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Lake CdA’s declining oxygen levels worry scientists

A jumper hits the chilly green water after leaping off the rocks of Tubbs Hill on Lake Coeur d'Alene in 2014.  (Jesse Tinsley / The Spokesman-Review)
A jumper hits the chilly green water after leaping off the rocks of Tubbs Hill on Lake Coeur d'Alene in 2014. (Jesse Tinsley / The Spokesman-Review)

Oxygen levels are declining in Lake Coeur d’Alene, raising concerns about the heavy metals buried in the mud at the bottom of the lake.

The lake is a repository for 75 million tons of sediment polluted with lead and other heavy metals that washed downstream from more than a century of mining activity in Idaho’s Silver Valley.

For 25 years, scientists have monitored the lake’s water chemistry, charting trends in oxygen levels that act as a cap on the metals.

“Deep down here at the bottom of the lake, the oxygen levels are decreasing. The changes we’re seeing are subtle and slow, but they are real,” said Craig Cooper, a lake scientist for the Idaho Department of Environmental Quality.

“Oxygen levels are going in a direction we don’t want to see them go,” he told a crowd at Tuesday’s Our Gem Symposium, a conference about Lake Coeur d’Alene’s water quality. “This is not opinion. This is fact, this is math.”

Cooper said the trend of lower oxygen levels is most pronounced at monitoring sites on the north and middle parts of the lake, and the oxygen levels are lowest during the summer and fall, when the lake is most productive biologically.

Scientists are also monitoring a 50-foot-deep hole near the lake’s outlet to the Spokane River, where oxygen levels already drop to zero during the summer. At the site, lead and other metals are being released into the water column.

The monitoring results should be a wake-up call for everyone who loves the lake, said Phil Cernera, director of the Coeur d’Alene Tribe’s lake department.

For years, people have known that phosphorus and other nutrients in the lake contribute to reduced oxygen levels, increasing the threat of the metals becoming re-suspended in the water, Cernera said. But action to protect the lake’s water quality hasn’t occurred, he said.

“Nobody thinks this will happen to Lake Coeur d’Alene. But it could. Nobody thinks this will happen in our lifetime. But it could,” Cernera said. “That’s the siren call I want to put out to folks.”

Water quality is more than an esoteric consideration, said Mark Solomon, interim director of the Idaho Water Resources Research Institute.

A recent University of Idaho study documented a correlation between water clarity, milfoil and lakefront home values. It looked at 614 waterfront sales between 2010 and 2014.

Resale values were $27,000 higher where people could see to depths of 16 feet in Lake Coeur d’Alene. Property values declined by an average of $65,000 where invasive milfoil was present.

“If we don’t take care of the lake, there will be a direct effect on property values and our ability to enjoy the lake,” Solomon said. “It’s not a disconnected issue.”

The lake’s water chemistry is complicated, but when phosphorus and other nutrients enter the lake, they spur plant growth, said Cooper, the DEQ scientist. When the plants die, they fall to the bottom of the lake, where bacteria feed on them. The process depletes oxygen levels in the lake bottom.

The Coeur d’Alene Tribe and DEQ are working together to quantify how nutrients are getting into the lake. Erosion from streams, septic tanks, manure from livestock and runoff from residential developments are some of the contributors.

The work will be out later this year, and it will help the agencies prioritize nutrient reduction projects, Cooper said.

“We want to protect the lake by limiting the nutrients that reduce oxygen,” he said.

Anything involving land use is likely to be controversial, he acknowledged.

Last year, Kootenai County commissioners considered relaxing shoreline regulations, allowing Lake Coeur d’Alene property owners more flexibility to clear native trees and shrubs and plant lawns near the water. The current proposal, which commissioners will vote on next month, calls for keeping a 25-foot buffer of native vegetation, while giving landowners more flexibility to work in the buffer zone.

While DEQ and the tribe are responsible for water quality in Lake Coeur d’Alene, they don’t have authority over land-use regulations that affect water quality, officials noted.

The nutrient work needs to happen quickly, Cernera said. In three years, Superfund cleanup in the Silver Valley will reduce the volume of zinc flowing down the Coeur d’Alene River and into the lake.

The present zinc levels violate water quality standards, but they also act like “chlorine in a swimming pool” to inhibit plant growth in the lake, said Scott Fields, the Coeur d’Alene Tribe’s water resources manager. Without the high zinc loads, the lake will become more productive, lowering oxygen levels.

“We’re concerned about what will happen if the pool turns a little green,” Fields said.