Manassas will connect to Fairfax County's water system as part of an exchange agreement with the Prince William County Service Authority.

"I think this is a major, major decision. It guarantees our water supply, and it makes the supply stable," Mayor Marvin L. Gillum (R) said after the unanimous vote Monday night by the City Council.

Although the summer's drought has not depleted Manassas's water supply, city officials favor the move because it gives the city greater flexibility in water consumption and a backup in case of disruptions to the Lake Manassas supply, which is the city's sole source for water.

"This opens up a whole new alternative water source for us: the Potomac," City Manager Lawrence D. Hughes said. "The strategic value of this new pipeline is to provide a backup if there ever is a serious mechanical problem at our water treatment plant" on Lake Manassas.

A new pipeline must be built, at a cost of $5.3 million. It will increase by 8 million gallons the amount of water swapped with Prince William County, making it less expensive to supply the city with water.

The 42-inch diameter pipeline, which will follow the alignment of the proposed Route 28 Bypass and will connect with Fairfax's water system in Bull Run, will be completed by August 2001.

"The cost of the project is very attractive to the city," said Allen P. Todd, director of utilities. "If we did not do this, we would have to construct another facility to get more water from the treatment facility to Manassas, which would cost us more and wouldn't provide the same amount of reliability.

"This agreement goes a long way toward securing the stability of our water system for many years," Todd added.

The project will most likely be funded by a bond issue in 2001 and by a 15 percent increase in residents' water bills starting that year, Todd said. Water rates were raised by 14 percent this year, increasing the base residential water rate to $1.50 per thousand gallons.

Under the water exchange agreement, which has been in negotiation for six months, the Prince William County Service Authority will increase the amount of water it swaps with Manassas from 2 million to 10 million gallons a day. This swap will allow Manassas to bring water from Prince William more cheaply than by piping it eight miles from Lake Manassas, and the water from Lake Manassas would likely supply communities in Gainesville.

Manassas also would have the right to purchase 4 million gallons of water a day from Fairfax if the city's future water needs grow beyond what Lake Manassas can supply. Manassas also is considering a plan to increase the amount of water it sells to Manassas Park from the current maximum of 750,000 gallons a day to 1 million.