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United States Department of Agriculture

Agricultural Research Service

Title: Multicomponent Transport Model for Variably-Saturated Porous Media: Application to the Transport of Heavy Metals in Soils

Authors
item Jacques, Diederik - SCK-CEN, BELGIUM
item Simunek, Jirka - ES DEPT, UC RIVERSIDE,CA
item Mallants, D. - SCK-CEN, BELGIUM
item Van Genuchten, Martinus

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: June 10, 2001
Publication Date: June 20, 2002
Citation: Jacques, D., Simunek, J., Mallants, D., Van Genuchten, M.T. 2002. Multicomponent transport model for variably-saturated porous media: application to the transport of heavy metals in soils. Book Chapter. In: Computational Methods in Water Resources, Hassanizadeh, S.M., Schotting,R.J., Gray, W.G., Pinder, G.F. (editors). 2002.Developments in Water Science, Elsevier Press, The Netherlands. 47:555-562.

Interpretive Summary: Transport of reactive solutes in soils is the result of the physical processes of water flow and solute transport, including diffusion and dispersion, and chemical interactions between solutes and the soil solid phase. When multiple solutes are present, they may be competing for the available sorption sites. Such competition cannot be modeled with classical approaches that treat solutes as chemically independent constituents and use simplified linear or nonlinear adsorption isotherms. The objective of this paper was to present a new reactive transport code based on the coupling of two existing models, one simulating water flow and solute transport (HYDRUS-1D), and a range of geochemical reactions (PHREEQC). After a short introduction of these two models, various features of the coupled HYDRUS1D-PHREEQC code are demonstrated for hypothetical problems dealing with the multicomponent transport of major cations (Al, Ca, K, Na, and Mg), an anion (Cl), and three heavy metals (Cd, Pb, and Zn) in both saturated laboratory soil columns and layered field soil profiles. Further research will concentrate on numerical testing of the coupled model, inclusion of kinetic reactions, application to more realistic transport problems in the field, and a comparison with experimentally obtained leaching data.

Technical Abstract: This paper presents a new coupled model for multicomponent, reactive transport during transient variably-saturated flow based on two comprehensive models: (1) HYDRUS-1D [1] and PHREEQC [2]. HYDRUS-1D is a one-dimensional finite element model simulating the movement of water, heat and multiple solutes in variably saturated heterogeneous or layered soils under various boundary conditions. PHREEQC is a computer program simulating the behavior of complex chemical systems including such reactions as speciation, ion exchange, surface complexation, mineral precipitation/dissolution. As a verification of the coupled HYDRUS1D-PHREEQC model, we compared results for a steady-state saturated flow problem obtained with both the coupled model and PHREEQC. To illustrate the possibilities of the new code for more complex problems we simulated the long-term leaching of heavy metals (Cd, Zn, Pb) in a contaminated soil profile. The example considers several levels of complexity that are included into the coupled model: multicomponent transport in homogeneous and physically and chemically heterogeneous (layered) soil profiles under steady-state, periodic or atmospheric flow conditions.

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