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Title: Modeling coupled water flow, solute transport and geochemical reactions affecting heavy metal migration in a podzol soil

Author
item JACQUES, D. - PERFORMANCE ASSESS UNIT
item SIMUNEK, J - UC RIVERSIDE
item MALLANTS, D - PERFORMANCE ASSESS UNIT
item Van Genuchten, Martinus

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2008
Publication Date: 6/1/2008
Citation: Jacques, D., Simunek, J., Mallants, D., Van Genuchten, M.T. 2008. Modeling coupled water flow, solute transport and geochemical reactions affecting heavy metal migration in a podzol soil. Geoderma. 145:449-461.

Interpretive Summary: Process-based models can be valuable tools for studying the subsurface fate and transport of organic or inorganic contaminants such as nutrients, toxic trace elements, radionuclides or pesticides. 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: HYDRUS-1D which simulates water flow and solute transport in unsaturated soils, and PHREEQC-2 which can be used to describe a broad range of geochemical reactions in soils. Various features of the coupled code are demonstrated for a hypothetical problem involving cadmium (Cd) transport in a layered sandy soil profile. Results show that temporal climatic variations in precipitation and evaporation can lead to significant changes in the soil geochemical conditions, especially near the soil surface. The example shows that numerical simulators such as HP1 are potentially very attractive tools for studying reactive transport processes in the vadose zone during transient unsaturated flow. The calculations indicate a close relationship between evaporation, upward flow, changing geochemical conditions and Cd accumulation near the soil surface.

Technical Abstract: Many or most subsurface pollution problems at the field scale involve such simultaneous processes as water flow, multicomponent solute transport, heat transport and biogeochemical processes and reactions. Process-based models that integrate these various processes can be valuable tools for investigating the mobilty of a wide range of inorganic and organic contaminants subject to different hydrologic and geochemical conditions. The HP1 reactive transport simulator, obtained by weak coupling the HYDRUS-1D and PHREEQC-2, was developed designed to address multicomponent geochemical transport processes in the vadose zone. In this paper we discuss a hypothetical HP1 application involving the transport of major cations and heavy metals in a soil during transient flow over a period of 30 years. Results show that variations in water content and water fluxes can significantly influence the speciation, and thus the mobility and availability, of the elements. Decreasing water contents near the soil surface decreased the pH and produced new cation exchange equilibrium conditions. The upward transport of Cl during summer due to increased evapotranspiration, and subsequent accumulation of Cl near the soil surface, caused an increase in the total aqueous Cd concentration because of the formation of Cd-Cl complexes. While the numerical example focused on relatively short-term (30 year) heavy metal mobility in soils, HP1 appears to be an equally promising tool also for studying more long-term pedogenetic processes, including the impact of natural processes and antropogenic activities on soil evolution.