Water Quality After Wildfire

07.08.2021
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Wildfires are devastating and can lead to long-term changes to the landscape. With a significant amount of our Nation’s drinking water sources originating in forested watersheds, evaluating the effect fires have on water quality is incredibly important. The USGS works with other federal and state land managers to assess water-quality after wildfires in order to help protect one of our most valuable resources.

BACKGROUND

Relatively new research has been aimed at the impacts of wildfires on the quality of drinking water and on water-dependent natural resources. In the last several decades, both the incidence of large wildfires and the duration of the wildfire season across much of the United States have increased. Approximately 80 percent of the U.S.'s freshwater resource originates on forested land, and more than 3,400 communities rely on public drinking-water systems located in watersheds on forest lands. Thus, the potential impacts from wildfire occurrence on the quantity and quality of runoff used for source water and to support fisheries and aquatic habitats are considerable. For example, water providers spent more than $26 million on water-quality treatment, sediment and debris removal, and related issues after two recent wildfires in Colorado.¹

Pre- and post-fire measurements of fire effects help ecologists, fire scientists, and managers determine how the severity of wildfires affects plants, animal habitat, and ecosystem services. (Credit: Rachel Loehman)

SOURCE WATER

Wildfire affects many facets of the source-water delivery system, ranging from immediate effects during a fire to long-term alteration of watersheds. During a fire, interruption of electrical power and access to water treatment plants, ambient water-quality monitoring equipment, and stream diversion and monitoring locations are common. Because existing water quality cannot be adequately determined, source-water suppliers are often forced to shift to stored water or other secondary water supplies. These sources tend to be of lower quality, which can necessitate increased pre-treatment actions and processes at considerable additional cost. Runoff from burned areas contains ash, which may have significant effects on the chemistry of receiving waters such as lakes, wetlands, reservoirs, rivers and. Runoff from burned areas also produces higher nitrate, organic carbon, and sediment levels, warmer temperatures, and flashier streamflows.

IMPLICATIONS FOR DRINKING-WATER TREATMENT

Drinking-water utilities strive to provide safe drinking water for their communities. Unfortunately, the unpredictable nature of wildfire makes it challenging to develop treatment-plant-specific strategies for treating source water degraded by the effects of wildfire. High-intensity rainfall events in steep, burned watersheds are likely to move large amounts of suspended and dissolved material into downstream water supplies. Potential effects of wildfire on municipal water supplies and downstream aquatic ecosystems include the following:

Changes in the magnitude and timing of snowmelt runoff, which influence filling of water-supply reservoirs
Increased sediment loading of water-supply reservoirs, shortened reservoir lifetime, and increased maintenance costs
Increased loading of streams with nutrients, dissolved organic carbon, major ions, and metals
Post-fire erosion and transport of sediment and debris to downstream water-treatment plants, water-supply reservoirs, and aquatic ecosystems
Increased turbidity (cloudiness caused by suspended material), or heightened iron and manganese concentrations, which may increase chemical treatment requirements and produce larger volumes of sludge, both of which would raise operating costs
Changes in source-water chemistry that can alter drinking-water treatment

A FUTURE OF CLIMATE EXTREMES

Wildfires are some of the world’s most deadly and expensive hazards. These events are incredibly distructive and are predicted to become bigger and more common in the future as extreme climate events become more frequent. Despite their critical importance, wildfires and their impacts on watersheds are challenging to study due to their unpredictability, the extensive damage they cause, and a lack of pre-existing information to which the response can be compared. What is know about them is they alter the surface soils, decrease infiltration during thunderstorms, and accelerate the movement of water, sediment, and debris from the landscape to nearby streams. This has a definitive impact on downstream water quality.

RELATED USGS RESEARCH

The USGS monitors water-quality and sediment characteristics in rivers and reservoirs affected by fires. These data helps scientists and water managers evaluate the cumulative effects of fire, and potential water-quality impacts on drinking-water supplies. Years after a wildfire, high-intensity storms can affect water quality. Monitoring source water downstream of burned watersheds allows water managers to minimize adverse effects on drinking-water quality, such as by temporarily diverting compromised water or changing source water.