GSI ENVIRONMENTAL INC. Houston, Texas (713) Workshop 1: Assessment and Evaluation of Vapor Intrusion at Petroleum Release Sites BioVapor: A 1-D Vapor Intrusion Model with Oxygen- Limited Aerobic Biodegradation

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

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

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

Conceptual Model Model Inputs Model Outputs Case Studies: Example Results Conceptual Model Model Inputs Model Outputs Case Studies: Example Results BioVapor: 1-D VI Model w/ Bio

What is BioVapor? Free, easy-to-use vapor intrusion model that accounts for oxygen-limited aerobic vapor intrusion. KEY POINT: 1-D Analytical Model Oxygen Mass Balance Version of Johnson & Ettinger vapor intrusion model modified to include aerobic biodegradation (DeVaull, 2007). Uses iterative calculation method to account for limited availability of oxygen in vadose zone. Simple interface intended to facilitate use by wide range of environmental professionals. User- Friendly O2O2 HC SIMPLE MATH Conceptual Model

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

BioVapor: Oxygen Mass Balance Conceptual Model Calculate oxygen demand: - depth of aerobic zone - HC vapor concentration - 1st order biodegradation Iterative Calculation Method Vapor Source anaerobic interface ?? O 2 demand = supply? Final Model Solution Yes No Increase or decrease depth of aerobic zone Calculations are cheap & quick KEY POINT:

BioVapor: Intended Application Conceptual Model YES 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. Predict hydrocarbon concentration in indoor air within <10x. - Site complexity - Temporal variability - Indoor background NO

Conceptual Model Model Inputs Model Outputs Case Studies: Example Results Conceptual Model Model Inputs Model Outputs Case Studies: Example Results BioVapor: 1-D VI Model w/ Bio

Model Inputs Data Requirements

Model Inputs Environmental Factors

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

Oxygen Boundary Condition Open Soil: (Constant O 2 Conc.) Solid Foundation: (Constant Air Flow) Constant oxygen concentration at top of vadose zone: - 21% oxygen in dirt crawl space - Measured oxygen concentration below solid foundation Constant oxygen flux across top of vadose zone: - Air flow from atmosphere to below building foundation User-specified depth of aerobic zone: - Based on measured vertical profile in vadose zone - No O 2 mass balance Fixed Aerobic Depth Model Inputs Dirt Crawl Space 21% O 2 Solid Foundation Aerobic Anaerobic

Human Health Risk Chemical Toxicity Exposed Dose COC Fate & Transport x = 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

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

Baseline Soil Respiration Rate Conceptual Model No Hydrocarbon Source Oxygen concentration WHAT: Rate of oxygen consumption in absence of hydrocarbon vapors (due to existing soil bacteria) OPTION 1: Enter directly OPTION 2: Estimate from soil organic carbon Base,O 2 (equation from, DeVaull, 2007 based on data from several studies) = 1.69 x f o c f oc > baseline respiration can be very high. f oc < baseline respiration variable, but generally low. LIMITATIONS:

Source Type: Soil gas or Groundwater Model Inputs 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 (AF GW-SG ) Software Calculation: C SG = C GW x H’ x AF GW-SG

Chemicals Model Inputs 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 All vapor-phase hydrocarbons must be included in model for proper oxygen mass balance. Can edit chemical database and add new chemicals. KEY POINTS:

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

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

Petroleum rapidly biodegrades in vadose zone with oxygen Geometric mean first- order rates: - BTEX = 0.79 /hr - Aliphatics = 71 /hr (DeVaull, 2007) Biodegradation occurs in pore water User can edit default biodegradation rates Biodegradation Rates Model Inputs

Conceptual Model Model Inputs Model Outputs Case Studies: Example Results Conceptual Model Model Inputs Model Outputs Case Studies: Example Results BioVapor: 1-D VI Model w/ Bio

Model Outputs Vapor Intrusion Risk Results

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

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

Model Outputs Detailed Results

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

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

Conceptual Model Model Inputs Model Outputs Case Studies: Example Results Conceptual Model Model Inputs Model Outputs Case Studies: Example Results BioVapor: 1-D VI Model w/ Bio

Case 1: Effect of Source Depth Case Study Fresh Gasoline Vapor Source CsCs CsCs CtCt CtCt Safe distance? Environmental Factors: - Residential building (slab-on-grade) - 21% O 2 below slab - Dry, sandy soil Petroleum Source: GRO TPH Conc. = 1.5% (40,000,000 ug/m 3 ) Benzene Conc. = 400,000 ug/m 3 (1% of TPH Conc.) Model Inputs QUESTION: Safe distance from source to building?

1.0E+02 Case 1: Effect of Source Depth Case Study Fresh Gasoline Vapor Source CsCs CsCs CtCt CtCt Safe distance? QUESTION: Safe distance from source to building? Benzene Concentration in Indoor Air (ug/m 3 ) Distance (feet) ANSWER: Model predicts sufficient attenuation w/ 2.8 ft of clean soil above source. However, may need safety factor to account for model uncertainty (e.g., where is top of source?) Risk Limit (3.1 ug/m 3 ) 2.8 ft

Case 2: Effect of Oxygen Concentration Case Study Fresh Gasoline Vapor Source CsCs CsCs CtCt CtCt 10 ft Environmental Factors: - Residential building (slab-on-grade) - Dry, sandy soil - Source depth = 10 ft Petroleum Source: TPH Conc. = 1.5% (40,000,000 ug/m 3 ) Benzene Conc. = 400,000 ug/m 3 (1% of TPH Conc.) Model Inputs QUESTION: How much oxygen required below foundation to protect building?

Case 2: Effect of Oxygen Case Study Fresh Gasoline Vapor Source CsCs CsCs CtCt CtCt 10 ft QUESTION: How much oxygen required below foundation to protect building? Benzene Concentration in Indoor Air (ug/m 3 ) Oxygen Concentration Below Foundation (%) Risk Limit 2.5% ANSWER: Model predicts 2.5% oxygen below foundation will protect building. (However, need may safety factor to account for model uncertainty.) Risk Limit (3.1 ug/m 3 ) 2.5 %

Software and Testing Testing Final Software Software evaluated by USEPA contractor. Verified accuracy of model math. Available from API web site: apor/index.cfm BioVapor Model

BioVapor Analytical Model: George DeVaull, Shell Global Solutions BioVapor Software Interface: Paul Newberry, GSI Environmental Project Funding, Review, Support: API Soil and Groundwater Task Force Harley Hopkins (now w/ Exxon) & Roger Claff BioVapor Analytical Model: George DeVaull, Shell Global Solutions BioVapor Software Interface: Paul Newberry, GSI Environmental Project Funding, Review, Support: API Soil and Groundwater Task Force Harley Hopkins (now w/ Exxon) & Roger Claff Acknowledgements Contact Information Roger Claff (202)