Numerical Modeling of Contaminant Transport Using HYDRUS and its Specialized Modules

Abstract

A broad range of numerical models have been developed during the past several decades to describe the fate and transport of agricultural, industrial, and other contaminants in soils and groundwater. Such models are now increasingly implemented in both research and engineering projects addressing subsurface pollution problems. Of particular concern is especially non-point source pollution stemming from plant and animal production. In this paper we first briefly review different types of mathematical models that are being used to describe the transport of agricultural chemicals in both the vadose zone and groundwater. We next review various versions of the HYDRUS computer software packages, including several specialized modules that were recently developed for simulating the movement of water, heat, and solutes in the subsurface.

Early versions of the HYDRUS models considered the transport of only one chemical species and assumed that the behavior of this solute was independent of other species present in the soil solution. Physical nonequilibrium transport could be accounted for in later versions of HYDRUS by assuming a two-region or dual-porosity type formulations that partition the liquid phase into mobile and immobile regions. Chemical nonequilibrium transport could be accounted for by assuming kinetic interactions between solutes in the liquid and solid phases. Physical and chemical nonequilibrium formulations were extended later also to particle transport by including provisions for filtration theory, and time- and/or depthdependent blocking functions.

Subsequent versions of the HYDRUS codes also considered the transport of multiple solutes, which either could be coupled by means of a unidirectional first-order degradation chain, or move independently of each other. While this approach proved effective for evaluating the subsurface transport of many chemicals (e.g., nitrogen species, pesticides, radionuclides),
many environmental problems require analyses of the transport of multiple chemical species that could interact mutually, create complexed species, precipitate, dissolve, and/or compete with each other for sorption sites. Several specialized modules have now been developed to simulate transport processes not accounted for in the earlier standard versions of HYDRUS. These include a wetlands module, the HP1/2/3 multicomponent
transport modules, the facilitated transport C-Ride module, a modulefor fumigants, and the major ion Unsatchem module. All of these modules simulate flow and transport processes in two-dimensional transport domains and are supported by the HYDRUS (2D/3D) graphical user interface. Many processes of these specialized modules are also available as part of the public domain HYDRUS-1D software. Brief overviews of these more recent modules are included in this manuscript.