BOISE, Idaho — Water quality in the Coeur d'Alene and Spokane river basins in northern Idaho and eastern Washington state is improving due to ongoing efforts to clean up one of the nation's largest Superfund sites, scientists say, a federal agency reported Monday.
The U.S. Geological Survey said concentrations of cadmium, lead and zinc decreased significantly since the U.S. Environmental Protection Agency began the cleanup in the 1990s.
"I was really pleased when I started looking at the data," said USGS hydrologist Greg Clark. "We just saw these dramatic decreases."
One of the major findings in the report that analyzed years of data is that between 1992 and 2013 the concentration of cadmium, lead and zinc decreased about 65 percent in the South Fork Coeur d'Alene River near Pinehurst.
"We still have a long way to go in our cleanup efforts, but it's nice to have scientific confirmation that we've made solid, measurable progress in reducing metal loads and improving area water quality," Rick Albright, EPA Supurfund cleanup director in Seattle, said in a statement.
Mining mainly for silver in northern Idaho starting more than a century ago — before pollution controls were put in place — resulted in millions of tons of ore and mine tailings. But those tailings, dumped along or even into streams up to 1968, still contain metals.
Over the years the metals continue to be transported downstream, with some of the material extending into Washington state. While the amount has decreased, the study also found that the concentration in some streams is above what's considered toxic to aquatic organisms.
It also found that Coeur d'Alene Lake annually receives about 5 tons of cadmium, 400 tons of lead and 700 tons of zinc. Some of that continues downstream into Washington.
"A lot of the trace metals are settling on the bottom of the lake," said Clark, noting it likely becomes trapped there but is also harmful to aquatic organisms.
The study found that the rate of decrease in metal concentrations in streams has slowed since 2003. Clark attributed that to workers taking care of some of the easier problems, such as capping or moving mine tailings so that water can't wash through and move the toxic metals into streams.
He said more difficult steps include doing work in sub-basins that are harder to reach to take care of mine tailings that continue to leach toxic metals. An even bigger project, he said, is building extraction wells near Kellogg in northern Idaho to prevent contaminated ground water from leaking into the river.
"You're never going to get back to pristine conditions," Clark said. "There was just so much spoil material (mine tailings) dumped into that valley that it's just a huge job."