A transient three dimensional numerical model to simulate vapor intrusion into buildings

A transient three-dimensional numerical model is developed and used to simulate subsurface air flow, contaminant transport, biodegradation and chemical vapor intrusion into buildings. This work is relevant to sites where hazardous volatile organic compounds are present in groundwater and subsurface soils, and buildings are present or will be constructed in the future. Buildings and other enclosed spaces can act to draw contaminant vapors present in the subsurface air into the indoor air. Depending on building air exchange rate and on contaminant entry rate, the indoor air can reach concentrations greater than the threshold allowed for human health safety. Soil vapor intrusion into buildings has been recognized for some time as a significant exposure pathway, but also one that is extremely complex and difficult to describe mathematically. Although advances have been made in modeling the pathway, the existing models are relatively simplistic analytical solutions. The model developed here presents a more sophisticated numerical solution to vapor intrusion scenarios and it was constructed to help gain a better understanding of the factors controlling this complex pathway. For example, the model is used as a tool to anticipate how indoor air concentration is affected by vapor source lateral distance from the building, vapor source concentration and depth, building construction characteristics (e.g., slab-on-grade vs. basement), and biodegradation rates. The numerical model allows for diffusive and advective transport, multi-component systems and reactions, spatially distributed foundation cracks, and transient indoor and ambient pressure fluctuations. To help visualize the air flow field and the chemical distribution in the subsurface, the model output can be presented as normalized contour plots for the air pressure field and contaminant concentration for both horizontal and vertical cross sections of the subsurface. Simulations involving different lateral separations between the vapor source and the building show decreasing indoor air concentration with increasing lateral separation. Simulations involving basement and slab-on-grade constructions produce similar trends for recalcitrant chemicals. For aerobically biodegradable chemicals the results show that under natural conditions biodegradation could play a significant role in reducing the vapor intrusion into buildings relative to the no-degradation case and that contaminant emissions to indoor air decreases with increasing vapor source depth, decreasing vapor source strength and increasing biodegradation rate. In addition, when buildings are over-pressurized to create outflow to soil gas on the order of 1--3 L/min, emissions to indoor air are reduced by over five orders-of-magnitude relative to intrusion rates at zero building under-pressurization.