Climate Change Alters Post‐Surge Recovery of Coastal Aquifers

Climate change is expected to increase storm‐surge intensity while reducing its frequency, posing complex challenges for the recovery of coastal aquifers subject to recurrent wave overwash events. This study quantifies the combined effects of these opposing trends using surface–subsurface integrated numerical simulations of a generalized island aquifer across 12 scenarios with varying storm‐surge frequency and intensity. Here, we show that two distinct long‐term regimes emerge: (a) full recovery, where the aquifer returns to pre‐surge conditions if storm intensity and frequency remain below critical thresholds, and (b) shifted equilibrium, characterized by persistent salt accumulation and depleted fresh groundwater availability if these thresholds are exceeded. Higher hydraulic conductivity and smaller island width exacerbate salt accumulation, the former by increasing the salt load introduced during each storm‐surge event, and the latter by decelerating subsequent flushing. The transition between recovery and shifted‐equilibrium regimes can be represented with a dimensionless number, , that integrates the effect of storm‐surge intensity and frequency on salt load. In a shifted equilibrium regime, the excess salt load at new dynamic equilibria is effectively approximated by linear functions of . This framework offers a straightforward tool for the preliminary assessment of climate‐change impacts on coastal groundwater systems, particularly for small islands with limited and vulnerable freshwater resources, thereby supporting proactive water security strategies against climate change.