UBC Theses and Dissertations
Field-based evaluation of processes and models for soil vapour intrusion into buildings Hers, Ian
This thesis presents the results of research on the validation of models used to evaluate the intrusion of subsurface volatile organic compounds (VOCs) into buildings, often referred to as the "soil vapour transport to indoor air pathway". Evaluation of this exposure pathway is important in the context of risk-based corrective action for contaminated sites. The study scope addresses both the fate and transport of VOCs in the unsaturated zone, and intrusion of VOCs through the subsurface building envelope (i.e., foundation). The primary approach adopted to investigate and validate models was to obtain extensive field monitoring data on VOC vapour transport and intrusion, obtained at a former petro-chemical plant site ("Chatterton" site). The Chatterton site, located near Vancouver, B.C., Canada, is contaminated with benzene, toluene and xylenes (BTX). To facilitate measurement of BTX intrusion, a small building (greenhouse) with controlled properties was constructed. A number of experiments were conducted to evaluate vadose zone processes and vapour intrusion for different greenhouse depressurization conditions. When the greenhouse was not subject to sustained fan-induced depressurization, there was significant aerobic biodegradation of BTX vapours between approximately 0.4 m and 0.8 m depth below the greenhouse, and subslab BTX vapour concentrations were low. When the greenhouse was depressurized to -10 pascals (Pa), the subslab BTX vapour concentrations were elevated, and significant vapour intrusion was measured using both tracer and flux chamber techniques, which was inferred to be a result of an upward BTX vapour flux that exceeded biodegradation capacity based on oxygen availability. A comprehensive analysis of case studies providing information on soil vapour intrusion was completed. This analysis, together with the Chatterton site results, were evaluated for key trends and factors affecting soil vapour intrusion. The vapour attenuation factors estimated based on field case studies were compared to those predicted using several screening level models, including the Johnson and Ettinger Model, a widely used model for this exposure pathway. The validation of screening models using field data is complicated in that model predictions can vary greatly depending on the model and input parameters used. Further, soil vapour fate and transport and intrusion into buildings is a complex process affected by numerous factors (soil properties, house conditions, and environmental factors); therefore, vapour intrusion will vary significantly depending on site specific conditions. The results of the model comparisons to field data indicated that screening level models, typically used for this exposure pathway, were found to generally yield conservative results (i.e., overpredict vapour intrusion); however, there were a few important exceptions. A multi-dimensional numerical model for vapour transport, which incorporated diffusion, biodegradation, sorption and soil gas advection, was also developed. Good comparisons between model predicted and measured vapour attenuation were obtained based on conditions observed at the Chatterton site.
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