The Ira A. Fulton School of EngineeringArizona State University Paul Johnson, Ph.D. Lilian Abreu Ph.D. Candidate Department of Civil and Environmental Engineering Ira A. Fulton School of Engineering Site-Specific Modeling in the Context of the OSWER Guidance? OSWER Guidance

The Ira A. Fulton School of EngineeringArizona State University Tier 3: Site-Specific Pathway Assessment “Modeling is considered to be useful for determining which combination of complex factors (e.g., soil type, depth to groundwater, building characteristics, etc.) lead to the greatest impact and, consequently, aid in the selection of buildings to be sampled. It is recommended that sampling of sub-slab or crawlspace vapor concentrations and/or sampling of indoor air concentrations be conducted before a regulator makes a final decision…” OSWER Guidance (11/29/02)

The Ira A. Fulton School of EngineeringArizona State University Tier 3: Site-Specific Pathway Assessment - Issues Why are you limited to near-foundation (e.g., sub-slab) soil gas data in Tier 3, when you can use soil gas data at any depth or groundwater data in Tier 2? Why is semi-site-specific J&E modeling used in Tier 2 to assess impacts, but site-specific J&E modeling is not allowed in Tier 3? If you allow site-specific modeling to decide on a subset of buildings does not need to be monitored - aren’t you using it to screen out sites? If you could do site-specific modeling in Tier 3 - who is qualified to perform it and who is qualified to review the output? Future use scenarios/no building currently present? OSWER Guidance (11/29/02)

The Ira A. Fulton School of EngineeringArizona State University Site-Specific Modeling Options… [What might we do if we ignored the current language in the OSWER guidance?] 1.Site-specific  -value determined from “tracer” input 2.Use of J&E model with site- specific inputs 3.Multi-dimensional numerical codes

The Ira A. Fulton School of EngineeringArizona State University Determination of, and use of, a site- specific  -value measure soil gas and indoor concentration of tracer (a conservative chemical not expected to be confounded by ambient or indoor air background sources; radon, 1,1 DCE, etc.) Derive site-specific  -value Estimation indoor air concentrations for chemicals of concern using that site-specific  -value measure soil gas and indoor concentration of tracer (a conservative chemical not expected to be confounded by ambient or indoor air background sources; radon, 1,1 DCE, etc.) Derive site-specific  -value Estimation indoor air concentrations for chemicals of concern using that site-specific  -value Option #1

The Ira A. Fulton School of EngineeringArizona State University enclosed space vapor migration Diffusion [pseudo- steady state] Well-Mixed Source [steady or transient] Vapor Source Use of J&E (1991) model for site-specific assessment Option #2 Layered settings? Measured D eff ? Perched water? Fresh- water lens? Site-specific  = Q soil /Q b

The Ira A. Fulton School of EngineeringArizona State University Generalized Sensitivity Assessment of the J&E (1991) Model * The output only depends on three parameters (A, B, C) If you understand sensitivity to those three parameters, you can quickly assess the sensitivity to any specific input. P.C. Johnson Sensitivity Analysis and Identification of Critical and Non-Critical Parameters for the Johnson and Ettinger (1991) Vapor Intrusion Model. API Technical Bulletin.

The Ira A. Fulton School of EngineeringArizona State University Generalized Sensitivity Assessment of the J&E (1991) Model

The Ira A. Fulton School of EngineeringArizona State University Generalized Sensitivity Assessment of the J&E (1991) Model If your analysis suggests “high” sensitivity to any inputs…you are probably using: -an inconsistent set of input values, or -an unreasonable set or unreasonable range of input values Most of the time critical*, but pretty well-constrained: [(V B /A B ), L T, D T eff, E B ] Sometimes critical, but data hints at their reasonable values: [(Q s /Q B )] Rarely critical, and any reasonable value works: [ , L crack, D crack eff ] Most of the time critical*, but pretty well-constrained: [(V B /A B ), L T, D T eff, E B ] Sometimes critical, but data hints at their reasonable values: [(Q s /Q B )] Rarely critical, and any reasonable value works: [ , L crack, D crack eff ]

The Ira A. Fulton School of EngineeringArizona State University Needed Improvements… Confusion stemming from (improper) use of the EPA spreadsheets could be minimized with the following changes: 1) Reformat the calculations in terms of: [(V B /A B ), L T, D T eff, E B, , (Q s /Q B ), L crack, and D crack eff ] 2)Eliminate the Q s calculation and input (Q s /Q B ) values based on empirical analysis. 3)Input moisture saturations instead of individual moisture contents and total porosities 4)Integrate the spreadsheet with the graphical flowchart for identifying critical parameters 5)Constrain users to reasonable ranges and combinations of inputs… 1) Reformat the calculations in terms of: [(V B /A B ), L T, D T eff, E B, , (Q s /Q B ), L crack, and D crack eff ] 2)Eliminate the Q s calculation and input (Q s /Q B ) values based on empirical analysis. 3)Input moisture saturations instead of individual moisture contents and total porosities 4)Integrate the spreadsheet with the graphical flowchart for identifying critical parameters 5)Constrain users to reasonable ranges and combinations of inputs…

The Ira A. Fulton School of EngineeringArizona State University small diameter tubing In Situ Diffusion Coeff. Measurements syringe tracer gas sample volume (≈ 9 cm radius) 1-L tedlar bag Time = 0Time = t 1 Time = t 2 Time Soil Gas Withdrawn % Mass Recovered Johnson et al. ES&T 1998

The Ira A. Fulton School of EngineeringArizona State University Effect of lateral separation between building and vapor source? Effect of building construction (slab vs. basement)? Sub-foundation vs. near-foundation soil gas sampling? Effect of aerobic biodegradation on  ? Near foundation soil characteristics Variation in  withconcentration, depth and soil type? Multi-dimensional multi-component numerical code Option #3 Effect of changing atmospheric conditions and occupant habits? Future use scenarios?

The Ira A. Fulton School of EngineeringArizona State University Sample details for simulations Grid spacing is variable - finer detail near cracks, source boundaries, and domain boundaries 10 m x 10m footprint 30 m x 30 m constant source (200 mg/L-vapor) Fine to medium sand 5 Pa constant building under-pressurization 1 mm wide full perimeter crack 12/d exchange rate

The Ira A. Fulton School of EngineeringArizona State University A Sample Pressure Field… Symmetrical Simulation - cross-section through plane of symmetry

The Ira A. Fulton School of EngineeringArizona State University Changes in  with Source Position and Depth… No biodegradation Alpha=1.2e-3, Qs=4.1 L/min

The Ira A. Fulton School of EngineeringArizona State University Changes in  with Source Position and Depth… No biodegradation Alpha=9.3e-6, Qs=4.1 L/min

The Ira A. Fulton School of EngineeringArizona State University No biodegradation Changes in  with Source Position and Depth… Source no longer under building

The Ira A. Fulton School of EngineeringArizona State University alpha=1.2e-3; Qs=4.0 L/min alpha=6.1e-4, Qs=5.1 L/min Changes in  with Building Cons- truction No biodegradation

The Ira A. Fulton School of EngineeringArizona State University Near- Foundation vs. Through- the- Foundation Sampling? alpha=1.4e-4 (w/biodegradation) alpha=1.2e-3 (no bio)

The Ira A. Fulton School of EngineeringArizona State University Changes in  with Depth with Bio- decay alpha=1.3e-18 (w/biodegradation) alpha=5.7e-4 (no bio)

The Ira A. Fulton School of EngineeringArizona State University In Progress… 1.Manuscript #1 – Model development and application to study of lateral distance and depths vs. impacts. 2.Manuscript #2 – Effects of aerobic biodegradation on impacts (source strength, depth, distance) 3.Study of role of sub-slab characteristics, pressure fluctuations, wind effects, etc. 4.Use of model to develop nomograph identifying sites where impacts may not be significant, based on Building footprint Depth to vapor source Vapor source strength

The Ira A. Fulton School of EngineeringArizona State University Final Thoughts… 1.Draft OSWER Guidance is inconsistent with respect to the role of modeling for site-specific pathway assessment (and the role of modeling in general..) 2.If the role of site-specific modeling is expanded, then we need to be prepared to address: What options are allowed? What data is required? How to ensure that the use of site-specific modeling is technically credible?

The Ira A. Fulton School of EngineeringArizona State University Discussion

The Ira A. Fulton School of EngineeringArizona State University Groundwater Data Interpretation Issues With samples collected across conventional well screen intervals, there are multiple realizations that would correspond to the same depth-averaged groundwater concentration (in other words, the measured concentrations do not correspond to a unique vapor transport scenario) C1 > C2C2 > C3

The Ira A. Fulton School of EngineeringArizona State University InputThoughtsReasonable* H, D air Tabulated Chemical PropertiesActual Value E d -1 (energy efficiency studies) 12 d -1 (V B /A B )2 - 3 m (= ceiling height) 2.5 m (Q soil /Q B )<0.01 (radon studies and Colorado field data) L T mactual value (  m /  T )10% - 50% (vadose zone + crack)0.10 L capillary cm5 cm (  m /  T )90% - 98% (capillary fringe)0.95  T L crack cm ( inches)15  (1=bare dirt floor)0.001 * reasonable conservative value Model Inputs - what do we know?

The Ira A. Fulton School of EngineeringArizona State University  T = 0.50  T = 0.35 Vadose zone soils capillary zone Sands/ Gravels Silts/ Clays Sensitivity of D T eff to Moisture Content.. D T eff not very sensitive to reasonable variation in moisture content for a given soil type. D T eff more sensitive to variation across gross changes in soil types (i.e. sands -> clay about 5X change). The most significant change occurs between vadose zone and capillary fringe soils **however** the magnitude depends on H (beware at small H!)