Wildfire burn scars threaten drinking water across the West

sediment may taint source water, even shut off intake valves

Many months after fires are extinguished, their effects continue to appear as rainfall have transported sediment from burned forests into sources of drinking water.

Sediment Contamination

In Colorado, mudslides choked parts of I-70 while sediment choked rivers that provide clean water to much of the state. Water in Glenwood Springs was so dark that inflow valves were closed to avoid overwhelming the filtration system. Residents were put on notice to minimize water use till the sediment, ash, and organic matter flowed further downstream.

Unseen in murky water may be viruses, parasites, bacteria and other contaminants that threaten public health. Water utilities have the major responsibility of detecting and treating these risks; experts say turbid water from burn scars is unlikely to make it to household taps. Source water can be disinfected with chemicals like chlorine. However, disinfectants themselves can interact with sediment / organic matter and create carcinogenic byproducts that are difficult and expensive to remove or treat. Operating costs for filters and chemicals can jump significantly.

Glenwood Springs Hanging Lake murky from wildfire debris flow | photo credit The Denver Post

photo of murky waters of Hanging Lake in Glenwood Springs, Colo. from 8/2021 | credit The Denver Post

Like Glenwood Springs, Greeley, Colo. experienced sediment contamination this year from 2020 burn scars that later flooded. In a rare instance, the municipal water system stopped intake from the Cache la Poudre River for 39 days. To lessen the supply shortage, the city has been trading water with a nearby agricultural company that owns and operates reservoirs for irrigation. In a uncommon arrangement, the turbid water is exchanged for cleaner reservoir water.

Intermittent water supply may arise if the intake is shut off. Water scarcity becomes more probable in burn scars. An increase frequency of droughts followed by heavy rainfall poses a threat to essential water supplies.

Proactive Preparation

Wildfires in usually wet western Oregon in 2020 surprised researchers. Now in the Cascade mountain range, the U.S. Forest Service is vigilantly monitoring burn scars above rivers and reservoirs that supply much of the state’s water. Specialists evaluate water supply risks posed by erosion and ash. Their data informs land managers and suggest tactics to reduce erosion -- thinning forests, dredging reservoirs, mulching and seeding. In some cases, forming a plan for alternative water sources is necessary.

sediment trap or sediment basin illustration by Tetra Tech for U.S. EPA and State of Kentuckey A few cities are taking proactive steps to protect watersheds and water treatment plants from sediment contamination. Helena, Mont. city and utility leaders are designing a basin to trap sediment before the water reaches intake valves. Since residents rely on surface water from the Upper Tenmile watershed, an alternative water supply is being identified in local groundwater wells. Treated water from nearby Missouri River is the capital’s current backup supply.

A silver lining to California’s drought is less sediment flow. Most burn scars are still intact, but storms can readily wash down years of debris. Unknown is the actual duration and amount of risks posed by landslides and sediment contamination.

Researchers cite that Alberta watersheds in the Canadian Rockies continued to see extremely turbid water for a decade after a 2003 fire.