Impact of permeable conduits on solute transport in aquitards: Mathematical models and their application

Abstract

The migration of contaminants through porous conduits (for example, sand layers) in aquitards, or any preferential flow feature within a low-permeability matrix, can be significantly retarded via diffusion into the matrix and processes such as sorption and decay. Previous solutions for simulating contaminant transport in these types of environments have been limited to a parallel-plate approach such as that used in studies of fractured rock aquifers. A major drawback of using these approaches is that they cannot account for the radial component of diffusion that occurs from circular- or elliptical-sectioned conduits. We have developed solutions for both of the latter cases and compared results from generic simulations to those obtained with a conventional parallel-plate model. The results demonstrated using a circular conduit approach can produce concentration profiles for both the conduit and surrounding matrix that are up to several orders of magnitude lower than those predicted using parallel-plate models. The solutions for elliptical-section conduits provide concentration profiles that are bounded by the circular- and parallel-plate models. We also present several generic simulations to demonstrate the effects of conduit radius and average groundwater flow velocity on concentration profiles. Finally, the circular conduit model is applied to a clay-rich till aquitard in southern Saskatchewan, Canada to provide insight into an anomalous dissolved chloride peak in the vertical pore water profile. The model demonstrates that the source of the high Cl- could be as far as 10-100 km from the site (although this distance may be unrealistic) and allows estimation of the Cl- concentration at the source. We envisage that the solutions for conduit diffusion will have far wider applications than just aquitard studies, and future uses may include tracer tests in karstic and fractured aquifers. Copyright 2007 by the American Geophysical Union.