Wildfires increase mercury in Pacific Northwest headwater streams, USGS study shows

Researchers with the U.S. Geological Survey found wildfires are increasing mercury concentrations in Pacific Northwest headwater streams.

Mercury is a naturally occurring metal found in rocks and soil. But under the right conditions, it can be converted into methylmercury, a highly toxic form that accumulates in organisms such as fish and humans, said Boise-based USGS research hydrologist Austin Baldwin, who led the study.

Baldwin said the purpose of the study was to fill the gaps of previous studies.

“A lot of the studies of the effects of wildfires on mercury previously focused on maybe one specific watershed, or have looked at the effects in a downstream lake or reservoir, but there hasn’t been a lot of work done on headwater streams,” he told the Idaho Capital Sun.

In the new study, which was published last month, Baldwin and his team of researchers sampled soils, water, sediment, instream leaf litter, streambed algae and aquatic invertebrates in 36 headwater-stream watersheds in Idaho, Washington and Oregon.

They sampled the sites between 2021 and 2022 – one year after they had been burned by wildfire. They also collected samples from 21 similar but unburned watersheds to compare mercury concentrations.

The amount of total mercury and methylmercury in water particles was 89% and 178% higher in the areas affected by fires.

So why is this a cause for concern? Collin Eagles-Smith, a USGS research ecologist, said the concentrations in the study are from headwater streams, which are at the top of watersheds.

“In general, the mercury is really low in those systems,” he said. “What we saw in vertebrates and streams weren’t terribly high concentrations, but they did increase with fire. In those environments, I don’t know that mercury poses a direct exposure threat to humans or wildlife or food webs.”

The concern, however, is more mercury being carried downstream.

“If that mercury is made more mobile and transports downstream, it could end up in reservoirs or larger rivers where there are predatory fish, top predators that humans would catch and consume,” he said. “In those scenarios, there could be an increase in risk of exposure if that work is transported downstream to those other types of environments.”

Surprising results, and why burn severity matters

Eagles-Smith, who sampled locations in the Oregon Cascades, said some of the results of the study were surprising.

“When there’s a wildfire, the soils get really hot, and we know that the organic matter is burned and vaporizes,” he said. “The question was – does all that mercury just volatilize and end up in the atmosphere, or is some of it left behind? And if it’s left behind, is it more mobile to be moved downstream?”

Eagles-Smith said the team was “a little surprised” to see that the concentrations were elevated in the water after the fire.

The study also found that concentrations of mercury increased with the severity of the fire.

Baldwin and his team sampled several locations in Idaho, including where the Buck Fire near Yellow Pine and the Grouse Fire near Featherville took place in 2020. They also collected samples from the sites of the 2021 Woodhead Fire near Council and the 2021 Snake River Complex Fire, south of Lewiston.

Because of climate change, there are more frequent and more severe wildfires, Baldwin said. But solutions exist.

“This practice of prescribed burns that has a lot of other benefits as well, might actually help keep mercury concentrations bound,” Baldwin said.

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