Report: Lake CDA improving

Lake Coeur d'Alene is beginning to recover from nearly a century of mining in its watershed, according to a new report from the National Academies of Sciences, Engineering, and Medicine.

“Overall, the water quality of the lake is improving, but it is possible for this progress to be reversed," said Samuel Luoma, research ecologist with the John Muir Institute of the Environment at University of California, Davis, and chair of the committee that wrote the report.

The committee found that in general, metal concentrations are slowly declining.

"There is no evidence that phosphorus levels are increasing, that oxygen levels are declining, or that the lake has become more anoxic over time," a press release said.

A century of mining is said to have contaminated over 75 million metric tons of lake sediment with lead, cadmium, arsenic, and zinc.

Those metals remain in the lake’s sediment and waters "at much higher concentrations than in most U.S. lakes."

While much of the lake’s watershed is a Superfund site managed by the U.S. Environmental Protection Agency, the lake’s environmental quality and cleanup is overseen by a Lake Management Plan, implemented by the Coeur d’Alene Tribe and the state of Idaho.

Concerns have been raised that development along the lake’s shores and other factors could increase the level of nutrients such as phosphorus entering the lake.

It is feared that could trigger increased algae growth and result in decreased oxygen levels that "could potentially lead to the release of metals trapped in the lake’s sediment, posing a major threat to ecosystems and human health."

But the report says most data show the lake has adequate oxygen and relatively low levels of nutrients. No evidence was found that oxygen depletion is getting worse in the deeper waters of the northern part of the lake, or in the deeper waters of the southern part.

The committee examined data collected in the two major rivers that flow to the lake, and found:

• Cadmium, lead, and zinc entering the south fork of the Coeur d’Alene River have declined over the last 30 years.
• Lead entering the lake from the Coeur d’Alene River is still substantial, and was 1.3 times higher in 2020 compared with the 1990s.
• Phosphorus at monitoring sites in the Coeur d’Alene River, St. Joe River, and Spokane River below the lake outlet has declined, by about 20 percent to 30 percent from 2010 to 2020.

Regarding the lake, the committee also found:

• Zinc concentration is trending downward significantly.
• Cadmium concentrations declined from 2004 to 2020, with virtually all of the decline occurring after 2014.
• Lead concentrations rose slowly between 2003 and 2012, but have declined since then.
• Reduced pH levels in the deeper lake waters could cause the release of zinc from the lake’s contaminated sediments.
• Metal concentrations in surface waters are declining.

Even if those trends are sustained, the report said it will take waters near the bottom of the lake at least 10 years, and possibly over 100 years, to reach target metal concentrations set by the Lake Management Plan.

The report says the Lake Management Plan would benefit from sampling the lake, its rivers, and watershed more often and in more places.

"Our report details improvements that should be made to lake and watershed monitoring, so that researchers and those managing the lake have more information to work with as the uncertainties of the future unfold," Luoma said.

The study — undertaken by the Committee on the Future of Water Quality in Coeur d’Alene Lake — was sponsored by the U.S. Environmental Protection Agency, Idaho Department of Environmental Quality, and Kootenai County.