Laboratory and numerical investigations on land-sourced solute transport in coastal fractured aquifers

Land-based pollutants threaten coastal aquifers, highlighting the need to protect groundwater and nearshore marine ecosystems. While aquifer heterogeneity has been recognized as a significant factor affecting solute behavior, the impact of fractures on land-sourced solute transport in coastal aquifers remains unclear. This study attempted to address this issue through laboratory experiments and discrete fracture matrix (DFM) models. The impact of horizontal fractures on the temporal and spatial characteristics of solute transport, spreading, and discharge under seawater intrusion was analyzed based on variations in fracture position and length. The results show that fractures/low-velocity zones (LVZ) can accelerate/delay solutes, dividing them into different transport modes and enhancing/prolonging their spreading/discharge duration. Changes in fracture position and length also affect its transport acceleration and path deviation abilities, which ultimately determine when solute discharge occurs. The mixing zone and unsaturated zone, in addition to the LVZ, hinder solute transport, reducing the rate and delaying the end of solute discharge. Meanwhile, fractures facilitate solute transport into the saltwater wedge, expanding the solute discharge zone. However, if solutes are initially within the LVZ, their entry into the fracture rely on their distance from the fracture's near-land edge and the size of the fracture's convergence zone.