The Central Arizona Project, Tucson's main drinking water source, is shut down after the first break in its concrete canal in the project's 27-year existence.

The canal rupture, spanning nearly 500 square feet and discovered early Sunday, could be repaired in less than three weeks but might take longer, depending on the cause, CAP officials said Tuesday.

The break allowed about 400 to 500 acre-feet, or 130 million to 160 million gallons, of Colorado River water to escape into a desert wash about 27 miles east of where the canal begins at Lake Havasu. The 336-mile-long CAP aqueduct ends at Pima Mine Road, 14 miles south of Tucson.

Because the water project stores 326,000 acre-feet of water in the Lake Pleasant reservoir northwest of Phoenix, the shutdown won't cause immediate water cutbacks to CAP customers in Tucson, Phoenix or Pinal County, officials said. The stored water - enough to serve more than 900,000 Tucson families for a year - will suffice for all customers through 2012, CAP spokesman Mitch Basefsky said Tuesday.

In addition, Tucson Water says it has at least 100,000 acre-feet of CAP water, about a year's city drinking supply, stored at three underground recharge basin facilities west and south of the city.

But "it's obviously a big concern when it's something that never happened before," Basefsky said. "We need to find out why this happened and ensure that whatever caused the break here won't happen elsewhere on the canal."

The break was discovered at about 7 a.m. Sunday, 4 1/2 hours after CAP operations crews noticed on computers that a section of canal spanning several miles near the town of Bouse in western Arizona was about a foot low. Once the water level stabilized, the CAP workers decided to wait until daylight to check out the situation, Basefsky said.

As they headed toward the canal, CAP workers were notified by the Arizona Department of Transportation that water was flowing under a bridge on Arizona 72, several miles from the canal, CAP general manager David Modeer wrote to the project's Governing Board Monday.

Shortly afterward, CAP crews discovered the break, and pumping into the canal stopped, Basefsky said. The 400 to 500 acre-feet of lost water was enough to serve 1,200 to 1,500 Tucson families for a year.

It's unusual for a drop of that size to occur when the canal isn't being pumped down deliberately, Basefsky said. But because the drop occurred in a large section of canal, it could have been difficult for CAP workers to find its cause in the dark, he said, adding, "I'd say they took appropriate steps."

The rupture wiped out a side panel, about 12 feet wide and 20 feet tall, and took out about two-thirds of a second panel. Concrete in the panels is about 3 1/2 inches thick and in some areas is reinforced with steel, CAP's website says. The area where the break occurred is not reinforced. Reinforcement is done only in areas where officials suspect problems such as fissures caused by subsidence, Basefsky said.

A CAP contractor was at the site of the break on Tuesday, conducting a soil analysis, Basefsky said. An analysis of the concrete, a chemical analysis and a probe of the canal's overall structural integrity are needed, he said. Project officials are arranging to get other experts there, he said.

"Was the soil not compacted enough? Is there evidence of some kind of hole that developed because of rain behind the panels?" Basefsky said. "Even though a lot washed out, there's enough there to look for clues as to what might have happened. It's like a crime scene, where we're saying, 'What do we see, and what can we infer from what we see?' "

There is no obvious evidence of foul play connected with the break, "but at this point we can't rule anything out," he said.

The canal is fenced and a CAP security force patrols the aqueduct by land and air, CAP's website says. Alarms are in place at all key structures, including pumping plants, turnouts and check structures.

Because Tucson Water has stored a lot of water at its recharge basins, city utility officials say they aren't seriously concerned about the impacts of the break. "These facilities continue to operate regardless of whether the canal is in operation or not," Tucson Water Director Alan Forrest said in a written statement. "With the current situation, our customers need to be aware that we can continue to provide reliability with deliveries of water."

Warren Tenney, an official with Metro Water, a suburban utility that has the region's third-biggest CAP allocation at 13,500 acre-feet, said this break needs to get fixed right away, but "I'm confident that staff is doing whatever it takes to get it done."

DID YOU KNOW?

Construction of the $3.6 billion Central Arizona Project was authorized by a 1968 law signed by President Lyndon B. Johnson, more than 20 years after U.S. Sen. Carl Hayden of Arizona first introduced CAP legislation.

Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746. Follow Davis on Twitter@tonydavis987.