PNNL

Abstract

Source zones containing volatile organic compounds (VOCs) in low permeability layers of the vadose zone may persist for long time periods and may provide a continuous supply of contamination to groundwater. At sites with low recharge rates where vapor migration is the dominant transport process, the impact of vadose zone sources on groundwater may be difficult to assess. Typical assessment methods include one-dimensional numerical and analytical techniques. The one-dimensional approaches do not consider groundwater coupling and yield artificially high mass fluxes because transport is assumed to occur by gas-phase diffusion between a source and an interface with a zero concentration boundary condition. Improvements in mass flux assessments for VOCs with vadose zone sources may be obtained by coupling vadose zone gas transport and dissolved contaminant transport in the saturated zone and by incorporating the inherent three-dimensional nature of gas-phase transport, including the potential of density-driven advection. In this paper, a series of three-dimensional simulations using data from the U.S. Department of Energy Hanford Site is described where carbon tetrachloride is present in a low permeability zone about 30 m above the groundwater. Results show that for most cases only a relatively small amount of the contaminant emanating from the source zone partitions into the groundwater and that density-driven advection is only important when relatively high source concentrations are considered. The introduction of vadose zone – groundwater coupling yields considerably lower mass fluxes than obtained with single-phase one-dimensional approaches.