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A Comparison of Hydrocarbon Vapor Attenuation in the Field with Predictions from Vapor Diffusion Models

Ririe, G.Todd, Sweeney, Robert E., Daugherty, Seth J.
Journal of soil contamination 2002 v.11 no.4 pp. 529-554
benzene, biodegradation, carbon dioxide, data analysis, gasoline, groundwater, methane, models, oxygen, prediction, risk, soil physical properties, vadose zone, vapors, California
This study used field data from three sites in Southern California to evaluate vapor phase transport from: (1) free product (die-sel and gasoline spill) on groundwater; (2) dissolved benzene (gasoline spill) in groundwater; and (3) hydrocarbon-impacted soil (gasoline spill) in the vadose zone. A sampling program to evaluate the vapor pathway included the following: vertical profile data, minimal purging prior to sample collection, field analysis of data, confirmation of field data using a fixed laboratory analysis, and soil physical property data. Comparison of hydrocarbon vapor concentrations measured in this field study with those calculated using vapor diffusion models suggest that an additional attenuation factor of between 500 and 35,000 is needed to account for observed concentrations. Comparison of hydrocarbon profiles with oxygen, carbon dioxide, and methane values is consistent with the interpretation that biodegradation is primarily responsible for the observed attenuation. Therefore, vapor pathway models that do not account for bioattenuation will result in a large overesti-mation of the risk at spill sites and will not be consistent with field data.