Three‐Dimensional Simulation of Volatile Organic Compound Mass Flux from the Vadose Zone to Groundwater

Low‐permeability layers of the vadose zone containing volatile organic compounds (VOCs) may persist as source zones for long time periods and may provide 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 only consider groundwater coupling options through boundary conditions at the water table and may yield artificially high mass flux results when 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 originating from 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. This paper describes a series of three‐dimensional simulations using data from the U.S. Department of Energy's Hanford site, 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.