But the reservoir is undergoing expansion work this winter and thus out of commission, leaving the city to test the water as it exits the treatment plant, where the chlorine has had only four hours to do its job, officials said.
Curphey said a more accurate test would have taken the samples closer to where the water flows into homes and businesses. That would give the water more time to mix with the chlorine.
"I think if they would have taken the samples in that other location, they would have been all right," he said.
The city did not have to wait 48 hours before lifting the ban on tap water because new results showed that the level of bacteria was safe and the level of chlorine in the water had increased, Curphey said.
Rob Shipley, a manager of the Ventura Water Treatment Plant, said the water samples could have been contaminated by the lab technician. He noted that bacteria can be carried on people's hands even after washing with soap and water.
"Even though you can use the best quality assurance you can, you can still get a contaminated sample," he said. "That's why we didn't take any chances."
Calkins said the city may never know what really happened at the treatment plant on Tuesday. "We could study it forever," he said, "but you're never going to be able to re-create what happened three days ago."
Times staff writers Mack Reed and Julie Fields and correspondent Paul Elias contributed to this story.
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Making Ventura's Water Drinkable:
Much of Ventura's water was deemed undrinkable Thursday after two sets of tests detected a bacteria that causes diarrhea and stomach cramps. About 65,000 residents were ordered to boil water before drinking, cooking or bathing. But retesting completed Friday showed no bacteria. Officials lifted the water ban after declaring the original tests in error.
1. Four wells up to 90 feet deep draw water from beneath the Ventura River.
2. The water is piped into a reservoir and injected with chlorine to kill bacteria. Purity samples are taken before the water is chlorinated.
3. In the treatment plant, more chlorine is added. So are polymers and alum to help particles bond and settle.
4. Pipes feed the water into tanks where the chemicals are allowed to work.
5. Sediment, dead bacteria, organic debris and other material sink to the bottom of settling tanks.
6. Treated water flows to 520-gallon filtration tanks, where mineral filters trap remaining material. More chlorine is added.
7. Normally, filtered water is pumped to a second reservoir, where chlorine works on it for several more hours. But that 17-million-gallon reservoir is closed for repairs, so water is now sent directly to exit pipes.
8. As the treated water enters city water mains, it is tested again for bacteria, sediment and foreign matter.
9. From intake to output, treatment takes 16 to 20 hours with the second reservoir. Water now leaves the plant within two to four hours.
SOURCE: Ventura Dept. of Public Works
Researched by MACK REED / Los Angeles Times