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Migration Of Soil Gas Vapors To Indoor Air

Determining Vapor Attenuation Factors Using A Screening-Level Model And Field Data From The CDOT-MTL Denver, Colorado Site

API Soil and Groundwater Research Bulletin Number 16

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With the advent of risk-based decision-making at petroleum and other chemical-release sites, screening-level models for the subsurface vapor-to-indoor air pathway are now being used to: a) identify scenarios for which there might be significant adverse impacts, b) determine when site-specific assessment is necessary, and c) develop target soil and groundwater screening levels. A range of approaches for addressing this pathway have been proposed and debated for the past decade. At the extremes are approaches relying solely on screening-level predictive models and approaches requiring indoor air sampling. Intermediate are approaches that combine screening level models and site-specific sampling, as presented in API Publication 4674 (Johnson et al., 1998).

To date, objective comparison of the strengths and weaknesses of the various approaches and screening models has been difficult because of the scarcity of data and experience. However, with the growing interest in this pathway, more extensive data sets are becoming available. One such data set comes from the Colorado Department of Transportation Materials Testing Laboratory (CDOT-MTL) Site located in Denver Colorado. This data set is of particular interest because of the large number (>1000) of groundwater, soil gas, and indoor air concentration data, and the fact that the multi-event sampling was conducted at a number of locations. Johnson et al. (2001) provides a detailed analysis of this data set, featuring a comparison between: a) empirically derived attenuation factors for the soil vapor –to- indoor air pathway using measured groundwater and indoor air concentrations, with b) attenuation factors estimated using the Johnson and Ettinger screening-level model and reasonable input values for this site. This bulletin provides a brief overview of the study followed by a summary of the key findings.