Guihéneuf N, Bour O, Boisson A, Le Borgne T, Becker M W, Nigon B, Wajiduddin M, Ahmed S, Maréchal J-C
BRGM, D3E, NRE, Indo-French Center for Groundwater Research, Uppal Road, 500007 Hyderabad, India; OSUR, Géosciences Rennes, UMR6118 CNRS - Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France.
OSUR, Géosciences Rennes, UMR6118 CNRS - Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France.
J Contam Hydrol. 2017 Nov;206:18-33. doi: 10.1016/j.jconhyd.2017.09.003. Epub 2017 Sep 15.
In fractured media, solute transport is controlled by advection in open and connected fractures and by matrix diffusion that may be enhanced by chemical weathering of the fracture walls. These phenomena may lead to non-Fickian dispersion characterized by early tracer arrival time, late-time tailing on the breakthrough curves and potential scale effect on transport processes. Here we investigate the scale dependency of these processes by analyzing a series of convergent and push-pull tracer experiments with distance of investigation ranging from 4m to 41m in shallow fractured granite. The small and intermediate distances convergent experiments display a non-Fickian tailing, characterized by a -2 power law slope. However, the largest distance experiment does not display a clear power law behavior and indicates possibly two main pathways. The push-pull experiments show breakthrough curve tailing decreases as the volume of investigation increases, with a power law slope ranging from -3 to -2.3 from the smallest to the largest volume. The multipath model developed by Becker and Shapiro (2003) is used here to evaluate the hypothesis of the independence of flow pathways. The multipath model is found to explain the convergent data, when increasing local dispersivity and reducing the number of pathways with distance which suggest a transition from non-Fickian to Fickian transport at fracture scale. However, this model predicts an increase of tailing with push-pull distance, while the experiments show the opposite trend. This inconsistency may suggest the activation of cross channel mass transfer at larger volume of investigation, which leads to non-reversible heterogeneous advection with scale. This transition from independent channels to connected channels when the volume of investigation increases suggest that both convergent and push-pull breakthrough curves can inform the existence of characteristic length scales.
在裂隙介质中，溶质运移受开放且连通裂隙中的对流以及可能因裂隙壁化学风化而增强的基质扩散控制。这些现象可能导致非菲克弥散，其特征为示踪剂到达时间提前、突破曲线上出现晚期拖尾以及运移过程中可能存在尺度效应。在此，我们通过分析一系列在浅部裂隙花岗岩中进行的收敛式和推 - 拉式示踪剂实验来研究这些过程的尺度依赖性，实验的调查距离范围为4米至41米。小距离和中等距离的收敛式实验呈现出非菲克拖尾，其特征为 -2幂律斜率。然而，最大距离实验并未呈现出明确的幂律行为，而是可能指示出两条主要路径。推 - 拉式实验表明，随着调查体积的增加，突破曲线拖尾减小，幂律斜率从最小体积到最大体积范围为 -3至 -2.3。这里使用Becker和Shapiro（2003）开发的多路径模型来评估流动路径独立性的假设。发现多路径模型可以解释收敛数据，即随着距离增加局部弥散度增大且路径数量减少，这表明在裂隙尺度上从非菲克运移向菲克运移的转变。然而，该模型预测拖尾会随着推 - 拉距离增加，而实验显示出相反的趋势。这种不一致可能表明在更大的调查体积下跨通道传质被激活，这导致了随尺度变化的不可逆非均匀对流。当调查体积增加时从独立通道向连通通道的这种转变表明，收敛式和推 - 拉式突破曲线都可以揭示特征长度尺度的存在。
