A chlorine gas leak at the Big Butte Springs water disinfecting plant created a scare for several hours Monday morning.

Alarms sounded at the plant near Fish Lake Road, indicating a leak, said Larry Rains, manager for the Medford Water Commission, which operates the plant. The building and nearby homes were evacuated.

Rains said the leak was initially treated as a serious incident because officials didn't know the extent of the problem. The leak was contained by mid-afternoon and deemed small, said Laura Hodnett, public information coordinator for the commission.

Hodnett said the water is safe to drink for everyone except 68 residential customers who obtain water directly from the Big Butte Springs pipeline. They have been cautioned to boil their water before using it.

The water commission serves 115,000 people in Medford, Central Point, Eagle Point, Jacksonville, Phoenix, Talent and White City. Hodnett said some customers may notice a slightly different taste in their water because a pump station off Foothill Road is chlorinating the water temporarily, but it's safe to drink.

The leak came just weeks before the commission is due to switch from chlorine gas to liquid chlorine for water purification. It's the first leak in the 40 years the commission has used the chlorine gas system, but officials have not found any connection between the pending switch and the leak.

"I don't know if there's any relationship," said Hodnett. "They're two separate systems. It may have just been ironic."

Chlorine gas is injected from cylinders into the water as part of the normal disinfection process. Hodnett said alarms went off Monday morning when monitors detected chlorine gas in the air of the disinfecting plant, but the gas was contained within the room where the leak occurred. The gas was sucked up and neutralized by a system designed to deal with chlorine gas leaks.

Hodnett said she didn't know exactly what caused the malfunction.

The monitor that detected the leak is also connected to a system that automatically turned off a chlorine valve, so the regular amount of chlorine was not being dispensed into the water flowing to customers' taps.

Butte Falls fire department evacuated homes around the disinfecting plant this morning. A hazardous materials response team also was dispatched to the area of the leak.

In 2006, the water commission board decided to convert its disinfecting system from chlorine gas to liquid chlorine. Many water systems are converting to liquid chlorine for safety and security reasons.

Chlorine gas is a deadly poison and was used in World War I.

The switch is planned at the Big Butte Springs facility and the Duff Water Treatment Plant on the Rogue River, the two sites where the region's tap water is treated. The new disinfecting system, which uses sodium hypochlorite (the same chemical in household bleach) costs $1.29 million.

The commission was facing a cost of $816,000 to upgrade the existing structures, so the change has a net price tag of $474,000.

Officials have said fees won't need to be boosted to cover the expense, but the commission will probably shift priorities to accommodate it.

The wastewater treatment plant, managed by Medford public works, also uses chlorine as a disinfectant before discharging treated wastewater into the Rogue River. It switched from chlorine gas to liquid chlorine around 2000.

Reach reporter Meg Landers at 776-4481 or e-mail mlanders@mailtribune.com.