Vapor Intrusion Evaluation Strategy and Modeling Developments Robert Ettinger Geosyntec Consultants California Industrial Hygiene Council 16th Annual Conference San Diego, CA December 4 – 6, 2006

Timeline for the Vapor Intrusion Pathway Radon Intrusion & Vapor Diffusion studies OSWER Draft Guidance CA, NY, NJ State Guidance Development Air-Superfund Guidance Draft RCRA EI Supplemental Guidance MA State Guidance J&E Model Revised OSWER Guidance ASTM RBCA Standard Response to Comments RCRA EI Guidance 1980’s 1989 1991 1992 1993 1994 1999 2001 2002 2004 2005 2007 Hillside School Hill AFB CDOT Redfields MEW Endicott

Vapor Migration to Indoor Air - General Conceptual Model

Vapor Intrusion Evaluation Strategy Utilize Tiered Approach Data collection and analysis increase in higher tiers Target Indoor Air Levels Risk-based levels, PELs, background Media Sampled & Locations Groundwater, soil gas, indoor air Near, next to, or beneath buildings Other Data Geologic characterization, building characteristics Modeling Options Empirical, screening level, site-specific Corrective Action Selection

Target Indoor Air Levels Typically, indoor air target levels based on risk-based concentrations developed using EPA risk methodology Need to consider Target risk level Occupational standards Background concentrations Example Target Indoor Air Levels Basis Benzene PCE 10-6 Risk 0.25 0.32 10-5 Risk 2.5 3.2 10-4 Risk 25 32 Background 3 - 5 1 - 5 PEL (8-hr TWA) 3200 170,000

Site Characterization What data are best to characterize vapor intrusion pathway? Indoor Air Groundwater Soil Soil Gas

Indoor Air Sampling Indoor air sampling may seem to be a direct assessment approach, but is typically conducted during higher tier of investigation Several challenges to indoor air sampling Occupant disruption Temporal and spatial variability Background effects May more practical to collect indoor air samples in occupational setting Indoor air sampling guidance Sample collection techniques Analytical methods Building survey examples

Source Characterization Groundwater Henry’s Law to evaluate partitioning Mass transport limitations due to vertical concentration gradients in saturated zone Soil Gas-water and water-solid partitioning Uncertainty in accuracy of partitioning equation Soil Gas Soil gas results can resolve uncertainty associated with groundwater or soil data Typically provide better source characterization for vapor intrusion pathway.

Soil Gas Sample Location Current regulatory focus on appropriate sampling locations Near source Exterior to building Sub-slab Soil gas profile may be affected by building More significant for biodegradable compounds

Soil Gas Sampling Soil gas sampling methods not as uniform as groundwater sampling methods, but approaches to meet investigation data quality objectives are available

Sub-Slab Soil Gas Sampling Requires building access Methods developed to limit intrusiveness (DiGiulio, 2004)

Vapor Intrusion Modeling Mixing in Breathing Zone Convective Transport into Bldg Diffusive Transport to Breathing Zone Impacted Soil and/or Groundwater in Equilibrium with Soil Gas Risk is proportional to (a) x (Csoil gas)

Empirical Attenuation Factor (Dawson, 2004)

Johnson-Ettinger (1991) Attenuation Factor Primary Parameters Deff = Effective diffusion coefficient LT = Depth to source AB = Building area in contact with soil QB = Building ventilation rate Qsoil = Soil gas convection rate Dcrack = Eff. diff. coeff. through cracks Lcrack = Crack thickness h = Building crack factor Secondary Parameters Deff = fn(H, Dwater, Dair, qT, qw) for each layer LT = S(Li) Qsoil = fn(k, DP, rcrack, zcrack, xcrack) >30 inputs, but only a handful that are really sensitive J & E Model has dozens of input parameters, how much data is required to use?

Common Screening Model Assumptions One-dimensional vertical transport Steady state conditions No preferential pathways Uniform mixing within building Slab on grade or basement construction No biodegradation Homogeneous vadose-zone Constant source concentration No gas generation (e.g., municipal waste) No barometric pumping Prior to using model results, you need to ensure that model assumptions and site conditions are consistent

Constrained Model Use Many problems with vapor intrusion modeling associated with improper inputs Updated EPA spreadsheets will limit values allowed for inputs Constraints based on Johnson, 2002

Biodegradation Modeling Methods to model vadose zone biodegradation have been developed Johnson et. al., 1999 – Dominant Layer Model Abreu and Johnson, 2004 – 3D Numerical Model Typically, additional site investigation data will be necessary to conduct biodegradation modeling Soil gas concentration profile data Analysis of biodegradation indicators (O2 , CO2) Tracer compounds Consider use of soil vapor profile data

Screening Level Biodegradation Model (Johnson, et al., 1999) Mixing in Breathing Zone Diffusive Transport Partitioning Convective Transport into Building Biodegradation Zone VOCs Source Requires additional data collection for bio indicators Calibrate model with site soil gas data to determine biodegradation parameters Reduce a by factor of 10 – 1000

Three-Dimensional Numerical Model (Abreu and Johnson, 2005) Model Description 3-D vadose zone F&T model Evaluate building type, source scenarios, and biodegradation kinetics Model Results Impact of biodegradation Significance of lateral migration

Investigation Approach for Complex Sites Soil surface CO2 VOCs O2 Soil gas profile sampling points Soil gas profile data recommended to assess biodegradation Biodegradation significantly affects petroleum compound vapor migration No common approach to use soil gas profile data to quantitatively evaluate vapor intrusion pathway

Soil Gas Profile Data Soil gas profile underneath building may be different than that outside building footprint. May need to assess potential exposure scenarios Evaluate soil gas data to address uncertainty in sub-surface transport (diffusion and biodegradation) Reassess vapor intrusion evaluation from subsurface source (include convection and ventilation effects) Soil gas samples

Example Modeling Results

Institutional Controls Engineering Controls Choice of Remedy Active Remediation Institutional Controls Engineering Controls “Radon System” HVAC Modifications Sealing Filtration Building Design (Brownfields)

Mitigation Options: Radon Sump Cheap and reliable. $2K per system lasts a decade. http://www.bre.co.uk/radon/reduce.html

Summary Selection of appropriate target levels is key factor in vapor intrusion assessment. Site investigation methods require careful planning. When modeling, assess whether site conditions are consistent with conceptual model assumptions and input parameters are reasonable. Corrective action planning may reduce scope of vapor intrusion investigation. Consider multiple lines of evidence to support conclusions. A balance of modeling and monitoring is typically appropriate.