Pedretti D, Molinari A, Fallico C, Guzzi S
Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC, Canada.
Dipartimento di Ingegneria Civile, Università della Calabria, Rende, CS, Italy.
J Contam Hydrol. 2016 Oct;193:86-95. doi: 10.1016/j.jconhyd.2016.09.005. Epub 2016 Sep 9.
A series of experimental tracer tests were performed to explore the implications of the change in the pressure status of a heterogeneous bimodal aquifer for scale-dependent dispersion and mass-transfer processes. The sandbox was filled with sands and gravel channels and patches to form an alluvial-like bimodal aquifer. We performed multiple injections of a conservative tracer from 26 different locations of the sandbox and interpreted the resulting depth-integrated breakthrough curves (BTCs) at the central pumping well to obtain a scale-dependent distribution of local and field-integrated apparent longitudinal dispersivity (respectively, α and α). We repeated the experiments under confined (CS) and unconfined (UNS) pressure status, keeping the same heterogeneous configuration. Results showed that α(associated with transport through gravel zones) was poorly influenced by the change in aquifer pressure and the presence of channels. Instead, α(i.e. macrodispersion) strongly increased when changing from CS to UNS. In specific, we found α≈0.03r for the CS and α≈0.15r for the UNS (being r the distance from the well). Second-to-fourth-order temporal moments showed strong spatial dependence in the UNS and no spatial dependence in the CS. These results seem consistent with a "vadose-zone-driven" kinetic mass-transfer process occurring in the UNS but not in the CS. The vadose zone enhances vertical flow due to the presence of free surface and large contrasts in hydraulic conductivity triggered by the desaturation of gravel channels nearby the pumping well. The vadose zone enhances vertical mixing between gravel and sands and generates BTC tailing. In the CS vertical mixing is negligible and anomalous transport is not observed.
进行了一系列实验示踪剂测试，以探究非均质双峰含水层压力状态变化对尺度相关弥散和传质过程的影响。砂箱填充有沙子、砾石通道和斑块，以形成类似冲积层的双峰含水层。我们从砂箱的26个不同位置多次注入保守示踪剂，并解释了中央抽水井处得到的深度积分突破曲线（BTCs），以获得局部和场积分表观纵向弥散率（分别为α和α）的尺度相关分布。我们在承压（CS）和无压（UNS）压力状态下重复实验，保持相同的非均质结构。结果表明，α（与通过砾石区的运移相关）受含水层压力变化和通道存在的影响较小。相反，从CS变为UNS时，α（即宏观弥散）显著增加。具体而言，我们发现CS时α≈0.03r，UNS时α≈0.15r（r为距井的距离）。二阶至四阶时间矩在UNS中表现出强烈的空间依赖性，而在CS中没有空间依赖性。这些结果似乎与UNS中发生但CS中未发生的“包气带驱动”动力学传质过程一致。由于自由表面的存在以及抽水井附近砾石通道脱饱和引发的水力传导率的巨大差异，包气带增强了垂直水流。包气带增强了砾石和沙子之间的垂直混合并产生BTC拖尾。在CS中，垂直混合可忽略不计，未观察到异常输运。
