Parts of Los Angeles are under water advisories. Wildfires can cause damage to drinking water systems, allowing contaminants such as benzene to compromise the water quality.
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The warning is stark: “Do not drink or cook with the tap water … Do not treat the tap water yourself,” typed in all caps and boldface. The alert, issued January 10 by the Los Angeles Department of Water and Power, also advises people not to take hot baths or long showers. It’s directed at residents of the Pacific Palisades and other neighborhoods near the Palisades Fire.
It’s the latest in a series of water advisoriesissued this week in places affected by the unprecedented Southern California wildfires, which are still burning. Wildfires like these can compromise drinking water, says Andrew Whelton, a professor of civil engineering at Purdue University who has studied other recent wildfires.
“They will find contamination,” he says. “The question is how high is it? Where is it? What parts of the communities will have to have restrictions?”
High levels of benzene, a chemical linked with cancer, were found in the drinking water after large wildfire disasters in the 2017 Tubbs Fire in Santa Rosa, and the 2018 Camp Fire in Paradise, Calif. Other volatile organic compounds are also known to contaminate tap water after fires.
During wildfires drinking water can get contaminated in two ways: via the water source and the system that delivers it.
So far, it seems that the water in the reservoirs that serve the Los Angeles area may be largely spared, since much of LA’s water comes from places far north and east of the county.
“There might be some ‘atmospheric deposition’ of ash on drinking water reservoirs, but this is generally not a serious concern” since it can be cleaned through water treatment, Randy Dahlgren, a professor of soil science at UC Davis, wrote in an email.
The greater concern is the water delivery system – from local water treatment plants, to water storage tanks, to the pipes that bring water to homes and businesses. When parts of it are malfunctioning, burned or broken, the water system is vulnerable to contamination.
Typically, the water flows in one direction: from the water treatment plant, out to people’s homes.
But in a situation when there’s a huge demand for water, the pressure in the system drops. Firefighters opening up dry hydrants, people encountering low water pressure at home – these are all signs of depressurization which creates a vacuum effect.
It’s especially problematic during an active wildfire. The soot and air pollution from burned trees and buildings gets pulled back into the water system though open fire hydrants and water storage tank vents and damaged pipes.
“That vacuum pulls contaminants, like gases from combustion, back into the distribution system. And this causes chemical contamination of the distribution system which is very difficult to get rid of,” says Jackson Webster, a professor of civil engineering at California State University in Chico.
The infrastructure of the water system itself can melt and contaminate the water too, especially when the parts are made from plastic. Webster points to examples from past fires in northern California: In the 2020 North Complex Fire, plastic liners in the water tanks melted. In the 2018 Camp Fire, plastic pipes and water meters melted. In all cases, melting plastic residue got pulled back into the water supply.
When it comes to water contamination, the situation can change a lot from fire to fire. It depends on the materials used in the water system, how the systems connect underground, and also what the fire burns, which can vary from place to place.
“Not everybody in all of Southern California needs to be concerned about their water right now. But there are certainly systems that have been damaged that probably will have lasting contamination issues,” says Webster of Chico State.
After the wildfires in 2017 and 2018 in northern California, the drinking water was heavily contaminated with benzene; after the 2023 wildfires on Maui, the water was contaminated, but less so: It was “unsafe for drinking water, but it wasn’t at a level that would cause health effects in the shower,” says Purdue’s Whelton.
He encourages people in affected areas to listen to warnings from local health officials, and to also ask them questions, as chemical contamination may not be detectable by sight or smell.
Whelton says utilities can help alleviate people’s exposure and health concerns by flushing the water systems promptly, testing the water repeatedly, and communicating problems and timelines with the public.
