A Groundwater Mass Flux Model for Groundwater-to-Indoor Air Vapor Intrusion

A groundwater mass flux screening-level model has been developed to evaluate potential indoor air exposures associated with contaminant volatilization from affected groundwater underlying occupied structures. Prior screening-level models have characterized potential groundwater-to-indoor-air exposures based on contaminant volatilization from an assumed infinite contaminant source mass within the groundwater that is in equilibrium with the overlying soil vapor. For these models, steady-state mass transfer into the overlying building is controlled by the rate of vapor diffusion and advection through the unsaturated soil zone and the building foundation. However, for many sites, this prior method of analysis can provide overly conservative groundwater screening levels that are inconsistent with mass balance considerations. Specifically, the assumed mass transfer into the building air space can greatly exceed the maximum groundwater to soil-vapor release that could occur during migration of the affected groundwater plume beneath the building. As an alternative conceptual model, the groundwater mass flux approach presented in this paper calculates an upperbound contaminant mass transfer into the overlying structure as a function of the vertical contaminant mass flux within the affected groundwater flowing beneath the building. This model assumes no impedance of vapor transport by the unsaturated soil zone or the building foundation. For a given groundwater flow condition and building configuration, this conservative mass flux relationship can be used to back-calculate a screening limit concentration for groundwater that is protective of a specified indoor air exposure level.