Asarco LLC will leave more than 3.2 billion gallons of water in the ground each year now that it has switched part of its Mission mine operations to Colorado River water.
The Tucson copper producer made the change from groundwater to Central Arizona Project river water last week.
Pumped from the Black Mountain Operating Reservoir, the water flows into Asarco's new pumping station on the Mission property, just west of Interstate 19 near Sahuarita.
It pumps between 2,700 and 4,700 gallons a minute to the mine's two mills and satisfies about 80 percent of the operation's water needs, said John D. Low, Asarco's vice president of mining operations.
"It's the right thing to do," Low said of the switch.
Initially, there were concerns that CAP water might be unsuitable for the copper-extraction process, but there has been no problem using it, said Dale C. Dixon Jr., manager of development projects for Asarco.
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Asarco buys the CAP water from the Tohono O'odham Nation as part of the Southern Arizona Water Rights Settlement Act. The company has not received its first bill and did not have an estimate as to how much water it will use each month.
Under the water agreement, approved by the U.S. Department of the Interior, the O'odham tribe's San Xavier and Shuk Toak districts receive 37,000 acre-feet of water from CAP annually.
The federal government also provides the districts with an additional 28,200 acre-feet annually from other sources. An acre-foot is about 324,000 gallons.
As part of the settlement, the water is provided without cost to the Tohono O'odham Nation, which also received a $15 million trust fund to develop its water resources. In return, the nation waived all future water-rights claims and related claims within the districts.
The settlement ended decades of litigation filed by tribal members on behalf of a group of San Xavier landowners to limit Tucson's water pumping, which they said was one reason the Santa Cruz River dried up.