SILVERTON — EPA Superfund officials trying to stop toxic mine contamination of the Animas River headwaters are preparing to close an underground dam, aiming to block a 300 gallon-per-minute discharge equal to a Gold King Mine disaster every week.

Shutting this Red and Bonita Mine bulkhead has emerged as a huge test on mountains here, where miners who penetrated fissures and groundwater pathways left behind the geologic equivalent of Swiss cheese.

Thousands of people in Colorado, New Mexico, Navajo country and Utah await action to clean up the Animas watershed. San Juan and La Plata county commissioners went to Washington last week and say Trump Administration officials promised that deep Environmental Protection Agency budget cuts won’t hit the work above Silverton.

But turning a valve and closing that bulkhead could trigger toxic leaks elsewhere, potentially spreading harm along already-contaminated headwaters. The EPA’s latest water data show widespread aluminum, cadmium, copper, iron and zinc contamination from mining and natural sources at levels too high for fish to survive.

EPA officials this week told The Denver Post — and assured local leaders — that the agency will use caution at the bulkhead, installed in 2004, and close it gradually next year while monitoring mountainsides for any new leaks. They’ve launched a data-gathering blitz, harnessing the local Mountain Studies Institute, to measure flows from dozens of mine tunnels and more than 97 mountainside seeps and springs.

“As soon as we feel we have a good handle on what the baseline is, then we’ll close the Red and Bonita bulkhead,” EPA project chief Rebecca Thomas said. “We would do it as a test initially and build up water behind the bulkhead.

“If we see a change we’re not comfortable with, if it is going to cause any further degradation of water quality, we’ll open up the bulkhead and drain it and treat it,” Thomas said in an interview.

“We need to understand how water flows before we close it. There are a lot of underground connections. Some are man-made. Some are natural,” she said. “We want to test whether or not closure of the bulkhead would help us improve water quality by stopping continued flow from the Red and Bonita.”

EPA hydrologist Ian Bowen said environmental gains could be big but emphasized unknowns. For example, the acid-metals muck draining from the Gold King, which is filtered before it mixes into the Animas, increased last year, reaching to 710 gallons per minute. The EPA can treat 1,200 gallons per minute at a plant below that mine.

Bulkheads also were installed inside Kinross Corp.’s Sunnyside Mine and American Tunnel. EPA crews plan to drill behind those bulkheads to test the pressure of pent-up mine wastewater — to make sure they will hold. American Tunnel bulkheads still leak 100 gallons a minute, and the Mogul Mine, where a bulkhead was installed in 2003, leaks 150 gallons a minute more unfiltered muck into headwaters.

Stopping the untreated Red and Bonita discharge would mark a first big fix in a Superfund cleanup following the Aug. 5, 2015, Gold King disaster, where an EPA crew accidentally triggered a 3 million gallon spill that turned the Animas mustard-yellow as it moved down the river and eventually reached the Grand Canyon of the Colorado River.

Installing more bulkheads to trap toxic mine muck inside mountains could mean taxpayers pay less for water cleaning at treatment plants built along headwaters.

Colorado “agrees with EPA’s plan,” state health department spokesman Warren Smith said.

Yet challenges loom. Mapping tunnels, fissures, seeps and springs increasingly occupies a legion of researchers and Deere & Ault engineering consultants tapped by the EPA. They anticipated in an April report that closing the Red and Bonita bulkhead would cause toxic overflows elsewhere. And EPA bureaucracy combined with uncertain funding from Congress has delayed a dozen or so other toxic mine Superfund cleanups around Colorado — let alone the tens of thousands of inactive mines contaminating water around the West.

East of Silverton, above Creede, the EPA’s Superfund cleanup of the Nelson Tunnel and various old mines — declared a national-priority disaster in 2008 — has yet to move beyond studies of tunnels and groundwater.

EPA officials on Wednesday said the agency “is evaluating the focused feasibility study for the (Creede) site and considering a range of alternatives for the proposed remedy” but that, because the EPA has not picked “a preferred alternative,” funds for cleanup aren’t available.

Southwestern Colorado leaders have invited EPA administrator Scott Pruitt to visit Silverton. Pruitt has said Americans deserve to have Superfund cleanups completed.

For years, Silverton and San Juan County officials opposed a federal cleanup of toxic mines in the Animas Basin, especially after the EPA caused the Gold King disaster. But they agreed to an EPA-run Superfund approach in 2016 after receiving assurances the cleanup would not stall.

That was locals’ greatest fear, San Juan County commissioner Scott Fetchenhier said. While Pruitt hasn’t responded to the invitation to visit, “his staff thought it was a good idea,” Fetchenhier said. “They said: ‘We have it by highest authority that Superfund cleanups are the No. 1 priority of the EPA’.”

“We took it a step further. We said we’d like our Superfund cleanup to be one of the highest priorities — we might not have endorsed that if EPA hadn’t caused the Gold King spill and put us in the national limelight,” Fetchenhier said. “The EPA does have some negative image issues from past cleanups, and this is a great opportunity for them to show times have changed.”

EPA crews seem to be working at sampling water and investigating hydrology before closure of the Red and Bonita bulkhead, said Fetchenhier, who is a geologist. “We said: ‘Before you close it, we want the data gathered on every spring, every seep, every tunnel so that you have a baseline. Anytime you put in a bulkhead, there is a chance something could come out someplace else.’ ”

EPA officials this week convened a forum in Silverton, an in-depth hydrology session with a brain trust of local scientists, mining engineers and others whose collective knowhow, federal officials said, exceeds the agency’s expertise.

Downriver in Durango, La Plata County leaders acknowledged a strong interest in stopping contamination after decades of enduring the toxic legacy of mining — because clean water is crucial for residents of Colorado and other western states.

“Obviously, sooner is good,” Commissioner Brad Blake said after a meeting with the EPA’s team. “But if you do close that bulkhead, maybe it’s going to cause a problem at another spot.”