Lake Cd'A oxygen levels may decline

COEUR d’ALENE — Scientists laid out new evidence that indicates dissolved oxygen levels in Lake Coeur d’Alene could be declining over time, which could eventually start releasing heavy metals trapped in the sediment.

They have also found evidence of increased nutrients making their way into the lake, which could speed up the oxygen depletion over time.

Dr. Craig Cooper, a limnologist with the Idaho Department of Environmental Quality, presented some of his findings at the Spokane River Forum Wednesday afternoon.

Cooper said the state has been analyzing water samples from the 1990s to present and found evidence of declining oxygen levels and increasing oxygen demands in deeper waters of Lake Coeur d’Alene.

“There is evidence of a long-term decline in the hypolimnion layer at the north end of the lake,” Cooper said, adding it could take decades before that becomes problematic. The hypolimnion layer is the bottom strata of the lake where very little mixing of the water occurs.

The problem, he said, is when the oxygen levels fall below six milligrams per liter, a complex chemical process occurs at the bottom of the lake which could cause some of the heavy metals trapped in the sediment to dissolve and release into the water.

He said with rising levels of nutrients, such as phosphorus, fertilizing the plant life in the lake, the oxygen levels will be depleted sooner as those plants eventually die and decompose.

Compounding that is the fact that the zinc levels in Lake Coeur d’Alene are expected to drop significantly due to Superfund cleanup activities upstream and that could further promote plant life in the lake.

“Zinc inhibits the growth of phytoplankton,” said Ben Scofield, a water resource specialist for the Coeur d’Alene Tribe.

Scofield said increased nutrients and decreased zinc could speed up the production of phytoplankton, which will create an even stronger demand for oxygen in the lake.

Scofield was presenting the findings of Dale Chess, the Tribe’s limnologist and chief research biologist. He said the Tribe is still working on developing a model that can simulate the stratification of the lake during the summer months.

When the Post Falls dam is closed to hold the lake levels up for recreation in the summer, the lake begins to stratify. The top layers of the lake begin to warm up but the bottom layers stay much colder and don’t get as much aeration.

Scofield said Chess is planning to model the impacts of climate change on the stratification process.

Both the Tribe and state are working together on the lake management plan that is designed to keep the metals in the lake stable.

Meanwhile the state is starting the process of setting Total Maximum Daily Loads, or TMDLs, for heavy metals in the Spokane River. Bob Steed, a surface water ecologist with IDEQ, has been testing the river at two points for heavy metals to determine where they may be entering the system.

Steed said the river exceeds the state water quality standards for heavy metals, which were listed as contaminants of concern in 1994. Under the Clean Water Act the state must now set the total amount of heavy metals that are allowed to be in system and set the amount wastewater dischargers can add to the river.

Steed set up two testing location at the mouth of the Spokane River and another test station where the river crosses the state line.

He said zinc is the largest loader in the system, but most of that is coming down the Coeur d’Alene River and into the lake.

Steed said his testing indicates that while the wastewater dischargers are contributing to the heavy metal levels in the river, the amount they contribute is miniscule compared to other sources.

Dan Redline, Coeur d'Alene Regional Office Administrator, said the TMDLs are necessary to give the dischargers an idea of how much they are allowed to discharge into the river.

He said the latest discharge permits that were issued to the cities of Kootenai County were delayed in part because the state has yet to set TMDLs for heavy metals.