A Hydrogeologic Tipping Point Controls Climate-Driven Saltwater Intrusion

Rising sea levels and shifting precipitation patterns are intensifying saltwater intrusion into coastal aquifers, threatening ecosystems, infrastructure, and water security. Here, using an ensemble of 800 high-fidelity three-dimensional simulations, we show that coastal aquifers have two regimes of salinization, controlled by a hydrogeologic tipping point in permeability. Below this threshold, aquifers are transport-limited and are comparatively resilient to climate disturbances. Once exceeded, however, coastal areas abruptly shift to a forcing-sensitive regime in which sea-level rise and recharge reduction nonlinearly amplify salinization. This tipping-point framework arises from novel physics-based simulations integrating compound processes of lateral saltwater intrusion, episodic inundation-driven vertical saltwater intrusion, vadose zone saltwater storage, precipitation-driven recharge, tides, and terrestrial groundwater. Applying the framework to national hydrogeologic and climate datasets, we find that forcing-sensitive areas expand by 109% nationally and pathway dominance shifts from overwhelmingly (97%) lateral-dominated to increasingly (53 $\pm$ 5%) mixed and vertical-dominated intrusion under late-century projections. These pathway shifts may bypass conventional lateral-focused prevention strategies, implying not only a larger exposed coastal area but also intensified salinization within exposed regions. These findings provide a first-order yet mechanistically grounded map of exposure and highlight the need for pathway-aware monitoring and adaptation in coastal areas worldwide.