Evaluation of vadose zone biodegradation of BTX vapours

Soil vapour transport to indoor air is an important potential exposure pathway at many sites impacted by subsurface volatile organic compounds (VOCs). The inclusion of biodegradation in vadose zone transport models for benzene, toluene and xylene (BTX) and fuel hydrocarbons has been proposed; however, there is still significant uncertainty regarding biodegradation rates and the local effects of buildings or ground surface cover on fate and transport processes. The objective of this study was to evaluate biodegradation processes through comprehensive monitoring at a site contaminated with BTX and model simulation. Study methods included extensive vertical profiling of BTX vapour and light gas (oxygen and carbon dioxide) concentrations and moisture content, and semi-continuous monitoring of oxygen and pressure below a building floor slab. Significant vadose zone biodegradation over a relatively small depth interval was observed. Based on the observed soil vapour profile, first-order biodegradation 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 (2-D) numerical model incorporating vapour-phase diffusion, advection, sorption and biodegradation was used to simulate the effect of a building floor slab on transport processes. Model results demonstrate the sensitivity of vapour-phase BTX and oxygen transport to partial barriers to diffusion (e.g. building foundation) and highlight the importance of using a model that ties biodecay to oxygen availability. In addition, depressurization within a building and advective transport is shown to have a potentially significant effect on BTX fate, in soil below.