High winds that sent huge dust clouds swirling last week caused Pima County to exceed federal air-quality standards for the first time in nearly four years, county officials said.
The worst dust pollution hit the Green Valley-Sahuarita area on April 9, causing levels of large dust particles at one monitor to average 450 parts per million over 24 hours, nearly three times the federal limit.
Because two copper mines adjoin the suburbs of Green Valley and Sahuarita, Pima County Department of Environmental Quality officials are investigating how much, if at all, the mine tailings contributed to the dust problem in violation of county air-quality laws.
Other potential sources of the dust including construction sites are also being investigated, officials said.
Mining companies Asarco and Freeport McMoRan Copper & Gold said they reported to the county that some tailings blew from their sites last week, but that other sources clearly contributed to the regional blowing dust.
On April 8 and 9, particle levels also slightly exceeded the standard at a county-run air monitor in South Tucson. On April 9, the standards also were exceeded at a monitor near Grant Road and Stone Avenue, showing 190 parts per million.
The particles can cause or aggravate lung or other respiratory problems, particularly in people with existing health problems such as asthma, the U.S. Environmental Protection Agency says.
The wind speed at Tucson International Airport peaked at 49 miles an hour late on the afternoon of April 8, the National Weather Service said. On April 9, the wind speed peaked at 30 miles an hour at about 3 p.m. In the surrounding mountains, winds exceeded 50 miles an hour on April 8.
The last time air-quality standards were exceeded in Pima County was June 22, 2009. That's when three county monitors, including those in South Tucson and near Grant and Stone, had particulate levels over the standards.
For a formal violation of federal Clean Air Act standards to occur, a single monitor must record particulate levels above standards four times within three years. That hasn't happened since 1999.
This week, county DEQ Director Ursula Kramer said it's too early to determine if individual businesses or other property owners violated the law. Under county laws, any person or business generating dust is supposed to keep dust levels no more than 20 percent opaque, blocking normal views of the air.
Tom Aldrich, an Asarco vice president, said the company has told the county that during the dust storms, dust blew off a tailings pile at its Mission Mine complex, just west of a Sahuarita residential area that has been hit by tailings dust repeatedly in recent years. The county is still investigating two alleged violations that it said occurred from the same Asarco tailings pile in December 2012 and January 2013.
Asarco has said it has had contractors working seven days a week capping the tailings. While that work is about 95 percent complete, the tailings don't have vegetation yet and haven't crusted over to prevent dust from blowing, said Aldrich, Asarco's vice president for environmental affairs. The company is "doing everything we possibly can" to fix the problem and meeting with residents, he said.
"If you look at the desert itself, when winds are up to 45 to 50 miles an hour, even the desert gets some dust off of it," Aldrich said. "Where are the worst dust spots along I-10? Not down by where the mines are; they're up by Picacho Peak."
Freeport, which runs the Sierrita Mine south of Mission, said in a written statement that it also noted intermittent dust blowing from Sierrita's tailings impoundment on April 8.
"Significant amounts of dust were also noted in the general Green Valley area, including the surrounding desert area and dirt roads throughout the valley," Freeport said, adding that its mine uses a comprehensive dust-management program. It took several precautions to prepare for the latest high winds and worked to manage the dust issues that day, the statement said.
For Sahuarita residents Jack and Eileen Belove, the tailings dust blowing from Asarco's neighboring Mission Mine complex was similar to past dust events. Dust landed on their patios in a thick layer and slid underneath their front metal security door. "We couldn't sweep it off. It would just get stuck in bricks. It's like clay, a thick substance, not just sand from the desert," said Jack Belove, who lives in Rancho Resort just east of the tailings. "It can only be taken off by high-pressure hoses," Eileen Belove said.
Kay Davin, another Rancho Resort resident, is on oxygen due to asthma. She said she had to wear a mask when she went outside on April 9, but that unlike past dust storms that blew tailings mainly from the Asarco site, "it looked like the whole desert was blowing everywhere. This time I couldn't differentiate - it was definitely coming off them but it seemed that everything was blowing."
Nancy Freeman, a community activist who lives in the Green Valley area, said that when the winds peaked last week, all cars in her condo complex parking lot were totally covered with a slurry dust. She said she believed the dust was from Freeport's tailings. She lives two miles east of Freeport's Sierrita Mine.
But Stan Riddle, also of Green Valley, said he believes most of the dust that blocked his view of the Santa Ritas was general dust from the desert - not from tailings. Riddle chairs the Green Valley Council.
"Our prevailing wind comes out of the south, going east to northeast, and Sierrita is almost directly west of us," Riddle said. "We get very little in the way of tailings here. Freeport has been very responsive to these issues. The winds that kicked up were so strong that Freeport or Asarco couldn't do anything about it."
If the county does violate air-quality standards over three years, the EPA can declare the county in formal noncompliance with the law and force it to submit an air-cleanup plan. If that plan is inadequate, the EPA could withhold highway funds from the county - an action that rarely occurs.
The county also could seek some relief from potential EPA action by declaring the pollution an exceptional event due to natural causes. That's what the county did, successfully, in 1999, to avoid sanctions. The county hasn't decided yet, Kramer said.
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Contact reporter Tony Davis at email@example.com or 806-7746.