By KATE SELIG NYTimes News Service
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As long-overdue rain sweeps through Los Angeles County on Sunday, the worry on many minds is that a heavy downpour on one of the region’s vast wildfire burn scars might cause flash flooding and mudslides.

But there is another, more insidious risk: rain that falls on a recently burned area, picks up contaminants from ash, charred vehicles and other debris, and then carries the pollutants into the ecosystem or the water supply.

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Officials have been racing to address the threat of toxic runoff, which they warned could harm the environment and human health.

Fires release contaminants from the things they burn, and their heat also creates new toxic compounds, according to Jackson Webster, a professor at California State University, Chico, who has studied the impact of wildfires on water systems.

The ash that the fires leave behind can be tainted with metals, pesticides, byproducts of burned plastic and all manner of other harmful materials, including long-lasting “forever” chemicals such as PFAS that can dissolve in rainwater. Larger debris such as pieces of charcoal can also be picked up by runoff.

Fires in natural wildlands can pose some of these risks to water systems, but when the works of humankind are caught up in the blaze as well, things can get particularly toxic. “Burned structures, vehicles, telephone poles, your local supermarket,” Webster said. “You’re adding a lot of complexity to the runoff.”

As the rain approached, Los Angeles city crews were installing concrete barriers, laying down sandbags and removing fire debris with heavy equipment in hopes of minimizing the flow of toxic runoff into watersheds and environmentally sensitive areas, and of diverting it instead into the sewer system, according to a news release from Mayor Karen Bass’ office.

Andrew Whelton, an environmental and ecological engineering professor at Purdue University, said toxic runoff threatens organisms that depend on clean water, such as fish, as well as humans who come into direct contact with the water or the contaminants that are left behind when it evaporates.

Whelton said the runoff was unlikely to directly affect the majority of the region’s drinking water supplies. Most people in the region, even in areas directly affected by the fires, get their water from sources far from the burn zones or from groundwater wells that are protected from runoff, he said.

But one important exception may be Castaic Lake, a reservoir that supplies water to millions of residents. The lake is adjacent to the Hughes fire, which was still burning in the northwestern part of Los Angeles County on Sunday, although it had been largely contained.

Whether the water in the lake is significantly affected will depend on factors such as how much runoff the lake receives, and where and how deep the drinking water intake is, Whelton said. He added that in any case, the water is treated and tested before it is distributed to the community.

Webster said the study of toxic runoff from a burned urban or suburban area is a subject of active scientific inquiry, and researchers have yet to develop a complete picture of the risks. As wildfires become more frequent, he added, researchers are gaining more data to better understand the issue.

“This won’t be the last one,” he said.

This article originally appeared in The New York Times.

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