The sediment in the Yukon River flowing through Alaska contains mercury leached from permafrost, posing a threat to the environment and the health of communities in the Arctic.
The Yukon River flows westward across Alaska towards the Bering Sea, eroding Arctic permafrost along its banks and transporting sediment downstream. This sediment contains a toxic metal, mercury. As the Arctic warms due to climate change—warming at a rate four times the global average—mercury accumulated in millennia-old permafrost is being eroded by river water and released into the environment.
Drone captures the Yukon River and downstream area from Beaver, Alaska. (Photo: Michael P. Lamb)
In a study published on August 15 in the journal Environmental Research Letters, researchers at the University of Southern California (USC) Dornsife introduced a more accurate method for measuring the amount of mercury released from permafrost through river water and estimating the total mercury waiting to be released. This toxic metal poses a threat to the environment and the health of 5 million people living in the Arctic, with over 3 million residents in permafrost regions projected to be entirely gone by 2050.
“A massive mercury bomb in the Arctic is ready to explode,” said Josh West, a professor of Earth Science and Environmental Research at USC Dornsife.
The natural atmospheric cycle of the Earth typically transports pollutants to high latitudes, leading to mercury accumulation in the Arctic, West explained. Permafrost accumulates so much mercury that it can exceed the total amounts found in the oceans, land, atmosphere, and biosphere combined.
In the Arctic, trees absorb mercury, which eventually die and become part of the soil, ultimately freezing into permafrost. Over thousands of years, mercury concentrations gradually increase in permafrost until thawing occurs, a growing problem due to climate change. The research team, which includes experts from the California Institute of Technology (Caltech), the Yukon River Inter-Tribal Watershed Council, the Massachusetts Institute of Technology, and Delft University of Technology in the Netherlands, focused their studies around two villages in the Yukon River basin in Alaska: Beaver, located 160 km north of Fairbanks, and Huslia, 402 km west of Beaver.
Previous methods for estimating mercury concentrations relied on core samples from the top 3 meters of permafrost, which had a fourfold margin of error and faced many limitations due to sample depth. To improve accuracy, the USC Dornsife-led research team analyzed mercury in sediment from riverbanks and river delta sandbars, reaching deeper soil layers. According to Isabel Smith, a PhD student at USC Dornsife, the river can quickly transport large amounts of mercury-laden sediment.
The research team found that mercury levels in sediment aligned with estimates from previous studies, confirming that sediment samples provide a reliable measure of mercury concentration and offer deeper insights into the potential risks in permafrost. Additionally, the team utilized remote sensing data from satellites to monitor how quickly the Yukon River is changing course. Changes in river flow are critical as they affect the amount of mercury-laden sediment eroded from riverbanks and deposited along sandbars, which helps researchers predict mercury movement.
Finer sediment contains more mercury than coarser sediment, indicating that certain types of soil may carry greater risks. Although mercury released from melting permafrost does not pose an acute toxic risk currently, its effects accumulate over time. Increased exposure occurs as mercury accumulates in the food chain, particularly in fish and animals consumed by humans. Long-term impacts could be severe, especially for Arctic communities reliant on hunting and fishing. The research team hopes that the tool they developed will enable more accurate assessments of the “mercury bomb.”
