General approach for modeling solute transport in structured soils

Classical convective-dispersive type transport models are often found to be of limited use for predicting in structured soils or fractured aquifer systems. Recently a number of deterministic two-region type models have appeared in the literature that consider transport in structured soils from a microscopic (macropore-scale) point of view. In these models, the chemical is assumed to be transported through a single pore or crack of known geometry, or through the voids between well-defined, uniformly-sized aggregates. In addition, diffusion-type equations are used to describe solute transfer from the larger pores into the soil matrix. This paper describes a method that extends the two-region modeling approach to more general conditions involving aggregates of arbitrary geometry. The method is based on the use of a geometry-dependent shape factor (f) that transforms an aggregate of given shape and size (platy, columnar, prismatic) into an equivalent sphere with similar diffusion characteristics as the original aggregate. Using conversions between known analytical solutions as test cases, the transformation was found to very accurate for most aggregate geometries commonly encountered in the field. A similar transformation was also used to quantify the unknown mass transfer coefficient in a previously employed first-order rate expression for solute exchange between mobile (interaggregate) and immobile (intra-aggregate) regions. 19 references, 8 figures, 1 table