Liu H H, Zhang Y Q, Zhou Q, Molz F J
Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States.
J Contam Hydrol. 2007 Feb 20;90(1-2):41-57. doi: 10.1016/j.jconhyd.2006.09.006. Epub 2006 Oct 25.
Matrix diffusion is an important process for solute transport in fractured rock, and the matrix diffusion coefficient is a key parameter for describing this process. Previous studies have indicated that the effective matrix diffusion coefficient values, obtained from a large number of field tracer tests, are enhanced in comparison with local values and may increase with test scale. In this study, we have performed numerical experiments to investigate potential mechanisms behind possible scale-dependent behavior. The focus of the experiments is on solute transport in flow paths having geometries consistent with percolation theories and characterized by multiple local flow loops formed mainly by small-scale fractures. The water velocity distribution through a flow path was determined using discrete fracture network flow simulations, and solute transport was calculated using a previously derived impulse-response function and a particle-tracking scheme. Values for effective (or up-scaled) transport parameters were obtained by matching breakthrough curves from numerical experiments with an analytical solution for solute transport along a single fracture. Results indicate that a combination of local flow loops and the associated matrix diffusion process, together with scaling properties in flow path geometry, seems to be an important mechanism causing the observed scale dependence of the effective matrix diffusion coefficient (at a range of scales).
基质扩散是溶质在裂隙岩石中运移的一个重要过程，而基质扩散系数是描述这一过程的关键参数。以往的研究表明，从大量现场示踪试验获得的有效基质扩散系数值，与局部值相比有所增大，并且可能随试验规模的增大而增加。在本研究中，我们进行了数值试验，以探究可能的尺度依赖行为背后的潜在机制。试验的重点是溶质在具有与渗流理论一致的几何形状且以主要由小尺度裂隙形成的多个局部流动环为特征的流动路径中的运移。通过离散裂隙网络流动模拟确定了通过一条流动路径的水流速度分布，并使用先前推导的脉冲响应函数和粒子追踪方案计算了溶质运移。有效（或放大后的）运移参数值是通过将数值试验的突破曲线与沿单一裂隙的溶质运移解析解进行匹配而获得的。结果表明，局部流动环和相关的基质扩散过程，以及流动路径几何形状的尺度特性，似乎是导致有效基质扩散系数在一定尺度范围内出现观测到的尺度依赖性的一个重要机制。
