Density‐driven flow of gas in the unsaturated zone due to the evaporation of volatile organic compounds

Falta, Ronald W.; Javandel, Iraj; Pruess, Karsten; Witherspoon, P.A.

doi:10.1029/wr025i010p02159

Cited by 216 publications

(130 citation statements)

References 26 publications

(14 reference statements)

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“…This relationship was derived by a number of researchers using related approaches (Bear 1972;Turner 1973;Cheng and Minkowycz 1977;Schery and Petschek 1983;Wilson et al 1987). Falta et al (1989) found that (3) was in close agreement with numerically simulated gas velocities within porous media partially saturated by a volatile organic liquid. The maximum vertical velocity may not be realized elsewhere because of contaminant diffusion, constraining boundaries and forced advection.…”

Section: Transport Processesmentioning

confidence: 88%

“…A number of these models have simulated forced advection of dilute gases in soils, including Mohsen et al (1980) and Metcalfe and Farquhar (1987), who address methane release from landfills; Loureiro et al (1990), who model radon transport into the basement of buildings, and a number of models related to the transport of volatile organic solvents in unsaturated soil (Abriola and Pinder 1985;Baehr and Corapcioglu 1987;Gierke et al 1990). With the recognition that volatile organic liquids become trapped in the subsurface following their release, gas transport models have included the effects of volatilization and buoyancy-induced advection (Sleep and Sykes 1989;Falta et al 1989;Mendoza and Frind 1990;Mendoza and McAlary 1990). These models differ in their representation of the source term, and in the two-dimensional geometry and boundary conditions.…”

Section: Introductionmentioning

confidence: 99%

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Buoyant Advection of Gases in Unsaturated Soil

Seely

Falta²

1994

J. Environ. Eng.

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In unsaturated soil, methane and volatile organic compounds can significantly alter the density of soil gas and induce buoyant gas flow. A series of laboratory experiments was conducted in a twodimensional, homogeneous sand pack with gas permeabilities ranging from 110 to 3,000 darcy. Pure methane gas was injected horizontally into the sand and steady-state methane profiles were measured. Experimental results are in close agreement with a numerical model that represents the advective and diffusive components of methane transport. Comparison of simulations with and without gravitational acceleration permits identification of conditions where buoyancy dominates methane transport. Significant buoyant flow requires a Rayleigh number greater than 10 and an injected gas velocity sufficient to overcome dilution by molecular diffusion near the source. These criteria allow the extension of laboratory results to idealized field conditions for methane as well as denser-than-air vapors produced by volatilizing nonaqueous phase liquids trapped in unsaturated soil.

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Section: Transport Processesmentioning

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Buoyant Advection of Gases in Unsaturated Soil

Seely

Falta²

1994

J. Environ. Eng.

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“…In that case, the application of the Stefan-Maxwell equations may be necessary for modelling gas phase diffusion (Baehr and Bruell, 1990;Van de Steene and Verplancke, 2006). Although some studies indicate that density-driven advection can play a role (Falta et al, 1989), this process was neglected. Gas phase diffusion also largely dominates over aqueous-phase diffusion (Schwarzenbach et al, 2003), and therefore the latter is neglected.…”

Section: Process Representation In the Modelmentioning

confidence: 99%

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

Bouchard¹,

Cornaton²,

Höhener³

et al. 2011

Journal of Contaminant Hydrology

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a b s t r a c tAnalytical models were developed that simulate stable isotope ratios of volatile organic compounds (VOCs) near a point source contamination in the unsaturated zone. The models describe diffusive transport of VOCs, biodegradation and source ageing. The mass transport is governed by Fick's law for diffusion. The equation for reactive transport of VOCs in the soil gas phase was solved for different source geometries and for different boundary conditions. Model results were compared to experimental data from a one-dimensional laboratory column and a radial-symmetric field experiment. The comparison yielded a satisfying agreement. The model results clearly illustrate the significant isotope fractionation by gas phase diffusion under transient state conditions. This leads to an initial depletion of heavy isotopes with increasing distance from the source. The isotope evolution of the source is governed by the combined effects of isotope fractionation due to vaporisation, diffusion and biodegradation. The net effect can lead to an enrichment or depletion of the heavy isotope in the remaining organic phase, depending on the compound and element considered. Finally, the isotope evolution of molecules migrating away from the source and undergoing degradation is governed by a combined degradation and diffusion isotope effect. This suggests that, in the unsaturated zone, the interpretation of biodegradation of VOC based on isotopic data must always be based on a model combining gas phase diffusion and degradation.

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“…3,6−9 Chlorinated ethenes are transported away from the NAPL source by gaseous diffusion and may induce groundwater contamination even if the NAPL has not reached the water table. 10 Mass transport between unsaturated and saturated zones and inside the unsaturated zone thus plays an important role in controlling the fate of these contaminants. 11 Stable isotope analysis is increasingly used to investigate the behavior of organic or inorganic contaminants in the subsurface.…”

Section: ■ Introductionmentioning

confidence: 99%

Can Soil Gas VOCs be Related to Groundwater Plumes Based on Their Isotope Signature?

Jeannottat

Hunkeler

2013

Environ. Sci. Technol.

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The isotope evolution of tetrachloroethene (PCE) during its transport from groundwater toward the soil surface was investigated using laboratory studies and numerical modeling. During air−water partitioning, carbon and chlorine isotope ratios evolved in opposite directions, with a normal isotope effect for chlorine (ε = −0.20‰) and an inverse effect for carbon (ε = +0.46‰). During the migration of PCE from groundwater to the unsaturated zone in a 2D laboratory system, small shifts of carbon and chlorine isotope ratios (+0.8‰) were observed across the capillary fringe. Numerical modeling showed that these shifts are due to isotope fractionation associated with air−water partitioning and gas-phase diffusion. Carbon and chlorine isotope profiles were constant throughout the unsaturated zone once a steady state was reached. However, depending on the thickness of the unsaturated zone and its lithology, depletion in heavy isotopes may occur with distance during the transient migration of contaminants. Additionally, variations of up to +1.5‰ were observed in the unsaturated zone for chlorine isotopes during water table fluctuations. However, at steady state, it is possible to link a groundwater plume to gas-phase contamination and/or to differentiate sources of contamination based on isotope ratios.

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Density‐driven flow of gas in the unsaturated zone due to the evaporation of volatile organic compounds

Cited by 216 publications

References 26 publications

Buoyant Advection of Gases in Unsaturated Soil

Buoyant Advection of Gases in Unsaturated Soil

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

Can Soil Gas VOCs be Related to Groundwater Plumes Based on Their Isotope Signature?

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