Tidal fluctuations relieve coastal seawater intrusion caused by groundwater pumping.

Coastal aquifers are vulnerable to seawater intrusion and salinization due to groundwater pumping. Most research on salinization vulnerability and pumping rate optimization considers constant seaside boundary. However, tidal fluctuations may propagate through aquifers and affect pumping-induced seawater intrusion. We numerically tested the combined effects of tidal fluctuations and groundwater pumping on the variable-density groundwater flow and salt transport in aquifers with pumping wells close to the coast. The simulations showed that tidal fluctuations relieved seawater intrusion due to groundwater extraction. Stronger tidal fluctuation created larger upper saline plume which inhibited the seawater intrusion in the lower aquifer. Compared with those without tides, the tidal fluctuations with a tidal amplitude of 1 m could increase maximum pumping rate by 35.2 % and 28.7 % in flux- and head-controlled systems, respectively. These results suggest that combined impacts of tides and pumping should be considered in assessing seawater intrusion and designing more sophisticated pumping optimization.