Zhou Quanlin, Liu Hui-Hai, Molz Fred J, Zhang Yingqi, Bodvarsson Gudmundur S
Earth Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA.
J Contam Hydrol. 2007 Aug 15;93(1-4):161-87. doi: 10.1016/j.jconhyd.2007.02.002. Epub 2007 Feb 22.
Matrix diffusion is an important mechanism for solute transport in fractured rock. We recently conducted a literature survey on the effective matrix diffusion coefficient, D(m)(e), a key parameter for describing matrix diffusion processes at the field scale. Forty field tracer tests at 15 fractured geologic sites were surveyed and selected for the study, based on data availability and quality. Field-scale D(m)(e) values were calculated, either directly using data reported in the literature, or by reanalyzing the corresponding field tracer tests. The reanalysis was conducted for the selected tracer tests using analytic or semi-analytic solutions for tracer transport in linear, radial, or interwell flow fields. Surveyed data show that the scale factor of the effective matrix diffusion coefficient (defined as the ratio of D(m)(e) to the lab-scale matrix diffusion coefficient, D(m), of the same tracer) is generally larger than one, indicating that the effective matrix diffusion coefficient in the field is comparatively larger than the matrix diffusion coefficient at the rock-core scale. This larger value can be attributed to the many mass-transfer processes at different scales in naturally heterogeneous, fractured rock systems. Furthermore, we observed a moderate, on average trend toward systematic increase in the scale factor with observation scale. This trend suggests that the effective matrix diffusion coefficient is likely to be statistically scale-dependent. The scale-factor value ranges from 0.5 to 884 for observation scales from 5 to 2000 m. At a given scale, the scale factor varies by two orders of magnitude, reflecting the influence of differing degrees of fractured rock heterogeneity at different geologic sites. In addition, the surveyed data indicate that field-scale longitudinal dispersivity generally increases with observation scale, which is consistent with previous studies. The scale-dependent field-scale matrix diffusion coefficient (and dispersivity) may have significant implications for assessing long-term, large-scale radionuclide and contaminant transport events in fractured rock, both for nuclear waste disposal and contaminant remediation.
基质扩散是溶质在裂隙岩石中运移的重要机制。我们最近针对有效基质扩散系数D(m)(e)开展了一项文献调研，该系数是描述现场尺度基质扩散过程的关键参数。基于数据的可得性和质量，我们对15个裂隙地质场地的40次现场示踪剂试验进行了调研并选入本研究。现场尺度的D(m)(e)值通过直接使用文献中报道的数据或重新分析相应的现场示踪剂试验来计算。利用示踪剂在线性、径向或井间流场中运移的解析或半解析解，对选定的示踪剂试验进行了重新分析。调研数据表明，有效基质扩散系数的尺度因子（定义为D(m)(e)与同一种示踪剂的实验室尺度基质扩散系数D(m)之比）通常大于1，这表明现场的有效基质扩散系数相对岩石芯尺度的基质扩散系数更大。这个较大的值可归因于天然非均质裂隙岩石系统中不同尺度的众多传质过程。此外，我们观察到尺度因子随观测尺度呈适度的、平均而言系统性增加的趋势。这一趋势表明有效基质扩散系数可能在统计上依赖于尺度。对于5至2000米的观测尺度，尺度因子值范围为0.5至884。在给定尺度下，尺度因子变化两个数量级，反映了不同地质场地裂隙岩石非均质性程度不同的影响。此外，调研数据表明现场尺度纵向弥散度通常随观测尺度增加，这与先前的研究一致。尺度依赖的现场尺度基质扩散系数（和弥散度）对于评估裂隙岩石中核素和污染物的长期、大规模运移事件可能具有重要意义，无论是对于核废料处置还是污染物修复。
