Quantifying diffusive mass transfer in fractured shale bedrock

Abstract

A significant limitation in defining remediation needs at contaminated sites often results from an insufficient understanding of the transport processes that control contaminant migration. The objectives of this research were to help resolve this dilemma by providing an improved understanding of contaminant transport processes in highly structured, heterogeneous subsurface environments that are complicated by fracture flow and matrix diffusion. Our approach involved a unique long‐term, steady state natural gradient injection of multiple tracers with different diffusion coefficients (Br, He, Ne) into a fracture zone of a contaminated shale bedrock. The spatial and temporal distribution of the tracers was monitored for 550 days using an array of groundwater sampling wells instrumented within a fast flowing fracture regime and a slow flowing matrix regime. The tracers were transported preferentially along strike‐parallel fractures, with a significant portion of the tracer plumes migrating slowly into the bedrock matrix. Movement into the matrix was controlled by concentration gradients established between preferential flow paths and the adjacent rock matrix. Observed differences in tracer mobility into the matrix were found to be a function of their free‐water molecular diffusion coefficients. The multiple tracer technique confirmed that matrix diffusion was a significant process that contributed to the overall physical nonequilibrium that controlled contaminant transport in the shale bedrock. The experimental observations were consistent with numerical simulations of the multitracer breakthrough curves using a simple fracture flow model. The simulated results also demonstrated the significance of contaminant diffusion into the bedrock matrix. The multiple tracer technique and ability to monitor the fracture and matrix regimes provided the necessary experimental constraints for the accurate numerical quantification of the diffusive mass transfer process. The experimental and numerical results of the tracer study were also consistent with indigenous contaminant discharge concentrations within the fracture and matrix regimes of the field site. These findings suggest that the secondary source contribution of the bedrock matrix to the total off‐site transport of contaminants is relatively large and potentially long‐lived.

Tracer transport in fractured crystalline rock: Evidence of nondiffusive breakthrough tailing

“…In lieu of direct evidence of matrix diffusion, the most compelling argument for diffusion-influenced breakthrough tailing comes from field tests employing multiple tracers with varying rates of molecular diffusion [Jardine et al, 1999]. A separation in the breakthrough curves produced by solute tracers of different diffusivity suggests that transport is in some way dependent upon molecular diffusion.…”

Field investigation into unsaturated flow and transport in a fault: model analyses

“…While effects of matrix diffusion on solute transport in saturated fractured rocks have been intensively investigated (e.g., Neretnieks, 1980Neretnieks, , 2002Moreno et al, 1997;Ostensen, 1998;Jardine et al, 1999;Shapiro, 2001;McKenna et al, 2001;Meigs and Beauheim, 2001), studies of these effects in unsaturated fractured rock are relatively limited in the literature. Compared with saturated systems, the effects of matrix diffusion in unsaturated systems are more complex, owing to the involved multiphase flow processes and the corresponding fracture-matrix interaction mechanisms.…”

Unsaturated flow and transport through a fault embedded in fractured welded tuff

“…This study differs from previous studies in that it involved multiple tracer transport and therefore allowed for a direct demonstration of the importance of matrix diffusion through unsaturated fractured rock. Compared with saturated systems [e.g., Neretnieks, 1980Neretnieks, , 2002Moreno et al, 1997;Jardine et al, 1999;Shapiro, 2001;Reimus et al, 2003], the effects of matrix diffusion in unsaturated systems are more complex, owing to the multiphase flow processes involved and the corresponding fracture-matrix interaction mechanisms.…”

“…However, while several field studies have investigated matrix diffusion processes in saturated fractured rock [e.g., Albelin et al, 1991;Novakowski and Lapcevic, 1994;Hadermann and Heer, 1996;Jardine et al, 1999;Callahan et al, 2000;Shapiro, 2001;Reimus et al, 2003], very few field studies have investigated these processes in unsaturated fractured rock environments [e.g., Hu et al, 2001].…”