Just call it Lake Rosemont.
Once the proposed Rosemont mine is exhausted of ore and shut down, look for a lake to fill part of the 2,000-foot-deep open pit where the copper came from, says a new report by a Rosemont Copper Co. consultant.
But don't think you or your grandkids will be able to enjoy a swim or boat ride there once the mine, planned for the Santa Rita Mountains southeast of Tucson, is closed after 20 to 25 years.
The lake will be well below the ground surface — 1,200 feet below, once 100 years have passed. The water will be 800 feet deep after that 100-year period, says the study by Montgomery & Associates. Most likely, the lake will be fenced off to keep people from falling in.
The lake will form because the company will no longer need to pump groundwater and rainfall runoff out of the pit, company consultants say.
Environmentalists warn that the lake may be toxic not just to people but to birds and other wildlife because of acidic compounds formed from sulfides in the underlying rock that react with air and water during or after mining.
Rosemont officials, however, say a soon-to-be-released study by a company consultant will quiet that concern.
The lake's presence also may be a positive sign that Davidson Canyon and Cienega Creek downstream won't suffer as much from Rosemont's pumping as some fear, said an independent hydrologist who reviewed the company-funded study.
A number of toxic pit lakes have formed elsewhere, particularly the Berkeley Pit in Butte, Mont., which is one of the country's largest Superfund cleanup sites. The highly acidic lake, an open-pit copper mine from the 1950s to the early 1980s, has been linked to hundreds of bird deaths since the mid-1990s.
In the late 1990s, a book written by the industry-run Society of Mining, Metallurgy and Exploration predicted that open-pit lakes would become one of the most significant environmental issues facing the global mining industry. With more mining companies employing open pits instead of underground mines for technological, efficiency and economic reasons, the number of open-pit lakes will grow, it said.
With sufficient advance planning, open-pit lakes could be used as recreation spots and wildlife habitat once mining is finished, the book said.
"At the opposite end of the spectrum, pit lakes with poor water quality pose potential risks to ecosystems and humans," said the book, titled "The Nature and Global Distribution of Pit Lakes." Besides the Montana example, other acidic pit lakes have formed in Nevada, California, South Carolina and Vermont, the book said.
The problem is that almost all ore bodies involving copper will be sulfide ore bodies, said Roger Featherstone of the Arizona Mining Reform Coalition, who opposes Rosemont, referring to the sulfur-based compounds found in many copper deposits.
"When that's mixed in the rocks over the eons, the sulfides are stabilized. But when you start mining and mix the sulfides with air and water, the sulfides react with them to create acid," said Featherstone, who said he's "sat at the knees of the leading experts" on such issues during his 30-plus years as an activist.
But Rosemont's study concluded that the underlying rock at the mine site contains enough limestone, which is basic, to offset any acids and produce a lake that is chemically neutral, a Rosemont official said last week.
"It's pretty clean water. It won't have any acid. It's not basic. It's neutral, you won't be seeing high metals or salts in the water like you see in some places," said Kathy Arnold, Rosemont's environmental affairs director.
She's not sure if the lake will be restricted from birds. The company evaluate that as it moves through the permitting process. The U.S. Forest Service and Fish and Wildlife Service and the Arizona Game and Fish Department will have something to say about this as well, Arnold said.
Still, people shouldn't think of this lake as a future recreation spot, a Rosemont consulting hydrologist said.
"Take a look at what the Twin Buttes pit looks like, or the Sierrita pit," said Hale Barter, of consulting firm Montgomery & Associates, speaking of the now-closed Twin Buttes mine and the still-active Sierrita mine south of Tucson near Green Valley. "I don't think the walls are going to be stable or safe where people will want to recreate."
But if a pit lake does form, that will reduce the drain on the aquifer feeding Davidson Canyon and Cienega Creek, downstream from Rosemont, the study found.
Thomas Meixner, a University of Arizona hydrologist who reviewed that study for the Star, agreed, because the presence of a pit lake means there will be less evaporation and more water can flow downstream.
A Pima County-commissioned study by Nevada hydrologist Tom Myers, however, said the pit will get only enough water from the underlying aquifer, rainfall and runoff to form a seasonal lake. Evaporation rates will exceed the rate of water flowing into the pit, Myers' study concluded.
Meixner said he doesn't know enough about the mineral content to say definitively that the pit lake's quality will be terrible, "but past experience indicates it is unlikely to be pleasant."