Wildfires linked to increased mercury in Pacific Northwest headwater streams, study finds

PORTLAND, Ore. ( KOIN ) – A new study from the United States Geological Survey found recent wildfires in the Pacific Northwest led to an increase in mercury levels in some streams, raising concerns over harmful levels of mercury in water and fish.

During the study — as first reported by the Idaho Capital Sun — researchers took samples of soil, water, bugs, and aquatic vertebrate from 36 headwater streams in Oregon, Washington, and Idaho one year after they were burned from wildfires in 2021 and 2022.

After gathering samples from areas including the western and eastern Cascades, and Blue Mountains, researchers found levels of mercury and methylmercury — a toxic form of mercury — were 89% and 178% greater in burned watersheds compared to unburned watersheds respectively.

While mercury is natural occurring and found in plants and soil, the researchers explained that wildfires destroy groundcover and roots. Layers of ash from fires can then lead to increased runoff and erosion of soil into streams, according to the study.

Lead researcher and USGS Research Hydrologist Austin Baldwin told KOIN 6 News that researchers are concerned if methylmercury — a toxic form of mercury — travels downstream.

“If you have more mercury coming in from headwater areas to a lake, to a reservoir, then the fish – especially the predator fish – are going to accumulate more mercury in their bodies, and then we eat those fish,” Baldwin said.

While the concentration of their mercury samples was low, Baldwin explained, “The thing with mercury, is you can have a very low concentration in water but because a bug that lives in the water will accumulate [mercury] in its body all of its life and then a fish eats all those bugs, it accumulates all the bug mercury. And then the fish above that, the bigger fish, eats that fish, and so it increases dramatically as you move up the food web.”

“The human health concern is not really the mercury in the water, it’s the mercury that accumulates in the second fish,” he continued.

According to the Centers for Disease Control and Prevention , all forms of mercury can affect the nervous system and kidneys. People who consume high levels of methylmercury have experienced tremors, incoordination, impaired vision, learning, and memory, along with mood changes.

Baldwin emphasized that this study shouldn’t deter people from eating fish.

“There’s still a lot of benefits to eating fish,” Baldwin said, noting the study is “most important for people who eat a lot of fish.”

He added, “I think it’s just something to be thinking about going forward as we see more wildfires and more severe wildfires.”

As climate change will likely increase the frequency and intensity of wildfires, the study also found higher mercury concentrations in streams in areas with higher burn severity.

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