Some of the nation's largest airlines yesterday agreed to stricter inspection and monitoring of the drinking water used on their aircraft, two months after a federal study of 158 planes found that 1 in 8 failed government health standards.

Under a two-year agreement, the airlines will test the water from each plane during the next year and report the results each quarter to the Environmental Protection Agency. The airlines also agreed to disinfect each plane's water tanks every three months.

Before the agreement, each airline had separate testing, disinfection and EPA reporting policies.

In September, the EPA found that drinking water in an estimated 12.6 percent of the nation's aircraft tested positive for various forms of bacteria, including coliform, and failed to meet EPA standards.

The new procedures are expected to be costly for the carriers. They require additional maintenance and that aircraft to be taken out of operation during testing and disinfecting, said Nancy Young, managing director of environmental programs for the Air Transport Association, the Washington-based airline trade group. Young said a cost estimate has not been made.

Tanks in an aircraft's belly supply water to the lavatories as well as the plane's galley. While most aircraft serve either bottled or canned water to passengers, the onboard supply is used for coffee and tea.

Twelve airlines agreed to abide by the new procedures: Alaska, Aloha, American, America West, ATA, Continental, Hawaiian, JetBlue, Midwest, Northwest, United and US Airways.

Southwest Airlines and Delta Air Lines did not participate. Southwest spokeswoman Julie Hatch said the airline has been developing its own water sampling and disinfection system for the past year and plans to implement it fully by the end of the month.

Southwest is also the only airline that drains the water tanks on all of its aircraft each night and refills them each morning, said Air Transport Association spokesman Jack Evans.

Delta spokesman Anthony Black said the airline's current water testing program exceeds the monitoring in the new agreement. Black declined to be specific but said the airline continues to work with the EPA to "provide quality onboard drinking water for our customers and crew."

In its earlier study, the EPA found that of the 158 planes that were tested, the water in 20 had coliform bacteria, which generally do not cause illness, while two planes tested positive for potentially harmful E. coli.

At the time, airline officials disputed the results of the EPA's test and argued that the carriers' water was safe and that no one had ever become sick from it. Yesterday, Young said the airlines agreed to strengthen their water sampling and disinfection programs.

"We worked on this agreement more quickly than any such major agreement within the industry sector," she said. "We did not focus on the costs. We focused on answering the [EPA's] questions."

EPA officials said the agency would continue to monitor aircraft drinking water.

"Today's actions will help the agency develop new regulations for monitoring and maintaining aircraft water systems," said Benjamin H. Grumbles, acting assistant administrator for EPA's Office of Water. "The new regulations will ensure safe drinking water for airline passengers while reflecting the unique characteristics of aircraft."