Researchers with the 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.
鈥淎 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鈥檛 been a lot of work done on headwater streams,鈥� he told the Idaho Capital Sun.
In the new study, which was published last , 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 鈥� 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.
鈥淚n general, the mercury is really low in those systems,鈥� he said. 鈥淲hat we saw in vertebrates and streams weren鈥檛 terribly high concentrations, but they did increase with fire. In those environments, I don鈥檛 know that mercury poses a direct exposure threat to humans or wildlife or food webs.鈥�
However, the concern is more mercury being carried downstream.
鈥淚f 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,鈥� he said. 鈥淚n those scenarios, there could be an increase in risk of exposure if that work is transported downstream to those other types of environments.鈥�
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.
鈥淲hen there鈥檚 a wildfire, the soils get really hot, and we know that the organic matter is burned and vaporizes,鈥� he said. 鈥淭he question was 鈥� does all that mercury just volatilize and end up in the atmosphere, or is some of it left behind? And if it鈥檚 left behind, is it more mobile to be moved downstream?鈥�
Eagles-Smith said the team was 鈥渁 little surprised鈥� 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.
鈥淭his practice of prescribed burns that has a lot of other benefits as well, might actually help keep mercury concentrations bound,鈥� Baldwin said.
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