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Dispersive processes that diminish contaminant concentrations originating from an unsaturated source zone on the way to groundwater, were investigated. Simulations using the numerical model MIN3P were performed for a non-volatile, non-degrading contaminant from a persistent source after reaching a steady state. A 2-D vertical cross-section was used as geometry. Two different types of sandy sediment were simulated: a rather coarse sand with a capillary rise of 90% water saturation to 4 cm above the water table, and a silty sand showing a capillary fringe of 30 cm height (90% water saturation). Major dispersive fluxes were found to take place below the water table, thus dilution and concentration reduction at and above the water table is not very significant.

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... In this approach, the assumption was made that the subsurface of the Hanford Site Z-9 Trench is sufficiently characterized such that a conceptual model as depicted in Fig. 1 could be derived with a reasonable level of confidence, including the hydraulic properties of the major porous media. In this context, it is recognized that the capillary fringe configuration may have a considerable effect on mass flux into the water table (Klenk and Grathwohl, 2002;Mccarthy and Johnson, 1993), including proper discretization of this zone in numerical models (Maier et al., 2008a(Maier et al., , 2008bOostrom et al., 2010). However, given that capillary fringe effects on contaminant transport are not yet fully understood, variations in capillary fringe configurations were not included in the current analysis approach. ...
Soil vapor extraction (SVE) is typically effective for removal of volatile contaminants from higher-permeability portions of the vadose zone. However, contamination in lower-permeability zones can persist due to mass transfer processes that limit the removal effectiveness. After SVE has been operated for a period of time and the remaining contamination is primarily located in lower-permeability zones, the remedy performance needs to be evaluated to determine whether the SVE system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. Numerical modeling of vapor-phase contaminant transport was used to investigate the correlation between measured vapor-phase mass discharge, MF(r), from a persistent, vadose-zone contaminant source and the resulting groundwater contaminant concentrations. This relationship was shown to be linear, and was used to directly assess SVE remediation progress over time and to determine the level of remediation in the vadose zone necessary to protect groundwater. Although site properties and source characteristics must be specified to establish a unique relation between MF(r) and the groundwater contaminant concentration, this correlation provides insight into SVE performance and support for decisions to optimize or terminate the SVE operation or to transition to another type of treatment.
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.
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The numerical model CompFlow is used to study the mechanisms controlling vacuum extraction, coupled with air sparging, as a means for remediation of heterogeneous formations contaminated with dense nonaqueous phase liquids (DNAPLs). Two dominant mechanisms are demonstrated to control this remediation technology. First, at early times, the gas phase directly contacts the DNAPL, particularly in the unsaturated zone, causing relatively rapid transfer of contaminant from the nonaqueous phase to the gas phase and subsequent removal by the vacuum extractor. Second, at later times, remediation is controlled by the transfer of contaminant from the nonaqueous phase to the aqueous phase below the water table. During this time the vacuum extractor pumps both liquid and vaporized water in the aqueous and gas phases. This causes the contaminant that is dissolved in the aqueous phase to migrate vertically upward across the permeability layers toward the vacuum extractor where it is removed. This intermediate to late time removal mechanism is shown to be controlled by contaminant dissolution, which is a slower transfer process than the direct DNAPL vaporization that occurs at early time. Our analysis indicates that as long as both air and water are actively flushed through the DNAPL zone, both early-time vaporization and intermediate- to late-time dissolution are effective mechanisms leading to the removal of the DNAPL. We show that it may be possible to design the remedial system so as to reduce its performance sensitivity to geologic heterogeneity. A lack of sensitivity of a remedial design to heterogeneity is highly desirable because a robust design implies that the degree of site characterization required for reasonable success will be less than that needed for a less robust scheme.