WASHINGTON — Coeur d’Alene Lake in northern Idaho is beginning to recover from nearly a century of mining in its watershed, says a new report from the National Academies of Sciences, Engineering, and Medicine. An original data analysis by the committee that wrote the report finds 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. However, an immense reservoir of metals, including lead, zinc, and cadmium, remain in the lake’s sediment, and future population growth, climate change, and other factors have the potential to slow or even reverse progress.

Coeur d’Alene Lake is an invaluable natural, recreational, and economic resource for communities in Idaho and eastern Washington. Starting in the late 1880s, areas in the lake’s watershed were mined for lead, silver, and zinc. Waste from this mining, laden with heavy metals, flows downstream and has contaminated over 75 million metric tons of lake sediment with lead, cadmium, arsenic, and zinc for nearly a century. These metals remain in the lake’s sediment and waters today, at or above ambient water quality standards and 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. In recent years, concerns have been raised that development along the lake’s shores and other factors may increase the level of nutrients such as phosphorus entering the lake, triggering increased algae growth and resulting in decreased oxygen levels or anoxia in the lake. Anoxic conditions could potentially lead to the release of metals trapped in the lake’s sediment, posing a major threat to ecosystems and human health.

“We found no evidence that the main body of Coeur d’Alene Lake is becoming more anoxic over time,” 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. “Overall, the water quality of the lake is improving, but it is possible for this progress to be reversed. 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.”

Lake Anoxia and Nutrient Levels

The report says most data show the lake is oligotrophic, meaning it 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, where anoxia does sometimes occur. Data from bays and shoreline waters of the lake, where signs of problems typically associated with population growth might first appear, are lacking.

There is no evidence that phosphorus concentrations in the main body of the lake are increasing. In the last decade, phosphorus levels in the lake’s surface waters have declined, as have phosphorus levels in the two major rivers entering the lake. The committee concluded that projecting future phosphorus levels in the lake is not feasible without developing a better understanding of the processes by which phosphorus moves from the watershed to the lake and of the lake’s nitrogen cycle.

The committee did not find evidence that zinc is suppressing levels of chlorophyll a in the lake. As zinc levels have decreased following Superfund remediation, concerns have been raised that algae growth might accelerate. The report says available field data do not support this concern, and recommends further research in this area.

Metals in the Lake and Watershed

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, and Superfund activities in former mining areas have likely contributed to this decline.
• 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. This is mainly due to physical processes in the lower basin that released lead between 2000 and 2020.
• 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.

Examining data from the lake itself, the committee found:

• Zinc concentration in the lake 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. Anoxia in those sediments could result in a release of arsenic and perhaps phosphorus.
• Metal concentrations in surface waters are declining. However, even if present trends are sustained, 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.

Climate Change

The report says climate change has the potential to slow or reverse progress made in the lake’s improvement in recent decades, but it recognizes the uncertainty in predicting specific climate change effects, especially for this region. For example, an increase in extreme precipitation events could result in more frequent and larger runoff events. Although this precipitation pattern has not been observed in the last 30 years in this region, such a shift could change how metals in former mining areas and the lake’s watershed enter the lake. The temperature of the lake’s surface waters are increasing, and the committee concluded these temperature changes could ultimately lead to longer periods of low oxygen levels near sediment and lower pH, increasing the potential for metals to be released from sediment. Monitoring, data synthesis, better collaboration among those monitoring the lake and watershed, and further study will be important in anticipating and managing future water quality changes.

Improvements to Lake and Watershed Monitoring

The report says the Lake Management Plan would benefit from sampling the lake, its rivers, and watershed more often and in more places. Sampling near the shoreline where the public uses the lake could create an early warning system for harmful or nuisance algal blooms and provide data for other uses. Deploying sensors could allow researchers to measure physical, chemical, and biological properties of the lake and watershed more often. Expanding ecological monitoring beyond phytoplankton could help researchers learn more about how ecosystems are responding to changes in metal and nutrient levels.

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, Idaho.

The National Academies of Sciences, Engineering, and Medicine are private, nonprofit institutions that provide independent, objective analysis and advice to the nation to solve complex problems and inform public policy decisions related to science, technology, and medicine. They operate under an 1863 congressional charter to the National Academy of Sciences, signed by President Lincoln.

Contact:
Megan Lowry, Media Officer
Office of News and Public Information
202-334-2138; e-mail news@nas.edu