Residents in southeast Napa looked on late Monday morning as 10 fire hydrants drained approximately 160,000 gallons of water onto residential streets due to a chlorine spill into the city's water supply.

Barry Martin, community outreach coordinator for the city of Napa, said a large amount of chlorine was accidentally added to the city's Imola water tank early Monday morning. The over-chlorinated water reached businesses and homes roughly located inside and around Shurtleff Avenue, Seville Drive, Imola Avenue and Cayetano Drive, reaching about eight square blocks, Martin said.

No other areas were affected. "When they recognized this happened, they did strategic shutdowns to make sure over-chlorinated water didn't go beyond that limited area," he said.

Water in the affected areas contained about the same chlorine concentration as swimming pool water, Martin said. "They would have to drink a lot of it for it to affect their health. Nevertheless, people need to know."

Those living in the affected area can flush out any excess chlorine by running a large spigot for about 10 minutes, said Martin.

Sodium hypochlorite, a liquid similar to household bleach, is used to kill bacteria in the city's water supply. Martin said the standard concentration of the chemical in city water is one part per million or less. Water in the affected areas contained more than four parts per million of sodium hypochlorite, he said. After treatment, sodium hypochlorite is the only additive present in the city's water.

While an investigation is pending, Martin said, the over-chlorination might be traced back to an automatic valve at the Imola tank.

"They were performing annual maintenance and inspection. As part of the process of bringing it back on-line, they washed the inside of the tank with chlorine and began refilling it. Somewhere during that process, this valve was (left) in an open position," said Martin.

Phillips Edison Partnership School on Shetler Avenue was among those affected by the over-chlorination, said Don Evans, school district administrator of general services and facilities. He said Pat Costello, a water resources specialist for the city of Napa, contacted the school at about 10 a.m. Monday.

"They were on the school site testing the water. … We initiated shutting down drinking fountains and brought in bottled water. An hour later, the city said to go ahead and flush the system for 10 minutes. We followed their directions but everything was fine. They were quick to respond, both physically and by phone. They opened up fire hydrants all over the area," Evans said.

Residents who notice a strong chlorine odor after running a large household faucet for 10 minutes should call the Napa Water Division at 257-9521.