Quantifying diffusive mass transfer in fractured shale bedrock

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doi: 10.1029/1999WR900043
Authors:Jardine, P. M.; Sanford, W. E.; Gwo, J. P.; Reedy, O. C.; Hicks, D. S.; Riggs, J. S.; Bailey, W. B.
Author Affiliations:Primary:
Oak Ridge National Laboratory, Environmental Science Division, Oak Ridge, TN, United States
Colorado State University, United States
Volume Title:Water Resources Research
Source:Water Resources Research, 35(7), p.2015-2030. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0043-1397
Publication Date:1999
Note:In English. 72 refs.; illus., incl. table, sketch map
Summary: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. Copyright 1999 by the American Geophysical Union.
Subjects:Breakthrough curves; Diffusion; Environmental analysis; Experimental studies; Fractured materials; Fractures; Ground water; Mass transfer; Migration; Models; Pollutants; Pollution; Preferential flow; Remediation; Soils; Solute transport; Spatial distribution; Transport
Coordinates:N180000 N190000 E0310000 E0290000
Record ID:1999059123
Copyright Information:GeoRef, Copyright 2018 American Geosciences Institute.
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