This megasuburb, Daybreak, is twice the size of San Francisco and will be the work of a mining company, Kennecott Utah Copper Corp., which has no experience in real-estate development.

The Utah company is a subsidiary of London-based Rio Tinto, a mining multinational and avowed convert to environmentalism, which decided to make a showcase out of its surplus Utah lands instead of just selling them off for cookie-cutter subdivisions.

Kennecott is helping build a pair of reverse-osmosis filter plants to clean tainted groundwater over the next 40 years, while providing fresh tap water for the southwest part of the Salt Lake valley. It dug other wells 300 feet deep to provide ground-source heating and cooling for a new elementary school and community center and contributed $400,000 to kick-start an environmental study of extending a light-rail line from downtown Salt Lake City to Daybreak.

“Sustainable” development is a term McMahon and other Kennecott executives often use to describe their venture. Daybreak, for example, will contain all of its own runoff, using it for irrigation for native grasses and 40 species of trees, said Greg L. Rasmussen, an engineer and Kennecott’s director of land development.