COEUR D'ALENE, Idaho (AP) — Scientists in northern Idaho say declining levels of oxygen in Lake Coeur d'Alene raise concerns about toxic heavy metals buried in the lake bottom getting free and becoming suspended in the water column.

Craig Cooper of the Idaho Department of Environmental Quality says oxygen levels are decreasing at the bottom of the lake. Scientists say oxygen acts as a cap on the toxic metals.

The lake holds about 75 million tons of sediment polluted with lead and other heavy metals washed downstream from more than a century of mining in Idaho's Silver Valley. The Spokesman-Review reports that Cooper spoke Tuesday at a conference about Lake Coeur d'Alene's water quality.