Biological transformation of volatile organic compounds is one of the key factors that influence contaminant-plume evolution and thus natural attenuation. In this study we investigate the effect of biological transformation on the transport of contaminants in the aqueous and gaseous phases. The analysis includes the study of the effect of density-driven advection of contaminants in the gaseous phase on multiphase and multispecies flow, fate and transport modeling in the subsurface. Trichloroethylene (TCE) and its two byproducts, dichloroethylene and vinyl chloride, are analyzed as the target contaminants. Our results indicate that density-driven advection of the gaseous phase, which is initiated by evaporation of TCE as a nonaqueous phase liquid, increases the downward and also the lateral migration of TCE within the unsaturated zone. This process also influences the location of high-concentration zones of the byproducts of TCE in the unsaturated and the saturated zones. Biotransformation of TCE contributes to the reduction

Once chlorinated volatile organic compounds (CVOCs) are released into the subsurface they are spread out through the porous soil matrix by complicated processes such as advection, dispersion, and sorption. CVOCs can be also biodegraded by indigenous microorganism via aerobic and anaerobic bioreactions, which generate distinct benign or harmful by-products. Heterogeneous subsurface characteristics and non-uniform distribution and consumption of oxygen in the subsurface may allow the dual bioreactions to coexist within a representative subsurface volume (RSV). The portion of each bioreaction within a RSV will depend on multiple factors such as oxygen, contaminant, and microorganism levels. Oxygen can be supplied into the contaminated zone through the dispersive and advective transport of oxygen in gas phase as well as the flow of the groundwater containing dissolved oxygen. As the dual bioreactions are coupled with multiple fluid flows (groundwater and gas) and multispecies transport, the analysis of the bioreactions is very complicated. However,