Wildfires Are Threatening Municipal Water Supplies

In recent decades, wildfire conflagrations have increased in number, size, and intensity in many parts of the world, from the Amazon to Siberia and Australia to the western United States. The aftereffects of these fires provide windows into a future where wildfires have unprecedented deleterious effects on ecosystems and the organisms, including humans, that depend upon them—not the least of which is the potential for serious damage to municipal water supplies.

In 2013, the Rim Fire—at the time, the third-largest wildfire in California’s history—burned a large swath of Stanislaus National Forest near Hetch Hetchy Reservoir, raising concerns about the safety of drinking water provided from the reservoir to San Francisco.

The 2018 Camp Fire not only burned vegetation but also torched buildings and the water distribution system for the town of Paradise in north central California, leaving piles of charred electronics, furniture, and automobiles scattered amid the ruins. Postfire rainstorms flushed debris and dissolved toxicants from these burned materials into nearby water bodies and contaminated downstream reaches. Residents relying on these sources complained about smoke-tainted odors in their household tap water [Proctor et al., 2020]. And in some cases, water utilities had to stop using water supplies sourced from too near the wildfire and supply alternative sources of water to customers.

Climate change is expected to increase the frequency and severity of wildfires, resulting in new risks to water providers and consumers. Water exported from severely burned watersheds can have greatly altered chemistry and may contain elevated levels of contaminants and other undesirable materials that are difficult to remove. For example, excess nutrients can fuel algal blooms and suspended soil erosion particles can clog water filters. Are water utilities in wildfire-affected areas prepared for these changes?

Our research team has conducted field studies after several severe wildfires to sample surface waters and investigate the fires’ effects on downstream water chemistry. In California, we have looked at the aftereffects of the 2007 Angora Fire, 2013 Rim Fire, 2015 Wragg Fire, 2015 Rocky-Jerusalem Fire, 2016 Cold Fire, 2018 Camp Fire, and 2020 LNU Lightning Complex Fire. We also studied the 2016 Pinnacle Mountain Fire in South Carolina and the long-term effects of the 2002 Hayman Fire in Colorado. These campaigns often involve hazardous working conditions, forcing researchers to wear personal protective gear such as respirator masks, heavy boots and gloves, and sometimes full gowns for protection from ash and dust. We also had to monitor the weather so as not to be surprised by the unpredictable and dangerous flash floods, debris flows, and landslides that can occur following fires.

The devastation of these burned landscapes is stunning and amplifies the urgency to better understand the fallout of fires on ecosystems and humans. Our field studies have provided important new insights about how surface water chemistry and quality are affected after fires—information useful in efforts to safeguard water treatment and water supplies in the future.

Wildfire Impacts on Water

Wildfires have well-documented effects on the quality of surface waters. Fires contaminate the rivers, streams, lakes, and reservoirs that supply public drinking water utilities with sediments, algae-promoting nutrients, and heavy metals [Bladon et al., 2014]. However, few researchers have addressed water treatability—the ease with which water is purified—or drinking water quality in water treated following wildfires (see video below).

Contaminants are mobilized in the environment as a result of the forest fires, which volatilize biomass into gases like carbon dioxide while producing layers of loose ash on the soil surface. Dissolved organic matter (DOM) leached from this burned, or pyrogenic, material (PyDOM) has appreciably different chemical characteristics compared with DOM from the unburned parent materials [Chen et al., 2020]. Although wildfires can destroy forest ecosystems within days, changes in DOM quantity and composition can persist in burned landscapes for decades [Chow et al., 2019].

DOM itself is not a contaminant with direct impacts on human health, but it creates problems for water treatment. It can cause off colorations and tastes and serve as a substrate for unwanted microbial growth or a foulant of membranes and adsorption processes. Also, DOM can increase treatment costs and chemical demand levels, that is, the amount of added chemicals, like chlorine and ferric iron, required to disinfect water and remove DOM. In addition, treatment efforts can introduce unintended side effects: Disinfection processes for DOM-contaminated water can form a variety of carcinogenic disinfection by-products (DBPs) such as chloroform, some of which are regulated by the EPA.

The characteristics, treatability, and duration of PyDOM from burned watersheds are poorly understood and require more study, but it is clear that this material poses several major challenges and health concerns related to municipal water supplies in wildfire-prone areas. In particular, it negatively affects treatability while increasing the likelihood of algal blooms and toxic chemical releases (Figure 1).

Figure illustrating threats to drinking water supplies from wildfires
Fig. 1. Threats to drinking water supplies from wildfires include releases of toxic chemicals from burned infrastructure, electronics, plastics, cars, and other artificial materials (left); releases of pyrogenic dissolved organic matter and toxic chemicals from ash deposits into source water supplies (middle); and postfire eutrophication and algal blooms in water supplies because of increased nutrient availability (right). Credit: Illustration, Wing-Yee Kelly Cheah; inset photos, Alex Tat-Shing Chow

Treatability of Pyrogenic Dissolved Organic Matter

Postfire precipitation can easily promote the leaching of chemicals from burned residues, and it can also transport lightweight ash to nearby surface waters. This deposition raises levels of DOM and total suspended solids, increases turbidity, and lowers dissolved oxygen levels in the water, potentially killing aquatic organisms [Bladon et al., 2014; Abney et al., 2019].

Our controlled laboratory and field studies demonstrated that DOM concentrations in leached water depend on fire severity. Burned residuals could yield DOM concentrations up to 6–7 times higher than those in leached water from the unburned parent biomass. DOM concentrations in stream water from a completely burned watershed were 67% higher than concentrations in water from a nonburned watershed in the year following a severe wildfire [Chen et al., 2020; Uzun et al., 2020].

High total suspended solid levels complicate drinking water treatment by increasing chemical demand and reducing filtration. We observed that PyDOM—which had a lower average molecular weight but greater aromatic and nitrogen content than nonpyrogenic DOM—was removed from water with substantially lower efficiency (20%–30% removal) than nonpyrogenic DOM (generally 50%–60% removal or more) [Chen et al., 2020].

Elevated levels of PyDOM in water mean that higher chemical dosages are needed for treatment, and higher levels of DBPs are likely to be formed during treatment. PyDOM is also more reactive, which promotes the formation of potentially harmful oxygenated DBPs. For example, chlorinating water that contains PyDOM produces haloacetic acids, whereas chloramination, another form of disinfection, produces N-nitrosodimethylamine [Uzun et al., 2020]. In addition, increased levels of bromide, another DBP precursor, released from burned vegetation and soils has been observed in postfire surface runoff, especially in coastal areas. This bromide may enhance the formation of more toxic brominated DBPs (e.g., by converting chloroform to bromoform) [Wang et al., 2015; Uzun et al., 2020].

Only a small fraction (less than 30%) of total DBPs generated from DOM have been identified in chlorinated or chloraminated waters. The unique chemical characteristics of PyDOM generated from wildfire may give rise to DBPs that do not typically occur in water treatment and have not been identified or studied.

Postfire Nutrient Releases and Algal Blooms

After wildfires, burned biomass, fire retardant, and suppression agents like ammonium sulfate and ammonium phosphate often release nutrients, including inorganic nitrogen and phosphorus, into source waters. Wildfire runoff is often alkaline (pH > 9), in part because of its interactions with wood ash and dissolved minerals. Under these conditions, high ammonia and ammonium ion concentrations can cause acute ammonia toxicity in aquatic organisms, especially in headwater streams where these contaminants are not as diluted as they become farther downstream.

Freshwater aquatic ecosystems tend to be phosphorus limited, meaning algal growth is naturally kept under control. But large phosphorus loads originating from burned watersheds, particularly phosphorus associated with sediments, can induce eutrophication (nutrient enrichment) and harmful algal blooms, particularly in lentic (still-water) ecosystems where nutrients accumulate. Blooms of cyanobacteria (blue-green algae) like Microcystis aeruginosa are especially hazardous for drinking water supplies because they produce neurotoxins and peptide hepatotoxins (liver toxins) such as microcystin and cyanopeptolin.

Algal organic matter is also nitrogen rich and contributes to the formation of a variety of carbonaceous and nitrogenous DBPs during drinking water disinfection [Tsai and Chow, 2016]. Although copper-based algicide treatments are options for controlling algal blooms, copper ions themselves catalyze DBP formation during drinking water disinfection [Tsai et al., 2019].

Releases of Toxic Chemicals

When forest vegetation burns, it can generate and directly release a variety of potentially toxic chemicals, including polycyclic aromatic hydrocarbons [Chen et al., 2018], mercury [Ku et al., 2018], and heavy metals [Bladon et al., 2014]. In addition, fires such as California’s 2017 Tubbs Fire and 2018 Camp Fire, which extended to the interfaces between wildlands and urban areas, have generated residues from burned infrastructure, electronics, plastics, cars, and other artificial materials, contributing a variety of toxic chemicals to source waters.

Fires can also burn plastic water pipelines in homes that are connected to municipal water distribution systems, potentially releasing hazardous volatile organic carbon into the larger water system. For example, up to 40 milligrams per liter of benzene, a known carcinogen, was reported in water distribution lines following the Tubbs Fire in an urban area of California [Proctor et al., 2020]. Benzene is one of many organic chemicals found in damaged water distribution networks, and experts worry there could be many other toxic chemicals released from burned pipes over time. Because these damaged pipes are downstream from the treatment facility, the best remediation option may be to replace the pipes entirely.

Climate Change and Wildfires Alter Watershed Hydrology

Our recent research demonstrates that the degree of water quality impairment increases markedly with increasing wildfire severity and with the proportion of the watershed area burned [Chow et al., 2019; Uzun et al., 2020]. Hence, as wildfires burn hotter and consume more fuel in future climates, water quality will progressively degrade.

Severe wildfires consume vegetative and soil cover and often cause soils to become more water-repellent, which greatly increases surface runoff at the expense of soil infiltration. In turn, these changes lead to enhanced soil erosion and sediment transport—carrying associated pollutants directly to downstream waters—and to reduced filtration of water in the soil profile [Abney et al., 2019].

Although water quality in riverine systems may recover quickly following successive storm-flushing events, pollutants can accumulate in lakes and reservoir systems, which are more sedentary, degrading water quality for decades as pollutants are recycled between the water column and sediments.

Other factors are also likely to influence postfire runoff, erosion, and contamination transport amid changing climates. For example, many forested watersheds today still receive much of their precipitation as snowfall, which is much less erosive than a comparable volume of rainfall. But as the climate warms and more precipitation falls as rain, postfire surface runoff and erosion and water quality impairment will increase considerably. In addition, as extreme weather events are expected to be more prevalent in the future, more intense rainfall could greatly increase postfire pollutant transport.

Burned trees line the banks of a creek.
Burned trees line the banks of a creek in the aftermath of the 2018 Camp Fire. A warming climate is expected to severely degrade water quality by contributing to larger burned areas and more severely burned watersheds. Credit: Alex Tat-Shing Chow

At present, portions of a watershed not burned during a wildfire serve to mitigate water pollution by providing fresh water that dilutes contaminants coming from burned areas. As wildfire sizes grow larger, this dilution will diminish. Furthermore, vegetation takes longer to recover after more severe wildfires, delaying recoveries in water quality.

Stream runoff dynamics will also be altered as increased surface runoff reduces soil water and groundwater recharge, leading to higher peak flows during storms and lower base flow conditions. The slow regeneration of vegetation, which will reduce consumption and transpiration of water by plants, will also lead to greater runoff following wildfires [Chow et al., 2019].

A warming climate is expected to severely degrade water quality by contributing to larger burned areas and more severely burned watersheds. The damage will be exacerbated by increases in rainfall compared with snow and by extreme storm events that enhance surface runoff and erosion.

Proactive and Prescribed Solutions

Wildfires can cause press (ongoing) and pulse (limited-duration) perturbations in forested watersheds, altering watershed hydrology and surface water quality and, consequently, drinking water treatability. Mitigating wildfire impacts on drinking water safety requires effective, proactive management as well as postdisaster rehabilitation strategies from the forestry and water industries.

Water quality impairment increases exponentially with increasing burn intensity and area burned, so reducing forest fuel loads is critical. From a forestry management perspective, forest thinning and prescribed fire are both well-established, effective fuel reduction techniques. However, the operational costs of thinning are usually high, and the residual foliar litter it produces can cause increased DOM concentrations in source waters.

By comparison, prescribed fire is an economic management practice to reduce loads of forest litter and wildfire hazard. These low-severity fires reduce the quantity of DOM (and DBP precursors) potentially released to waterways while not appreciably affecting its composition or treatability in source water [Majidzadeh et al., 2019] (see video below). Establishing landscape-scale firebreaks is another effective management strategy, providing defensible corridors within forests to limit the rapid spread of fire and reduce the size of burned areas.

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Water utilities, particularly those in fire-prone areas, should develop risk analysis and emergency response plans that combine multiple approaches. Such approaches include identifying alternative source waters, extensive and long-term postfire source water quality monitoring, and modifications in treatment processes and operations, such as using adsorbents and alternative oxidants that reduce taste and odor problems, remove specific contaminants, and decrease the formation of regulated DBPs.

Other research and preventive efforts should be encouraged as well. Such efforts include research studying the fates and effects of fire retardants in source water and the effects of postfire rehabilitation practices (e.g., mulching) on water chemistry, and the use of different pipeline and construction materials in newly developed housing near the wildland-urban interface. Furthermore, a collaborative system and an effective communication network between the forestry and water industries linking forest management to municipal water supplies will be critical in assessing and addressing wildfire impacts on drinking water safety.


The fieldwork efforts described in this article were supported by National Science Foundation RAPID Collaborative Proposal 1917156, U.S. EPA grant R835864 (National Priorities: Water Scarcity and Drought), and National Institute of Food and Agriculture grant 2018-67019-27795.

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