Gaseous transport in the vadose zone : computer simulations using the discrete state compartment model

Abstract

Past disposal practices of TricNoroethylene (ICE) and other halogenated hydrocarbons have resulted in the contamination of groundwater in part of the Tucson Basin, Tucson, Arizona. At the Carranza site, known to overlie a ICE groundwater contamination plume, a nest of gas sampling piezometers was constructed to measure the vertical distribution of TCE vapor in the vadose zone. The distribution of TCE vapor in the vadose zone was found to be nonmonotonically decreasing from the water table to the atmosphere. To investigate this ICE concentration profile, simulation studies were performed using the Discrete State Compartment model to test various hypotheses concerning the transport mechanisms of TCE vapor in the vadose zone. The studies showed that unless a high permeable column by which diffusing gas could by-pass low permeable layers was included in the simulation molecular diffusion alone could not produce the concentrations measured at the Carranza site. The simulation also showed that a nonmonotonic concentration profile similar to the measured concentration profile could be produced if multiple sources are assumed in the vadose zone. Soil gas advection by barometric pressure fluctuations was shown to increase concentrations at all depths in the vadose zone but the effect was minor compared with the effect of the high permeability column bypass for TCE diffusional transport.