The soil gas transport to indoor air pathway is an important potential exposure pathway at many sites impacted by subsurface volatile organic compounds (VOCs). The inclusion of biodegradationin vadose zone transport models for BTX and fuel hydrocarbons has been proposed; however, there is still significant uncertainty regarding biodegradation rates, and local effect of a building on processes. The objective of this study is to evaluate biodegradation processes through comprehensive monitoring at a site contaminated with benzene, toluene and m&p-xylene (BTX) and model simulation. Vertical profiling of soil gas concentrations indicated significant attenuation over a relatively small depth interval. This attenuationwas inferred to be due to biodegradation. Based on the measured soil gas profile, first-order boidegradation rates were estimated by fitting an analytical solution for diffusion and biodecay to the data. Degradation rates were found to compare well to other reported laboratory and field data. A two-dimensional numerical model incorporating gas-phse diffusion, sorption and biodegradation was used to simulate the effect of a building slab on transport processes. Model results demonstrate the sensivitity of gas-phase BTX and oxygen (O_2) transport to surface barriers and highlight the importance of using a model that ties biodecay to O_2 availability.
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