ALTERNATIVE MODEL-BASED APPROACH FOR ASSESSING VAPOR INTRUSION AT PETROLEUM HYDROCARBON SITES

Recently, the spatial and temporal variability of volatile organic compound concentrations in soil gas has been well documented. This variability could potentially limit the use of these data for risk-based assessment purposes unless the risk is bounded through the collection of substantial quantities of soil-gas data. Alternatively, the risk can be predicted through the application of transport modeling. The most common vapor intrusion model, however, has been demonstrated to be overly conservative when applied to petroleum hydrocarbons. The conservatism exists primarily because the model does not account for biodegradation, which can be a critical attenuation mechanism, especially at dissolved-phase contamination sites. A simple transport model was developed to assess risk-based target levels in groundwater that considers biodegradation. The model is based on assuming a conservation of mass and steady state, one-dimensional transport in the unsaturated zone. The model is used to develop a dimensionless type curve, which can be applied to estimate risk-based target concentrations in ground water associated with vapor intrusion. The type curve is developed from knowledge of the mass flux across the building foundation; determined from the risk-based screening level in indoor air, building size (area), and ventilation rate. Risk-based concentrations in ground water are extrapolated from the mass flux estimate on the basis of soil type (effective diffusion coefficient), depth to ground water, air-water partitioning (Henry's Law constant), and biodegradation rate. The model does not consider physical transport across the building foundation or gaseous-phase advection. The model is therefore conservative at sites where vapor intrusion is limited by the rate of transport across the foundation and the model may not appropriate at sites where advective transport is significant (e.g., shallow sources, highly permeable soils).