Scale Dependency of the Effective Matrix Diffusion Coefficient

H. H. Liu, G. S. Bodvarsson, and Guoxiang Zhang

Contact: H.H. Liu, 510/486-6452, hhliu@lbl.gov

Research Objectives
The exchange of solute mass (through molecular diffusion) between fluid in fractures and fluid in rock matrix is called matrix diffusion. Owing to the order-of-magnitude slower flow velocity in the matrix compared to fractures, matrix diffusion can significantly retard contaminant transport in fractured rock. The effective matrix diffusion coefficient is an important parameter for describing this matrix diffusion, in that it largely determines overall contaminant transport behavior in fractured rock (in many cases). Such diffusion coefficient values measured from small-scale rock samples in the laboratory have been directly used for modeling large-scale radionuclide transport at the proposed Yucca Mountain nuclear waste repository site (and many other sites). However, some preliminary studies have indicated that field-scale matrix diffusion coefficient values are different from local values. The major objective of this work is to determine if a relationship exists between the effective matrix diffusion coefficient and test scales.

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