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Workshop 1: Assessment and Evaluation of Vapor Intrusion at Petroleum Release Sites. BioVapor: A 1-D Vapor Intrusion Model with Oxygen-Limited Aerobic Biodegradation. GSI ENVIRONMENTAL INC. Houston, Texas www.gsi-net.com (713) 522-6300 temchugh@gsi-net.com. Vapor Intrusion Models.
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Workshop 1: Assessment and Evaluation of Vapor Intrusion at Petroleum Release Sites BioVapor: A 1-D Vapor Intrusion Model with Oxygen-Limited Aerobic Biodegradation GSI ENVIRONMENTAL INC. Houston, Texas www.gsi-net.com (713) 522-6300 temchugh@gsi-net.com
Vapor Intrusion Models Types of Vapor Intrusion Models Predictions based on observations from other sites (e.g., attenuation factors). Empirical (Tier 1) Analytical (Tier 2) Mathematical equation based on simplification of site conditions (e.g., Johnson and Ettinger). SIMPLE MATH Numerical models:- Abreu and Johnson, Bozkurt et al.Mass flux model, foundation transport model, etc. Others (Tier 3) Wide range of approaches to vapor intrusion modeling, varying in complexity and specificity. KEY POINT:
Groundwater-Bearing Unit 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 3 “Site-specific” predictions based on soil type, depth to groundwater, and building characteristics. KEY POINT: H = Henry’s Law Constant
Vapor Intrusion Models J&E Model: Key Assumptions Does not account for heterogeneities, preferential pathways, or temporal variation. 1-D Steady-State Model No mass balance; mass flux into building can exceed available source mass. Infinite Source soil vapor Does not account for biotransformation in the vadose zone No Bio-degradation Affected GW Plume J&E model is generally conservative, but model error can be very large (orders-of-magnitude). KEY POINT:
BioVapor: 1-D VI Model w/ Bio nConceptual Model nModel Inputs nModel Outputs nCase Studies: Example Results
O2 HC Conceptual Model What is BioVapor? Version of Johnson & Ettinger vapor intrusion model modified to include aerobic biodegradation (DeVaull, 2007). 1-D Analytical Model SIMPLE MATH Oxygen Mass Balance 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 Free, easy-to-use vapor intrusion model that accounts for oxygen-limited aerobic vapor intrusion. KEY POINT:
3 Advection, diffusion, and dilution through building foundation aerobic zone 2 Diffusion & 1st order biodegradation in aerobic zone 1 Diffusion only in anaerobic zone Conceptual Model BioVapor: Conceptual Model Ct anaerobic zone Cs Vapor Source
Conceptual Model BioVapor: Oxygen Mass Balance Iterative Calculation Method Calculate oxygen demand:- depth of aerobic zone- HC vapor concentration- 1st order biodegradation ?? No Increase or decrease depth of aerobic zone anaerobic interface O2 demand = supply? ?? Yes Vapor Source KEY POINT: Calculations are cheap & quick Final Model Solution
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 • Predict hydrocarbon concentration in indoor air within <10x. - Site complexity - Temporal variability - Indoor background NO
BioVapor: 1-D VI Model w/ Bio nConceptual Model nModel Inputs nModel Outputs nCase Studies: Example Results
Model Inputs Data Requirements
Model Inputs 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.
Model Inputs Oxygen Boundary Condition Constant oxygen concentration at top of vadose zone: - 21% oxygen in dirt crawl space - Measured oxygen concentration below solid foundation Open Soil: (Constant O2 Conc.) 21% O2 Dirt Crawl Space Constant oxygen flux across top of vadose zone: - Air flow from atmosphere to below building foundation Solid Foundation: (Constant Air Flow) 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
START W/ COC CONC START W/ RISK LIMIT Model Inputs Forward and Backward Calculations Human Health Risk COCFate &Transport Exposed Dose Chemical Toxicity x = x RISK = ? Baseline Risk Calculation SSTL = ? Risk-Based Cleanup Level Calculation COC = Chemical of Concern; SSTL = Site-Specific Target Level
OPTION 1: OPTION 2: Model Inputs Backward Calculations: Conc. Vs. Risk Human Health Risk COCFate &Transport Exposed Dose Chemical Toxicity x = x Calculation based on target indoor air concentration (from BioVapor database) Calculation based on target indoor air risk limits (enter by user)
WHAT: OPTION 1: OPTION 2: Enter directly Estimate from soil organic carbon Base,O2 Conceptual Model Baseline Soil Respiration Rate Rate of oxygen consumption in absence of hydrocarbon vapors (due to existing soil bacteria) Oxygenconcentration = 1.69 x foc No Hydrocarbon Source (equation from, DeVaull, 2007 based on data from several studies) LIMITATIONS: foc >0.02 - baseline respiration can be very high. foc <0.001 - baseline respiration variable, but generally low.
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
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.
Model Inputs Typical Vapor Composition: NAPL Source ModeratelyWeathered Gasoline Fresh Gasoline Weathered Crude Oil Benzene 0.25 - 1% 1 - 2 % <0.02 – 0.5% T, E, X 1 - 4% 5 - 15% <0.02 – 2% Other Aromatic HCs <1% 0.01 – 2% <0.1% Aliphatic HCs* 85 - 90% 96 – 99.8% 95 - 99% * More than 90% of aliphatic hydrocarbons are pentane, methylated butanes and pentanes, and n-hexane. nVapor composition can be estimated based on i) product type and ii) either BTEX or total TPH data. nMay 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.
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
Model Inputs Biodegradation Rates nPetroleum rapidly biodegrades in vadose zone with oxygen nGeometric mean first-order rates: - BTEX = 0.79 /hr - Aliphatics = 71 /hr (DeVaull, 2007) nBiodegradation occurs in pore water nUser can edit default biodegradation rates
BioVapor: 1-D VI Model w/ Bio nConceptual Model nModel Inputs nModel Outputs nCase Studies: Example Results
Model Outputs Vapor Intrusion Risk Results
Model Outputs Vapor Intrusion Risk Results nModel sometimes, but not always, predicts high attenuation factors. KEY POINT:
Aerobic zone Aerobic/anaerobic interface Anaerobic zone Model Outputs Vapor Intrusion Risk Results Source
Model Outputs Detailed Results
Model Outputs Detailed Results: VOC Attenuation Conclusion: For this model scenario, most VOC attenuation occurs in aerobic zone.
Model Outputs Detailed Results: Oxygen Demand Conclusion: For this model scenario, most oxygen demand is from baseline soil respiration.
BioVapor: 1-D VI Model w/ Bio nConceptual Model nModel Inputs nModel Outputs nCase Studies: Example Results
Safe distance? Ct Cs QUESTION: Fresh Gasoline Vapor Source Safe distance from source to building? Case Study Case 1: Effect of Source Depth Model Inputs • Environmental Factors: - Residential building (slab-on-grade)- 21% O2 below slab- Dry, sandy soil • Petroleum Source:GRO TPH Conc. = 1.5% (40,000,000 ug/m3)Benzene Conc. = 400,000 ug/m3(1% of TPH Conc.)
Safe distance? Ct Cs Fresh Gasoline Vapor Source QUESTION: Safe distance from source to building? Case Study Case 1: Effect of Source Depth 1.0E+02 ANSWER: 10-5 Risk Limit (3.1 ug/m3) 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?) Benzene Concentration in Indoor Air (ug/m3) 2.8 ft Distance (feet)
10 ft Ct Cs Fresh Gasoline Vapor Source Case Study Case 2: Effect of Oxygen Concentration QUESTION: How much oxygen required below foundation to protect building? Model Inputs • Environmental Factors: - Residential building (slab-on-grade)- Dry, sandy soil- Source depth = 10 ft • Petroleum Source:TPH Conc. = 1.5% (40,000,000 ug/m3)Benzene Conc. = 400,000 ug/m3(1% of TPH Conc.)
10 ft Ct Cs Fresh Gasoline Vapor Source Case Study Case 2: Effect of Oxygen QUESTION: How much oxygen required below foundation to protect building? 10-5 Risk Limit (3.1 ug/m3) ANSWER: 10-5 Risk Limit Model predicts 2.5% oxygen below foundation will protect building. (However, need may safety factor to account for model uncertainty.) Benzene Concentration in Indoor Air (ug/m3) 2.5% 2.5 % Oxygen Concentration Below Foundation (%)
BioVapor Model Software and Testing Testing • Software evaluated by USEPA contractor. • Verified accuracy of model math. Available from API web site: http://www.api.org/ehs/groundwater/vapor/index.cfm Final Software
Acknowledgements nBioVapor Analytical Model: George DeVaull, Shell Global Solutions nBioVapor Software Interface: Paul Newberry, GSI Environmental nProject Funding, Review, Support: API Soil and Groundwater Task ForceHarley Hopkins (now w/ Exxon) & Roger Claff Contact Information www.api.org/viRoger Claff (Claff@api.org) (202) 682-8399