Scale Dependence of the Effective Matrix Diffusion Coefficient Evidence and Preliminary Int.pdf

Scale Dependence of the Effective Matrix Diffusion Coefficient: Evidence and Preliminary Interpretation Hui-Hai Liu and Yingqi Zhang Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA Fred J. Molz Department of Environmental Science and Engineering, Clemson University, SC 1. Introduction The exchange of solute mass (through molecular diffusion) between fluid in fractures and fluid in the rock matrix is called matrix diffusion. Owing to the orders-of- magnitude slower flow velocity in the matrix compared to fractures, matrix diffusion can significantly retard solute transport in fractured rock, and therefore is an important process for a variety of problems, including remediation of subsurface contamination and geological disposal of nuclear waste. The effective matrix diffusion coefficient (molecular diffusion coefficient in free water multiplied by matrix tortuosity) is an important parameter for describing matrix diffusion, and in many cases largely determines overall solute transport behavior. While matrix diffusion coefficient values measured from small rock samples in the laboratory are generally used for modeling field-scale solute transport in fractured rock (Boving and Grathwohl, 2001), several research groups recently have independently found that effective matrix diffusion coefficients much larger than laboratory measurements are needed to match field-scale tracer-test data (Neretnieks, 2002; Becker and Shapiro, 2000; Shapiro, 2001; Liu et al., 2003, 2004a). In addition to the observed enhancement, Liu et al. (2004b), based on