At the November meeting of the California Water Quality Monitoring Council, a comprehensive presentation shed light on the growing challenges posed by wildfires to drinking water systems. The discussion covered wildfire trends and behavior in California, the contamination that can occur in the aftermath of fires, the critical importance of post-fire water quality monitoring and the evolving best practices to address these issues. As wildfires become more frequent and severe, the presentation highlighted actions the State Water Board’s Division of Drinking Water is taking to protect public health and ensure the safety of drinking water systems in fire-affected communities.
Wildfire trends and behavior in California
California has been facing a growing number of large wildfires in recent decades. The chart illustrates wildfire activity from the 1950s to the present. While wildfire impacts were relatively low in the 1990s, they have steadily increased since then, with a significant surge observed between 2000 and 2020.
Several factors contribute to the rise in wildfires. Prolonged drought conditions, often spanning multiple years, have created hotter, drier environments. Additionally, annual air temperatures have been climbing. The chart below presents an 11-year rolling average of ambient temperatures since the 1900s, indicating an approximate 2-degree rise in average temperatures over the past century.
This translates to increasingly challenging fire seasons, with 2018 serving as a prime example. That year saw the Carr Fire and the Mendocino Complex Fire, both massive wildfires that erupted in July. Later in November, California faced two more devastating events: the Woolsey Fire and the Camp Fire, marking the end of what was then the most destructive fire season on record—a record that would soon be surpassed by even more severe seasons in the years that followed.
In the aftermath of wildfires, a growing concern has emerged regarding chemical contamination in drinking water pipelines. This issue primarily involves volatile organic compounds (VOCs), though other contaminants have been detected on occasion. Among these, benzene stands out as the most prevalent contaminant exceeding the maximum contaminant limit (MCL). In California, benzene has proven to be a reliable indicator of whether water systems have been impacted by wildfire events.
“This type of contamination primarily occurs in service lines,” said Yvonne Heaney, Fire Research Engineer with the State Water Board. “It originates in the distribution system; it is not a source water issue.”
Under normal conditions, water flows steadily through mains and service lines. However, during a fire, open lines and heavy firefighting water use can create a negative-pressure event, which allows surrounding combustion byproducts—such as air, smoke, and debris—to be drawn into empty pipelines. Once water service is restored, these contaminants can spread throughout the distribution system.
Low or no-pressure conditions leave pipelines empty, creating an opportunity for smoke, gases, and other combustion byproducts to infiltrate. Additionally, heat damage from fires can release chemicals from plastics and other synthetic materials, which can adhere to pipes and circulate through the system. While flushing has proven effective in some cases, pipe replacement may be necessary.
This type of contamination does not occur after every fire; several factors influence whether it happens. Key factors include the timing of pressure loss and the presence of open water lines within the system. System hydraulics, infrastructure characteristics—such as type, material, and age—and the distance from the contamination source also play significant roles. Additionally, pressure zones, piping configurations, fire temperature, and soil burn intensity can determine the extent of infrastructure damage and the potential for contamination to spread.
The timing of repressurization is another critical factor. The longer a water system remains depressurized, the greater the likelihood of contamination and its associated impacts. Site-specific conditions further influence whether contamination occurs and how challenging it may be to remove if it does.
California has experienced several notable wildfire events that resulted in contamination, including the Tubbs Fire of 2017, the Camp Fire of 2018, the CZU Lightning Complex Fire in 2020, the Caldor Fire in 2021, and, most recently, the LA fires of 2025. Similar contamination has also been observed outside California, including during the Oregon fires of 2020, the Colorado Marshall Fire of 2021, and the Hawaii Lahaina Fire of 2023.
However, it’s important to note that wildfires don’t always lead to contamination. For instance, no contamination was detected following the Carr Fire of 2017, the Kincaid Fire of 2019, the North Complex Fire of 2020, or the Mountain Fire of 2024.
How this type of contamination was discovered – Tubbs Fire
This type of contamination was first identified in 2017 following the Tubbs Fire. The affected area included approximately 300 homes, of which only 13 remained standing after the fire. Shortly after residents began returning, a complaint was made to the city about water that smelled like gasoline, prompting an investigation.
The investigation revealed that benzene had infiltrated the pipelines within the Santa Rosa water system. Benzene levels were alarmingly high, with some detections exceeding 40,000 parts per billion (ppb). For context, California’s maximum contaminant level (MCL) for benzene is one ppb, while the federal MCL is five ppb. Persistent benzene detections were found throughout the water quality advisory area.
While the highest detection was 40,000 ppb, not all samples were at such extreme levels. Many samples fell within the low range of 1 to 10 ppb, with a subset measuring under 100 ppb. A smaller number of samples showed levels in the hundreds or thousands of ppb. These extreme cases are rare, but they do occur, underscoring the potential severity of wildfire-related contamination.
Key takeaways from the Tubbs Fire monitoring efforts include the extensive investigation of both water and soil samples. This was the first time this type of contamination had ever been identified, and initially, there was little understanding of why it occurred. Early theories suggested a potential spill in the surrounding area, prompting soil sampling. However, all soil samples came back negative. Further investigation revealed that the contamination was actually inside the water pipelines.
Santa Rosa tested over 8,000 samples and confirmed that volatile organic compound (VOC) contamination was the primary issue. A forensic investigation into the causes identified several contributing factors: debris was found inside the pipelines, back siphonage occurred during the event, and the absorptive properties of the infrastructure allowed contaminants to adhere to and infiltrate the pipes. These findings provided critical insights into the mechanisms of wildfire-related water contamination.
Large-scale contamination after the Camp Fire
The next case study focuses on the large-scale contamination event following the 2018 Camp Fire. As shown on the slide, areas marked in orange represent destroyed or damaged structures, while those in gray indicate structures that remained standing. The fire caused extensive structural damage, destroying or damaging 19,000 structures, leaving only about 1,800 standing.
This was a significant, large-scale event that resulted in widespread drinking water contamination across four community water systems. The contamination was primarily attributed to plastic service lines, which were heavily impacted by the fire.
Following the Camp Fire, over 5,000 water samples were collected over 17 months, including duplicate samples. The highest contamination rates were found in destroyed service lines, with over 24% of them exceeding the maximum contaminant level (MCL). In contrast, main lines showed very low contamination rates, with less than 1% exceeding an MCL. Notably, the few instances of contamination in main lines occurred in smaller-diameter pipes, measuring less than six inches.
Standing structures also had minimal contamination, with only about 1% of samples exceeding an MCL. It’s worth noting that these samples were taken closer to the fire, a time when contamination was likely more active and moving through the system.
Appurtenances—components of the water system other than pipelines, such as valves and hydrants—also had low contamination rates, with approximately 6% exceeding an MCL. Many of these cases came with caveats, suggesting specific factors or explanations contributed to the slightly higher detection rates in these components.
AB 541
In response to the devastating wildfires and the discovery of chemical contamination in 2017, the California legislature passed Assembly Bill 541 in 2023. This legislation granted the Division of Drinking Water (DDW) the authority to mandate chemical testing in public water systems following a wildfire, provided certain conditions were met. Governor Gavin Newsom signed AB 541 into law on October 8, 2023, codifying it under Section 116596 of the California Health and Safety Code.
Section 116596 requires benzene testing whenever a wildfire exceeding 300 acres impacts a water system and damages or destroys structures connected to that system. If benzene is detected, DDW will likely mandate additional testing, potentially including other analytes. The agency may also prescribe flushing and remediation measures, along with customer notifications, to address contamination concerns.
It’s important to note that benzene contamination typically occurs when a depressurization or low-pressure event coincides with the loss of structures. This combination is a primary factor behind benzene detections in wildfire-impacted water systems.
Post-fire sampling protocols
After a wildfire, the Division of Drinking Water (DDW) typically requires water systems to test for volatile organic compounds (VOCs) using Method 524.2, with specific modifications tailored for post-fire contamination. Under normal circumstances, this method involves flushing the sample line for 10 minutes before sampling. However, this approach is not suitable for detecting wildfire-related contaminants. Instead, post-fire sampling requires water to stagnate in the system for at least 72 hours before collection. Flushing the lines before sampling is strictly prohibited to ensure accurate detection of contaminants. Additionally, strict protocols for handling samples must be followed to prevent contamination and to avoid the loss of VOCs, which can volatilize at ambient temperatures. Sample site selection must also adhere to specific requirements.
Water systems must use an ELAP-certified laboratory for testing. The laboratory uploads the results to the California Laboratory Intake Portal (CLIP), ensuring transparency, as these results are publicly accessible. Furthermore, water systems must notify customers of the test results to keep them informed.
When this issue was first identified in 2017, limited information was available about the contributing factors, highlighting the urgent need for research to better understand the underlying causes.
“To this day, we still don’t have a full understanding of how and why this contamination occurs, meaning that we don’t have specific chemical pathways that we understand today to be able to follow some of the products that are introduced in and possibly determine what may come out of it,” said Ms. Heaney. “So there’s quite a bit of unknown, and that does relate to fire specifically; the temperature that a fire burns at will dictate the byproducts that come out of it. So it is something that is temperature specific, it is fire specific, it is material specific.”
Study: Fire and Water Study
The Fire and Water Study was led by Dr. Gina Solomon of the Public Health Institute, who also serves as a department chief at UCSF. Initiated in 2020 and published in 2021 in Environmental Science: Water Research & Technology, the study aimed to investigate water quality directly at customers’ taps following the 2018 Camp Fire.
The research was conducted inside customers’ homes and included both spatial and chemical analyses to identify unknown contaminants. The process began with broad screening and targeted sampling, ultimately involving 150 homes. From these, a smaller subset of approximately 10 homes with detected contaminants underwent further testing for non-targeted analytes.
“The study was very interesting and gave us an inside look at the health of water at customers’ taps after the fire, and then it also gave us some information about what potentially can impact standing homes,” said Ms. Heaney.
Study: Fire-damaged service line study
The study, Organic Chemical Contaminants in Water System Infrastructure Following Wildfire, focused on a fire-damaged service line that was heavily contaminated. Led by Dr. William Draper of the California Department of Public Health, the research began in 2020 and was published in 2022 in Environmental Science: Water Research & Technology. The primary goal was to analyze contaminants within a severely impacted service line from a wildfire-affected water system.
The study examined a high-density polyethylene (HDPE) service line that had a benzene concentration exceeding 2,000 parts per billion. Key findings revealed the presence of over 90 different compounds. These byproducts were traced back to sources such as polyethylene, polyvinyl chloride (PVC), and synthetic fuels found in ambient vegetation. This research provided critical insights into the chemical impacts of wildfires on water system infrastructure.
“This study also revealed that post-fire reactions were taking place,” said Ms. Heaney. “Typically, water systems will use chlorine as a disinfectant. There are other compounds outside of chlorine that can cause these reactions, but it’s primarily chlorine that, mixed with the introduction of increased organics from the combustion byproducts that enter the service lines, will potentially cause an increase in disinfection byproducts.”
In summary …
Ms. Heaney concluded by summarizing her key points, emphasizing the critical takeaways from addressing wildfire-related water contamination.
First, wildfires are expected to continue impacting drinking water systems across California and beyond. Contamination events are not only time-consuming but also challenging and costly to remediate. While significant progress has been made through ongoing research, there is still much to learn about the full scope of these impacts.
Importantly, it has been 8 years since this type of contamination was first identified, and emerging best practices are now guiding responses. These practices are continually updated as new information and insights become available. Above all, protecting public health remains the top priority when evaluating and addressing potential wildfire impacts on water systems.
LA Wildfire response
Last January, the Palisades Fire swept through much of the Pacific Palisades on the west side of Los Angeles, extending toward Malibu and burning over 23,000 acres. The fire destroyed more than 6,800 structures, primarily within the city of Los Angeles. The Los Angeles Department of Water and Power (LA DWP) was the water system most severely impacted by this event.
Further east, the Eaton Fire burned through 14,000 acres in Altadena and north of Pasadena, destroying 9,400 structures. Together, these fires affected a vast area of Southern California, forcing the evacuation of approximately 200,000 residents during and after the events.
Both fires ignited on January 7 and burned for several days, leaving widespread destruction in their wake. Repopulation efforts were delayed due to the extensive damage, as roads needed to be cleared, downed power lines addressed, and water systems assessed. Many areas remained inaccessible for days, preventing a complete evaluation of the water systems and the remaining infrastructure.
Repopulation officially began on January 18 and continued for several months until all affected areas were fully reopened. The fires were ultimately declared 100% contained on January 31. Between the two fires, nine community water systems were impacted. The Los Angeles Department of Water and Power (LA DWP) lost approximately 4,000 connections due to the Palisades Fire. In Altadena, three mutual water companies serving the community lost about 5,000 connections, with 56% of their customers losing their homes.
When homes are destroyed during a wildfire, the water pipes supplying them often break, leading to leaks and disrupted service connections. This damage extends to the broader water system infrastructure, including storage tanks and booster pumps. In Altadena, several water systems experienced catastrophic damage to their water tanks, with roofs collapsing into the tanks, rendering them completely unusable. Electrical equipment was also heavily impacted, with power outages at pumping stations caused by the fire itself or by Edison’s public safety power shutoffs. These disruptions significantly hindered the movement of water and the system’s ability to provide service during the event.
In some areas, four water systems experienced partial or complete dewatering of their distribution systems. This dewatering is believed to be a leading cause of VOC contamination, as depressurized and empty distribution lines can draw in combustion gases through broken service lines at damaged homes.
As a result, nine water systems were affected by the fires. Between January 9 and 12, DDW worked closely with these systems to issue “Do Not Drink, Do Not Boil” notices to all customers. This initiated a coordinated response to restore water service, ensuring that customers with standing homes had access to clean water when repopulation began. Additionally, DDW collaborated with the water systems to develop sampling and recovery plans to address contamination and support long-term recovery efforts.
The first of the nine “Do Not Drink, Do Not Boil” notices was lifted by the Division of Drinking Water (DDW) on January 21. The final notice for Las Flores Water Company in Altadena was lifted in May.
Following the Tubbs Fire, the Camp Fire, and now the Maui wildfires, the risk of benzene and VOC contamination in water systems has become a critical concern that must be thoroughly investigated after a wildfire impacts a water system. Contamination is believed to stem from two primary sources: service line contamination, where dewatering of the system draws combustion gases through broken service lines, and thermal impacts on plastic materials. Pipes, appurtenances, and other components that were softened or damaged by the heat of the fire can release VOCs into drinking water.
In such situations, it is essential to issue not only a “Do Not Drink” notice but also a “Do Not Boil” notice. Boiling water containing VOCs can increase their vaporization, creating a significant inhalation hazard within the home.
Given the uncertainty surrounding the location and severity of VOC contamination, the Division of Drinking Water (DDW) takes precautionary measures by issuing “Do Not Drink, Do Not Boil” notices for systems impacted by wildfires. This is especially critical when there is a loss of homes, damage to the drinking water system, and a loss of pressure, as these factors significantly increase the risk of contamination.
Sampling results
Following these fires, water systems collected over 1,500 samples for volatile organic compounds (VOCs) in a relatively short period, with 95% of the samples taken at service lines. A significant number of water distribution mains were also sampled, with approximately 12% showing at least one VOC detection. In total, 19 different VOCs were identified, with benzene being the most common, detected in about 8% of the samples. Other VOCs found in at least 1% of the samples included toluene, naphthalene, ethylbenzene, styrene, and xylene. However, benzene, followed by toluene, accounted for the majority of detections.
The table highlights the most commonly detected VOCs during this fire event, along with vinyl chloride and 1,2-dichloromethane, which were the only other compounds to exceed their Maximum Contaminant Levels (MCLs). Benzene had the highest number of detections above the MCL and was the only compound to surpass the higher health advisory levels. Notably, one sample exceeded the EPA’s 10-day health advisory level. All other contaminants remained below their MCLs, except when benzene was also present. This underscores the significance of benzene as the primary public health risk during the LA fires, as it consistently posed the most significant threat compared to other contaminants, which were rarely detected without its presence.
“I think that’s important to note, at least for the LA fires, benzene did show to be the greatest risk to public health,” said Sean McCarthy, Assistant Deputy Director for the Southern California Field Branch. “There are others that could be found, but also not without the presence of benzene.”
The data indicate that contamination is not a system-wide issue but rather localized to specific service lines, requiring targeted sampling to identify potential hotspots. The location of these hotspots depends on factors such as the timing of the fire and the manner of depressurization during the event. This can be influenced by the gradient of the pressure zone—steeper zones may dewater more quickly. Additionally, mapping the results revealed that contamination tends to concentrate around dead ends and the edges of the pressure zone.
“It’s just a general observation of the data that we found in this fire. You know, why it happens in certain areas, and not others, is still, we may never know from this fire or future ones as well, because just the lack of research on the mechanisms of why this happened in the first place.”
He also noted that water systems affected by wildfires must account for changes in hydraulics and flow patterns, as the loss of homes in impacted areas significantly reduces water demand.
“Water systems need to be vigilant and watch the way water quality is maintained throughout their system. What they did before the fire may not be the same thing they need to do after the fire.”
NOTE: This post is not full coverage of the wildfire water quality portion of the meeting. To view the entire meeting, click here. Agenda and presentation slides are available here (under 2025 meetings).
