Effect of evolving recirculation zones on anomalous solute transport in rough single fractures

Understanding the anomalous solute transport in single fractures is important for many hydrogeologic processes and subsurface applications. Recirculation zones (RZs) and corresponding main flow zones (MFZs) have been widely recognized as low-velocity regions and preferential pathways that could explain the simple anomalous solute transport, i.e., heavy tailings and early arrival. However, the direct relation between RZs and more complex anomalous transport phenomena, e.g., multi-modal peaks and fluctuating tailings, has been elusive. This may be due to the limited understanding of the evolution of RZs and the mass transfer process between RZs and MFZs, i.e., the monotonically increasing RZs volume (S_v) and defaulted diffusion-dominated mass transfer. In this study, we systematically generate a series of 2D/3D rough single fractures with different geometric properties to investigate the evolution of RZs and its influence on anomalous transport across a wide Re range of 0–426.88. Three-stage evolution of RZs with increasing Re was identified by using the growth rate of S_v (dS_v/dRe), the rapid growth stage (Stage I) where dS_v/dRe increase, the slow growth stage (Stage II) where dS_v/dRe decrease, and the fully developed stage (Stage III) where dS_v/dRe is a constant. The mass transfer mode between recirculation and main flow zones is shifted from diffusion-dominated in Stage I to convection-dominated in Stage II due to the enhanced convection in RZs. This shift of mass transfer mode enhances the mass transfer rate (α) between RZs and MFZs by 5–20 times. In Stage II, the solute was trapped around the interface between RZs and MFZs before entering RZs, i.e., the solute “film”. The coexistence of the solute “film” and the solutes trapped by RZs induces multi-modal peaks and strengthened tailings of BTCs. In Stage III, the solute “film” cannot form due to the rapid dissipation of detained solutes driven by stronger convection-dominated mass transfer around the RZs-MFZs interface, which in turn leads to the disappearance of multi-modal peaks and induces monotonically shortened tailings. This study fills the gap in the RZs evolution and the associated mass transfer process in the microscopic flow fields, which deepens our understanding of the anomalous transport mechanism.