As Earth’s rising temperature causes sea levels to rise, coastal communities have more to worry about than disappearing beaches—they could lose up to 50 percent more of their fresh water supplies than previously thought, a new study suggests.
Scientists had previously assumed that as the ocean's salty waters gradually invaded the shore, they would penetrate underground only as far as they did above ground.
But new simulations of the sea level rise predicted by the Intergovernmental Panel on Climate Change (IPCC)—23 inches in the next 100 years—show that saltwater can mix with fresh groundwater, turning aquifers into undrinkable zones of brackish water.
"Most people are probably aware of the damage that rising sea levels can do above ground, but not underground, which is where the fresh water is," said study leader Motomu Ibaraki of Ohio State University.
The results of the study were presented on Oct. 30 at the Geological Society of America annual meeting.
Just how far the saltwater will penetrate underground depends on the texture of sand found along a coastline—fine sands are more tightly packed, and so allow less water through than coarser sands.
Coastlines typically have layers of different types of sands, and the simulations run by the hydrologists showed that the more layers present, the more the saltwater and fresh water mix together. This mixing creates convection that stirs the two types of water into a brackish mixture with salt levels that are too high to drink.
Water that has more than 250 milligrams of salt per liter, which brackish water would have, is considered unsafe to drink because it causes dehydration.
According to U.S. Geological Survey estimates, about half of the country depends on groundwater supplies for drinking water, and these sources would be endangered as sea levels crept inland.
While desalinating the brackish water would create more freshwater, it's still a very expensive process, Ibaraki said.
"To desalinate, we need energy, so our water problem would become an energy problem in the future," he said.
Areas at risk
The areas of the United States most likely to be flooded as sea levels rise are along the East Coast and the Gulf of Mexico, especially low-lying Florida and Louisiana. (The West Coast is less vulnerable to sea level rise because it has more high ground along its coast.)
Worldwide, vulnerable areas include Southeast Asia, the Middle East and northern Europe.
"Almost 40 percent of the world population lives in coastal areas, less than 60 kilometers from the shoreline," said study team member Jun Mizuno, an Ohio State graduate student. "These regions may face loss of freshwater resources more than we originally thought."