Phosphorus numbers troubling
Staff Writer | May 5, 2021 1:00 AM
In the second series of the National Academies of Sciences discussions, water quality and environmental specialists took a closer look at the 200% increase in Lake Coeur d'Alene's phosphorus levels.
The two-year, $770,000 state-directed water quality study by the NAS and national, state, and local groups was created late last year to take an in-depth look at the health of Lake Coeur d'Alene.
Craig Cooper, a limnologist with the IDEQ, kicked off the three-day sessions with an overview of the sources and trends contributing to the phosphorus inventory in the Coeur d'Alene Basin.
According to the EPA, high concentrations of phosphorus can cause increased algae and aquatic plant growth — which can cause eutrophication or decreased levels of oxygen. These produce algae blooms which negatively "impact water quality, food resources, and habitats and reduce the oxygen that fish and other aquatic life need to survive."
The basin, which spans about 3,700 square miles, connects the two major rivers that create the heart of Kootenai County — the Coeur d'Alene and St. Joe rivers. Added into the mix are roughly 450 square miles of ungauged tributaries that drain directly into the lake.
Breaking down a study by the IDEQ, Cooper explained that roughly 70% of the phosphorus load from the rivers, tributaries, and other input sources is retained in the lake. The two rivers alone, Cooper noted, provide between 75 to 80 tons of phosphorus each per year — making up 85.5% of the total 180 tons deposited annually.
Based on watershed monitoring data by the U.S. Geological Services and the U.S. Environmental Protection Agency, the North Fork of the Coeur d'Alene River — the largest watershed in the system — produced the highest flow of phosphorus. This wasn't surprising to Cooper, who pointed out that "larger watersheds equals more flow equals more load," but there was one exception.
"There is a relatively high load coming from the small section between Kellogg and Smelterville, which is adjacent to the Bunker Hill Central Impoundment Area," Cooper said.
Looking at overall yield, Cooper said the North Fork of the Coeur d'Alene River has a relatively low phosphorus content. The highest output, he noted, comes from the Silver Valley between Kellogg and Pinehurst.
"These three sections between Kellogg, Smelterville, and Pinehurst provide about 11 to 25 tons per year or 15% to 35% of the total load for the Coeur d'Alene River," Cooper said.
Cooper said the watershed covering that approximately two-mile strip from Kellogg to Smelterville is a critical groundwater source. It is important, the expert noted, because it pulls nutrients from around the Bunker Hill Central Impoundment Area — which is currently being moderated through an expanded central treatment plant and groundwater capture system.
As mentioned in the last NAS Committee on the Future of Water Quality in Lake Coeur d'Alene meetings, the Bunker Hill Mine and Smelting Complex in Kellogg released thousands of toxic elements into water runoff, seeped into the Lake Coeur d'Alene tributaries. The adverse effects of these elements became nationally recognized in the 2000s leading to its Superfund site designation.
"Phosphorus is increasing in the northern lake, north of the Coeur d'Alene River, at approximately 0.2 to 0.4 micrograms per liter per year," Cooper said. "We know that reduced metals alone can influence a lake's internal phosphorus cycle, and perhaps more coming into the lake is now staying in the ecosystem and not going into the sediment."
Comparing the annual average discharge from two studies, one documented between 1991 and 1992, the other from 2004 to 2013, watershed load more than doubled. In the 1990s study, the St. Joe River Basin reported an annual average load of 30 tons — 163.3% less than the 2000s study total of 79.
The Coeur d'Alene River Basin's annual average load grew by 200%, from 25 in the 90s to 75 in the 2000s.
"That is remarkable change and loading over 25 years, and it is definitely a cause for concern," Cooper said.
Changes Cooper said specialists suspect could impact phosphorus are:
• changes in soil chemistry
• land use
• climate-related changes
• large flood events
• hazardous metal deposits
Moving forward, Cooper said IDEQ plans to continue a multiyear study with the EPA and the Coeur d'Alene Tribe to develop an action plan for phosphorus management on the Coeur d'Alene River.