The U.S. Environmental Protection Agency is installing a barrier and valve inside an inactive Colorado mine to prevent another surge of wastewater like a 2015 blowout that contaminated rivers in three states.
The 12-inch (30-centimeter) valve will regulate wastewater pouring from the Gold King Mine in the San Juan Mountains of southwestern Colorado, where the EPA inadvertently triggered a wastewater spill while excavating at the mine entrance in August 2015.
That spill released 3 million gallons (11 million liters) of wastewater containing aluminum, iron and other heavy metals and instantly became a major embarrassment for the EPA.
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Rivers in Colorado, New Mexico and Utah were tainted. Irrigators, water utilities and rafting companies temporarily stopped using the Animas and San Juan rivers. The EPA says water quality quickly returned to pre-spill levels.
The valve will be mounted in a steel-and concrete barrier about 70 feet (20 meters) inside the mine. The barrier will have water-tight access doors so workers and equipment can get deeper into the mine for cleanup and investigation.
The EPA is also drilling a 170-foot (50-meter) horizontal well into another part of the Gold King to drain any water building up there. That water would be routed through a temporary treatment plant below the mine where wastewater draining from the main entrance is cleaned up.
The EPA said it can control the flow of wastewater from the new drain to avoid another blowout.
The documents did not say say how much the work will cost and the EPA did not immediately respond to emails and a phone call Wednesday seeking comment.
The work is expected to be completed next month.
Peter Butler, a leader of the volunteer Animas River Stakeholders Group, which works to improve water quality in the area, said he agreed with the EPA's decision to install the barrier and drainage well.
"It's probably a good idea," he said. "They are showing an abundance of caution."
Wastewater has flowed from the Gold King for years, and since the 2015 blowout, it has poured out at a rate of about 500 gallons (1,900 liters) a minute.
Mine waste flows are unpredictable in the San Juan Mountains, where underground water flows through an interconnected warren of mine tunnels and natural faults.
Precautions such as the barrier, valve and horizontal drain will make it safer for investigators to enter the mines and try to figure out the water flows, Butler said.
The Gold King and dozens of other mining-related sites in the region were designated a Superfund district in 2016.