Tucson hopes to start construction next year on a $10 million plant to clean up another cancer-causing chemical from south-side groundwater, more than 15 years after authorities began cleanup of TCE and TCA from that aquifer.

This time authorities are targeting 1,4-dioxane, which has not been removed from the groundwater as the other solvents were being cleaned up.

Treated water containing low amounts of that carcinogen has been delivered for years to homes and businesses in a wide area of Tucson (see map, Page A4). Federal officials say the health risk from drinking that water is "exceedingly low," but the compound is significantly more toxic than previously believed.

City officials hope to get reimbursed for money needed to build and to operate the plant - pegged at another $250,000 annually - by the Air Force. It was Air Force contractors who dumped the polluting solvents into the groundwater starting more than a half-century ago.

Andrew Quigley, interim Tucson Water director, and U.S. Environmental Protection Agency officials said they're also hopeful of getting an agreement with the Air Force soon, perhaps by July 15, to allow negotiations to move more quickly toward a commitment to reimburse city cleanup costs.

Officials at Davis-Monthan Air Force Base had no immediate comment on the city's statements. The matter of Air Force reimbursement has been forwarded for review to Air Force headquarters at the Pentagon, said Master Sgt. Heather Legg, a D-M spokeswoman.

City officials discussed the plans for the treatment plant Wednesday before the City Council and in interviews this week with the Arizona Daily Star.

Quigley and Deputy City Manager Richard Miranda said a federal study concluded last August that the dioxane compound is nine times more toxic than previously thought.

The city money for the treatment plant would come from previously approved bonds that had not been targeted for any specific projects but were included in the city's current, fiscal 2011-12 financial plan.

The plant would be built next to the existing Tucson Airport Remediation Project (TARP) plant. The existing plant has been removing TCE and TCA from the groundwater for many years - but can't remove the dioxane.

The current federal drinking-water health standard for 1,4-dioxane is 3 parts per billion. The treated south-side groundwater now contains about 1 to 1.15 parts per billion, city officials have said.

The new federal study on 1,4-dioxane essentially means the maximum level of the compound in drinking water should be about 0.3 of a part per billion, a little less than one-third the level currently found in the treated south-side groundwater, an EPA toxicologist said Wednesday. The federal report stems from a 2009 Japanese study of the effects of the chemical on laboratory mice and rats.

The city's equipment can't detect the dioxane at levels below 1 part per billion, meaning the city will also have to use new equipment able to detect the dioxane at lower levels once the new plant is built, officials said.

Officials hope to have the new treatment plant designed by the end of this year, to start construction in early 2012 and to have it operating in about 20 months, Miranda and Tucson Water administrator Jeff Biggs said.

"We felt it was important that we start the project and deal with the bureaucracy later," Miranda said. "The legal issue is that we feel the Air Force needs to pick up the tab on this one."

Currently, the south-side groundwater that comes out of the TARP treatment plant is served at the rate of about 7 million gallons daily to about 30,000 homes and businesses outside the south side. It's served in a V-shaped area bounded by 22nd Street on the south, Silverbell Road on the west, First Avenue on the east and River Road on the north, said Fernando Molina, a Tucson Water spokes-man. But there is also a significant amount of additional dilution taking place in this area, because that TARP water is mixed with 8 million gallons of Central Arizona Project canal water from the Colorado River, Molina said.

Also, the area getting the treated water is also served by other wells, he said.

The risk from drinking the groundwater is "exceedingly low" at the 1,4-dioxane levels found in it, EPA toxicologist Gerald Hiatt said.

"Certainly in a situation such as where the TARP water is being delivered as drinking water, we really want to stay as low as we can reasonably," Hiatt said. "We think Tucson Water is being very protective and very responsible in trying to drive the 1,4-dioxane concentrations as low as possible."

Yolanda Herrera, a lifelong south-side resident who chairs a community board that deals with the area's water issues, said she's happy the city is taking a "proactive" approach by building the plant now rather than waiting for the contamination to become a bigger problem.

At the same time, "We want to hold their feet to the fire" to make sure the water is cleaned up," said Herrera, who chairs the United Community Advisory Board.

"The fact that we are actually monitoring this whole situation is a great thing," said Herrera, who said she saw many of her friends and family members get cancer from drinking the TCE-tainted south-side water in the past. "I also think it is good that the EPA is taking down the maximum level of dioxane (allowed) in the water."

Authorities also want to start removing it from south-side groundwater because it's starting to show up in areas of the TCE pollution plume where it hadn't been before, city officials said. With that fact plus the health study, the city can no longer keep the dioxane at low enough levels simply by blending it with other city water as it has until now, officials said.

In recent years, dioxane levels at the southern half of the TARP wellfield's aquifer were stabilizing at about 11 parts per billion. But dioxane levels showed up this past winter in measurable concentrations for the first time - about 1.25 parts per billion. One part per billion is about the same as a drop of water in 22,000 gallons, Tucson Water has said.

Star reporters Rob O'Dell and Carol Ann Alaimo contributed to this story. Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.