By Kelly A. Reynolds, MSPH, PhD

According to the USGS, about 80 percent of the fresh water supplies in the United States originate on forestlands. Protection of these supplies from both natural and man-made threats is essential for sustainability of drinking water quality.1 In the past 30 years, more than 12 million acres of land in the southwestern United States have burned due to wildland fires. Post-fire conditions may threaten water quality due to flooding, erosion and the introduction of fire-deterrent chemicals.

Wildfire history
One of the largest fires in California’s history occurred in the summer of 2013. The Rim Fire burned over 256 thousand acres including > 400 square miles of land encompassing two major watersheds in central California. In addition, Yosemite National Park was filled with smoke for weeks while the fire consumed 112 structures. After more than two months, the fire was fully contained but ecological burdens continued in the aftermath. One major concern was how the fire impacted two local watersheds that were critical providers of domestic and agricultural water to the San Francisco Bay and Central Valley areas. The Hetch Hetchy reservoir, serving up to 85 percent of the San Francisco water supply (including 2.6 million customers), was feared impaired. As a precaution, utility managers began diverting incoming water from the reservoir, while monitoring protocols were initiated to track potential contamination of the supply.2

The Rodeo-Chediski wildfire of 2002 burned over 468 thousand acres of forestland, destroying 491 structures in east-central Arizona, including areas in the Fort Apache Indian Reservation, the Apache-Sitgreaves National Forests and the Tonto National Forest.3 Environmental impacts related to peak water flows following this fire increased 235,000 percent during the intensive monsoon rain season. (This ‘first flush’ of rainwater across burned regions greatly impacted downstream areas and watersheds as large amounts of debris, sediment and chemicals rapidly accumulated.)

Another concern relative to the accumulation of post-fire debris in lakes, streams and watersheds is the use of fire-suppression chemicals. For the southwest (and California in particular), 2017 was one of the worst years on record for wildfires. One of the more destructive events during this time was the Tubbs Fire in Sonoma County, burning 137 square miles of land and more than 5,600 structures. Likewise, the use of chemical flame retardants to suppress those fires also reached record highs.

According to local media and CalFire (California’s Department of Forestry and Fire Protection), 19 million gallons of fire-suppression chemicals were used in all of 2016 to control wildland fires around the country.4 In a single week of 2017, CalFire applied 2.7 million gallons of these chemicals to control a series of wildfires in northern California. Air tankers flying overhead dropped massive loads of fire-retardant chemicals, which are typically a mixture of water and fertilizer compounds aimed at protecting trees from the flames and encouraging regrowth. The plumes, usually dyed with bright colors, helped firefighters track where they were being applied. While generally considered environmentally friendly, problems associated with the growing use of these chemicals has not been well studied and negative impacts to aquatic life have been commonly reported.5

Environmental impacts
Depending on the extent of the wildfire, rainfall activity and land characteristics, watershed volume and quality may be effected. Continuous burning of forestland creates an abundance of ash that can impact nearby lakes and reservoirs, ultimately affecting stream water quality and drinking water supplies. Subsequent rainfall events act to consolidate ash in stormwater runoff, which can further impact drinking, irrigation and wildlife water supplies. Rates of flow can increase or decrease water accumulation rates, sediment and nutrient loads, turbidity and source-water chemistry. One study found that levels of iron, lead, nickel and zinc were elevated above criteria established for aquatic life in streams near Los Angeles following the 2009 Station Fire in the Angeles National Forest nearby. Here, high mortality of rainbow trout and potentially other aquatic organisms was documented.6

In addition, wildfires can affect changes in soil infiltration rates, nutrient availability and trace metals (i.e., arsenic, lead, iron, cyanide) or polycyclic aromatic hydrocarbon (PAH) concentrations,6 chemicals that form during the incomplete burning of organic substances like trees and leaves. Although ubiquitous in the outdoor environment, they are often insoluble in water and an overabundance can cause a decline in water quality and create ecological imbalances that are harmful to aquatic animals. These changes are often most notable soon after the fire is contained but can continue for months to years.

Use of fire retardants have been found to cause the most dramatic impacts on aquatic populations where fish kills and changes to spawning patterns have been documented. Although deemed safe for human and terrestrial organisms, research is underway via support from the USGS Columbia Environmental Research Center to evaluate the toxicity of fire-suppressant chemicals to fish and amphibians.

Drinking water risks
The greatest risks to drinking water supplies occur soon after wildfire activities begin, when contaminant levels or quality indicators in the source water often exceed recommended limits. Studies show that contaminant exceedances are generally of short duration. Thus, in an abundance of caution, protocols should be in place to apply monitoring protocols, implement additional water treatment measures or utilize alternative drinking water sources, if needed.7

Data on the impact wildland fires have on drinking water quality is scarce. California’s Rim Fire gave stakeholders the opportunity to monitor hydrological and water quality changes over time following a major event. Data collected in the Rim Fire region, along with other US locations, is publically available within the USGS National Water Information System (NWIS) portal. Linking with the USGS WaterWatch, website researchers can compare results over 30-year historical data as well as real-time information on current streamflow and drought/flooding conditions. This information includes past flow/runoff information in the form of visual maps, graphs and tables (

The northern California fires of 2017 impacted hundreds of waterways that feed downstream watersheds. Increased water quality monitoring ensued after the fires and before and after the first rainfall events to ensure typical post-fire contaminants did not exceed recommended exposure levels or impair drinking water treatment efficacy.

Practical solutions
Wildfires often lead to dramatic and severe ecological changes that can impact source water quality. These impacts, however, tend to normalize over short periods of time. Utility managers have many tools available to manage these variations in source water quality, including treatment modifications or source substitutions. Basic carbon filtration can remove many of the contaminants of concern post-fire, while more advanced technologies provide options for broader spectrum control.


  1. California Water Science Center Water Quality after a Wildfire. Available online: (accessed on Feb 13, 2018).
  2. Kearney, L. Hetch Hetchy Threatened As Rim Fire Creeps Closer. Available online: (accessed on Feb 13, 2018).
  3. Tecle, A.; Neary, D. Water Quality Impacts of Forest Fires. J. Pollut. Eff. Control 2015, 3, doi:10.4172/2375-4397.1000140.
  4. Weiser, M. Fire Retardant Use Explodes as Worries About Water, Wildlife Risk. Available online: (accessed on Feb 16, 2018).
  5. Kalabokidis, K.D. Effects of wildfire suppression chemicals on people and the environment–a review. Glob. Nest Int. J. 2000, 2, 129–137.
  6. Burton, C.A.; Hoefen, T.M.; Plumlee, G.S.; Baumberger, K.L.; Backlin, A.R.; Gallegos, E.; Fisher, R.N. Trace Elements in Stormflow, Ash, and Burned Soil following the 2009 Station Fire in Southern California. PLoS One 2016, 11, e0153372, doi:10.1371/journal.pone.0153372.
  7. Smith, H.G.; Sheridan, G.J.; Lane, P.N.J.; Nyman, P.; Haydon, S. Wildfire effects on water quality in forest catchments: A review with implications for water supply. J. Hydrol. 2011, 396, 170–192, doi:10.1016/j.jhydrol.2010.10.043.

About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]


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