Denver (AP) Workers have begun drilling a relief well into a collapsed mine-drainage tunnel above Leadville.

The environmental protection agency said Wednesday that the work is expected to take two or three weeks. When the drilling is done, a pump will be installed to remove contaminated water that has built up in the tunnel.

A cave-in detected in 1995 has caused an estimated 1 billion gallons of water to back up inside mine shafts that the tunnel was designed to drain.

The EPA hopes to start pumping out water by mid-June before treating it and releasing it into the Arkansas river.