Research shows that fossil groundwater, which hasn’t seen the surface of the Earth for thousands of years, has trace amounts of radioactive isotopes as a result of 20th-century nuclear testing.

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Scientists have long believed groundwater to be largely protected from the problems that plague water on Earth’s surface, including pollution and its many effects on our health. Now they’ve got some bad news: Groundwater might not be all that much better.

There is roughly 100 times more water stored underground than in all the lakes and rivers that criss-cross the Earth’s surface. This groundwater, located deep underground in the fissures in rocks and the spaces between soil particles, is often exceptionally old; much of it has been stored in the Earth’s crust for at least 12,000 years. A new study published in Nature Geoscience suggests that this fossil water may still be mixing with younger, and potentially contaminated, water sources—bad news for the billions of humans who rely on groundwater for agriculture and drinking water.

“Groundwater is an immense resource,” says Scott Jasechko, an assistant professor of geography at the University of Calgary and lead author on the new study. “About a third of human water uses are derived from groundwater, so it’s really an important and precious resource that’s already widely used.”

Jasechko and his colleagues looked at groundwater samples from nearly 6,500 wells around the world, and used carbon dating to determine the age of the water within those wells. They found that fossil groundwater makes up as much as 85 percent of the water in aquifers within the upper kilometer of the Earth’s crust. The researchers also showed that the vast majority of water pumped from wells that tapped into groundwater 250 meters or more below the surface had been underground for at least 12,000 years.

“One might imagine that these deep wells may be less vulnerable to modern land use changes and contamination,” Jasechko says. However, half of those deep wells contained tritium, a radioactive hydrogen isotope that researchers use to identify recently recharged water. Nuclear testing in the 1950s left its mark on groundwater in the form of an overabundance of tritium, meaning water with detectable levels of the isotope were almost certainly recharged after 1950.

“Even though these wells may be deep, they still often contain a small amount of recent rain and snowmelt,” Jasechko explains, “implying that even deep wells are vulnerable to modern era contamination.”