What killed hundreds of fish in a Boulder-area creek?

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BOULDER COUNTY, Colo. - A release of contaminated water from an old gold mine near Boulder is responsible for killing a few hundred fish this week, and is not a threat to the drinking water supply, according to state and federal engineers.

The Problem Solvers discovered the Environmental Protection Agency and Colorado Department of Public Health and Environment released the water from the Captain Jack Mill - a Superfund site - on purpose, saying they were worried about it coming out of the mine uncontrolled.

To understand why that happened, it's important to know the history of the site.

It started back in the 1860's as a gold mine.

It was shut down in 1992 and became a Superfund site in 2003 because it contained contaminated rocks, groundwater, hazardous materials and more.

"It looked a lot different before we started doing work here," the EPA's Joy Jenkins told the Problem Solvers. "This particular adit was draining orange water that was draining over a very large waste rock pile here."

The EPA and CDPHE cleaned up the surface area. And that was the easier job.

"Addressing discharging mine water is a lot more difficult, because it’s an on-going source and an on-going generation," said Mary Boardman an engineer with the CDPHE. "So you have to address it continuously, as opposed to a once and done."

Typically, cleaning contaminated groundwater is an expensive process.

There wasn't a huge flow of it at the Captain Jack Mill, so the EPA and CDPHE tried an innovative process - they installed a concrete barrier and large amount of limestone to naturally filter out the metals, including zinc and copper and decrease the acidity.

But the space inside mine is small and the water quickly built up.

"We knew at some point we would probably have to release some of the water back out, because we weren’t going to be able to continuously store up the water," Boardman said. "It was going to reach some elevation where if we don’t do a controlled release of the mine pool, it’s going to flow out in some other uncontrolled fashion."

On September 6th, the EPA and CDPHE released the contaminated water. It flowed at a rate of about 25 to 30 gallons per minute for about a month and a half.

It flowed down to the Left Hand Creek and killed a few hundred fish in a five to seven mile stretch of the water.

When the agencies realized what happened, they shut the flow off and called in an EPA's emergency response and preparedness team.

They said they tried to mitigate the problems as quickly as possible and created two pools to let the metals sink out of the water.

"The good news is the river is starting to recover and everything we do should be helping that," said the team's coordinator Craig Myers.

The EPA and CDPHE are now trying to improve the limestone system to decontaminate water that's inside the Captain Jack Mill.

They told the Problem Solvers there's a good chance they'll have to release contaminated water again, but are working to make it safer for fish.

They said none of the water is a danger to the drinking water supply.

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