(The Hill) — Alaska’s Yukon River may be shuttling much more than just water as it traverses the state and empties into the Bering Sea.

A California-led research team has now found evidence that climate change-induced permafrost melting is unleashing long-sequestered mercury deposits into this critical freshwater artery.

The release of the metal, which has been stored in the permafrost for millennia, now poses an environmental and public health hazard to about 5 million people living in the Arctic zone, according to the study, published on Thursday in Environmental Research Letters.

More than 3 million of those individuals reside in areas where permafrost is expected to disappear entirely by 2050, the researchers noted.

“There could be this giant mercury bomb in the Arctic waiting to explode,” said co-author Josh West, professor of Earth sciences and environmental studies at the University of Southern California, in a statement.

Earth’s natural atmospheric circulation tends to transport pollutants to higher latitudes, resulting in a northbound buildup of mercury — which then moves from air to ground to water, West explained.

“A lot of mercury pollution ends up in the Arctic,” he said. “Permafrost has accumulated so much mercury that it could dwarf the amount in the oceans, soils, atmosphere and biosphere combined.”

Arctic plants that absorb mercury ultimately die and become part of the soil, freezing into permafrost. Once that soil starts thawing, however, it discharges the highly concentrated mercury into the environment.

While previous methods of assessing mercury levels have involved sampling cores from the top 10 feet of permafrost, the researchers decided to dig deeper — into riverbank and sandbar sediments. 

“The river can quickly mobilize large amounts of sediment containing mercury,” lead author Isabel Smith, a doctoral candidate at USC Dornsife, said in a statement.

The researchers concluded that because the levels they detected were consistent with past estimates, this new measurement technique could constitute a reliable gauge for mercury content.

Although the mercury accumulation doesn’t pose an acute threat to residents today, the risks stand to increase over time and will primarily impact fish and game consumption, the authors noted.

Mercury deposits may also fluctuate, as the river both mobilizes mercury-laced sediments and redeposits them onto sandbars and beaches, according to the study.

“Decades of exposure, especially with increasing levels as more mercury is released, could take a huge toll on the environment and the health of those living in these areas,” Smith said.