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Title: REVIEW AND COMPARISON OF MODELS FOR DESCRIBING NON-EQUILIBRIUM AND PREFERENTIAL FLOW AND TRANSPORT IN THE VADOSE ZONE

Author
item SIMUNEK, JIRKA - UC RIVERSIDE, CA
item JARVIS, NICK - SWEDISH UNIV OF AG SCI
item Van Genuchten, Martinus
item GARDENAS, ANNEMIEKE - SWEDISH UNIV OF AG SCI

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2002
Publication Date: 10/2/2002
Citation: Simunek, J., Jarvis, N.J., Van Genuchten, M.T., Gardenas, A. 2002. Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone. Journal of Hydrology. Vol 272:14-35

Interpretive Summary: The problem of subsurface preferential flow has received much attention in the soil and agricultural sciences because of its implications in accelerating the movement of agricultural contaminants (fertilizers, pesticides, pathogenic microorganisms, toxic trace elements) through the unsaturated zone to underlying groundwater. The potentially rapid migration of radionuclides from low- and high-level nuclear waste disposal facilities, and the preferential movement of non-aqueous liquids or other pollutants from underground storage tanks, waste disposal sites and mine tailings, has also become a concern for hydrologists, geophysicists, and environmental scientists. Preferential flow is probably the most frustrating process in terms of hampering accurate predictions of contaminant transport in soils and fractured rocks. Preferential flow, as opposed to uniform flow, results in irregular wetting of the soil profile as a direct consequence of water moving faster in certain parts of the soil profile than in others. In this paper, we review various approaches for modeling preferential flow in the vadose zone between the soil surface and the groundwater table. Existing approaches range from relatively simplistic models to more complex physically based dual-porosity, dual-permeability, and multi-region type models. Several models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures. Advantages and disadvantages of the different models are discussed, and the need for inter-code comparison is stressed, especially against field data that are sufficiently comprehensive to allow calibration/validation of the more complex models and to distinguish between alternative modeling concepts. Several examples and comparisons of equilibrium and various nonequilibrium flow and transport models are also provided. Results are important to better understand and predict the fate and transport of agricultural contaminants in the subsurface.

Technical Abstract: In this paper, we review various approaches for modeling preferential and non-equilibrium flow and transport in the vadose zone. Existing approaches differ in terms of their underlying assumptions and complexity. They range from relatively simplistic models to more complex physically based dual-porosity, dual-permeability, and multi-region type models. A relatively simple dual-porosity flow model results when the Richards equation is combined with composite (double-hump type) equations for the hydraulic properties to account for both soil textural (matrix) and soil structural (fractures, macropores, peds) effects on flow. The simplest non-equilibrium flow model, a single-porosity model which distinguishes between actual and equilibrium water contents, is based on a formulation by Ross and Smettem [Soil Sci. Soc. Am. J. 64 (2000), 1926] that requires only one additional parameter to account for non-equilibrium. A more complex dual-porosity, mobile-immobile water flow model results when the Richards or kinematic wave equations are used for flow in the fractures, and immobile water is assumed to exist in the matrix. We also discuss various dual-permeability models, including the formulation of Gerke and van Genuchten [Water Resour. Res. 29 (1993a), 305] and the kinematic wave approach as used in the MACRO model of Jarvis [Technical Description and Sample Simulations, Department of Soil Science, Swedish University of Agricultural Science, Uppsala, Sweden (1994), 51]. Both of these models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures. Advantages and disadvantages of the different models are discussed, and the need for inter-code comparison is stressed, especially against field data that are sufficiently comprehensive to allow calibration/validation of the more complex models and to distinguish between alternative modeling concepts. Several examples and comparisons of equilibrium and various nonequilibrium flow and transport models are also provided.