Solute Transport With Linear Reactions in Porous Media With Layered Structure: A Semianalytical Model

Layered heterogeneous media widely exist in subsurface environment. They commonly give rise to preferential flow that provides a fast path for water and solute transport in soils and aquifers. One-dimensional (1-D) double-region models are frequently used to simulate solute transport in heterogeneous media by adopting a mass transfer coefficient, rather than transverse dispersion, to represent interaction between two zones. A two-dimensional (2-D) solute transport model considering transverse dispersion and linear reactions in a layered medium where water in the two regions is mobile was presented, and its semianalytical solution was derived. The solution was tested using a numerical solution and an existing analytical solution. The results indicated that the mass exchange between the two zones was induced by the contrast in properties of the two zones and determined by the transverse dispersion across the interface. When solutes pass through the two-layer medium, part of the solutes in the fast flow zone will migrate into the slow flow zone in the beginning, and later this mass exchange will be reversed; that is, part of the solutes in the slow flow zone will migrate back to the fast flow zone. The magnitude of the mass exchange was determined by the property difference in the two zones. The reaction of the solute has significant impacts on the mass interaction between the two zones. The solution was applied to previous laboratory experiments and compared to the 1-D double-region model as well. The 2-D model matched the observed breakthrough curves better than the 1-D model.