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Title: THE HYDRUS-PHREEQC MULTICOMPONENT TRANSPORT MODEL FOR VARIABLY-SATURATED POROUS MEDIA: CODE VERIFICATION AND APPLICATION

Author
item JACQUES, DIEDERIK - SCK-CEN, BELGIUM
item SIMUNEK, JIRI - UC RIVERSIDE, CA
item MALLANTS, DIRK - SCK-CEN, BELGIUM
item Van Genuchten, Martinus

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/17/2003
Publication Date: 9/19/2003
Citation: Jacques, D., Simunek, J., Mallants, D., Van Genuchten, M.T. 2003. The hydrus-phreeqc multicomponent transport model for variably-saturated porous media: code verification and application. Meeting Proceedings. "MODFLOW and More 2003: Understanding through modeling". Int Ground Water Modeling Center, Colorado School of Mines, Golden, CO. Sept 16-19, 2003.

Interpretive Summary: The subsurface transport of many contaminants is affected by a multitude of complex and often interactive physical, chemical, mineralogical and biological processes. Simulation of these processes requires a coupled reactive transport code that integrates the physical processes of water flow and solute transport with the prevailing biogeochemical processes. In this paper we describe a new reactive transport code based on the coupling of two existing models: HYDRUS-1D simulating water flow and solute transport in the vadose zone between the soil surface and the groundwater table, and PHREEQC which considers a large number of biogeochemical reactions. The combined model allows one to simultaneously simulate transient variably-saturated water flow, multicomponent solute transport, speciation and geochemical processes, including a variety of mixed equilibrium and kinetic reactions. Simulations were compared with results obtained with a different reactive transport model for several benchmark problems. One problem was limited to chemical speciation and cation exchange, while two second problems also involved mineral dissolution and precipitation. Excellent agreement between the two codes was obtained. The new capabilities of the model are demonstrated by simulating long-term heavy metal (cadmium) transport in a soil profile in Belgium subject to complex transient weather conditions. The example showed reduced leaching of Cd from the top soil layer when transient precipitation-evaporation cycles were included in the model as compared to using simplified steady-state (constant in time) flow conditions.

Technical Abstract: A new coupled model for multicomponent, reactive transport during transient variably-saturated flow is presented. The model combines two comprehensive existing models: HYDRUS-1D and PHREEQC. 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 subject to a variety of atmospheric and other boundary conditions. PHREEQC is a computer program simulating the behavior of complex chemical systems, including such reactions as speciation, ion exchange, surface complexation, and mineral precipitation/dissolution. The accuracy of the coupled HYDRUS1D-PHREEQC model was verified by comparing simulation results with calculations obtained with an independent model (CRUNCH, using both the operatoring splitting and global implicit coupling modes) for two steady-state flow problems. One problem considered the transport of heavy metals in a layered soil profile having a pH-dependent cation exchange complex. Another problem simulated the intrusion of a high-pH solution (pH 13) into a compacted clay core leading to kinetic dissolution of primary minerals and precipitation of secondary minerals. Simulation results of HYDRUS1D-PHREEQC were in close agreement with those by CRUNCH for both problems. We illustrate the possibilities of the new code by predicting the long-term leaching of heavy metals (Cd, Zn, and Pb) in a contaminated soil profile. The example showsis used to demonstrate several levels of complexity that can be considered with the coupled multicomponent transport model.