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SILVERTON – Deep down, hundreds of feet below the Earth’s surface, the Environmental Protection Agency is looking for answers to one of the biggest mysteries in the Superfund site of mines around Silverton polluting the headwaters of the Animas River.

EPA crews last week started to bore into the ground in what is expected to be a more than 500-foot journey to reach the American Tunnel in hopes of better understanding a complex network of mines in the upper Cement Creek basin, a tributary of the Animas.

It’s these mines that are considered the worst polluters of heavy metals seeping into the Animas River.

“This is the first piece of the puzzle,” said Kerry Guy, an emergency response on-scene coordinator with the EPA.

The American Tunnel was first started in 1900 beneath the Gold King Mine but was never finished.

In 1959, however, when Standard Metals announced it was going to reopen the Sunnyside Mine, the now-defunct company also said it was going to extend the American Tunnel from the vast mine network to Gladstone, an old mining community about 10 miles north of Silverton.

Extending the tunnel solved two costly problems for previous mining companies: It allowed for ore to be easily taken out for further processing, and it created a better system for groundwater to exit the mine.

The move led to a three-decade period of prosperity, said Bev Rich, Silverton native and director of the San Juan County Historical Society.

“They discovered some really good gold, and a good reserve of it,” she said.

In 1991, however, the Sunnyside Mine, which had been taken over by Sunnyside Gold Corp., closed as a result of depressed gold prices. What was left behind, in terms of the American Tunnel, was a never-ending pathway for acidic discharges.

Sunnyside Gold initially pulled water coming out of the American Tunnel into a treatment plant, a costly yet effective method that took metals out of Cement Creek and greatly improved the quality of the Animas River.

But, in a move hoping to end its financial involvement in the Animas River basin, Sunnyside Gold entered an agreement in 1996 with the state of Colorado to shut down the treatment plant and instead install three bulkheads that essentially function as plugs to stem the acidic flow.

By 2001, though, it was thought the water had backed up and reached capacity within the Sunnyside Mine network, which has led some researchers and experts familiar with the basin to believe that water is spilling out into adjacent mines, like the Gold King.

Sunnyside Gold, which was purchased by international mining conglomerate Kinross Gold Corp. in 2003, has adamantly denied that its mine pool is the cause of discharge from other mines, saying there is no factual evidence for the assertion.

Kevin Roach, with Sunnyside Gold, wrote in an email to The Durango Herald that it was expected, including by the regulators who approved the action, that bulkheading would cause the water table to return to natural levels in the mountain and that water would discharge at some points.

“However, water from the interior workings of the Sunnyside Mine is not discharging from these points,” he wrote. “The bulkheads successfully isolated the water in the interior workings of the Sunnyside Mine, resulting in improved water quality in the Animas River.”

Peter Butler, with the Animas River Stakeholders Group, said it’s likely the situation is much more complicated.

Historic records show the Gold King Mine, for instance, experienced discharges before the American Tunnel was drilled. After the tunnel was drilled, the Gold King went dry, leading some to believe water was finding an easier way out. And, once again, after the tunnel was plugged, the Gold King started to leak mine waste.

“Did the bulkheads in the tunnel change the groundwater hydrology? Yes,” Butler said. “But is the mine pool water affecting those mines or did you just back up the water table and it’s going somewhere else?”

One thing is clear: After the Bonita Peak Mining District Superfund site was officially listed in fall 2016, EPA made it a priority to figure out what was happening with water movement underground. This past winter, a helicopter carrying an electromagnetic mapping device made the rounds around Silverton to try to understand the geological makeup of the San Juan Mountains, and hopefully, its groundwater workings.

This desire to know what lies beneath is what ultimately led to EPA drilling into the American Tunnel.

Rory Cowie, a hydrologist with Mountain Studies Institute, said pressure valves were not installed when the bulkheads where put in during the 1990s, and as a result, there’s no way to know just how much water is backed up behind the plugs.

“Right now, they have no idea how much it’s holding back,” he said. “And there’s no way to get that information, short of putting wells in the mine tunnel.”

Guy, with the EPA, said it could take almost a month to reach the American Tunnel, boring through 20 to 30 feet of hard rock per day. The intent is to reach a portion of the tunnel between bulkheads 2 and 3, but it’s going to take more wells and more research to form a better grasp on how water moves underground in this geological puzzle.

Butler said the project plays into the larger question surrounding the Bonita Peak Superfund site: What is the ultimate strategy to fix issues in the upper Cement Creek area? EPA, for its part, has said that question warrants further investigation and time before being answered.

“There’s a real question, and a real debate, whether to spend an enormous amount of money to make a hydrological model … or spend that money on building a better treatment plant,” Butler said. “I think there’s a real debate within EPA on what’s the best route to go.”

Cowie, for his part, said EPA’s studies are necessary so the agency can come up with and decide on the best solution possible. And that may take time.

“Miners spent 80 years digging holes through that mountain, and it’s been sitting still for 50 years in different states of degradation,” he said. “People can’t expect it to be fixed in two or three years. It’s a long process. That’s just the reality.”

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