Forest fires impact on water quality

Day two of the National Academies of Sciences discussions focused on how climate change, notably forest fires, could affect the water quality of Lake Coeur d’Alene. 

Guillaume Mauger, a member of the University of Washington Climate Impacts Group, noted that “of course” global warming is “a big one” in terms of climate change impacts that are likely to affect the Coeur d’Alene Watershed. However, more noticeable is the loss of snowpack. 

“Declining snowpack is really one of the big hallmarks of climate change in our region,” Mauger said. “We’ve already seen big changes in snowpack. It’s about a 25% decline in the last 50 years, so, pretty massive.” 

Part of this is due to the increase in wetter winter weather and heavy rainstorms — a trend Mauger expects to become more “intense” in the future. There have also been significantly higher, drier summers with less precipitation. 

Those dry summers, in turn, are prime conditions for higher forest fire rates, University of Idaho Department of Soil and Water Systems Associate Professor Erin Brooks said. Forest fires have a long history across the Pacific and Inland Northwest, Brooks noted, recently mentioning the flames that carried smoke across California, Oregon, Washington, and Idaho in the late summer of 2020. 

“We’re starting to see fires in areas that we have never seen fires before,” Brooks said. 

Brooks said there are water quality consequences for areas like Seattle and Portland that get their drinking water from unfiltered reservoirs. Fires, he explained, don’t just alter the air but affect the soil. But, the loss of ground cover wildfires cause lead to extreme flooding, debris flows, and suspended sediment that catches toxic chemicals from residential and urban areas can impact aquatic habitat. 

“So it’s not a big leap to say that the susceptibility of the ground after a wildfire is highly, highly susceptible,” Brooks said.

Recent reports from 159 fires show that as a burn rages and consumes organic matter, there is a 32% increase in phosphorus concentration on average, Brooks explained. The export of phosphorus from those fires, he said, is overall about 4,000%. 

Here in Kootenai County, the likelihood of a large fire like those in California is less than 10% to 20%, Brooks said. So while it is not a notable concern, he said the probability of erosion and groundwater contamination in the area would be more significant from a post-fire rainstorm event.

Water studies also show an increase in pH levels following a burn, which can cause issues with phosphorus retention, he said. 

University of Idaho students took soil samples in the north end of the Coeur d’Alene Basin, Fernan, Wolf Lodge, and Lake Creek to look at total phosphorus concentration and landscape distribution. 

“We are able to retain a lot of phosphorus and soils in the (Coeur d’Alene) basin,” Brooks said. “Forest soils actually have higher total phosphorus than agriculture (soils). A lot of that is tied up in the organic matter.”

Brooks said eroded sediment is a concern for the Coeur d’Alene Basin as phosphorus is present in the soil. The more phosphorus the lake contains can cause increased algae and aquatic plant growth. Too much of this growth can cause eutrophication — decreased levels of oxygen — which the U.S. Environmental Protection Agency studies say can negatively impact water quality, food resources and impact other aquatic life.

Much of the mitigation efforts Brooks has seen to avoid wildfires, and the subsequent runoff of toxins compare prescribed burns and thinning to ground coverage. However, part of his research with the University of Idaho is to work with specialists around the Northwest United States. 

Some of the questions he and other specialists are posing:

Where are burns likely to occur most rapidly?

What are the most sensitive soils that will distribute phosphorus into watersheds?

How do we come up with a protection plan to mitigate that issue? 

“In this new era, the trends that are showing here (in North Idaho) are not where a wildfire is going to happen, it is when it is going to happen, and what we are going to do about it when it does,” Brooks said.