Laboratory observations for two-dimensional solute transport in an aquifer-aquitard system

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.