NASA-DOD Study: Saltwater to Widely Taint Coastal Groundwater by 2100

Seawater will infiltrate underground freshwater supplies in about three of every four coastal areas around the world by the year 2100, according to a recent study led by researchers at NASA’s Jet Propulsion Laboratory in Southern California. In addition to making water in some coastal aquifers undrinkable and unusable for irrigation, these changes can harm ecosystems and corrode infrastructure.

Called saltwater intrusion, the phenomenon happens below coastlines, where two masses of water naturally hold each other at bay. Rainfall on land replenishes, or recharges, fresh water in coastal aquifers (underground rock and soil that hold water), which tends to flow below ground 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.

Now, two impacts of climate change are tipping the scales in favor of salt water. Spurred by planetary warming, sea level rise is causing coastlines to migrate inland and increasing the force pushing salt water landward. At the same time, slower groundwater recharge — due to less rainfall and warmer weather patterns — is weakening the force moving the underground fresh water in some areas.

Worldwide Intrusion

The study, published in Geophysical Research Letters in November, evaluated more than 60,000 coastal watersheds (land area that channels and drains all the rainfall and snowmelt from a region into a common outlet) around the world, mapping how diminished groundwater recharge and sea level rise will each contribute to saltwater intrusion while estimating what their net effect will be.

Considering the two factors separately, the study’s authors found that by 2100 rising sea levels alone will tend to drive saltwater inland in 82% of coastal watersheds studied. The transition zone in those places would move a relatively modest distance: no more than 656 feet (200 meters) from current positions. Vulnerable areas include low-lying regions such as Southeast Asia, the coast around the Gulf of Mexico, and much of the United States’ Eastern Seaboard.

Meanwhile, slower recharge on its own will tend to cause saltwater intrusion in 45% of the coastal watersheds studied. In these areas, the transition zone would move farther inland than it will from sea level rise — as much as three-quarters of a mile (about 1,200 meters) in some places. The regions to be most affected include the Arabian Peninsula, Western Australia, and Mexico’s Baja California peninsula. In about 42% of coastal watersheds, groundwater recharge will increase, tending to push the transition zone toward the ocean and in some areas overcoming the effect of saltwater intrusion by sea level rise.

All told, due to the combined effects of changes in sea level and groundwater recharge, saltwater intrusion will occur by century’s end in 77% of the coastal watersheds evaluated, according to the study.

Generally, lower rates of groundwater recharge are going to drive how far saltwater intrudes inland, while sea level rise will determine how widespread it is around the world. “Depending on where you are and which one dominates, your management implications might change,” said Kyra Adams, a groundwater scientist at JPL and the paper’s lead author.

For example, if low recharge is the main reason intrusion is happening in one area, officials there might address it by protecting groundwater resources, she said. On the other hand, if the greater concern is that sea level rise will oversaturate an aquifer, officials might divert groundwater.

Global Consistency

Co-funded by NASA and the U.S. Department of Defense (DOD), the study is part of an effort to evaluate how sea level rise will affect the department’s coastal facilities and other infrastructure. It used information on watersheds collected in HydroSHEDS, a database managed by the World Wildlife Fund that uses elevation observations from the NASA Shuttle Radar Topography Mission. To estimate saltwater intrusion distances by 2100, the researchers used a model accounting for groundwater recharge, water table rise, fresh- and saltwater densities, and coastal migration from sea level rise, among other variables.

Study coauthor Ben Hamlington, a climate scientist at JPL and a coleader of NASA’s Sea Level Change Team, said that the global picture is analogous to what researchers see with coastal flooding: “As sea levels rise, there’s an increased risk of flooding everywhere. With saltwater intrusion, we’re seeing that sea level rise is raising the baseline risk for changes in groundwater recharge to become a serious factor.”

A globally consistent framework that captures localized climate impacts is crucial for countries that don’t have the expertise to generate one on their own, he added.

“Those that have the fewest resources are the ones most affected by sea level rise and climate change,” Hamlington said, “so this kind of approach can go a long way.”