Influence of Tide-Induced Unstable Flow on Seawater Intrusion and Submarine Groundwater Discharge

Recent studies have confirmed the instability of tide-induced upper saline plume (USP) in tidal beach aquifers. A systematic understanding of how the unstable USP contributes to groundwater flow and salinity distribution is still lacking. Here we used laboratory experiments and numerical simulations to examine the responses of the salinity distribution and submarine groundwater discharge (SGD) to tide-induced unstable flow. The results revealed that changes in physical forcing conditions (tides and inland freshwater input) drove the transition of the USP from stable to unstable states. Meanwhile, the phase-averaged SGD and its components fluctuated over time, and their oscillation amplitudes depended on the physical forcing conditions. There was a correlation between USP instability and the fluctuation of water exchange across the aquifer-sea interface. The fluxes across the aquifer-sea interface under unstable conditions usually varied considerably over time with a lower frequency in comparison to stable conditions. The extent of seawater intrusion (SWI) measured by the total salt mass stored in the aquifer also varied over time, especially under unstable conditions. The vibration tendency of the total salt mass was opposite to that of the sum of inland freshwater discharge and tide-induced circulation efflux. For coastal aquifers affected by tides, SWI was dominated by the sum of inland freshwater discharge and tide-induced circulation efflux, followed by the sum of inland freshwater input and tide-induced circulation influx. These findings lay the foundation for gaining a better understanding of tide-induced unstable flow and its potential implications.

Automated summary

Recent studies have shown that tide-induced unstable flow affects groundwater flow and salinity distribution, with unstable upper saline plume states impacting submarine groundwater discharge and seawater intrusion. The study also found that variations in physical forcing conditions lead to transitions in the stability of the upper saline plume and fluctuations in water exchange across the aquifer-sea interface.