NASA’s Jet Propulsion Laboratory in Southern California recently conducted a study projecting that seawater will infiltrate underground freshwater supplies in about 77% of coastal watersheds worldwide by 2100. This phenomenon, known as saltwater intrusion, will result from the combined effects of sea level rise and slower replenishment of groundwater supplies due to warmer, drier regional climates. The study, funded by NASA and the U.S. Department of Defense, was published in Geophysical Research Letters in November 2024.
It evaluated more than 60,000 coastal watersheds around the world, mapping how diminished groundwater recharge and sea level rise will each contribute to saltwater intrusion and estimating their combined effects. Saltwater intrusion occurs deep below coastlines, where masses of fresh and saltwater meet. Rainfall on land replenishes fresh water in coastal aquifers, which tends to flow underground toward the ocean.
Meanwhile, seawater, backed by ocean pressure, pushes inland. Typically, this balance keeps the water fresh on one side and salty on the other. However, climate change is tipping this balance.
Melting ice sheets and glaciers are causing sea levels to rise and coastlines to migrate inland, increasing the force pushing underground saltwater landward. At the same time, reduced rainfall and warmer weather patterns are decreasing groundwater recharge, weakening the force behind fresh water in some areas. By 2100, rising sea levels alone will drive saltwater inland in 82% of coastal watersheds, affecting regions such as Southeast Asia, the Gulf of Mexico, and the U.S. Eastern Seaboard.
On the other hand, slower groundwater recharge alone will cause saltwater intrusion in 45% of coastal watersheds, with regions like the Arabian Peninsula, Western Australia, and Mexico’s Baja California peninsula being most affected.
Saltwater intrusion and groundwater recharge
Remarkably, in about 42% of coastal watersheds, an increase in groundwater recharge will mitigate the effects, pushing the transition zone between fresh and saltwater toward the ocean.
Overall, by century’s end, saltwater intrusion will occur in 77% of the coastal watersheds evaluated. “Depending on where you are and which factor dominates, your management implications might change,” said Kyra Adams, a groundwater scientist at JPL and the paper’s lead author. For areas where low recharge is the main cause, officials might focus on protecting groundwater resources, while regions threatened by sea level rise might consider diverting groundwater.
Saltwater intrusion can render water in coastal aquifers undrinkable and useless for irrigation. It can also harm ecosystems and damage infrastructure. The study aims to evaluate how sea level rise will impact the DOD’s coastal facilities and infrastructure.
It utilized data from HydroSHEDS, a World Wildlife Fund-managed database, and modeled projections considering variables like groundwater recharge, water table rise, and coastal migration due to sea level rise. Study coauthor Ben Hamlington, a climate scientist at JPL and a coleader of NASA’s Sea Level Change Team, likened the global picture to coastal flooding scenarios. “As sea levels rise, there’s an increased risk of flooding everywhere.
With saltwater intrusion, sea level rise is raising the baseline risk, making changes in groundwater recharge a serious factor.”
Hamlington emphasized that such a global framework is vital for countries lacking the expertise to create one. “Those with the fewest resources are the most affected by sea level rise and climate change,” he said. “This approach can make a significant difference.”
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