DENVER -- The EPA began drilling a relief well into a mine drainage tunnel Wednesday in an effort to stop a potentially catastrophic wave of contaminated water from flooding the town of Leadville.

The drilling began Wednesday morning with a 40-foot drill rig that was mounted on a truck and a 24-inch drill bit.

The tunnel lies approximately 350 feet below ground and contains an estimated 500 million to one billion gallons of water that has become trapped behind various blockages over the past decades.

Drilling the relief well is expected to take two to three weeks, and the installation of a pump will follow.

The EPA estimates the well will be operational by mid-June.

Water pumped from the well will be transported via a pipeline to a water treatment plant operated by the U.S. Bureau of Reclamation, which has agreed to treat the water before it is released into the Arkansas River.

"Drilling the relief well is an important step towards addressing any risks that this water may present," said EPA regional administrator, Robert E. Roberts. "We continue to work towards the day when we begin pumping water out of the tunnel and sending it to the treatment plant."