Climate change alters post-surge recovery of coastal aquifers

Climate change is projected to increase storm-surge intensity while reducing 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 across 12 scenarios with varying storm-surge frequency and intensity. Here, we show that two distinct long-term regimes emerge: (i) full recovery, where the aquifer returns to pre-surge conditions if storm intensity and frequency remain below critical thresholds, and (ii) 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 the salt accumulation by enhancing the salt load introduced during each storm-surge event and decelerating the subsequent flushing processes. The transition between the regimes is captured by a single dimensionless number, E, which integrates recurrent storm-surge characteristics affecting salt load (intensity and frequency). In the second regime, the excess salt load and freshwater volume at new dynamic equilibria are effectively approximated by linear functions of E. 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.