Effects of Downward Intrusion of Saline Water on Nested Groundwater Flow Systems

Nested groundwater flow systems (NGFS) are commonplace in various hydrogeological environments, including endorheic basins and coastal aquifers. The subsystems of the NGFS can be spatially separated by streamlines around the internal stagnation points. At the discharge zones of the topographic depressions, saline water often emerges due to high evaporation in endorheic drainage basins or the combined effects of evaporation and intermittent seawater submersion in coastal areas. To date, there are limited studies that have considered the impact of local-scale downward migration of saline plumes in the topographic depressions on the NGFS. In this study, the classic NGFS are revisited by considering saline water in their discharge zones. To quantify the effects of salinity in the discharge zones on the NGFS, scenarios of various salinities in the discharge zones are simulated. The displacements of the internal stagnation points are used to quantify the evolution of the NGFS in response to salinity changes in the discharge zones. The results show that, as the salinity in the discharge zones increases, the hydraulic gradient near the discharge zone can be significantly reduced, the internal stagnation points shift upward, and the local groundwater flow systems retreat upward so that their original spaces are replaced by intermediate or regional flow systems. The discharge zone is expanded, and the overall groundwater flow velocity magnitude of the entire system decreases with salinity. This study may shed light on the management of saline wetlands, for example, the control of groundwater salinization, evolution of saline groundwater basins, and seawater intrusion.