The freshwater-saltwater mixing zone is of great importance in the study of the land-ocean interaction processes and coastal groundwater environments. This paper considers a coastal multilayered system which consists of an unconfined aquifer and a confined aquifer with an aquitard between them. The aquitard is assumed to have a breached zone (preferential zone). 2D density-dependent numerical simulations with the MARUN code were conduced to explore effects of the preferential zone on the movements of the freshwater-saltwater mixing zone. The results show that an inland preferential zone caused the freshwater-saltwater mixing zone in the confined aquifer to move landward, and induced seawater upcoming around the zone. An intertidal preferential zone caused the freshwater-saltwater mixing zone in the confined aquifer to move close to the preferential zone, while an offshore preferential zone would enhance the water exchange in the system. The findings provide insights into the management of coastal environment.
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