The Environmental Protection Agency will temporarily close a bulkhead at one of the worst polluting mines near Silverton as an experiment to better understand hydrology in the region.

Bulkheads are essentially plugs placed at the entrance of a mine to hold back water that’s usually laced with heavy metals.

In 2015, a bulkhead was installed at the Red & Bonita Mine, one of the worst loaders of heavy metals located about 8 miles north of Silverton up Cement Creek, a tributary of the Animas River.

Later that summer, however, an EPA-contracted crew dug too deeply into a pile of rock and dirt that had collapsed over the entrance of the Gold King Mine, releasing about 3 million gallons of mine wastewater into the Animas River.

In fall 2016, the Bonita Peak Mining District Superfund site was declared.

As a result, plans to close the valve to the bulkhead at the Red & Bonita Mine were delayed, that is, until this year, said Ian Bowen, a hydro-geologist for the EPA.

Bowen said the EPA spent the last few years fine-tuning the plan for closing the bulkhead, installing numerous wells and monitoring zones to have a firm grasp on what impact the closure will have on surrounding mines and springs.

“It’s been a long run from 2015 to now,” Bowen said. “But we really think we’re in a good spot to both monitor any potential effects from the bulkhead closure, and we’re ready to test the bulkhead itself.”

The network of mines around Cement Creek is considered the most vexing polluter into the Animas River, which includes the Gold King, Red & Bonita and Mogul mines, as well as the American Tunnel and the vast Sunnyside Mine pool.

By plugging the Red & Bonita, the EPA can monitor to see if other mines start discharging more water or if nearby seeps increase flows. That way, the agency can get a better understanding of how groundwater moves in the mountains.

“One of our biggest needs is to evaluate the potential connection with the Sunnyside Mine pool,” Bowen said.

James Hou, a remedial project manager with EPA, said the agency will close the valve July 13, and through a phased approach, start raising the mine pool behind the Red & Bonita bulkhead over a four-week period.

As water fills up, crews will be dispatched weekly around a carefully sketched out area that includes 35 sampling sites to see if water is seeping out.

The water will then be drained around mid-September for about five weeks, and treated at the EPA’s temporary water treatment plant, which only takes in discharges from the Gold King Mine at this time. After the mine pool is drained, discharges out of the Red & Bonita will be returned untreated to Cement Creek.

What’s learned this summer, Hou said, will also help inform the EPA about whether a permanent closure of the bulkhead at the Red & Bonita is a possible long-term treatment plan for the mine.

“Maybe this is an option we’d like to employ,” he said. “But that will take at least a year to evaluate.”

Asked if one summer season is sufficient to have enough data to make that decision, Bowen said future monitoring projects are possible, but not planned at this time.

“We might take a longer crack at it,” he said. “But that decision will be informed after the data we obtain this year.”

Peter Butler, chairman of the Bonita Peak Mining District Community Advisory Group, said critics of bulkheads say plugging a mine just causes water to seep out in other areas of the watershed.

But Butler said bulkheads can force water to take paths with less mineralized rock, and it can limit the interaction between water and oxygen, which causes water to become acidic and potentially toxic.

“It really varies on location,” Butler said of the success of bulkheads. “Some are really successful, some are not.”

Scott Fetchenheir, a San Juan County commissioner and former miner, said a local task force that advises EPA on Superfund matters is supportive of closing the Red & Bonita bulkhead.

Originally, Fetchenheir’s main concern was having a good database of surrounding seeps and springs, and he said it appears the EPA, as well as Sunnyside Gold Corp., have accomplished that inventory.

Closing the bulkhead, he said, holds virtually zero risk, while at the same time, allows for a potential better understanding of how groundwater moves in one of the most complicated mining areas around Silverton.

And, Fetchenheir said it could lead to the permanent closure of the mine, which is a low-cost treatment option.

“I think it’s a great experiment,” he said. “This (mine) is one of the bad boys, as far as zinc loading, and if you can cut zinc by 20 to 30%, you’ve accomplished a lot while not having to treat the water (in perpetuity).”

jromeo@durangoherald.com