Quantifying diffusive mass transfer in fractured shale bedrock

A significant limitation in defining remediation needs at contaminated site s often results from an insufficient understanding of the transport process es that control contaminant migration. The objectives of this research were to help resolve this dilemma by providing an improved understanding of con taminant transport processes in highly structured, heterogeneous subsurface environments that are complicated by fracture flow and matrix diffusion. O ur approach involved a unique long-term, steady state natural gradient inje ction of multiple tracers with different diffusion coefficients (Br, He, Ne ) into a fracture zone of a contaminated shale bedrock. The spatial and tem poral 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 prefe rentially along strike-parallel fractures, with a significant portion of th e tracer plumes migrating slowly into the bedrock matrix. Movement into the matrix was controlled by concentration gradients established between prefe rential flow paths and the adjacent rock matrix. Observed differences in tr acer mobility into the matrix were found to be a function of their free-wat er molecular diffusion coefficients. The multiple tracer technique confirme d that matrix diffusion was a significant process that contributed to the o verall physical nonequilibrium that controlled contaminant transport in the shale bedrock. The experimental observations were consistent with numerica l simulations of the multitracer breakthrough curves using a simple fractur e flow model. The simulated results also demonstrated the significance of c ontaminant diffusion into the bedrock matrix. The multiple tracer technique and ability to monitor the fracture and matrix regimes provided the necess ary experimental constraints for the accurate numerical quantification of t he diffusive mass transfer process. The experimental and numerical results of the tracer study were also consistent with indigenous contaminant discha rge concentrations within the fracture and matrix regimes of the field site . These findings suggest that the secondary source contribution of the bedr ock matrix to the total off-site transport of contaminants is relatively la rge and potentially long-lived.