Effect of vapor source-building separation and building construction on soil vapor intrusion as studied with a three-dimensional numerical model.

A three-dimensional numerical model of the soil vapor-to-indoor air pathway is developed and used as a tool to anticipate not-yet-measured relationships between the vapor attenuation coefficient, alpha (indoor air concentration/source vapor concentration), and vapor source-building lateral separation, vapor source depth, and building construction characteristics (depth of building foundation) for nondegrading chemicals. The numerical model allows for diffusive and advective transport, multicomponent systems and reactions, spatially distributed foundation cracks, and transient indoor and ambient pressure fluctuations. Simulations involving different lateral separations between the vapor source and building show decreasing alpha values with increasing lateral separation. For example, alpha is 2 orders of magnitude less when a 30 m x 30 m source located 8 m below ground surface is displaced from the edge of the building by 20 m. The decrease in alpha with increasing lateral separation is greater for shallower source depths. For example, alpha is approximately 5 orders of magnitude less when a 30 m x 30 m source located 3 m below ground surface is displaced from the edge of the building by 20 m. To help visualize the effects of changing vapor source-building separations, normalized vapor concentration contour plots for both horizontal and vertical cross sections are presented for a sequence of lateral separations ranging from the case in which the 30 m x 30 m source and 10 m x 10 m building footprint centers are collocated to shifting of the source positioning by 50 m. Simulations involving basement and slab-on-grade constructions produce similar trends. In addition, when buildings are overpressurized to create outflow to soil gas on the order of 1-3 L/min, emissions to indoor air are reduced by over 5 orders of magnitude relative to intrusion rates at zero building underpressurization. The results are specific to simulations involving homogeneous soil properties, nondegrading chemicals, steady source concentrations and building underpressurizations, and the geometries studied in this work.