Biodegradation of organic compounds in unsaturated soil: role in reducing risk from vapor intrusion

Emissions of volatile contaminants from groundwater sources to indoor air, represent a critical risk driving process in urban environments with shallow groundwater. Risks are normally evaluated through risk assessment models. There is however ample evidence from field measurements of indoor air concentrations that current model predictions generally provide a one to three orders of magnitude overestimation. Biodegradation of organic compounds in the unsaturated zone is a process that potentially interrupts exposure to humans as the final receptor, but so far knowledge is lacking about the precise parameters controlling biodegradation. Here we present results from model simulations researching the role of biodegradation. The model simulations use the finite difference numerical model STOMP. It simulates the flow of the gas and water phases and the diffusive and advective transport of solutes in both phases. The model simulates vertical transport through a soil column towards a crawl space in a shallow groundwater area, as well as air exchange in the crawl space. An oxygen dependent biodegradation mechanism is incorporated in the model, and different scenarios, neglecting or including biodegradation are compared.