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Evaluation of the Johnson and Ettinger Model for Prediction of Indoor Air Quality
Authors:Ian Hers  Reidar Zapf-Gilje  Paul C Johnson  Loretta Li
Institution:Ian Hers;is completing Ph.D. studies at the University of British Columbia and is a senior consultant with Colder Associates Ltd. () in Vancouver, British Columbia. His research interests include subsurface remediation technologies for organic chemicals and development and field verification of models for risk-based decision making. Reidar Zapf-Gilje;is an adjunct professor at the University of British Columbia () where he teaches graduate courses in the Department of Civil Engineering. He is a contaminated site expert and consults on investigations, human health and ecological risk assessment, and site remediation.
Abstract:Screening level models are now commonly used to estimate vapor intrusion for subsurface volatile organic compounds (VOCs). Significant uncertainty is associated with processes and models and, to date, there has been only limited field-based evaluation of models for this pathway. To address these limitations, a comprehensive evaluation of the Johnson and Ettinger (J&E) model is provided through sensitivity analysis, comparisons of model-predicted to measured vapor intrusion for 11 petroleum hydrocarbon and chlorinated solvent sites, and review of radon and flux chamber studies. Significant intrusion was measured at five of 12 sites with measured vapor attenuation ratios (αm's) (indoor air/source vapor) ranging from ∼1 × 10−6 to 1 × 10−4. Higher attenuation ratios were measured for studies using radon, inert tracers, and flux chambers; however, these ratios are conservative owing to boundary conditions and tracer properties that are different than those at most VOC-contaminated sites. Reasonable predictions were obtained using the J&E model with comparisons indicating that model-predicted vapor attenuation ratios (αp's) were on the same order, or less than the αm's. For several sites, the (m were approximately two orders of magnitude less than the a 's indicating that the J&E model is conservative in these cases. The model comparisons highlight the importance in using appropriate input parameters for the J&E model. The regulatory implications associated with use of the J&E model to derive screening criteria are also discussed.
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