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Evaluation of Soil Vapor Extraction Data to Characterize Mass Flux at the Vapor Intrusion Boundary
Citation:
Stewart, L., R. Truesdale, C. Lutes, B. Schumacher, AND J. Zimmerman. Evaluation of Soil Vapor Extraction Data to Characterize Mass Flux at the Vapor Intrusion Boundary. AEHS Foundation 35th Annual International Conference on Soils, Sediments, Water, and Energy, Amherst, MA, October 21 - 24, 2019.
Impact/Purpose:
Human exposure to vapors of chlorinated volatile organic compounds in indoor air increases risk of neurological, immunological, reproductive, and developmental effects. Current methods for assessing contaminant mass flux for vapor intrusion (VI) rely primarily on two approaches. First, mass removal is measured during operation of mitigation systems, either in basements or beneath building slabs. However, climate variables as well as building usage introduce significant uncertainties. The second approach evaluates upward vapor diffusion from persistent sources in the vadose zone and/or contaminated groundwater using measured vapor concentration profiles and assumed vapor diffusion coefficients. Consistent measures of soil gas concentration can usually be attained; however, these profiles are usually measured under steady conditions. The flux calculation assumes an average, unmeasured diffusion coefficient that can vary by order of magnitude in native soils. Contaminated groundwater requires assumptions about mass transfer across the capillary fringe. To overcome these limitations, this project demonstrated the utility of transient concentration profiles measured after ceasing soil vapor extraction (SVE) for VI assessments.
Description:
Results are presented from a field study that included three periods of rebound between active SVE operations over a three-year period at an industrial site with modestly stratified soil and sources of PCE and TCE in the vadose zone and in groundwater. The SVE data yielded estimates of diffusion-limited mass transfer along with a volume-averaged maximal mass flux and its decay with mass removal during subsequent periods of SVE. The estimated mass flux of PCE and TCE before SVE was found to be reduced by more than two orders of magnitude at the end of SVE. The results provide a relationship for assessing acceptable source vapor concentrations at the close of SVE that are sufficient to obviate future active mitigation of vapor intrusion. The results are supported with measures of indoor air before, during and after SVE. In addition, the transient vapor concentration profiles identified shallow, low-level residual contaminant vapor sources within the overlying concrete slab.