Two mining companies have agreed to plant one million trees on barren Silver Valley slopes as part of a plan to stabilize soil contaminated by heavy metals.

The Hecla Mining Co. and Gulf Resources and Chemical Co. agreed to carry out the huge erosion-control project, officials with the U.S. Environmental Protection Agency said this week.The five-year project, which could cost $5 million, is part of a larger plan to clean up the Bunker Hill Superfund site, a 21-square-mile area that ranks as one of the nation's largest areas of industrial pollution.

The work is intended to reduce the amount of contaminated dust that blows from the hillsides and sediment that rain and melting snow carry into streams.

"The ugliness is certainly a nice thing to resolve," EPA project manager Nick Ceto said. "But the reason we're doing this is for the environmental benefits of sediment control and dirt control."

Starting next year, seedlings will be planted on 3,100 acres of hillside that were left barren by heavy metals and chemical contamination from decades of mining and smelting operations. That inhibited plant growth, making it difficult for trees to recover from logging and fires.

Most of the planting will be done on the south side of the valley, where the defunct Bunker Hill smelter operated. It is also where the hills are most barren and the soil is the most contaminated, Ceto said.

The companies also will build sediment catch basins in streams; regrade and plant grasses in 22 major gullies; rebuild terraces in the hillside to slow soil erosion; and plant shrubs and grass behind homes in Smelterville and Wardner.