01 1.0E+01 1.0E+03 1.0E+05 1.0E+07 1.0E+09 TPH-gro, ug/m3
Conclusions from Models - UNDER-predict subsurface attenuation by 10xxxx - OVER-predict PVI by 10xxxx
Developing Screening/Exclusion Criteria to Screen Out PVI Low-Risk Sites
Method for Developing Screening Criteria for Dissolved Sources Multi-Depth Vapor Monitoring Well Feet bgs 0 Beaufort, SC, Lahvis et al 1999 NJ-VW-2 3 ft Benzene vapor concentrations at depth <1 ug/m 3 4 ft 2,300 ug/m 3 5 8 feet Clean overlying soil 7 ft 16,700 ug/m 3 Estimated Contaminated soil zone 10 DTW ~11 ft 11 ft 145,000 ug/m 3 Benzene in GW 16,000 ug/l 15 METHOD FORMULA: 11 ft 3 ft = 8 ft clean overlying soil
Screening Criteria for Dissolved Benzene & TPH (Exterior + Sub-Slab) Benzene: Soil Vapor & Dissolved Paired Measurements TPH: Soil Vapor & Dissolved Paired Measurements Near-Slab Multi-Depth, Sub-Slab Benzene: 199 exterior/near-slab + 37 sub-slab = 236 total All Soil Types Near-Slab Multi-Depth, Sub-Slab TPH: 73 exterior/near-slab + 24 sub-slab = 97 total All Soil Types Thickness Clean Soil Required to Attenuate Benzene Vapors, feet 10 9 8 7 6 5 4 3 2 1 0 1 10 100 1,000 10,000 100,000 Benzene, dissolved, ug/l Thickness Clean Soil Required to Attenuate TPH Vapors, feet 10 9 8 7 6 5 4 3 2 1 0 1 100 10,000 1,000,000 TPH, dissolved, ug/l 5 ft Clean Overlying Soil Attenuates Vapors Associated with Dissolved Benzene <1,000 ug/l, TPH <10,000 ug/l
Method for Developing Screening Criteria for LNAPL & Soil Sources 0 VW-7 Hal s, Green River, VW7, 6/26/07 6/26/07 Utah (UDEQ) O2 & CO2 (% v/v) 0 5 10 15 20 Oxygen Carbon Dioxide Benzene Depth feet bgs 5 10 15 contaminated soil zone Top of contamination to top clean soil = feet CLEAN soil needed to attenuate vapors 20 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 Benzene (ug/m3)
Results for LNAPL & Soil Sources Benzene 48 exterior/near-slab + 23 sub-slab = 71 total Benzene SV Sample Event over LNAPL & Soil Sources TPH 17 exterior/near-slab + 19 sub-slab = 36 total TPH SV Sample Event over LNAPL & Soil Sources Near-Slab Multi-Depth, Sub-Slab Near-Slab Multi-Depth, Sub-Slab Thickness of Clean Soil Overlying LNAPL Required to Attenuate Vapors, feet 10 9 8 7 6 5 4 3 2 1 0 Sites Refineries Thickness of Clean Soil Overlying LNAPL Required to Attenuate Vapors, feet ~8 ft CLEAN overlying soil attenuates vapors associated with LNAPL/Soil Sources 10 9 8 7 6 5 4 3 2 1 0 Sites
Reference Screening Criteria Published & Cited Values (after Lahvis & DeVaull, 2011) Database & Site Type Benzene Soil Gas Screening Level (ug/m3) Screening/Exclusion Distance (feet) Screening/Exclusion Concentration Benzene (ug/l) Other Criteria Davis, R.V. (2009, 2010) International Petroleum Vapor Database Non-detect 5 <1000 5 feet for TPH <10,000 ug/l 8 LNAPL 30 ft poorly-characterized sites Lahvis et al (2012) R.V. Davis & J. Wright (retail sites only, no refineries) 100 0 <15,000 Dissolved phase only, BTEX <75,000 ug/l 15 LNAPL McHugh et al (2010) various publications, professional judgement 10 Dissolved phase only 30 LNAPL Peargin & Kolhatkar (2011) Chevron, all sites 300 0 <1000 Wright, J. (2011) Australia & U.S. sites, all sites + refineries 15 >1000 10, 50, 100, 1000 5 <1000 30 LNAPL California various references, R.V. Davis, McHugh et al 50, 100 5 <100 no SG Oxygen measured 5 <1000 with SG Oxygen measured >4% 10 <1000 no SG Oxygen measured 30 LNAPL Indiana various references, (RV Davis 2009-2010, McHugh et al 2010) 5 <1000 no SG Oxygen requirement AFs for GW & SG 30 LNAPL Distances apply vertically & horizontally New Jersey various uncited references 5 <100 no SG Oxygen measured 5 <1000 with SG Oxygen measured >2% 10 <1000 no SG Oxygen measured 100/30 LNAPL/Gasoline Horizontal & vertical distance Wisconsin Davis, R.V., 2009, Luo et al 2009, McHugh et al, 2010 NONE 5 <1000 20 >1000 Exclusion distances apply vertically & horizontally 30 LNAPL
Screening Criteria EPA OUST PVI Guide (3-15-12 draft)
Conclusions PVI pathway not complete when following criteria apply: Dissolved Sources - 5 feet CLEAN soil overlying Benzene <1,000 ug/l, TPH <10,000 ug/l - >5 feet CLEAN soil overlying Benzene >1,000 ug/l, TPH >10,000 ug/l` LNAPL Sources - 8 feet CLEAN soil overlying top of LNAPL smear zone or soil sources Soil Sources - 5 feet CLEAN soil = TPH <100 mg/kg, PID <100 ppm-v (gasoline), <10 ppm-v (diesel) Vapor Sources - Petroleum vapors are attenuated below the receptor - If measuring soil vapor, analyze ALL COCs, O2, CO2, methane, others - Oxygen to Carbon Dioxide ratios demonstrate petroleum biodegradation
Flow chart in 3-15-12 draft EPA OUST PVI guide
Recommendations Fully characterize sites, determine full extent, degree of contamination Collect basic field data to assess PVI pathway Apply Screening/Exclusion Criteria in deciding if PVI investigation is necessary