Tidal influence on seawater intrusion in unconfined coastal aquifers

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.