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A mine the size of the one proposed by Rosemont Copper normally would use enough water every year to supply 40,000 households.

But officials with Rosemont Copper say they will only use half that much — 5,000 to 8,000 acre-feet a year — by using techniques developed for mining in extremely arid climates.

Residents who live near the company's well sites worry those numbers are both too good to be true and too high to to live with.

They question how well-tested the techniques are and why the company has an additional 25 wells registered in its name.

And they also worry what effect so much additional groundwater pumping will have on the small, private wells serving nearly 250 people living east of Sahuarita.

"This is going to drastically affect the community," said Robert Robuck, a resident of the area whose concern about the mine's water use led him to run against District 2 Supervisor Ramón Valadez. "It's ridiculous that they'll be allowed to pump up our drinking water without thinking about the impact."

Officials with Rosemont Copper say they are thinking very much about the impact.

In addition to offering to pay for a pipeline to bring Central Arizona Project water to the area for recharge, they recently sent a letter to a group called the United Sahuarita Well Owners offering to pay for any damage to private wells — including deepening wells or drilling new wells — that develop after the mine starts pumping.

"They're looking for security and they're looking for confidence that a big bad mining company won't leave them stranded," said Jamie Sturgess, vice president for projects and environment with Rosemont Copper. "If a well owner's well were not deep enough to stay in the water table, we will make sure that homeowner is not stranded. We will monitor the impact and take responsibility for the impact."

Rosemont Copper plans to mine 27 million tons a year of copper, molybdenum and silver on 4,400 acres, much of it Coronado National Forest land in the Santa Rita Mountains. The company is going through the Forest Service's environmental impact study process.

In January, the Arizona Department of Water Resources issued Rosemont a withdrawal permit for 6,000 acre-feet a year.

Most of the water will come from wells on a 53-acre site off Santa Rita Road, south of Sahuarita Road and east of the pecan groves, and a 12-acre site near South Alvernon and Dawson roads. Rosemont has a contract to buy a third property in the area for use as a well site.

Farmers Investment Company objected to the permit, but department officials said state law doesn't allow them to consider the possible impact on other water users when granting a withdrawal permit for mining.

Rosemont's plan of operations calls for using dry stacking of tailings, a process developed for use in extremely dry climates, like the deserts of northern Chile.

Instead of allowing water to evaporate from tailings ponds, mine companies mechanically remove the water, creating tailings with a very low moisture content. Water can be reused multiple times.

Rosemont officials believe their mine will be the largest to date to use dry stacking.

Fred Tahse, a retired geologist and opponent of the mine, said he doesn't believe dry stacking has been used enough to guarantee Rosemont's water use predictions.

"How realistic is that 6,000 acre-feet?" he said. "That's up in the air."

He also questioned how safe the tailings will be in a mountain location that gets between 18 and 24 inches of rain a year.

But Kathy Arnold, Rosemont's director of environmental and regulatory affairs, said dry stacking has been used safely in areas that get far more rain than the Santa Ritas, like Alaska.

She said the difference between Rosemont and other mines is only one of scale.

"It's just more equipment, more money," she said. "We wouldn't have put it in our plan of operations if we were not very confident it will work."

Sturgess, the Rosemont VP, said that confidence is reflected in the withdrawal permit application filed with the state.

"We could have applied for any amount we could show we would need," Sturgess said. "We could have applied for 16,000 acre feet, but we didn't."

In addition to its withdrawal permit, seven monitoring wells and 21 wells for mineral exploration, Rosemont has 25 wells registered in its name or in the name of its parent company, Augusta Resource Corp. Four of those wells are non-exempt wells outside the Tucson Active Management Area and capable of producing more than 35 gallons per minute.

Sturgess said the company acquired those wells as it bought up ranch land for the mine and has no intention of using them for anything other than watering livestock.

Sturgess is quick to point out that Rosemont Copper's water use will be a fraction of the other major users in the area.

FICO's pecan groves and the Sierrita Operations of Freeport McMoRan Copper and Gold each use more than 20,000 acre-feet a year. Several private water companies also draw from the same aquifer, and there is no recharge south of Pima Mine Road, where the Central Arizona Project pipeline ends.

The water table in the Upper Santa Cruz Basin is dropping between 2 and 4 feet a year. Satellite data from the last year show 3 centimeters of subsidence centered in the pecan groves, and the Arizona Department of Water Resources is monitoring the situation to see if the subsidence is an ongoing problem.

Rosemont has offered to let the department put a transducer in one of its wells to provide more data on any subsidence.

The company's interest in pre-existing water woes makes some residents nervous. Robuck worries the company will blame future water problems on past pumping.

Sturgess said the company's detailed hydrological studies are aimed at understanding the water table and preventing problems, for the sake of mine operations as well as for the neighbors.

Sturgess said the company won't quibble about why a well went dry.

"Some wells may already be low, but we'll give them the benefit of the doubt," he said. "If someone's well would have gone dry anyway in five years, they'll luck out, because we'll replace it."

Thomas Perry, a founding member of United Sahuarita Well Owners, said the company's promises are an important first step in protecting access to water for people who depend on small private wells.

He said the most important part of the agreement is that Rosemont will pay for a detailed hydrology study it will make available to independent experts.

That information will allow well owners to know if the proposed mitigation will be enough before they sign on to the agreement, Perry said. Each well owner can decide individually whether to participate in the agreement, but Perry warned that those who don't participate will not have very many legal options if their wells run dry.

"There are no legal protections against being pumped dry and there is court precedent that they can suck us dry and do nothing," he said. "Most of the people in this group, if you took their land away from them, they would have nothing and no ability to sue anyone or replace their well."

Just as the Green Valley water company has drawn criticism for working with Rosemont, Perry said he has heard from people who don't want to participate because they don't want to make the company look socially responsible or environmentally sustainable.

The group has no official position on the mine.

"We have purposely not taken a position," Perry said. "One, we need copper. Two, many of our members are hard-rock miners. Three, many of our members are low-income and need jobs. Four, we don't know if the mine will happen or not."