SPOKANE, Wash. — Motorists traveling through northern Idaho’s Silver Valley will see the signs of a $48 million federal project to keep heavy metals out of the South Fork of the Coeur d’Alene River.

The work to trap and divert polluted groundwater to a wastewater treatment plant gets underway this spring and will last through 2021, The Spokesman-Review reported last week. Contractors will dig ditches to install an 8,000-foot-long underground clay barrier so the polluted water can be piped to the plant. The work will leave large dirt piles next to Interstate 90 at times.

“This is the big crunch year,” said Rod Zion, a project manager for the U.S. Army Corps of Engineers. “We’ll see a lot of work going on out there.”

It’s all part of an effort to get cleaner water in the South Fork and ultimately Lake Coeur d’Alene. Groundwater in the region becomes polluted as it percolates through historical mine waste in the Silver Valley.

Mining left behind lead, arsenic, cadmium and zinc, polluting 160 miles of the Coeur d’Alene River and water downstream such as Lake Coeur d’Alene and the Spokane River, which stretches into Washington state.

Much of the zinc comes from old workings of the Bunker Hill Mine and Smelter complex and the Central Impoundment Area, a repository that holds 20 million cubic yards of mine waste.

“Zinc is an eco-toxin. It kills aquatic life,” said Ed Moreen, a U.S. Environmental Protection Agency manager. “It’s extremely toxic to the benthic community — the bugs that fish eat.”

Removing the zinc will help the ecosystem, allowing aquatic insects to rebound and support stronger populations of cutthroat trout in the river, Moreen said.

A four-year contract to design and build the treatment plant was awarded last year to AMEC Foster Wheeler Environmental & Infrastructure of Pennsylvania. The company will operate the new treatment facility for a year. The state will take over in 2021 and has set up an endowment to run it.

Bunker Hill Mine built an existing plant in 1974 to treat water from the Kellogg mine.

The new plant will be much more efficient, Moreen said. The clay barrier to keep heavy metals out of the South Fork of the Coeur d’Alene River will extend 25 feet deep in places.

The money for the contract comes from a $263 million settlement that Hecla Mining Co. reached with the U.S. Justice Department, the Coeur d’Alene Tribe and Idaho over its past releases of mining pollution in the Coeur d’Alene basin. The 2011 settlement resolved one of the nation’s largest Superfund lawsuits.

Information from: The Spokesman-Review, http://www.spokesman.com