Rosemont Copper says it has designed its proposed mine to avoid or reduce impacts on federally regulated watercourses as much as feasible.

In a recently filed permit application to the Army Corps of Engineers, the company said it has located various mine structures, including tailings areas, to avoid wetlands and other aquatic sites.

The amount of material that the company plans to discharge into various federally regulated washes is large: over the 20-year life of the mine, nearly two billion tons in the mine's waste rock, leach pad and dry-stack tailings facilities, and nearly six million tons of additional material associated with waste rock, roads, concrete and other work.

This information comes from Rosemont Copper's application for a federal Clean Water Act permit from the Army Corps of Engineers. It's one of many key permits the company needs to start construction of the $1 billion project in the Santa Rita Mountains southeast of Tucson.

This permit process is likely to be contentious since unlike the U.S. Forest Service, the corps has asserted the legal right to deny a mining company's permit application.

For now, Pima County officials and the leading opposition group to the mine, Save the Scenic Santa Ritas, have concerns but no formal comments on the permit application, filed in October, because they haven't fully reviewed it or because it is early in the corps' process.

The corps has said it will make a decision after the Forest Service publishes its final environmental impact statement on the mine.

Known as a 404 permit - named after Section 404 of the Clean Water Act - the corps permit covers dredging and filling of regulated watercourses with material from the mine property.

Rosemont Copper needs the permit to build diversion structures to reroute water now running in washes around mine facilities, including the open pit, tailings and waste rock storage area. Such permits are required to discharge fill into rivers, washes and adjacent wetlands under the Corps' control.

In its application and in an associated environmental analysis, the copper company says:

• The mine will impact 31 to 53 acres of federally regulated wash areas, depending on which of six mine alternatives is chosen. That's out of a total of 101 acres of washes there under the corps' jurisdiction.

• The acreage affected grows when the company's analysis expands to all riparian areas, whether federally regulated or not - to a range of 83 to 238 acres depending on the design alternative.

• Alternative 3, the Barrel alternative that is the Forest Service's preferred option, would directly or indirectly affect nearly 40 acres of federally regulated washes - the second lowest of the alternatives studied. It would affect 207 acres of total riparian area - also the second lowest total.

• The Barrel alternative would reduce rainfall runoff flows into Davidson Canyon downstream by the second lowest percentage - 33 percent compared to up to 45 percent for other design alternatives.

Suzanne Shields, the county's flood control director, said the county will review the permit application closely, looking at it jointly with other Rosemont permit applications to the Forest Service and to the Arizona Department of Environmental Quality, to determine the combined impacts of these activities.

County officials are concerned about how much the mine will reduce flows to Davidson Canyon, a tributary of Cienega Creek and - in areas downstream of the Rosemont Mine - a county open-space preserve.

They're concerned, for instance, that the mine structures could cut off stream flows that percolate through a fissure in the limestone rock underlying the area that later rises above ground as a spring.

"There are many things that can disrupt that. One is digging a hole into the ground, where water will seep into the pit rather than continuing downstream," said Shields, of the Regional Flood Control District.

County officials are also concerned about potential impacts to birds, mammals, bugs and other wildlife upstream of the mine that depend on areas downstream for food, Shields said. "They may roost in the upper watershed but forage and feed in the area that is being disturbed," Shields said.

A Rosemont Copper official said that while the mine may cut off up to 45 percent of the storm runoff from the mine area, that doesn't mean Davidson Canyon will lose that percentage of its total stream flow. Davidson draws upon a much larger watershed than just the mine area, said Kathy Arnold, Rosemont Copper's environmental and regulatory affairs director.

The company's proposed mine plan would cut off about 10 percent of Davidson's total water sources while the Forest Service's preferred Barrel Canyon alternative would cut off 5 percent, Arnold said.

The mine's impact on wildlife living upstream of it won't be determined until the U.S. Fish and Wildlife Service performs a biological analysis of the mine's effects on endangered and threatened species, Arnold said, adding, "I don't want to hazard a guess on that one."

Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.