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

Abstract

[1] 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 À could be as far as 10-100 km from the site (although this distance may be unrealistic) and allows estimation of the Cl À 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.

Using excess 4He to quantify variability in aquitard leakage

“…Two dimensional effects have been shown to be significant within aquitards, (e.g. Harrington et al, 2007), and could affect concentrations in the aquifers bounding the aquitard. For the core profiles, little evidence of advective mass transport was observed, thus 2- Fig.…”

“…Increased fluid velocity in fractures and high permeability zones will have a large impact on solute transport through the aquitard, and environmental tracer profiles can reveal local scale heterogeneity in fluid flux (Harrington et al, 2007;Hendry and Wassenaar, 2009). Fluid transport through aquitards can vary at the regional scale (Gerber et al, 2001;Marty et al, 2003); however, most studies to date have focused on single environmental tracer profiles to characterize mass transport through aquitards (e.g.…”

“…Aquitards generally contain heterogeneities such as sand lenses and fractures which can increase hydraulic conductivity (Gerber et al, 2001;Harrington et al, 2007;Hendry and Wassenaar, 2009;McKay et al, 1993). Increased fluid velocity in fractures and high permeability zones will have a large impact on solute transport through the aquitard, and environmental tracer profiles can reveal local scale heterogeneity in fluid flux (Harrington et al, 2007;Hendry and Wassenaar, 2009).…”

Inferring Heterogeneity in Aquitards Using High‐Resolution δD and δ18O Profiles

“…The presence of geological heterogeneities such as fractures (Hendry 1983;McKay et al 1993;Gerber et al 2001) and sand streaks (Harrington et al 2007;Gerber et al 2001) is commonly encountered in clay aquitards. These heterogeneities can be spatially extensive or discontinuous and can create perturbations in transport flow paths and rates.…”

Cl− as a Chemical Fingerprint of Solute Transport in the Aquitard-Aquifer System of the North Jiangsu Coastal Plain, China

“…Previous researches on solute transport in the thick, regional aquitards have suggested that thin, sand-filled layers or permeable conduits might result in facilitating Cl − "halos" laterally through an aquitard [56] and major deviations from one-dimensional diffusion profiles [12]. In this study, the sand layers (aquifers or sand streaks) are commonly encountered in the aquitards.…”