When wildfires devastated a wide swath of Los Angeles last winter, officials warned residents of several ZIP codes not to drink the water, or boil it first if they must. They worried that soot, ash, and other debris from the blazes might have infiltrated the groundwater, or that damaged pipes might allow toxins into the supply. The last of these “do not drink” orders was lifted last month.

But the first large-scale study of post-wildfire water quality has found that pollution created by such a blaze can threaten water supplies for eight years — far longer than previous studies indicated. Researchers at the Cooperative Institute for Research in Environmental Science, or CIRES, at the University of Colorado Boulder analyzed 100,000 samples from 500 watersheds across the western United States. They found “contaminants like organic carbon, phosphorus, nitrogen, and sediment” throughout those that had burned. At their peak, those pollutants can be found at levels up to 103 times higher than before the fire. There also can be 9 to 286 times as much sediment in water after a fire. 

The findings have great implications for water systems as they prepare for a world in which fires like those that burned in Los Angeles and, more recently, North Carolina and a great swath of Canada, grow more common. One in six people in the United States lives in a wildfire risk zone, and forested watersheds provide water to almost two-thirds of municipalities in the U.S., making water systems everywhere vulnerable. 

“I’ve had a lot of conversations with different utilities and water managers in the West, and every single one of them are concerned about wildfire impacts,” said Carli Brucker, lead author of the study, published Tuesday. But, she added, what they don’t have is longer-term data. “I’m hoping that this research provides these concrete numbers that can really back up water managers’ concerns, and turn those concerns into real funding that they can start putting towards climate resilience. Strong evidence can be really helpful in securing funding.”

Water utilities in the LA area addressed the threat posed by the fires that burned in January in the short term by flushing water mains and pipes. Officials with the Los Angeles Department of Water & Power said they are conducting ongoing water testing in the Palisades area, and are offering free water quality testing to any resident that wants it.

“These urban fires are creating these unprecedented challenges that treatment plants can’t really deal with,” Brucker said. “Burning buildings and businesses and roads and cars, it creates all these contaminants that are just way more dangerous and way more difficult to deal with.”

Across the locations the researchers analyzed, contamination levels varied widely. In general, post-fire pollution was worse in heavily forested or heavily urbanized areas. The “most dramatic spikes” in pollutants like phosphorus, nitrate, organic carbon and sediment generally occurred in the first few years after a fire, according to researcher Ben Livneh. 

“We found the impacts to be really persistent,” Livneh wrote in The Conversation. “We saw significantly elevated levels of nitrogen and sediment for up to eight years following a fire.” Even years after a fire, a major rainfall can trigger a mudslide, unearthing contaminants. Beyond polluting groundwater, that can cause unexpected environmental issues. “Nitrogen and phosphorus act like fertilizer for algae. A surge of these nutrients can trigger algal blooms in reservoirs, which can produce toxins and create foul odors,” Livneh said. 

There are several ways to fight these threats to water supply.

“The first line of defense is just diversifying water sources,” Brucker said. Ideally, a utility would draw from several watersheds, so it has a backup in the event one of them is impacted by a fire, she said. They also can build additional sedimentation basins to increase their capacity for sediment handling. 

“But all of these things cost a lot more,” Brucker said. And it’s difficult to convince strained utilities in Western states – already dealing with things like water shortages – to spend money on wildfire mitigation without numbers. Rural communities, in particular, often rely on single-source water systems and limited funding, which makes responding to emergencies much more challenging. 

“Utilities don’t usually have these sorts of process improvements in place, unless they have a good reason,” she said. “I’m hoping this research can point to – this is a pretty good reason to start planning for and trying to budget for those resilience improvements.”