Modeling Volatile Chemical Transport, Biodecay, and Emission to Indoor Air |
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| Authors: | Jack C Parker |
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| Institution: | Jack Parker;is a distinguished research scientist at Oak Ridge National Laboratory in Oak Ridge, Tennessee. Previously, he served as president of Environmental Systems &Technologies Inc. for 12 years and as a professor of contaminant hydrology at Virginia Tech for 15 years. He has directed many research and consulting projects involving the modeling of subsurface contaminant transport in aqueous, gaseous and/or NAPL phases. His research has focused on the development and testing of models for multiphase flow and transport, remedial assessment and design optimization, risk assessment, inverse modeling, and analysis of model reliability. He has authored more than 200 technical publications and served on numerous expert panels, advisory and review boards, and delegations for government agencies, professional groups, private industry, and others. He has taught academic and short courses and presented workshops and invited seminars on modeling of subsurface organic contaminant transport in some 15 countries. |
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| Abstract: | A model is presented for estimating vapor concentrations in buildings because of volatilization from soil contaminated by non- aqueous phase liquids (NAPL) or from dissolved contaminants in ground water. The model considers source depletion, diffusive- dispersive transport of the contaminant of concern (COC) and of oxygen and oxygen-limited COC biodecay. Diffusive-advective transport through foundations and vapor losses caused by foundation cross-flow are considered. Competitive oxygen use by various species is assumed to be proportional to the product of the average dissolved-phase species concentration and a biopreference factor. Laboratory and field data indicate the biopreference factor to be proportional to the organic carbon partition coefficient for the fuel hydrocarbons studied. Predicted indoor air concentrations were sensitive to soil type and subbase permeability. Lower concentrations were predicted for buildings with shallow foundations caused by flushing of contaminants by cross-flow. NAPL source depletion had a large impact on average exposure concentration. Barometric pumping had a minor effect on indoor air emissions for the conditions studied. Risk-based soil cleanup levels were much lower when biodecay was considered because of the existence of a threshold source concentration below which no emissions occur. Computed cleanup levels at NAPL-contaminated sites were strongly dependent on total petroleum hydrocarbon (TPH) content and COC soil concentration. The model was applied to two field sites with gasoline-contaminated ground water. Confidence limits of predicted indoor air concentrations spanned approximately two orders of magnitude considering uncertainty in model parameters. Measured contaminant concentrations in indoor air were within model-predicted confidence limits. |
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