DURANGO – The Environmental Protection Agency plans to temporarily seal a bulkhead at one of the most polluted mines in a Colorado region as part of a study of the region’s water.

The EPA plans to close a bulkhead valve at the Red & Bonita Mine, The Durango Herald reported Tuesday.

Bulkheads are placed at mine entrances to hold back water that is usually laced with heavy metals.

A bulkhead was installed in 2015 at the mine 8 miles north of Silverton along Cement Creek, an Animas River tributary.

The accidental release of about 3 million gallons of mine wastewater into the river by a work crew resulted in the EPA establishing a Bonita Peak Mining District Superfund site in 2016.

The declaration of the Superfund site delayed the closure of the Red & Bonita valve until now, EPA hydro-geologist Ian Bowen said.

Bowen said the EPA spent the last few years finalizing plans to close the bulkhead, installing numerous wells and monitoring zones to understand the impact of the closure on surrounding mines and springs.

By plugging the Red & Bonita mine, the EPA can monitor whether other mines begin discharging more water or if groundwater flow increases in the area.