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IDEM has reviewed the recent literature on the use of the Johnson & Ettinger (J&E) model for evaluation of the vapor intrusion pathway. Based on this evaluation, the IDEM Vapor Intrusion Workgroup does not recommend that the results of site-specific vapor intrusion modeling be accepted without appropriate site-specific data for sensitive model parameters, and actual sub-slab/soil gas and indoor air data to calibrate and validate the model.
Soil gas, sub-slab, and/or indoor air samples should be collected to determine if contaminant concentrations exceed health-protective levels, particularly in residential areas where soil or groundwater contaminant concentrations exceed IDEM's screening levels (IDEM 2006).
Site closure requests based only on modeling in the absence of site-specific parameter data and confirmatory sub-slab/soil gas and indoor air samples will not be accepted.
The Johnson & Ettinger model is sensitive to a number of parameters for which site-specific data are typically not available. The EPA (2002) states:
"If the J&E model is deemed applicable to the site, critical model parameters from site data are needed. We recommend that site-specific information [emphasis added] include soil moisture, soil permeability, building ventilation rate, and sub-slab as well as deep vapor concentrations."
None of the recommended site-specific data are typically obtained for use in the model. In the absence of reliable site-specific data for model parameters, results from J&E modeling cannot be accepted.
In addition, the EPA (2005) has conducted an evaluation of parameter uncertainty in the Johnson & Ettinger model. The EPA (2005) notes that:
"Typical use of this model relies on a suite of estimated data, with few site-specific measurements...An apparent increase in simulated cancer risk caused by the uncertainty introduced from the input parameters was as much as 1285%."
"Use of the J&EM in screening of sites for vapor intrusion should account for input parameter uncertainty. Simulation using default parameters, generally picked at the midpoint of their possible ranges, does not correctly represent the possible model outputs."
The report concludes:
"Standard approaches for application of models...indicate that a necessary step in model application is calibration of results to field data. In situations where the model is not calibrated to measured indoor air data, and subsequently demonstrated to have predictive capability, the input parameters cannot be assured to represent the properties of the flow system. By performing an uncertainty analysis...a range of potential outputs is revealed to the decision maker."
The J&E model applications seen by IDEM are typically not calibrated, do not contain appropriate site-specific parameter information, and the model inputs are usually not tested for sensitivity. Without this data, the results of modeling do not demonstrate that indoor air will not be impacted at concentrations that exceed health-protective levels.
The Johnson & Ettinger model as described by the EPA (2002) is not intended to be used to close potential vapor intrusion sites, or to demonstrate that further investigation is not required, in the absence of confirmatory indoor air data. The model is intended to be used to identify the most potentially affected buildings so that those buildings can be preferentially tested for vapor intrusion by actual indoor air sampling. Once the buildings that are suspected of having the greatest susceptibility for vapor intrusion are identified, indoor air sampling efforts can be focused on the more susceptible buildings. The EPA (2002) states:
"The recommended site-specific modeling is intended to be complementary to the more direct building-related measurements collected from a selected subset of the potentially impacted buildings" [emphasis added].
"...Mathematical modeling may be useful in determining which combination of factors leads to the greatest impact and, consequently, aid in identifying appropriate buildings to be sampled." [emphasis added].
Therefore, IDEM will generally request appropriate site-specific indoor air samples, coupled with sub-slab samples where possible, before site closure will be considered.
The IDEM position on the J&E model is supported by recent guidance from other states with active vapor intrusion research programs (e.g. New York State Department of Health, 2006), which found that:
"...soil vapor results alone typically cannot be relied upon to rule out sampling at nearby buildings...Therefore, exposures are evaluated primarily based on sub-slab, indoor air, and outdoor air sampling results..."
The New York State Department of Health (2006) further notes that:
"...use of modeling as the sole means of evaluating potential exposures should be avoided. The limitations of modeling...introduce uncertainty as to whether human exposure is occurring, in the absence of actual field data. Conclusions drawn from modeling should be verified with actual field data. For example, if modeling results indicate indoor air concentrations are predicted to be below applicable guidelines or levels of concern, indoor air and/or sub-slab vapor sampling would be appropriate to verify a conclusion that mitigation or other actions are not needed."
Based on a review of the recent literature on vapor intrusion and modeling, IDEM has concluded that the use of modeling in the absence of confirmatory indoor air samples is an unacceptable method to demonstrate that the vapor intrusion pathway is incomplete.
The results of vapor intrusion modeling without appropriate site-specific data for model parameters and actual soil gas and indoor air data to calibrate and validate the model will not be accepted as a stand-alone justification for site closure. For sites where contaminant concentrations exceed screening levels, soil gas, sub-slab, and/or indoor air samples should be collected to determine if contaminant concentrations exceed health-protective levels, particularly in areas where residential exposure is possible.
EPA, 2002. Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils. Office of Solid Waste and Emergency Response.
EPA, 2005. Uncertainty and the Johnson-Ettinger Model for Vapor Intrusion Calculations. Office of Research and Development, EPA/600/R-05/110.
IDEM, 2006. Draft Vapor Intrusion Pilot Program Guidance.
New York State Department of Health, 2006. Guidance for Evaluating Soil Vapor Intrusion in the State of New York. Center for Environmental Health, Bureau of Environmental Exposure Investigation.