Wildfires, which have intensified with climate change, litter the ground with debris that can contaminate drinking water supplies after a heavy rain.

By Alex Urquhart and Tanya Petach, Yale Climate Connections (CC BY-NC-ND 2.5)

The largest wildfire in New Mexico’s state history burned over 300,000 acres in the summer of 2022 and came within a mile of the town of Las Vegas. The flames ultimately spared the town of 13,000, but months later, ash and soot left by the Calf Canyon/Hermits Peak wildfire fouled drinking water there when monsoon rains blanketed the region, paradoxically slamming Las Vegas with both flooding and a municipal water shortage.

Four people drowned in flash floods, and residents were forced to erect sandbag barriers to protect their houses. Meanwhile, the inundation overwhelmed the town’s water filtration system with ash contamination, forcing mandatory restrictions to cut water consumption by about two-thirds. Swimming pools went empty, and restaurants resorted to disposable dishes and utensils to cut back on dishwashing.

In September, New Mexico spent $2 million to rapidly install a temporary pre-treatment system. It is still propping up the overstrained filtration system while the town applies for federal funds for a permanent water treatment facility that the mayor estimates could cost as much as $100–200 million.

Climate change is worsening wildfires

Around the world, more extreme wildfires have become a shocking signal that the effects of climate change are here. Wildfires are now more common and more destructive, making their damage more expensive.

Climate models have predicted this worsening trend for years and suggest it will continue as long spells of hot and dry weather become more common. In California, 12 of the 20 largest fires since 1932 occurred in the last five years. In the Mediterranean, the frequency of so-called “fire weather”—hot and dry weather that leads to large wildfires—is projected to increase by up to 30% by the end of the century.

Toxic runoff dirties drinking water

Although the dramatic violence of wildfires attracts intense media coverage, long-term impacts on water quality have gone largely unreported. The problem is alarming in the U.S. West, which has wrestled with regional water shortages for years. Researchers are finding that heavy rains in areas affected by wildfires can contaminate watersheds and overwhelm municipal drinking water systems. Municipalities must often pay astronomical costs to augment, repair, or replace entire water distribution systems. With risks growing, researchers say at-risk areas must plan ahead to act quickly and communicate clearly about water issues to fire-hit residents.

Wildfires lead to increased flooding and sediment erosion into rivers because a healthy forest is no longer there to slow stormwater runoff and increase water absorption. During storms, ash from the wildfire will be carried unchecked directly into streams, where it can easily flow to a municipal water intake and overwhelm treatment plants, leading to water shortages or even total failure of municipal water systems.

Following the Rocky and Wragg fires in California, researchers studying the affected watersheds recorded drastic increases in dissolved organic carbon, dissolved organic nitrogen and ammonium. It took over a year for these levels to return to normal.

When fires burn through developed areas, toxic runoff is created from the destruction of building materials, electronics, appliances, and vehicles. Rain transports these dangerous chemicals into groundwater, contaminating private wells and municipal systems. This can force months of boil water advisories, or even do not drink/do not boil orders, where drinking water must be brought in from other areas.

Even the water distribution system itself can become a source of contamination. Following the Tubbs Fire and the Camp Fire in California, both of which burned through developed areas, researchers found that municipal drinking water exceeded exposure limits for volatile organic compounds such as benzene. The source of this contamination may have been fire damage to plastic pipes and other synthetic components of the distribution system. With so many potential sources and causes of contamination, it is challenging for public officials to define an appropriate response. This has led to conflicting or variable recommendations in the aftermath of a fire, damaging public trust in official guidance.

Can we build fire-resilient water systems?

As wildfires worsen globally, water quality problems will affect millions of people who live in threatened watersheds. In addition to cutting planet-heating emissions, specific solutions are needed to protect public health and safety from the inevitable fires to come.

Researchers who studied the aftermath of the Tubbs and Camp Fire have called for standardized and streamlined water quality monitoring following wildfires. They recommend a “do not use” order following any wildfire that burns through developed areas. Other recommendations include updated building codes to limit the spread of contaminated water within damaged distribution systems.

Clear health and safety guidance in the aftermath of a fire is crucial. In the months following the Camp Fire, surveys of 233 households within the affected community showed 54% had some level of anxiety about water contamination, and 85% were seeking alternative water sources. The public needed clear recommendations about drinking water safety, including how to conduct at-home testing. Following a fire, clear and regular communication may be required for months or years, depending on the scope of contamination.

Municipalities may also identify standard operating procedures and fire response policies before disaster strikes. A new study examining the 2021 Marshall fire in Colorado outlined potential mitigation procedures that municipalities could implement, from emergency planning to post-fire flushing protocols.

“There are very simple straightforward actions that municipalities can take today to prevent wide-scale water distribution system contamination,” said Andrew Whelton, a lead author of the study. For example: “isolating your water distribution center into zones so that if one part of the system is damaged it doesn’t spread to the other parts of the system.”

Having a plan in place will reduce confusion and increase trust and efficiency in the wildfire response, recent research suggests. One vital consideration is the level of water contamination that constitutes acceptable or unacceptable health risks.

“There are certain conditions that would indicate that your water is lightly contaminated and you should not use it,” Whelton said. “The Marshall Fire case study identifies those conditions, and another study identifies conditions of contamination in private wells. Your water can be chemically contaminated after a fire, and you have to do testing to determine if it is safe or not.”

Understanding these thresholds will lend clarity and speed to post-fire decision-making. And with climate change accelerating, the need for standardized practices that will educate the public about water safety and ensure access to clean water will only grow.

Alex Urquhartis the research and modeling manager at Energy Innovation Policy and Technology LLC® and Tanya Petachis the Climate Science Fellow at the Aspen Global Change Institute. Both organizations are Yale Climate Connections content-sharing partners.

*This post was updated Feb. 3, 2023, to reflect the correct spelling of Andrew Whelton’s name.