The Environmental Protection Agency said Wednesday work is expected to resume at the American Tunnel Mine, one of the biggest problem areas in the Superfund site north of Silverton.

In coordination with the Bureau of Land Management, the EPA will resume work started in 2017 to stabilize and maintain access to the American Tunnel, said EPA spokesman Cynthia Peterson.

The American Tunnel, which travels about 11,000 feet, served as a transportation route for ore, as well as a drainage for mine runoff, from the vast Sunnyside Mine workings to facilities at Gladstone, north of Silverton.

When Sunnyside Mine closed for good in 1991, attention turned to what to do with acidic discharges out of the American Tunnel. Sunnyside initially pulled the water into a treatment plant but ultimately decided with the state of Colorado to install three bulkheads (essentially plugs) to stem the flow of acid drainage.

But in recent years, researchers believe the Sunnyside mine pool behind the American Tunnel has reached capacity and the water is spilling into other mine networks, such as the Gold King and Red & Bonita.

The EPA’s work this summer will include:

Installation of a flume and piping just inside the adit to maintain and measure discharge.Sump and piping outside the adit to convey drainage.Grading on the hillside and area around the portal structure to stabilize slopes.Maintenance of the settling pond and drainage outside the adit.The work may cause sediment to stir in the water being discharged from the American Tunnel, Peterson said.

In an attempt to keep the sediment out of Cement Creek, a tributary of the Animas River, the EPA will catch the drainage in existing sedimentation ponds outside the tunnel.

If the sediment ponds reach capacity, Peterson said the EPA will temporarily direct the discharged water into the temporary water-treatment plant at Gladstone, which is currently treating only mine waste out of the Gold King.

But despite these precautions, Peterson said some sediment could potentially reach Cement Creek, causing cloudiness or discoloration.

In March, the EPA ordered Sunnyside Gold to drill a well into the American Tunnel as part of a larger project to investigate groundwater at its former mine, one of the largest in the Silverton area.

Peterson said: “Sunnyside is evaluating drilling locations, and we are working with them to approve a work plan under the unilateral administrative order.”

Sunnyside Gold has challenged the order, arguing it is not responsible for anymore cleanup costs in the region.

jromeo@durangoherald.com

An earlier version of this story quoted Bill Cooper as a spokesman with the Environmental Protection Agency. Cooper is a contractor for the EPA who sends news releases on behalf of Cynthia Peterson.