Impact of nonaqueous phase liquid (NAPL) source zone architecture on mass removal mechanisms in strongly layered heterogeneous porous media during soil vapor extraction
An existing multiphase flow simulator was modified in order to determine the effects of four mechanisms on NAPL mass removal in a strongly layered heterogeneous vadose zone during soil vapor extraction (SVE): a) NAPL flow, b) diffusion and dispersion from low permeability zones, c) slow desorption from sediment grains, and d) rate-limited dissolution of trapped NAPL. The impact of water and NAPL saturation distribution, NAPL type (i.e., free, residual, or trapped) distribution, and spatial heterogeneity of the permeability field on these mechanisms were evaluated. Two different initial source zone architectures (one with and one without trapped NAPL) were considered and these architectures were used to evaluate seven different SVE scenarios. For all runs, slow diffusion from low permeability zones that gas flow bypassed was a dominant factor for diminished SVE effectiveness at later times. This effect was more significant at high water saturation due to the decrease of gas-phase relative permeability. Transverse dispersion contributed to fast NAPL mass removal from the low permeability layer in both source zone architectures, but longitudinal dispersion did not affect overall mass removal time. Both slow desorption from sediment grains and rate-limited mass transfer from trapped NAPL only marginally affected removal times. However, mass transfer from trapped NAPL did affect mass removal at late time, as well as the NAPL distribution. NAPL flow from low to high permeability zones contributed to faster mass removal from the low permeability layer, and this effect increased when water infiltration was eliminated. These simulations indicate that if trapped NAPL exists in heterogeneous porous media, mass transfer can be improved by delivering gas directly to zones with trapped NAPL and by lowering the water content, which increases the gas relative permeability and changes trapped NAPL to free NAPL.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 946665
- Report Number(s):
- PNNL-SA-61530; 25598; 830403000
- Journal Information:
- Journal of Contaminant Hydrology, 100(1-2):58-71, Journal Name: Journal of Contaminant Hydrology, 100(1-2):58-71 Journal Issue: 1-2 Vol. 100; ISSN JCOHE6; ISSN 0169-7722
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ARCHITECTURE
DESORPTION
DIFFUSION
DISSOLUTION
DISTRIBUTION
Environmental Molecular Sciences Laboratory
GAS FLOW
MASS TRANSFER
MULTIPHASE FLOW
PERMEABILITY
REMOVAL
SATURATION
SEDIMENTS
SIMULATORS
SOILS
WATER
WATER INFLUX
WATER SATURATION
contaminant transport
nonaqueous phase liquid