Numerical Simulation of Radial Gas Flow: Effects of Soil Anisotropy, Well Placement, and Surface Seal
Publication: Journal of Hydrologic Engineering
Volume 3, Issue 1
Abstract
A variety of methods have been used to control gas flow in the vadose zone in order to remove volatile organic compounds. The effects of well configuration and engineered surface seals on gas flow are recognized but have not been thoroughly analyzed for the design of a vapor extraction well. This paper presents a numerical model to simulate axisymmetric gas flow toward an extraction well in the unsaturated zone. Simulations were conducted to analyze the behavior of gas flow under a variety of system conditions. Results indicate that soil anisotropy, length of the well screen, depth of the gas extraction well, and surface sealing need to be considered in the analysis of a vapor well. Anisotropic soils can significantly affect the area of influence (AI) where the upper surface is open to the atmosphere but generally play a lesser role in a confined soil zone. Setting screens deeper can generate a larger AI, while a longer screen can reduce the pressure drop at the extraction well without causing a major impact on the AI. Simulations also show that surface seals for a shallow well and homogeneous soils are more effective in enhancing well efficiency when compared to a deep well and heterogeneous soils. Application of surface seals reduces air leakage from the ground surface and increases the horizontal gas flow.
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References
1.
Baehr, A. L., and Hult, M. F. (1988). “Determination of the air-phase permeability tensor of an unsaturated zone at the Bemidji, Minnesota Research Site.”Water-Resour. Investigation Rep. 88-4220, U.S. Geological Survey, 55–62.
2.
Baehr, A. L., and Hult, M. F.(1991). “Evaluation of unsaturated zone air permeability through pneumatic tests.”Water Resour. Res., 27(10), 2605–2617.
3.
Beckett, G. D., and Huntley, D.(1994). “Characterization of flow parameters controlling soil vapor extraction.”Ground Water, 32(2), 239–24.
4.
Benson, D. A., Huntly, D., and Johnson, P. C.(1993). “Modeling vapor extraction and general transport in the presence of NAPL mixtures and nonideal conditions.”Ground Water, 31(3), 437–445.
5.
Chen, X. H. (1994). “Finite element transport modeling of biodegradable contaminants in heterogeneous porous media,” PhD dissertation, Dept. of Geology and Geophys., Univ. of Wyoming, Laramie, Wy.
6.
Chen, X. H.(1996). “Estimate of soil permeability and porosity using an analytical solution considering gas compressibility.”J. Envir. Sys., 25(1), 29–40.
7.
Chen, X. H., Jayne, L. (1994). “Estimating soil permeability from partially penetrating wells.”Geological Soc. of Am. Abstract with Programs, 26(7), A–444.
8.
Cho, J. S., and DiGiulio, D. C.(1992). “Pneumatic pumping test for soil vacuum extraction.”Envir. Progress, 11(3), 228–233.
9.
Edwards, K. B., and Jones, L. C. (1994). “Air permeability from pneumatic tests in oxidized till.”J. Envir. Engrg., ASCE 120(2), 329–347.
10.
Falta, R. W.(1996). “A program for analyzing transient and steady-state soil gas pump tests.”Ground Water, 34(4), 750–755.
11.
Freeze, R. A., and Cherry, J. A. (1979). Groundwater. Prentice-Hall, Inc., Englewood Cliffs, N.J.
12.
Gamliel, A., and Abdul, A. S.(1993). “Numerical investigation of optimal well spacing and the effect of screen length and surface sealing on gas flow toward an extraction well.”J. Contaminant Hydrol., 12, 171–191.
13.
Johnson, P. C., Kemblowski, M. K., and Colthart, J. D.(1990). “Quantitative analysis for the cleanup of hydrocarbon-contaminated soils by in-situ soil venting.”Ground Water, 28(3), 413–429.
14.
Massmann, J. W.(1989). “Applying groundwater flow models in vapor extraction system design.”J. Envir. Engrg., ASCE, 115(1), 129–150.
15.
Massmann, J. W., and Madden, M.(1994). “Estimating air conductivity and porosity from vadose-zone pumping tests.”J. Envir. Engrg., 120(2), 313–328.
16.
McWhorter, D. B.(1990). “Unsteady radial flow of gas in the vadose zone.”J. Contaminant Hydrol., 5, 387–402.
17.
Mohr, D. H., and Merz, P. H.(1995). “Application of a 2D gas flow model to soil vapor extraction and bioventing case studies.”Ground Water, 33(3), 433–444.
18.
Pandit, A., and Aoun, J. M. A.(1994). “Numerical modeling of axisymmetric flow.”Ground Water, 32(3), 458–464.
19.
Pinder, G. F., and Gray, W. G. (1977). Finite element simulation in surface and subsurface hydrology. Academic Press, Inc., New York, N.Y.
20.
Reilly, T. E. (1984). “A Galerkin finite-element flow model to predict the transient responses of a radially symmetric aquifer.”Water-Supply Paper 2198, U.S. Geological Survey.
21.
Sepehr, M., and Samani, Z. A.(1993). “In situ soil remediation using vapor extraction wells, development and testing of a three-dimensional finite-difference model.”Ground Water, 31(3), 425–436.
22.
Shan, C., Falta, R. W., and Javandel, I.(1992). “Analytical solutions for steady state gas flow to a soil vapor extraction well.”Water Resour. Res., 28(4), 1105–1120.
Information & Authors
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Published In
Journal of Hydrologic Engineering
Volume 3 • Issue 1 • January 1998
Pages: 52 - 61
Copyright
Copyright © 1998 American Society of Civil Engineers.
History
Published online: Jan 1, 1998
Published in print: Jan 1998
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