BioVapor Application of BioVapor to Petroleum Vapor Intrusion Sites

BioVapor Application of BioVapor to Petroleum Vapor Intrusion Sites
A 1-D Vapor Intrusion Model with Oxygen-limited Aerobic Biodegradation Application of BioVapor to Petroleum Vapor Intrusion Sites 1 1

Course Outline n Overview of Petroleum Vapor Intrusion (60 min)
n Introduction to BioVapor Model (45 min) n Break (15 min) n Case Study 1: GW Screening Values (30 min) n Case Study 1: Dissolved Hydrocarbon Plume (30 min) n Case Study 2: Gasoline Vapor Source (30 min) n Questions (30 min)

Gettin’ the Goods How Who BioVapor Model
Download at: (Registration information used so we can notify users of updates. No spam.) Roger Claff (202) • Bruce Bauman (202) Who

Meet the Trainers Thomas McHugh George DeVaull Jim Weaver
Introduction Meet the Trainers Thomas McHugh GSI Environmental Developer of BioVapor Interface Shell Global Solutions Developer of BioVapor Model George DeVaull US EPA, Office of Research and Development Petroleum Vapor Intrusion Research and Policy Jim Weaver

Overview of Petroleum VI
n General VI Conceptual Model n Vadose Zone Attenuation of Petroleum Vapors n Oxygen Below Building Foundations n Framework for Evaluation of Petroleum VI

Groundwater-Bearing Unit
Conceptual Model for Vapor Intrusion: Regulatory Framework BUILDING Building Attenuation Due to Exchange with Ambient Air 3 Air Exchange Advection and Diffusion Through Unsaturated Soil and Building Foundation Unsaturated Soil 2 Affected Soil Affected GW Partitioning Between Source and Soil Vapor Groundwater-Bearing Unit Slide Topic: Conceptual model for vapor intrusion Diffusion from source and advection into building does not match industry experience. The standard conceptual model of vapor intrusion has the following key elements: Partitioning between the source and soil vapors. An infinite source of VOCs is assumed (i.e., no mass flux or mass balance considerations) Diffusion and advection (but not biodegradation) through unsaturated soils and the building foundation Attenuation in the building due to exchange with ambient air. This conceptual model is based on the J&E model. The U.S. EPA VI guidance was developed using this conceptual model of vapor intrusion. Slide Presentation: Describe the conventional conceptual model for vapor intrusion. Key Points: This conceptual model does not distinguish between petroleum and chlorinated VOCs. The model assumes an infinite source of VOCs The model does not account for biodegradation in the vadose zone. The model does not reflect industry experience with petroleum vapor intrusion impacts (see next slide). 1 KEY POINT: Regulatory guidance focused on building impacts due to vapor migration.

Physical Barriers to Vapor Intrusion
Slide Topic: Conceptual model for vapor intrusion The standard conceptual model of vapor intrusion has the following key elements: Partitioning between the source and soil vapors. An infinite source of VOCs is assumed (i.e., no mass flux or mass balance considerations) Diffusion and advection (but not biodegradation) through unsaturated soils and the building foundation Attenuation in the building due to exchange with ambient air. This conceptual model is based on the J&E model. The U.S. EPA VI guidance was developed using this conceptual model of vapor intrusion. Slide Presentation: Describe the conventional conceptual model for vapor intrusion. Key Points: This conceptual model does not distinguish between petroleum and chlorinated VOCs. The model assumes an infinite source of VOCs The model does not account for biodegradation in the vadose zone. The model does not reflect industry experience with petroleum vapor intrusion impacts (see next slide). 7 7

Petroleum Biodegradation Conceptual Model
CHmin Comax Aerobic Biodegradation Possible Co>Comin  Oxygen L No Aerobic Biodegradation Co<Comin Hydrocarbon Comin CHmax Hydrocarbon Source Vapor Concentration KEY POINT: Correlation between oxygen consumption and hydrocarbon attenuation. From Roggemans et al., 2001, Vadose Zone Natural Attenuation of Hydrocarbon Vapors: An Empirical Assessment of Soil Gas Vertical Profile Data, API’s Soil and Groundwater Technical Task Force Bulletin No. 15.

Conceptual Model: Biodegradation
KEY POINT: Rapid vapor attenuation is observed within the “clean soil” layer above top of dissolved or NAPL source.

What is “Clean Soil”? Dirty Soil O2 HC Clean Soil Watch-out
Conceptual Model What is “Clean Soil”? Dirty Soil Soil impacted by mobile NAPL, residual NAPL, NAPL smear zone, or Groundwater fluctuation zone. O2 HC Clean Soil Soil with low hydrocarbon concentrations (e.g., <100 mg/kg TPH). Consider potential for shallow sources or other unexpected vadose zone impacts. Watch-out “Clean soil” means low hydrocarbons, not zero hydrocarbons. KEY POINT:

VOC Concentration vs. Depth Biogenic Gases vs. Depth
Petroleum Biodegradation: Real Site Data Diesel Release Site, North Dakota VOC Concentration vs. Depth Biogenic Gases vs. Depth

Petroleum Biodegradation: Real Site Data
Oxygen Carbon Dioxide Benzene Beaufort, SC Coachella, CA Salina, UT Ubiquitous vadose zone attenuation of petroleum hydrocarbons. KEY POINT: Vadose zone profiles compiled by Robin Davis, UDEQ.

Petroleum Soil Vapor Database: Robin Davis, Utah DEQ
2/13 ~170 sites & ~1000 Soil Vapor Sample Events United States, Canada, Australia 54/307 KEY USEPA data base compiled by EPA, clumped together chlorinated and petroleum hydrocarbons. 112/608 Perth Sydney Tasmania 54 # Geographic Locations Evaluated 307 # Subsurface paired benzene soil vapor & GW sample events

Robin Davis Soil Vapor Database: Observed Soil Vapor Attenuation
73 exterior/near-slab + 24 sub-slab = 97 total 199 exterior/near-slab + 37 sub-slab = 236 total USEPA data base compiled by EPA, clumped together chlorinated and petroleum hydrocarbons. KEY POINT: For dissolved sources (<1000 ug/L benzene or <10,000 ug/L TPH), 5 ft of clean soil is sufficient for vadose zone attenuation to non-detect benzene in soil gas.

Robin Davis Soil Vapor Database: Observed Soil Vapor Attenuation
73 exterior/near-slab + 24 sub-slab = 97 total 199 exterior/near-slab + 37 sub-slab = 236 total USEPA data base compiled by EPA, clumped together chlorinated and petroleum hydrocarbons. KEY POINT: For UST site NAPL sources, 10 ft of clean soil is sufficient for vadose zone attenuation.

Petroleum Vapor Intrusion vs Chlorinated Solvent VI
Slide Topic: Industry experience with petroleum vapor intrusion Based on industry experience, petroleum vapor intrusion impacts are generally associated with: Direct NAPL impacts on a building foundation NAPL or dissolved hydrocarbon impact on building sump NAPL impact on preferential flow pathway or Diffusion of vapors from subsurface NAPL source Slide Presentation: Discuss industry experience with petroleum vapor intrusion impacts Key Points: Current USEPA VI guidance provides GW screening concentrations for benzene and other petroleum hydrocarbons in the low ug/L range (i.e., 5 ug/L for benzene). These low screening concentrations are not consistent with industry experience that vapor intrusion impacts are not associated with low concentrations of petroleum hydrocarbons dissolved in groundwater. We believe that the science available today is sufficient to support the development of separate attenuation factors and screening criteria for petroleum and chlorinated VOCs KEY POINT: Increasing regulatory acceptance that petroleum releases have much lower vapor intrusion risk. Graphic from: USEPA OUST Petroleum Vapor Intrusion Workgroup, 2010, Petroleum Vapor Intrusion Information Paper, June 2010 Draft

Correlation Between Groundwater Concentration and Indoor Air??
IA ?? Petroleum Hydrocarbons Chlorinated Solvents GW Indoor Air Concentration ( ug/m3) Indoor Air Concentration ( ug/m3) CORRELATION ? YES (p <0.001) CORRELATION ? NO (p = 0.11) USEPA data base compiled by EPA, clumped together chlorinated and petroleum hydrocarbons. GW Concentration (ug/L) GW Concentration (ug/L) Observable Relationship Cia vs. Cgw ? n Petroleum Hydrocarbons: No n Chlorinated Solvents: Yes - Direct Cgw = COC conc. In groundwater; Cia = COC conc. In indoor air; (p = 0.11) = Probability = 11% that slope of best-fit line = 0 (I.e., no trend).

Oxygen Under Building Foundation
Key Question n Is there enough oxygen below building foundations to support aerobic biodegradation? aerobic zone Ct anaerobic zone Cs Vapor Source

Oxygen Under Foundation: Model Prediction
Numerical model predicts oxygen shadow below building, but….. n Very strong vapor source (200,000,000 ug/m3) n All flow into building is through perimeter crack n No advective flow below building Model may not account for key oxygen transport processes. KEY POINT: From Abreu and Johnson, ES&T, 2006, Vol. 40, pp 2304 to 2315.

Petroleum Hydrocarbons
Aerobic Biodegradation: Oxygen Mass Balance bacteria Hydrocarbon + Oxygen Carbon dioxide + Water 1 kg CxHy + 3 kg O2 3.4 kg CO kg H2O New Cells Electrons & Carbon + Petroleum Hydrocarbons Energy Electrons Electron Acceptor (e.g., O2)

Aerobic Biodegradation: Oxygen Mass Balance
n Atmospheric air (21% Oxygen) = 275 g/m3 oxygen > Provides capacity to degrade 92 g/m3 hydrocarbon vapors (= 92,000,000 ug/m3) KEY POINT: Even limited migration of oxygen into subsurface is sufficient to support aerobic biodegradation.

Bi-Directional Flow Across Foundation
Transport of Oxygen Under Foundation Bi-Directional Flow Across Foundation Wind Driven Advection +/- +/- KEY POINT: Advection drives oxygen below building foundation.

Transport of Oxygen Under Foundation
Conceptual Model Field Data 0.0 Depth (m) 0.5 Wind-driving building ventilation isoP 1.0 CH4 CO2 Wind Loading 02 1.5 Advection of subslab soil gas into bldg. 0.0 0.5 Biodegradation 1.0 Upwind-downwind advection in soil gas Diffusion from deep sub-slab soil gas 1.5 0.0 Subslab VOC source 0.01 0.1 1 10 100 1000 Concentration (g m-3) KEY POINT: Conceptual model and field data indicate common presence of oxygen under building foundation. From Fisher et al., 1996 Environmental Science and Technology, Vol. 30 No. 10, p

Soil Column Attenuation Transport of Oxygen Under Foundation Nitrogen Flooding Experiment: Purge sub-foundation soils with nitrogen gas and observe oxygen recovery Low Oxygen Time = 0 Time > 0 3 m N 1.1 0.8 1.0 0.9 concrete garage Injection wells % O2 (shallow) % O2 After Flood Oxygen Recovery Below Building Slide Topic: Transport of Oxygen Under Foundation, Case Study This slide and the next slide present the results of a field experiment conducted to measure the transport of oxygen below a building foundation. The test building was a residential structure in Santa Maria, CA built on fill material. For the experiment, nitrogen was pumped under the building, displacing the air (which contained relatively high oxygen levels). Following the nitrogen flooding, monitoring points (separated from the N2 injection points) showed that the air had been displaced, resulting in very low oxygen levels below the building. Oxygen sensors were then used to monitor the recovery of oxygen under the slab. The sensors (shown on the graph) indicate that oxygen levels recovered at many locations in less than two days and at all locations within two weeks. The sudden recovery in oxygen levels at just under time = 2 days corresponds to a high wind event, supporting the conceptual model that wind facilitates oxygen transport under the building foundation. Slide Presentation: Describe the experiment and the results. Key Points: The results of this experiment indicate rapid oxygen transport under the building foundation and support the conceptual model of wind-driven oxygen transport. Data from Lundegard, Johnson, and Dahlen. “Sub-slab Nitrogen Flood-Oxygen Re-entry Test.”

Soil Column Attenuation Transport of Oxygen Under Foundation Nitrogen Flooding Experiment: Purge sub-foundation soils with nitrogen gas and observe oxygen recovery Low Oxygen Time = 0 High Oxygen Time = 2 weeks KEY POINT: 1.1 0.8 1.0 0.9 concrete 3 m N garage 16.6 18.4 19.4 15.4 14.0 15.2 12.2 14.5 13.7 15.9 3 m N garage concrete Rapid recovery of oxygen below building foundation supports petroleum biodegradation. % O2 After Flood Injection wells % O2 (shallow) Data from Lundegard, Johnson, and Dahlen. “Sub-slab Nitrogen Flood-Oxygen Re-entry Test.”

Advective Transport Processes
Lower building pressure Residence in winter (chimney effect); bathroom, kitchen vents EXAMPLES Gas flow from subsurface into building Higher building pressure Building HVAC designed to maintain positive pressure Gas flow from building into subsurface Variable building pressure Barometric pumping; variable wind effects Bi-directional flow between building and subsurface Flow in Flow out Reversible flow Low Pressure High Pressure UPWARD TRANSPORT High Pressure Low Pressure DOWNWARD TRANSPORT

Pressure Gradient Measurements: School Building, Houston, Texas
KEY POINTS: Pos. Pressure (Flow out of Bldg) • Pressure gradient frequently switches between positive and negative within a single day. • Continuous inward flow does not occur. Differential Pressure (Pascals) Neg. Pressure (Flow into Bldg) Time (July 14-15, 2005)

Advection Through Building Foundation: Field Evidence
n VOCs from indoor air typically detected in sub-slab samples: - alpha pinene - limonene - p-dichlorobenzene n Oxygen transported below foundation by same mechanism INDOOR AIR S BELOW SLAB KEY POINT: Reversing pressure gradient drives air (and VOCs and oxygen) through building foundation. S

Fill, “crust,” sandy silt Fill, dredged river sand
Chatterton Research Site, British Columbia, Canada (Hers, et al 2000) Building Feet Below Grade SG-BC, 10/1/97 SG-BR 5/14/97 <1000 Fill, “crust,” sandy silt 11% 10 % <1000 80,000 10% 8 % 1.0% 50,000,000 Oxygen, % Benzene, ug/m3 Sub-Slab vapor sample point Sub-Surface vapor sample point KEY SG-BC Vapor sample point identifier 55,000 3% 6% 25,000,000 50,000,000 Fill, dredged river sand 1.0% 50,000,000 5 60,000,000 Small building lies ~8 feet above benzene-rich LNAPL. Vapors fully attenuated with ~7 feet clean soil. Strong source puts strong demand on oxygen regardless of building presence. Building does not occlude oxygen. 1.0% 20 Feet, horizontal 10 LNAPL, benzene-rich Slide from Robin Davis, UDEQ

Hal’s, Green River, Utah (Utah DEQ, 8/26/06)
Feet Below Grade Motel Office Breezeway Café/Bar VW-7 Asphalt 8.4 850 Basement Basement 14% Clayey Silt 7.0 380 VW-4 VW-5 20% 51 22 10 2800 1600 7.7 9.5% 18% Silt 250 12% 570 87 70,000 12,000 260,000 4.1% 11% 33,000,000 2.5% Benzene in GW 1,000-5,000 ug/L 20 Small- and Medium-sized buildings overlie very strong source of soil and groundwater gasoline contamination. Aerobic biodegradation of vapors with ~5 feet of clean overlying soil regardless of building presence. Buildings do not occlude oxygen or impede aerobic biodegradation if sufficient thickness of clean soil overlies the source. Vapor intrusion not likely. Exterior soil vapor sample points are adequately representative of sub-building conditions. KEY 260,000 2.5% Benzene, ug/m3 Oxygen, % VW-7 Multi-depth vapor monitoring well Sub-Surface vapor sample point TPH-gro, ug/m3 LNAPL, gasoline Feet, horizontal 20 33,000,000 Slide from Robin Davis, UDEQ

Perth, Australia (B. Patterson & G. Davis, 2009)
Very Large Building 410 Feet Below Grade Uncovered open ground <2 19,000,000 <50,000 <50,000 <50,000 <0.5% 10.7% 19.9% 18.8% Lateral Extent of Oxygen & Biodegradation Sand 35,000,000 <50,000 <50,000 <50,000 5 <0.5% 8.2% 14.5% 15.9% 35,000,000 1,200,000 <50,000 <50,000 <0.5% 8.2% 4.5% 4.6% Very large building overlies very strong source of kerosene LNAPL. For ~30 feet laterally from center of building to building edge, ~5 feet of clean overlying attenuates vapors associated with very strong source as evidenced by low soil vapor and high Oxygen concentrations. Building >30 feet wide/deep occludes oxygen (“casts a shadow”) and impedes aerobic biodegradation. Interior and Exterior soil vapor sample points may be necessary for sites with very large buildings. NOTE: Indoor air TPH concentration does not exceed health-based criteria for kerosene VOCs. Conclusion: Very strong source in close vertical proximity to a very large building produces very high vapor concentrations that cannot degrade perhaps due to the size of the building and occlusion of oxygen. 10 KEY LNAPL Kerosene (very low BTEX) Outdoor air sample Indoor air sample Sub-slab vapor sample Sub-surface vapor sample 20 Feet, horizontal 1,200,000 Total Petroleum Hydrocarbons, ug/m3 8.2% Oxygen, % Slide from Robin Davis, UDEQ

Oxygen Under Building Foundation
Summary n Wind and building pressure drive atmospheric air below building foundation n Even modest oxygen transport sufficient aerobic biodegradation

Petroleum Vapor Intrusion: Field Experience BUILDING 2 NAPL directly impacts building wall or floor. 3 Preferential pathway allows vapors to enter building. Unsaturated Soil NAPL NAPL Sump draws NAPL or dissolved hydrocarbons into building. Affected GW 1 4 Vapors from NAPL diffuse through vadose zone (large releases). NAPL Slide Topic: Industry experience with petroleum vapor intrusion Based on industry experience, petroleum vapor intrusion impacts are generally associated with: Direct NAPL impacts on a building foundation NAPL or dissolved hydrocarbon impact on building sump NAPL impact on preferential flow pathway or Diffusion of vapors from subsurface NAPL source Slide Presentation: Discuss industry experience with petroleum vapor intrusion impacts Key Points: Current USEPA VI guidance provides GW screening concentrations for benzene and other petroleum hydrocarbons in the low ug/L range (i.e., 5 ug/L for benzene). These low screening concentrations are not consistent with industry experience that vapor intrusion impacts are not associated with low concentrations of petroleum hydrocarbons dissolved in groundwater. We believe that the science available today is sufficient to support the development of separate attenuation factors and screening criteria for petroleum and chlorinated VOCs Groundwater-Bearing Unit For petroleum sites, vapor intrusion is generally associated with two factors acting together - shallow sources and preferential pathways. KEY POINT:

Framework for Evaluation of Petroleum Vapor Intrusion Sites: Based on Modeling and Empirical Data (McHugh et al.1) 1 LNAPL or Dissolved Plume in Contact with Foundation: HIGHER RISK BUILDING NAPL Affected GW GW-Bearing Unit Unsaturated Soil Proposed Action: Test hydrocarbon concentrations inside structure. Mitigate as needed. 1) Adapted from McHugh, Davis, DeVaull, Hopkins, Menatti, and Peargin, Evaluation of Vapor Attenuation at Petroleum Hydrocarbon Sites: Considerations for Site Screening and Investigation, Soil and Sediment Contamination: An International Journal, November/December 2010, Vol. 19, No. 5. 37 37

Framework for Evaluation of Petroleum Vapor Intrusion Sites: Based on Modeling and Empirical Data (McHugh et al.1) 2 LNAPL present 3 to 10 m (10 to 30 ft) below building foundation: MEDIUM RISK Groundwater-Bearing Unit BUILDING Unsaturated Soil LNAPL ? 3 to 10 m Subsurface LNAPL: Vapor intrusion observed at a few large release sites (refineries) but not at UST sites. (10 to 30 ft) Proposed Action: Test for hydrocarbons in shallow soil gas below or directly adjacent to building foundation. 1) Adapted from McHugh, Davis, DeVaull, Hopkins, Menatti, and Peargin, Evaluation of Vapor Attenuation at Petroleum Hydrocarbon Sites: Considerations for Site Screening and Investigation, Soil and Sediment Contamination: An International Journal, November/December 2010, Vol. 19, No. 5. 38 38

Framework for Evaluation of Petroleum Vapor Intrusion Sites: Based on Modeling and Empirical Data (McHugh et al.1) 3 LNAPL present >10 to 30 ft below building foundation: LOWER RISK Groundwater-Bearing Unit BUILDING Unsaturated Soil >3 to 10 m LNAPL (>10 to 30 ft) Proposed Action: Evaluate presence of preferential flow pathways or other site-specific risk factors. Testing for hydrocarbons in shallow soil gas below or directly adjacent to building foundation may be appropriate. 1) Adapted from McHugh, Davis, DeVaull, Hopkins, Menatti, and Peargin, Evaluation of Vapor Attenuation at Petroleum Hydrocarbon Sites: Considerations for Site Screening and Investigation, Soil and Sediment Contamination: An International Journal, November/December 2010, Vol. 19, No. 5. 39 39

Framework for Evaluation of Petroleum Vapor Intrusion Sites: Based on Modeling and Empirical Data (McHugh et al.1) 4 Dissolved hydrocarbon plume 5 to 10 ft below building: LOWER RISK BUILDING Unsaturated Soil >1.5 to 3 m Affected GW (>5 to 10 ft) Proposed Action: Evaluate presence of preferential flow pathways or other site-specific risk factors. 1) Adapted from McHugh, Davis, DeVaull, Hopkins, Menatti, and Peargin, Evaluation of Vapor Attenuation at Petroleum Hydrocarbon Sites: Considerations for Site Screening and Investigation, Soil and Sediment Contamination: An International Journal, November/December 2010, Vol. 19, No. 5. 40 40

Types of Vapor Intrusion Models
Predictions based on observations from other sites (e.g., attenuation factors). Empirical (Tier 1) Mathematical equation based on simplification of site conditions (e.g., Johnson and Ettinger). Analytical (Tier 2) SIMPLE MATH Numerical models: - Abreu and Johnson, Bozkurt et al. Mass flux model, foundation transport model, etc. Others (Tier 3) KEY POINT: Wide range of approaches to vapor intrusion modeling, varying in complexity and specificity.

Vapor Intrusion Models Johnson and Ettinger Model (Tier 2) Building Attenuation Due to Exchange with Ambient Air 1 RESIDENTIAL BUILDING Air Exchange Advection and Diffusion Through Unsaturated Soil and Building Foundation Unsaturated Soil 2 Equilibrium Partitioning Between GW and Soil Vapor Csv = Cgw x H’ source area Groundwater-Bearing Unit 3 KEY POINT: “Site-specific” predictions based on soil type, depth to groundwater, and building characteristics. H = Henry’s Law Constant

J&E Model: Key Assumptions
Vapor Intrusion Models J&E Model: Key Assumptions 1-D Steady-State Model Does not account for heterogeneities, preferential pathways, or temporal variation. No mass balance; mass flux into building can exceed available source mass. Infinite Source soil vapor No Bio-degradation Does not account for biotransformation in the vadose zone Affected GW Plume KEY POINT: J&E model is generally conservative, but model error can be very large (orders-of-magnitude).

BioVapor: 1-D VI Model w/ Bio
n Conceptual Model n Model Inputs n Model Outputs n Example Model Validation

What is BioVapor? 1-D Analytical Model Oxygen Mass Balance O2 HC
Conceptual Model What is BioVapor? 1-D Analytical Model Version of Johnson & Ettinger vapor intrusion model modified to include aerobic biodegradation (DeVaull, 2007). SIMPLE MATH Oxygen Mass Balance Uses iterative calculation method to account for limited availability of oxygen in vadose zone. O2 HC Simple interface intended to facilitate use by wide range of environmental professionals. User-Friendly Easy-to-use vapor intrusion model that accounts for oxygen-limited aerobic vapor intrusion. Free download at: KEY POINT:

BioVapor: Conceptual Model
3 Advection, diffusion, and dilution through building foundation aerobic zone 2 Diffusion & 1st order biodegradation in aerobic zone Ct anaerobic zone 1 Diffusion only in anaerobic zone Cs Vapor Source

Calculations are cheap & quick
Conceptual Model BioVapor: Oxygen Mass Balance Iterative Calculation Method Calculate oxygen demand: - depth of aerobic zone - HC vapor concentration - 1st order biodegradation ?? No anaerobic interface Increase or decrease depth of aerobic zone O2 demand = supply? ?? Yes Vapor Source KEY POINT: Calculations are cheap & quick Final Model Solution

BioVapor: Intended Application
Conceptual Model BioVapor: Intended Application Obtain improved understanding of petroleum vapor intrusion. Calculate oxygen concentration/flux required to support aerobic biodegradation. Identify important model input parameters and evaluate model sensitivity. Yes Simplifying Assumptions 1-D Model: Does not account for spatial variability Steady State: Does not account for temporal variability Single Source: Does not account for indoor sources and other background sources of petroleum VOCs

Model Inputs Data Requirements

Baseline Risk Calculation Risk-Based Cleanup Level Calculation
Human Health Risk Chemical Toxicity Exposed Dose COC Fate & Transport x = Baseline Risk Calculation Risk-Based Cleanup Level Calculation RISK = ? SSTL = ? START W / COC CONC COC = Chemical of Concern; SSTL = Site-Specific Target Level START W / RISK LIMIT Forward and Backward Calculations Model Inputs

Backward Calculations: Conc. Vs. Risk
Model Inputs Backward Calculations: Conc. Vs. Risk COC Fate & Transport Human Health Risk Exposed Dose Chemical Toxicity x x = OPTION 1: OPTION 2: Calculation based on target indoor air concentration (from BioVapor database) Calculation based on target indoor air risk limits (enter by user)

Environmental Factors
Model Inputs Environmental Factors

Environmental Factors
Model Inputs Environmental Factors KEY POINT: Model inputs similar to J&E, plus a few new inputs related to oxygen-limited biodegradation: - New inputs can be measured or estimated.

Oxygen Boundary Condition
Model Inputs Oxygen Boundary Condition Open Soil: (Constant O2 Conc.) Constant oxygen concentration at top of vadose zone: - 21% oxygen in dirt crawl space - Measured oxygen concentration below solid foundation 21% O2 Dirt Crawl Space Solid Foundation: (Constant Air Flow) Constant oxygen flux across top of vadose zone: - Air flow from atmosphere to below building foundation Solid Foundation User-specified depth of aerobic zone: - Based on measured vertical profile in vadose zone - No O2 mass balance Fixed Aerobic Depth Aerobic Anaerobic

Baseline Soil Respiration Rate
Conceptual Model Baseline Soil Respiration Rate WHAT: Rate of oxygen consumption in absence of hydrocarbon vapors (due to existing soil bacteria) Oxygen concentration OPTION 1: Enter directly OPTION 2: Estimate from soil organic carbon Base,O2 = x foc No Hydrocarbon Source (equation from, DeVaull, 2007 based on data from several studies) LIMITATIONS: foc > baseline respiration can be very high. foc < baseline respiration variable, but generally low.

Source Type: Soil gas or Groundwater
Model Inputs Source Type: Soil gas or Groundwater Soil Gas: Enter VOC concentrations in soil gas. Soil gas data available - NAPL source Groundwater: Enter VOC concentrations in groundwater. Dissolved VOC plume, no NAPL Requires use-specified groundwater to soil gas attenuation factor (AFGW-SG) Software Calculation: CSG = CGW x H’ x AFGW-SG

Source Depth (L): Foundation to Source
Separation Distance (L) From Bottom of Foundation (basement, slab-on-grade, crawlspace floor) to Top of residual petroleum (actual or suspected) ‘dirty soil’. Water table or capillary fringe.

Model Inputs Chemicals Risk Drivers: Vadose zone transport/oxygen demand and indoor concentration/risk. Other Hydrocarbons: Only vadose zone transport/oxygen demand - Not considered risk drivers - No well accepted tox. values Hydrocarbon Surrogates: Only vadose zone transport/oxygen demand - One surrogate can represent multiple hydrocarbons KEY POINTS: All vapor-phase hydrocarbons must be included in model for proper oxygen mass balance. Can edit chemical database and add new chemicals.

Moderately Weathered Gasoline
Model Inputs Typical Vapor Composition: NAPL Source Moderately Weathered Gasoline Fresh Gasoline Weathered Crude Oil Benzene T, E, X Other Aromatic HCs Aliphatic HCs* % 1 - 2 % 1 - 4% 5 - 15% <0.1% <1% % % <0.02 – 0.5% <0.02 – 2% 0.01 – 2% 96 – 99.8% * More than 90% of aliphatic hydrocarbons are pentane, methylated butanes and pentanes, and n-hexane. n Vapor composition can be estimated based on i) product type and ii) either BTEX or total TPH data. n May need to consider methane. Source concentrations can be in percent-range (>10,000 ppmv). KEY POINTS: * = Value based on MCL, risk-based number would be lower.

? Chemicals Concentrations Option 1: Individual COCs
Model Inputs Chemicals Concentrations Collect source vapor sample and analyze for individual COCs: TO-15 w/ modified data processing to quantify C5 & C6 aliphatics. Option 1: Individual COCs Measure Source BTEX Concentration: - Dissolved source = mostly BTEX - NAPL Source = estimate TPH concentration (e.g., benzene x 100). ? Option 2: BTEX Data For NAPL source, measure TPH Concentration: - Estimate BTEX concentrations (e.g., benzene = TPH/100) Option 3: TPH Data

Biodegradation Rates Model Inputs
n Petroleum rapidly biodegrades in vadose zone with oxygen n Geometric mean first-order rates: - BTEX = 0.79 /hr - Aliphatics = 71 /hr (DeVaull, 2007) n Biodegradation occurs in pore water n User can edit default biodegradation rates This slide defines the “alpha” value in terms of an empirical “pseudo” first-order biodegradation rate. Biodegradation is presumed in the water phase. The term includes moisture “theta-w” in soil, and Henry’s law coefficient (vapor/water partitioning). In diffusive transport, the effective diffusion coefficient in soil is included. There’s a lot of data confirming petroleum biodegradation. This shows published data for non-amended (not fertilized) aerobic native soils, where either consistently defined rates were included in the publication, or could be derived (in the water phase) with the published data. This is extensive good supporting data for selecting rates. The first-order biodegradation rate is defined conditional on aerobic soil conditions (oxygen present) [therefore a “pseudo” first-order rate], and derived from data. Available data is shown, here broken down by chemical and chemical class. This is experimental data from laboratory microcosms, diffusive columns, convective flow columns, soil reactors, field data. Points on a distribution are shown. Most of the distributions are log-normal (skewed – a few high rates), so the median is much less than the arithmetic average. Chemical-specific trends are evident. Long-chain paraffins (alkanes) degrade faster than short-chain or highly branched alkanes. Single ring aromatics (BTEX) are the slowest, although all of the observed rates are relatively fast (compared to diffusion rates). Methane is shown. Naphthalene is shown. Our application is in soil vapor, so the observed rate trend does shift when presented in the vapor phase. Henry’s law for BTEX is approximately 0.3, while for some of the alkanes it is about 30 or 40. So the observed reaction rate for BTEX can be faster [degradation occurs over less distance] than for isooctane (trimethylpentane), in air-filled soils. The final caveat is that soils are not always aerobic, especially because these petroleum chemicals can have high oxygen demand at high concentrations. There are qualitative arguments on molecular structure relating to degradation rate. Here it is plotted. Also note that “first-order” biodegradation may not always match the data. Other equation forms are proposed, and some other equation forms are better data fits for selected chemicals, sometimes over limited data sets. The biodegradation data is discussed in DeVaull, Env. Sci & Technol., Additional published data has since been added to the data set and the presentation is different here. Tabulated rates are for vadose-zone soils - not directly comparable for other environmental media surface water, ‘ready-biodegradability’ testing, for example. Averaging volume adjustments over very large changes in volumes (lengths) may need consideration. Thiele efficiencies adjustments at smaller scale (flocs, biofilms, microbes), heterogeneity at larger scales (mesoscales, watersheds, etc.). Water-phase rates Also: van Groenestijn, J. W.; Hesselink, P. G. M., Biotechniques for air pollution control, Biodegradation, 1993, 4, Conventional techniques such as biofilters have low elimination capacities for hydrophobic compounds caused by a poor mass transfer from the gas to the aqueous phase. Add oil and/or activated carbon to biofilter (this would increase the residence time)

Vapor Intrusion Risk Results
Model Outputs Vapor Intrusion Risk Results

Vapor Intrusion Risk Results
Model Outputs Vapor Intrusion Risk Results KEY POINT: n Model sometimes, but not always, predicts high attenuation factors.

Aerobic/anaerobic interface
Model Outputs Vapor Intrusion Risk Results Aerobic zone Anaerobic zone Aerobic/anaerobic interface Source

Model Outputs Detailed Results

Detailed Results: VOC Attenuation
Model Outputs Detailed Results: VOC Attenuation Conclusion: For this model scenario, most VOC attenuation occurs in aerobic zone.

Detailed Results: Oxygen Demand
Model Outputs Detailed Results: Oxygen Demand Conclusion: For this model scenario, most oxygen demand is from baseline soil respiration.

n Conceptual Model n Model Inputs n Model Outputs Example Model Validation

Gas Station Site, Beaufort SC
1 Measured benzene (and other hydrocarbons) plus oxygen in vertical profile (Data from Lahvis et al., 1999) 2 Compared field measurements to BioVapor model prediction using model default input values Hydrocarbon Source Field data from Lahvis et al., Water Resources Research, Vol. 35, No. 3, pp Model validation work conducted by Robin Davis, UDEQ

BioVapor Model: Gas Station Site, Beaufort SC
Foundation Type: Bare soil 21% Oxygen at top of vadose zone Source Concentration (GW Source): Benzene: 16 mg/L Ethylbenzene: 5.7 mg/L Toluene: 26 mg/L Xylenes: 25 mg/L Other Hydrocarbons (TPH): 65 mg/L Field data from Lahvis et al., Water Resources Research, Vol. 35, No. 3, pp Model validation work conducted by Robin Davis, UDEQ

BioVapor Model: Gas Station Site, Beaufort SC
Field Data Field Data Conclusion: Using defaults, model under-predicts observed vadose zone attenuation. Similar findings for other sites. Field data from Lahvis et al., Water Resources Research, Vol. 35, No. 3, pp Model validation work conducted by Robin Davis, UDEQ

BioVapor Acknowledgements
n BioVapor Analytical Model: George DeVaull, Shell Global Solutions n BioVapor Software Interface: Paul Newberry, GSI Environmental n Project Funding, Review, Support: API Soil and Groundwater Task Force Roger Claff (API) & Harley Hopkins (now w/ Exxon) BioVapor Software Available from API web site:

BioVapor Application of BioVapor to Petroleum Vapor Intrusion Sites

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BioVapor Application of BioVapor to Petroleum Vapor Intrusion Sites

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