MULTICOMPONENT GEOCHEMICAL TRANSPORT MODELING USING HYDRUS-1D AND HP1

Authors: SIMUNEK, JIRKA - UC, RIVERSIDE; JACQUES, DIEDERIK - SCK, MOL BELGIUM; Van Genuchten, Martinus; MALLANTS, DIRK - SCK, MOL BELGIUM

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2006
Publication Date: 12/15/2006
Citation: Simunek, J., Jacques, D., Van Genuchten, M.T., Mallants, D. 2006. Multicomponent geochemical transport modeling using HYDRUS-1D and HP1. Journal of the American Water Resources Association. Vol.42:1537-1547

Interpretive Summary: Concern is increasing about the appearance of agricultural chemicals (salts, pesticides, pathogenic micro-organisms, fertilizers, heavy metals) in soils and groundwater. Predicting the fate and transport of these chemicals is generally difficult since they are often affected by a large number of interactive physical, chemical, and biological processes. Simulating these processes requires an comprehensive computer code that couples the physical processes of water flow and solute transport with a range of biogeochemical processes. This paper summarizes two recently developed codes that account for a broad range of geochemical processes in the subsurface. Both are based on the HYDRUS-1D software package simulating water and solute transport in the unsaturated zone between the soil surface and the groundwater table. One model resulted from coupling HYDRUS- 1D with the UNSATCHEM module for major ion chemistry involving such cations as sodium, calcium and magnesium. This program enables quantitative predictions of such problems as analyzing the effects of salinity on plant growth, or estimating the amount of water and chemical amendments needed to reclaim salt-affected soils. The second model, HP1, resulted from coupling HYDRUS-1D with the popularly used PHREEQC biogeochemical code. The latter program accounts for a wide range of chemical and biological reactions applicable to the transport of pesticides, chlorinated hydrocarbons, heavy metals, radionuclides, and bacteria and viruses. Two HP1 example applications are discussed: the leaching of heavy metals (zinc, lead and cadmium) from a contaminated soil profile, and the transport of the explosive TNT and its degradation products. The latter example shows that groundwater may be more vulnerable to the leaching of TNT degradation products, than of the parent compound itself. The two public-domain reactive transport models provide theoretical and applied scientists and engineers with much-needed tools for predicting water and chemical transport procesess in the subsurface.

Technical Abstract: The transport of reactive contaminants in the subsurface is generally affected by a large number of nonlinear and often interactive physical, chemical, and biological processes. Simulating these processes requires a comprehensive reactive transport code that couples the physical processes of water flow and advective-dispersive transport with a range of biogeochemical processes. Two recently developed coupled geochemical models that are both based on the HYDRUS-1D software package for variably saturated flow and transport are summarized in this paper. One model resulted from coupling HYDRUS- 1D with the UNSATCHEM module. While restricted to major ion chemistry, this program enables quantitative predictions of such problems as analyzing the effects of salinity on plant growth and the amount of water and amendments required to reclaim salt-affected soil profiles. The second model, HP1, resulted from coupling HYDRUS-1D with the PHREEQC biogeochemical code. The latter program accounts for a wide range of instantaneous or kinetic chemical and biological reactions, including complexation, cation exchange, surface complexation, precipitation dissolution and/or redox reactions. The versatility of HP1 is illustrated in this paper by means of two examples: the leaching of toxic trace elements and the transport of the explosive TNT and its degradation products.