Wastewater flows through a retention pond built to contain and remove heavy metals from the Gold King Mine outside Silverton, Colo., Aug. 12, 2015

Brennan Linsley/AP

The U.S. Environmental Protection Agency plans to drill a test well into an inactive mine tunnel in southwestern Colorado to get information for a Superfund cleanup, the agency said Friday.

The EPA said it will drill into the American Tunnel next month to measure water levels and investigate how the passage is connected to other shafts.

The agency is looking for ways to stop or treat contaminated water pouring into rivers from old mine sites in the Bonita Peak Superfund area north of Silverton.

The agency designated the Superfund site after it inadvertently triggered a spill while doing excavation work at the inactive Gold King Mine in August 2015. The spill released 3 million gallons of wastewater, polluting rivers in Colorado, New Mexico and Utah.

The Gold King is part of the Superfund cleanup.

The EPA said it would follow strict safety guidelines when drilling the test well into the American Tunnel.

The agency also said it has modified an order to Sunnyside Gold Corp. to pay for some of the cleanup investigation, but no details of the changes were released.

Sunnyside owns property in the area, and the EPA wants the company to help pay for the cleanup. Sunnyside says it's not responsible for the pollution and shouldn't have to pay.

Sunnyside reclamation director Kevin Roach said the company is reviewing the revised order.