Draft- Dual Domain Reaction module Clement, 10/28/981 RT3D Reaction Module for Modeling Contaminant Transport in Dual Domain Porous Media

In soil physics literature, it is often common to conceptualize soils as aggregated porous media consisting of slowly and rapidly conducting pore sequences. The aggregates might have many micro pores in which displacement is dependent only upon diffusion, because convection in these smaller pores is usually negligible (van Genuchten and Wierenga, 1976). Solute transport in such aggregated media could lead to slow and incomplete mixing. Dual domain models are useful for modeling solute transport in such aggregated porous media systems. In addition, dual domain models may also be used for modeling transport in fractured porous formation if the continuum approximation is assumed to valid (Berkowitz, 1988; Dykhuizen, 1990). The dual domain transport model can also be extended to account for biodegradation in different domains. For example, Sun et al. (1998) formulated a kinetic model to predict microbial transport and biodegradation in a dual-porosity system. The kinetic model was coded as a reaction package and was solved by the RT3D code. Zheng and Wang (1998) describe a three-dimensional formulation for modeling transport and first order decay in a dual-porosity system. The mathematical equations used for describing dual porosity systems are very similar to those used for describing the rate-limited sorption systems [see Clement (1997) discussions for reaction module #4], but with a few subtle differences. Based on the formulation described by Zheng and Wang (1998), the governing equations for modeling contaminant transport in a dual-porosity domain can be written as: ( ) ()CCCs qs+Cvi xi x