Rosemont Copper is backing off from a recent statement that its proposed mine will use less water than originally thought due to changes in its operations.
In a new letter to Coronado National Forest Supervisor Jim Upchurch, Rosemont Vice President Kathy Arnold describes a potential tradeoff:
• Water use will be reduced by more than 10 percent due to the company's new plan to eliminate heap leaching and solvent extraction facilities to process copper oxides.
• But that could be offset by a need for more water for increased dust control requirements. The dust control could be required for air- quality purposes and "longer haulage roads," the letter said, without elaborating.
That contrasts with Arnold's July 8 letter to Upchurch, in which she wrote that because of eliminating plans for heap leaching and solvent extraction, "the quantity of water used for mineral processing will be reduced, reducing the project's overall water demand and groundwater pumping in the Santa Cruz Sub-Basin."
In the new letter, dated Sept. 12 - and sent because Upchurch asked for more details - Arnold said the company wants the Forest Service to assume that the Rosemont Mine's water use will stay at its legally permitted level of 6,000 acre feet a year. That way, Rosemont Copper can "ensure (that) maximum potential impacts are disclosed." An acre-foot is 326,851 gallons, or enough to serve three families homes for a year.
Upchurch said his staff is reviewing Arnold's new letter for accuracy.
Rosemont Mine opponents are skeptical of the letter's conclusions about water use, and still want the Forest Service to do an updated or new draft environmental impact statement to analyze the changes. The Forest Service has said it will decide that question by December.
Arnold said the company has given more details to the Forest Service about mitigation measures that haven't been made public yet. Until it is, outsiders' statements about the mine's impacts are "speculative," she said.
Rosemont Copper officials would not answer most questions from the Star about Arnold's letter. One area that her letter didn't address in most cases was whether the predictions took into account the impacts of increases in copper production now planned for the mine, due to a newly discovered, large increase in copper sulfide reserves at the site.
Changes in plan
The changes in Rosemont's planned mining operations stem first from the Forest Service's proposed layout for the mine in the Santa Rita Mountains southeast of Tucson.
Known as the Barrel Alternative, it's a layout the Forest Service prefers over other possible alternatives because it would keep the mine's tailings from covering a major canyon.
The second reason for the changed plans is that the company recently found larger copper sulfide reserves than expected and will up its production levels by 25 percent.
In her new letter to Upchurch, Arnold didn't discuss water use impacts from the copper sulfide production boost. To a question about that from the Star, Arnold replied in part, "Water balances in any process are more complex that simply factoring the water use against a product."
According to a state study of other mines, and to an outside expert, it's not unreasonable to think that Rosemont will have to use more water - rather than less - now that it's scrapping the heap leaching process.
A 2010 study by the Arizona Department of Mines and Mineral Resources concluded that Arizona's two most water-thrifty mines use heap leaching: Asarco's Silverbell and Freeport McMoRan's Morenci mines. Rosemont came in a close third in water-thriftiness, with its heavy reliance on recycling water and its plan for dry stack mine tailings that would use less water than conventional tailings.
Gary Kordosky, a retired mining industry chemist in Tucson who still works as a metallurgy consultant, said that typically, heap leaching uses less water than does processing copper sulfide into copper concentrate for shipment to smelters.
It's hard to know for sure how much water would be used without more detail about Rosemont's operations, said Kordosky, who said he's neither strongly for nor against the mine. But he said the mine's dry stack tailings could make it possible for the company to save lots of water, possibly enough to offset the increase in sulfide production.
Fergus Graham is a retired mineral exploration geologist active in the Rosemont opposition group called Save the Scenic Santa Ritas. He said that while the company could well save water by scrapping heap leaching in the mine's first six years or so, he expects water use to go up afterward, when the amount of copper ore processed is scheduled to increase.
Did you know?
Rosemont will pump and pipe groundwater to its mine from wells in the Sahuarita area. It is paying to have a pipeline built to carry Colorado River water from the current terminus of the Central Arizona Project south of Tucson to a recharge basin near its wells, to offset effects of its pumping on the aquifer.
Critics fear the CAP could eventually run dry due to climate change and growing demands on river water in other states, forcing Rosemont to rely totally on groundwater.
Contact reporter Tony Davis at firstname.lastname@example.org or 806-7746.