School of Earth and Environment, Anhui University of Science and Technology, Huainan, 232001, China.
School of Environmental Studies, China University of Geosciences, NO. 68 Jincheng Street, East Lake High-Tech, 17 Development Zone, Wuhan, Hubei, 430074, People's Republic of China.
Environ Sci Pollut Res Int. 2021 Aug;28(29):38664-38678. doi: 10.1007/s11356-021-13123-1. Epub 2021 Mar 18.
Low-permeability media such as clay appear in nearly all hydrogeological systems. To date, although significant efforts have been put forward by hydrologists, transport mechanism is still not well understood in such media, especially in an aquifer-aquitard system. In this study, two-dimensional experiments of groundwater flow and solute transport were conducted in a clay-sand two-layer system to investigate the characteristics of flow and transport in such a system. Sodium chloride (NaCl) (a conservative tracer) from a tank was injected after passing by the pre-inlet reservoir where the mixing effect and flow transiency were analyzed. A new numerical model considering the mixing effect and flow transiency was developed to interpret the experimental data based on the finite-element COMSOL Multiphysics platform. Transport parameters were assessed by best fitting the observed breakthrough curves (BTCs). Several important results were obtained. Firstly, aquitard advection was found to be non-negligible and should be considered in a proper mathematical model for describing the transport process. Secondly, advective velocities were temporally variable and showed decreasing trends in the sand and clay layers, mainly due to the impacts of physical and biological clogging. Thirdly, the mixing effect in the pre-inlet reservoir led to a lower tracer concentration in the sand layer at early times. Finally, the observed BTCs exhibited early arrivals in the clay layer, possibly resulting from preferential flow pathways. These findings can provide hints for contamination remediation works in aquifer-aquitard systems.
低渗透性介质如粘土几乎存在于所有水文地质系统中。迄今为止，尽管水文学家已经做出了重大努力，但在这种介质中，特别是在含水层-隔水层系统中，传输机制仍未得到很好的理解。在这项研究中，在粘土-砂双层系统中进行了二维地下水流动和溶质运移实验，以研究这种系统中流动和运移的特征。储层中的混合效应和流动暂态性进行了分析。基于有限元 COMSOL Multiphysics 平台，开发了一个考虑混合效应和流动暂态性的新数值模型，以解释实验数据。通过最佳拟合观测到的穿透曲线 (BTC) 来评估传输参数。得到了几个重要的结果。首先，发现隔水层的对流不可忽略，在描述传输过程的适当数学模型中应该考虑到这一点。其次，对流速度是时间变化的，在砂层和粘土层中呈下降趋势，主要是由于物理和生物堵塞的影响。第三，预进口储层中的混合效应导致早期砂层中示踪剂浓度降低。最后，观测到的 BTC 在粘土层中出现较早的到达，可能是由于优先流途径造成的。这些发现可以为含水层-隔水层系统中的污染修复工作提供线索。
