When the U.S. Environmental Protection Agency and its contractors excavated the Gold King Mine on Aug. 5, 2015, crews were attempting to assess water releases from the mine with the aim of guiding future mine remediation.

Instead, they struck rock and soil plugging the mine, releasing more than 3 million gallons of water laden with heavy metals such as lead, arsenic and cadmium into Cement Creek north of Silverton and eventually the Animas River.

Much of the subsequent focus was on the culpability of the EPA and its contractors, but much less attention was paid to the bulkheads that a federal investigation by the U.S. Bureau of Reclamation would later fault for partly causing the release.

Bulkheads remain relatively obscure except to those involved in mine remediation, but their purpose is to plug mines and limit the release of mine waste while reversing the chemical processes that contribute to acid mine drainage. They can be simple fixes for extraordinarily complex mining systems and produce unintended consequences. But they are also a critical tool for the EPA and those working to improve water quality and reduce the lingering effects of more than a century of mining in the Bonita Peak Mining District.

“They have great value and perform a really important function in mine remediation,” said Ty Churchwell, mining coordinator for Trout Unlimited, a fish conservation group. “In that sense, they are important and need to remain in the toolbox.”

In its October 2015 technical assessment of the incident, the U.S. Bureau of Reclamation argued that bulkheads were at least partially responsible for the Gold King Mine spill.

The Gold King Mine is a maze of tunnels, faults and fissures located at different elevations inside Bonita Peak and the surrounding mountains in Gladstone. The mine opening that drained when the EPA crews struck a plug holding back water was actually what’s known as the “Upper Gold King Mine,” or Gold King Mine Level 7.

A short distance away lies the “Gold King Mine,” which refers to a mine adit called American Tunnel.

American Tunnel served as a primary haulage point for the Sunnyside Mine, which was operated by the Sunnyside Gold Corp. for about a decade and a half until 1991, according to a 2015 report from the Colorado Division of Reclamation, Mining and Safety. It also drained water inside the mountain to allow for mining operations at lower elevations.

With oversight from the Colorado Division of Reclamation, Mining and Safety, Sunnyside Gold Corp. first installed a bulkhead in American Tunnel in 1995 to stop mine drainage from entering Cement Creek. The company closed the valve on the first bulkhead in October 1996 and would go on to install two other bulkheads in American Tunnel.

With the installation of the bulkheads, the flow of toxic mine waste into Cement Creek decreased from 1,700 gallons per minute to about 100 gallons per minute.

But as the impounded water rose behind the bulkheads, the water rose elsewhere, including in Gold King Mine Level 7, which sits about 750 feet above American Tunnel, according to the Bureau of Reclamation’s assessment.

“We don’t exactly know, but what we believe happened was that the water raised inside the mountain through the installment of those bulkheads, causing the water to pool up inside Gold King Mine Level 7, which was then released back in 2015,” said Christina Progess, the EPA’s remedial project manager for the Bonita Peak Mining District Superfund site.

According to the Bureau of Reclamation’s assessment, which was peer reviewed by the U.S. Geological Survey and U.S. Army Corps of Engineers, the EPA assumed that Gold King Mine Level 7 was not full of water because of previous excavations and planned for 5 to 6 feet of water.

The EPA had previously used a drilling rig to bore into the Red and Bonita Mine in 2011 to check on water levels, but for whatever reasons chose not to do so for Gold King.

It turned out that EPA’s assumption was wrong, and when crews excavated the entrance to Gold King Mine Level 7 they released a rush of toxic mine water.

The EPA has yet to determine if it was faults and fractures in the rock or other internal mine workings that carried water from American Tunnel to Gold King Mine Level 7, but the EPA and the Bureau of Reclamation have both said the spill was in part the result of this buildup from the bulkheads in American Tunnel.

“By the time that system was installed, you can imagine it starts to raise the groundwater inside the mountain and that starts to fill up other workings,” Progess said. “We believe that there’s a hydraulic connectivity between the American Tunnel (and) Gold King Mine Level 7 and part of our evaluation of the Bonita Peak groundwater system is going to be understanding these hydraulic connectivities a little bit better.”

At its most basic, a bulkhead is a concrete wall that prevents water from escaping a mine opening.

They can be anywhere from 10 to 20 feet long and some possess a valve at the bottom, according to the Colorado Department of Natural Resources and Churchwell.

They are designed to stop acid mine drainage and control the flow of metal-laden water (through the valve) for treatment. They also prevent unplanned releases and can return groundwater levels to their previous levels. Those installed by Sunnyside Gold Corp. were designed for a 100-year life span, Progess said.

“If you have a concrete plug with no valve, your intention is to try to hold back all the groundwater in the mountain so that you don’t have to treat it,” she said. “If you have a flow-through bulkhead, the intention is to meter out the flow to minimize unplanned releases, but also to allow you to treat water downstream or maintain that water level within the mountain at a certain height.”

Bulkheads have been used in mine remediation efforts in Colorado for more than three decades, and there are about 40 installed across the state, said Jeff Graves, director of Colorado Division of Reclamation, Mining and Safety’s Inactive Mine Reclamation Program.

“The biggest issue that bulkheads solve is management of uncontrolled releases from mine workings,” Graves said in an email. “Discharge from mine portals can be controlled, and in many cases significantly reduced by installing bulkheads. Bulkheads can also improve water quality by re-establishing pre-mining flow paths and groundwater levels.”

In addition to impounding or directing the flow of water, remediation agencies and mining companies use bulkheads to counteract the chemical reaction that creates toxicity in mine drainage.

Gold, silver and other important metals are often extracted from rocks containing sulfides. When these rocks are exposed to both air and water, they leach their sulfides into the water through a process known as oxidative dissolution. This process can turn water orange in the case of the Animas River after the Gold King Mine spill.

Bulkheads back up water and fill mine tunnels. When they do so, they limit the air rocks can come into contact with, preventing the chemical reaction that creates acid mine drainage.

“The idea of a bulkhead is to try to flood the workings to change the chemical reaction that’s happening within a typical acid mine drainage scenario, where we’re cutting off the oxygen to the workings with the goal of reducing the acid mine drainage that’s generated,” said Rob Parker, a Superfund remedial project manager with the EPA for the Bonita Peak Mining District.

The chemical reaction can still occur, and water can escape the mountain through other faults and fissures, but the volume is limited so that the EPA and other operators can treat the water in other ways.

While bulkheads serve an important role in the remediation of hardrock mines, their use does carry risks.

Managers cannot see behind bulkheads, making it incredibly difficult to monitor what is going on inside the mine. Pressure transducers included in newer walls can give them some idea, but as is the case of the Red and Bonita Mine, drilling bore holes hundreds of feet into the mountainside is often the only way to get a definitive sense for water levels.

Acid mine drainage can also still make its way into river systems. Water naturally moves through rock and can turn into acid mine drainage when exposed to oxygen, though in smaller volumes.

“The idea is to try to put a bulkhead in a location where you’re going to have minimal exit points through faults or fractures,” Progess said. “And you need to monitor that because you need to understand what the water quality is when you do flood the workings. Are you seeing seeps that used to be clean that are now dirty because that water is now being oxygenated as it hits the seeps and springs?”

This movement of water is perhaps the greatest concern for remediation managers. Mines are complex networks of tunnels, but there also fissures and faults that a bulkhead can exploit to send pollution to other areas.

In the case of the Gold King Mine spill, the EPA had no idea that a bulkhead about 750 feet below Gold King Mine Level 7 was causing the adit to fill up in its entirety.

“We’re not talking about just connectivity between actual mine workings where people have drilled holes and where they’ve actually connected,” Parker said. “There’s natural fractures and faults in (Bonita Peak Mining District) that have really high hydraulic conductivity where it’s a little bit of a water superhighway between areas.”

The Bonita Peak Mining District has 48 mines, but almost half of all the toxic metals that end up in the Animas River and its tributaries come from just four mines located along Cement Creek, Churchwell said.

The EPA has been considering installing bulkheads to address the pollution coming from these mines, and the Bonita Peak Mining District Community Advisory Group, which provides community input to the EPA and on which Churchwell sits as vice chairman, has supported the agency.

“The CAG believes EPA’s initial main focus should be on the four big sources of metals in the Gladstone area – the mine drainages of the Gold King, Red & Bonita, American Tunnel, and Mogul,” wrote Peter Butler, chairman of the Bonita Peak CAG, in a letter to Progess and the EPA in 2019. “To address these sources, we support the presumptive remedy for Upper Cement Creek ... re-establishing the groundwater table by installing bulkheads at various portals.”

Churchwell said evidence from other mines in the district has strengthened the Bonita Peak CAG’s view that bulkheads should be used to address mine pollution in the area.

Koehler Tunnel sits over a ridge of mountains to the west of American Tunnel. In 2003, the Gold King Mines Corp., a mine operator in the area, installed a bulkhead in the tunnel.

Over the last about two decades, Churchwell and others have noticed a significant improvement in water quality to the point that trout, an indicator species for clean water, have returned to a stretch of Mineral Creek downstream from Koehler Tunnel where they have been absent for decades.

“The water quality improvements on Mineral Creek have been dramatic and it is largely because of that really successful bulkheading project,” Churchwell said.

The benefits of bulkheading are perhaps most vivid in the case of Mineral Creek and the Koehler Tunnel, but bulkheads throughout Bonita Peak Mining District have limited the release of toxic wastewater and allowed trout to return.

“We can point to several places on the Animas (River) where we do have trout today that are in part a response to improvements to water quality that have been gained through bulkheading,” Churchwell said.

The EPA and mine remediation groups have a few other techniques at their disposal besides bulkheads.

“Passive treatment” involves the creation of wetlands or other natural systems that can filter out metals without equipment. Passive treatment solutions can be left alone and do not require routine maintenance.

With “active treatment,” water is pumped from mines to a treatment plant where it can be filtered before returning to rivers. While active treatment is the most effective way to remove heavy metals and clean water, it is also the most costly.

Bulkheads serve as an in-between. Bulkheads can cost hundreds of thousands of dollars, but by metering the flow of water, they allow mine remediation managers to use limited active treatment or wetlands and passive treatments that require few, if any, long-term costs.

“Ideally, if you can limit the amount of water through bulkheading and have a much smaller volume of discharge to then potentially treat, then you hold down maintenance costs substantially,” Churchwell said.

Ultimately, any maintenance costs at a Superfund site like the Bonita Peak Mining District are paid by the American taxpayer, he said.

The EPA has only installed one bulkhead in the Bonita Peak Mining District so far. Of the more than a dozen bulkheads in the district, many were installed by Sunnyside Gold Corp. in the 1990s and early 2000s.

In Gold King Mine Level 7, the agency has placed a “flow control structure,” which is like a temporary bulkhead that crews can get behind.

Any decision to install additional bulkheads will likely take years.

Learning from the Gold King Mine spill, the EPA is studying the mines around Gladstone like American Tunnel to understand the hydrology of the mines, including how Gold King Mine Level 7 filled with water and how additional bulkheads would affect that hydrology.

“At this point, you know, we really have a lot of investigative work that we want to get through,” Progess said. “... It’s going to be several years before I think we can really make any affirmative decisions about what to do with any of these mines, especially the major draining mines that we have (in) Upper Cement Creek.”

Whether bulkheads can be a part of the solution for Bonita Peak Mining District and Gold King Mine Level 7 will depend on this yearslong investigation.

But if Churchwell and the Bonita Peak CAG have any say, bulkheads will remain in the EPA’s toolbox.

“If there were a bulkhead in the Gold King Mine, guess what? It would not have blown out,” he said.

ahannon@durangoherald.com