The Fairfax County Water Authority shut down one of its major water treatment plants for five hours yesterday after poisonous chlorine gas began leaking into the facility from two one-ton tanks, county water and fire officials said.

Officials said water quality was not affected, and the leak was confined to one part of the Potomac Water Treatment Plant, in the Herndon area. Customers were asked to conserve water in case it became necessary to make up for water normally processed by the plant.

The Potomac plant, which treats and pumps 150 million gallons of water a day, is responsible for most of the 230 million gallons generated by the water authority every day. Together, the Potomac plant and three plants in Lorton serve 1.3 million residents in parts of Loudoun, Prince William and Fairfax counties.

One employee, who was exposed to the gas while trying to shut down plant operations, was taken to a hospital, treated and released, water officials said.

Two others were evaluated for mild exposure at the plant, said Lt. Raul Castillo, spokesman for the Fairfax County Fire and Rescue Department.

"There was some gas that accumulated in lower areas of the facility," said Castillo, who added that 30 emergency units, including a hazardous materials team, responded to the incident.

Water authority customers, especially those at higher elevations, initially were warned that they might experience low water pressure while the plant was closed. That problem never arose, however, and the plant was reopened by 4 p.m., water officials said.

The leak was believed to be the result of a mechanical failure, although the exact cause had not been determined yesterday, said Jeanne Bailey, a spokeswoman for the water authority.

"Chlorine is a very difficult chemical to work with," she said. "It's very necessary for water treatment, but you need to take special precautions when you're dealing with it, which is what we do."

Fire and rescue personnel arrived at the plant after receiving a 911 call at 10:13 a.m., Castillo said. The plant was shut down and its front entrance closed about 11 a.m. Employees were moved to a section of the facility not affected by the leak, and the rear entrance remained open.

The leak was contained quickly, Castillo said. Inspectors, however, spent most of the afternoon trying to fix the leak and reopen the plant.

Chlorine is a yellowish-green corrosive gas that at high levels can cause chest pain, vomiting and death, according to the Centers for Disease Control and Prevention.

At low levels, it can irritate eyes and lungs. During World War I, it was used as a choking agent, although now it is commonly used to kill bacteria in drinking water and swimming pools and in manufacturing.

Coincidentally, the plant had provided a chlorine-handling refresher course for many of its employees Thursday, which a water official said probably improved their response yesterday.