Zhang Yong, Green Christopher T, Tick Geoffrey R
Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487, United States; Hohai University, Nanjing 210098, China.
U.S. Geological Survey, Menlo Park, CA 94025 United States.
J Contam Hydrol. 2015 Jun-Jul;177-178:220-38. doi: 10.1016/j.jconhyd.2015.04.001. Epub 2015 Apr 8.
This study evaluates the role of the Peclet number as affected by molecular diffusion in transient anomalous transport, which is one of the major knowledge gaps in anomalous transport, by combining Monte Carlo simulations and stochastic model analysis. Two alluvial settings containing either short- or long-connected hydrofacies are generated and used as media for flow and transport modeling. Numerical experiments show that 1) the Peclet number affects both the duration of the power-law segment of tracer breakthrough curves (BTCs) and the transition rate from anomalous to Fickian transport by determining the solute residence time for a given low-permeability layer, 2) mechanical dispersion has a limited contribution to the anomalous characteristics of late-time transport as compared to molecular diffusion due to an almost negligible velocity in floodplain deposits, and 3) the initial source dimensions only enhance the power-law tail of the BTCs at short travel distances. A tempered stable stochastic (TSS) model is then applied to analyze the modeled transport. Applications show that the time-nonlocal parameters in the TSS model relate to the Peclet number, Pe. In particular, the truncation parameter in the TSS model increases nonlinearly with a decrease in Pe due to the decrease of the mean residence time, and the capacity coefficient increases with an increase in molecular diffusion which is probably due to the increase in the number of immobile particles. The above numerical experiments and stochastic analysis therefore reveal that the Peclet number as affected by molecular diffusion controls transient anomalous transport in alluvial aquifer-aquitard complexes.
本研究通过结合蒙特卡罗模拟和随机模型分析，评估了在瞬态异常输运中佩克莱数受分子扩散影响所起的作用，瞬态异常输运是异常输运中的主要知识空白之一。生成了两个分别包含短连接或长连接水相的冲积环境，并将其用作水流和输运模拟的介质。数值实验表明：1）佩克莱数通过确定给定低渗透层中的溶质停留时间，影响示踪剂突破曲线（BTCs）幂律段的持续时间以及从异常输运到菲克输运的转变速率；2）由于洪泛平原沉积物中的速度几乎可以忽略不计，与分子扩散相比，机械弥散对后期输运异常特征的贡献有限；3）初始源尺寸仅在短运移距离处增强了BTCs的幂律尾。然后应用 tempered稳定随机（TSS）模型来分析模拟的输运。应用结果表明，TSS模型中的时间非局部参数与佩克莱数Pe相关。特别是，由于平均停留时间的减少，TSS模型中的截断参数随Pe的减小而非线性增加，并且容量系数随分子扩散的增加而增加，这可能是由于不动颗粒数量的增加。因此，上述数值实验和随机分析表明，受分子扩散影响的佩克莱数控制着冲积含水层-隔水层复合体中的瞬态异常输运。
