"We've handed back our Superfund membership card," company President Andrew Harding said Wednesday.

Fourteen years ago, the company struck a deal with the U.S. Environmental Protection Agency and the Utah Department of Environmental Quality to avoid being forced to do a cleanup on regulators' terms.

Instead, the company designed its own long-term strategy for removing mining contamination such as sulfate and acid from groundwater in southwestern Salt Lake County.

More than $400 million has been spent on contamination collection, a reverse-osmosis plant and other cleanup efforts. The company has signed a long-term legal agreement with regulators to continue the work and a similar accord is being drafted for the now-under-way North Zone cleanup.

- Judy Fahys