[1] Studies of seawater intrusion in unconfined coastal aquifers typically neglect oceanic oscillations such as tides and assume a static seaward boundary condition defined by the mean sea level. Laboratory experiments and numerical simulations were conducted to investigate the influence of tidal oscillations on the behavior of the saltwater wedge. For the conditions examined, the experiments showed that an upper saline plume formed in the intertidal zone due to tide-induced seawater circulation. The presence of the upper saline plume shifted the fresh groundwater discharge zone seaward to the low-tide mark and restricted the intrusion of the saltwater wedge. The overall seawater intrusion extent, as indicated by the wedge toe location, was reduced significantly compared with the nontidal (static) case. Results from the numerical model matched these experimental observations and further demonstrated the similar type of tidal influence on the saltwater wedge in a field-scale aquifer system. The Glover (1959) solution for predicting the saltwater wedge was modified to account for the tidal effect by including the tide-induced circulation as a ''recharge'' to the aquifer. The findings highlight the significant impact of the tide in modulating the groundwater behavior and salt-freshwater dynamics, not only within but also landward of the intertidal zone.

Tides and seasonally varying inland freshwater input, with different fluctuation periods, are important factors affecting flow and salt transport in coastal unconfined aquifers. These processes affect submarine groundwater discharge (SGD) and associated chemical transport to the sea. While the individual effects of these forcings have previously been studied, here we conducted physical experiments and numerical simulations to evaluate the interactions between varying inland freshwater input and tidal oscillations. Varying inland freshwater input was shown to induce significant water exchange across the aquifer‐sea interface as the saltwater wedge shifted landward and seaward over the fluctuation cycle. Tidal oscillations led to seawater circulations through the intertidal zone that also enhanced the density‐driven circulation, resulting in a significant increase in the total SGD. The combination of the tide and varying inland freshwater input, however, decreased the SGD components driven by the separate forcings (e.g., tides and density). Tides restricted the landward and seaward movement of the saltwater wedge in response to the varying inland freshwater input in addition to reducing the time delay between the varying freshwater input signal and landward‐seaward movement in the saltwater wedge interface. This study revealed the nonlinear interaction between tidal fluctuations and varying inland freshwater input will help to improve our understanding of SGD, seawater intrusion, and chemical transport in coastal unconfined aquifers.

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