A recent study led by researchers at NASA's Jet Propulsion Laboratory in Southern California that seawater will infiltrate underground fresh water supplies in about 77% of coastal watersheds around the world by the year 2100, as illustrated in this graphic. Called saltwater intrusion, the phenomenon will result from the combined effects of sea level rise and slower replenishment of groundwater supplies due to warmer, drier regional climates, according to the study, which was funded by NASA and the U.S. Department of Defense and published in Geophysical Research Letters in November 2024.

In the graphic, areas that the study projected will experience the most severe saltwater intrusion are marked with red, while the few areas that will experience the opposite phenomenon, called saltwater retreat, are marked with blue.

Saltwater intrusion happens deep below coastlines, where two masses of water naturally run up against each other. Rainfall on land replenishes, or recharges, fresh water in coastal aquifers (essentially, underground rock and dirt that hold water), which tends to flow underground toward the ocean. Meanwhile, seawater, backed by the pressure of the ocean, tends to push inland. Although there's some mixing in the transition zone where the two meet, the balance of opposing forces typically keeps the water fresh on one side and salty on the other.

Spurred by melting ice sheets and glaciers, sea level rise is causing coastlines to migrate inland and increasing the force pushing underground salt water landward. At the same time, slower groundwater recharge resulting from reduced rainfall and warmer weather patterns is weakening the force behind the fresh water in some areas.

Saltwater intrusion can render water in coastal aquifers undrinkable and useless for irrigation. It can also harm ecosystems and damage infrastructure.