Water samples from the city's water system that tested positive for coliform bacteria in early December were found to be clean after additional tests.

City Engineer Larry White said the city tests water samples at 10 residential sites each month, five one day and five the following day. He said test results are normally returned within four to five days, but the lab called the city within one or two days after receiving the positive sample to report its findings.

White said two of the 10 samples tested positive for coliform bacteria. The two sites, and additional residences adjacent to those, were re-tested within days and received a negative test.

White theorized, and said he hoped, the positive coliform results were due to improper testing procedures. He said each collection site, which could even be an outside faucet, is cleaned and disinfected before the sample is taken. He said perhaps the samples were contaminated by fingers accidentally touching the water - but that's just a guess.

He said all municipal water users were notified of the presence of coliform in their water bills. The city also placed a legal advertisement that was published Dec. 29, 2006, in the Lahontan Valley News.

White said coliform bacteria is treated with chlorine at the arsenic treatment plant. He said the chlorine is added to the water and the mix adjusted until a level of 0.20 milligrams per liter residual chlorine is detected.

White said he doubts inadequate chlorine led to positive coliform tests because sufficient chlorine levels were detected at the testing sites furthest from the treatment plant.

On Wednesday, repair work was wrapped up at the treatment plant to repair the base of the backwash tank. The bottom of the 30,000-gallon steel tank showed signs of corrosion, so the supplier replaced the base since the tank is still under warranty.

White said water in the backwash tank is "aggressive" in that it is very acidic with high levels of arsenic and iron.

Before repairs to the tank were begun on Tuesday, the city coordinated with water crews to ensure all treatment reservoirs were full. White said two city reservoirs on Rattlesnake Hill and one at the plant were full, plus two reservoirs at NAS Fallon for a total of 5,700,000 gallons of backup water supply.

He said Tuesday's accident, when a city water main was broken by electrical repair crews, did not affect the treatment plant because of the stored water. White said the city specifically chose this time of year to perform the repair work because "we couldn't take down the tank at the summer peak." He said water users should not have been affected.

Tuesday's broken water main at the intersection of Maine Street and Williams Avenue disrupted traffic for about 12 hours while the street was dug up to access two water mains buried beneath the street. White said he didn't have accurate numbers on the amount of treated water lost, but his rough estimate was 300,000 to 400,000 gallons - about half a day's worth of water. He said city and contractor crews were on scene until about midnight repairing the line.

Christy Lattin can be contacted at clattin@lahontanvalleynews.com