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In layered permeable deposits with flow predominately parallel to the bedding, advection causes rapid solute transport in the more permeable layers. As the solute advances more rapidly in these layers, solute mass is continually transferred to the less permeable layers as a result of molecular diffusion due to the concentration gradient between the layers. The interlayer solute transfer causes the concentration to decline along the permeable layers at the expense of increasing the concentration in the less permeable layers, which produces strongly dispersed concentration profiles in the direction of flow. The key parameters affecting the dispersive capability of the layered system are the diffusion coefficients for the less permeable layers, the thicknesses of the layers, and the hydraulic conductivity contrasts between the layers. Because interlayer solute transfer by transvetse molecular diffusion is a time-dependent process, the advection-diffusion concept predicts a rate of longitudinal spreading during the development of the dispersion process that is inconsistent with the classical Fickian dispersion model. A second consequence of the solute-storage effect offered by transverse diffusion into low-permeability layers is a rate of migration of the frontal portion of a contaminant in the permeable layers that is less than the groundwater velocity. Although various lines of evidence are presented in support of the advection-diffusion concept, more work is required to determine the range of geological materials for which it is applicable and to develop mathematical expressions that will make it useful as a predictive tool for application to field cases of contaminant migration. where c is the solution concentration (M/L3), vi is the linear groundwater velocity (L/T) and Dij is the hydrodynamic dispersion tensor (L2/T). An early development of the advection-dispersion equation is given by Scheidegger [1954], and its applications are described in detail by Bear [1972] and Fried [1975]. Gillham and Cherry [1982] discuss several limitations of the model and critically appraise its utility as a predictive tool. The hydrodynamic dispersion tensor is assumed to be the 369 sum of two components, which for isotropic media can be represented as [Bear, 1972] 0 "' 30 ß ß ß ß •_• ß ß 40-2::50 2::50 225 225 ß "' 220 • ; 220 ,• 215 215 u) 210 ' J ,.u 21o I• ß ß ß ß I-ß LU 205 ' ' ' 205 200 I00 50 0 I00 m 200 ß 195 VERT. EXAG. = I0 195 ß SAMPLE POI•IT Cross section through the Borden aquifer along the direction of flow showing the chloride plume and the location of sampling points.

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Deep‐well injection into fractured sandstone is an option for the disposal of contaminated mine dewatering discharge from an open pit uranium mine. As part of the assessment of potential contaminant migration from deep‐well injection, the effect of matrix diffusion was evaluated. An analytical mathematical model was developed for the simulation of the radial movement of a contaminant front away from an injection point under steady flow conditions in a planar fracture with uniform properties. The model includes the effects of advection in the fracture, diffusion of contaminants from the fracture into the rock matrix, and equilibrium adsorption on the fracture surface as well as in the rock matrix. Effective diffusion coefficients obtained from laboratory experiments on 11 intact core samples varied from 3.4 × 10−8 to 3.2 × 10−7 cm2/s. Model simulations were made with diffusion coefficient values in this range and with single‐fracture injection rates estimated from fracture frequencies in boreholes, and from bulk hydraulic conductivity values obtained from field tests. Because of matrix diffusion, the rate of outward movement of the front of the nonreactive contaminants from the injection well is much slower than the rate of water flow in the fractures. Simulations of the movement of contaminants that undergo adsorption indicate that even a small distribution coefficient for the rock matrix causes the contaminants to remain very close to the injection well during the one‐year period. The results of the simplified model demonstrate that matrix diffusion is an important process that cannot be neglected in the assessment of a waste disposal scheme located in fractured porous rock. However, in order to make a definitive assessment of the capability of matrix diffusion and associated matrix adsorption to significantly limit the extent of contaminant migration around injection wells, it would be necessary to conduct field tests such as a preliminary or experimental injection.