When the Oceans Come for Your Tap

The oceans are changing, but they are not changing alone. Many evident impacts of changing sea level occur on the surface, and we oftentimes miss the silent companion beneath the surface. Saltwater intrusion, the underground salinization of coastal fresh groundwater, is already—and will be—widely rampant across global coastlines. Recent analysis shows that by 2100, more than 75% of the world's coastal regions will experience some form of saltwater intrusion, driven by changing rain patterns on land or sea level (Figure 1; Adams et al. 2024). Saltwater intrusion is detrimental to coastal communities in various aspects. It collapses fresh or transitional ecosystems, as evidently shown in many of US east coast cities and nearshore agricultural lands (Mondal et al. 2023). It corrodes and damages underground pipes, tunnels, building foundations, and sewer/septic systems (Habel et al. 2024). Perhaps most importantly, it can salinize our freshwater reserves (Zamrsky et al. 2024). Apart from sea level rise, storm surges are another source of groundwater salinization by seeping into the ground through pockets of trapped seawater (Paldor and Michael 2021). Additionally, increased reliance on groundwater for human or agricultural use has led to larger pumping rates, potentially drawing in extra seawater inland (Peters et al. 2022). Here, the groundwater wells act like straws, while our rising freshwater consumption takes ever larger gulps from the invisible subsurface. Conversely, in regions with ample, near-surface groundwater, rising seas can “push” the shallow groundwater above the surface due to increasing pressure, causing bottom-up flooding that leads to other hazards (Befus et al. 2020). With 2 billion people residing within 50 km of global coasts, where population growth rates are also faster than inland communities (Cosby et al. 2024), freshwater and habitat security in coastal regions are increasingly pertinent and critical. Studying saltwater intrusion poses situational challenges due to its occurrence at depth. Cross-sectional salinity sampling in the field is cumbersome and labor-intensive, and requires many data points to render a full picture. Patterns and rates of recharge and sea level change vary spatially and temporally. Coastal aquifers are highly heterogeneous and encompass a spectrum of different geologies, making it difficult to measure or predict salinization pathways. Lastly, pumping and other human activities (e.g., canals, shoreline reinforcements) are difficult to quantify in terms of their hydrologic impact and require detailed models to fully account for their role against the backdrop of other environmental drivers (Helton et al. 2025). To this end, understanding and mitigating saltwater intrusion requires coordinated data acquisition plans and creative solutions. Use of remote sensing such as airborne electromagnetic flights (Gottschalk et al. 2020), satellite-based data proxies such as vegetation (White Jr and Kaplan 2021), or fast analytical models (Adams et al. 2024) are recent advancements within the space. Numerical groundwater models are also often employed to study saltwater intrusion in coastal aquifers. Based on numerical approximations of groundwater flow and its associated processes, the models can simulate past, present, and future groundwater conditions. Recent technological advances and access to high-performance computing have enabled us to study saltwater intrusion at various scales and from different angles. It is always important, however, to understand the limitations and uncertainties of these numerical methods, often due to the lack of data and local information. Uncertainties in the hydrogeologic setting, such as hydraulic conductivity, aquifer thickness, or microtopography, propagate to the uncertainty in saltwater intrusion or freshwater salinization estimates. Model choice also contributes to uncertainty. Therefore, uncertainty quantification and result contextualization are critical parts of leveraging models to understand saltwater intrusion and coastal groundwater vulnerabilities. Improving the quality and reliability of numerical models and applying those on continental or global scales will be another key factor in better understanding future threats posed by groundwater salinization. By zooming out, we will be able to identify hotspots most imminently threatened by groundwater salinization, potentially saving enough time for water management bodies to enact effective policies and countermeasures in time. As pathways and impacts of saltwater intrusion are most relevant and best managed at local to regional scales, quick identification at large scales followed by prevention and mitigation at local to regional scales can be an effective way to approach saltwater intrusion rather than a pervasive policy mandate. Preserving coastal fresh groundwater resources is essential to protect human and ecological communities worldwide. Even small rates of groundwater salinization can render the previously potable water resources unsuitable for drinking or agricultural purposes, causing public health risks (e.g., heart or digestive diseases) and crop failures. Informing and involving the public will be key to raising support in implementing effective local mitigation and adaptation measures and thus ensure safe, sustainable conditions for future coastal generations. The research was partially carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). The authors declare no conflicts of interest. The authors have nothing to report.