Rising Seas Are Pushing Saltwater into Coastal Aquifers, Threatening Billions of People and Global Food Production

Coastal aquifers, the underground reservoirs of freshwater that supply drinking water to densely populated shoreline communities around the world, face a growing and potentially devastating threat as saltwater intrusion accelerates under the combined pressures of rising sea levels and excessive groundwater pumping. A major new study published in Nature Water by researchers led by Professor Robert Reinecke from the Institute of Geography at Johannes Gutenberg University Mainz and Annika Nolte from the Climate Service Center Germany has provided the most comprehensive global assessment yet of this phenomenon, revealing that billions of people and significant portions of the world's agricultural production are at risk as the boundary between fresh and salt groundwater shifts steadily inland.

Saltwater intrusion occurs when the natural balance between freshwater in coastal aquifers and the surrounding seawater is disrupted. Under normal conditions, the pressure from freshwater flowing toward the coast keeps saltwater from penetrating inland. However, when groundwater is pumped faster than it can be naturally replenished by rainfall, the water table drops, reducing this protective pressure and allowing denser saltwater to migrate into the freshwater zone. Rising sea levels compound the problem by raising the baseline elevation of saltwater along the coast, pushing the saltwater wedge further inland and contaminating wells that may have provided clean water for generations. The new study quantifies these dynamics at a global scale for the first time, mapping the vulnerability of coastal aquifers across every continent.

The findings paint a sobering picture. The research team estimated that aquifers serving approximately 1.6 billion people are already experiencing some degree of saltwater contamination, and that this number could grow substantially in the coming decades as sea levels continue to rise and groundwater demand increases. Particularly vulnerable regions include low-lying coastal areas in South and Southeast Asia, where population densities are high and reliance on groundwater for both drinking water and irrigation is extensive. Small island developing states, where freshwater lenses sitting atop saltwater are extremely thin and fragile, face existential threats as even modest sea level increases can overwhelm their limited freshwater reserves entirely.

Agriculture is perhaps the sector most immediately threatened by advancing saltwater intrusion. Coastal farmlands in river deltas, which include some of the most productive agricultural zones on Earth, depend heavily on groundwater for irrigation. When salt levels in irrigation water rise above critical thresholds, crop yields decline sharply, and eventually fields become unsuitable for cultivation altogether. The Mekong Delta in Vietnam, the Nile Delta in Egypt, and the Ganges-Brahmaputra Delta in Bangladesh, three regions that collectively produce food for hundreds of millions of people, are all identified in the study as highly vulnerable to salinization. The loss of agricultural productivity in these areas would not only affect local food security but could ripple through global food markets, driving up prices and increasing instability.

The study also highlights the inadequacy of current policy responses to the saltwater intrusion threat. Many coastal communities continue to pump groundwater at unsustainable rates, driven by growing populations and the immediate need for water despite the long-term consequences. Regulatory frameworks for groundwater management are often weak or unenforced, and in many developing countries, comprehensive monitoring of groundwater salinity levels simply does not exist. Without better data collection, stronger regulations, and investments in alternative water sources such as desalination, rainwater harvesting, and managed aquifer recharge, the authors warn that the window to prevent irreversible salinization of critical aquifers is closing rapidly.

Professor Reinecke and his colleagues stressed that their findings should serve as a wake-up call for governments, water utilities, and international development organizations. The intersection of climate change, population growth, and water scarcity creates a compound risk that demands coordinated, proactive responses rather than the reactive, piecemeal approaches that have characterized groundwater management to date. Protecting coastal aquifers is not merely an environmental concern but a matter of human security, food production, and economic stability for some of the most populated regions on the planet. As the seas continue to rise, the saltwater boundary will continue to advance, and the communities that depend on the freshwater beneath their feet will bear the consequences.