A proposed detailed study of the Occoquan watershed, Northern Virginia's primary source of drinking water, has been stymied by the refusal of Prince William County to share in the program's cost.

The Prince William Board of Supervisors recently voted 3 to 2 in favor of contributing $7,929 to the $72,000 study but at least four votes are necessary for an appropriation. Two members were absent at the time. Supervisor Kathleen Seefeldt, a principal backer of the project, has been unable to secure the necessary fourth vote to allow the proposal to be brought up again and passed by the Board.

The Northern Virginia Planning District Commission has been working for several years to develop a model of the environmentally critical watershed and reservoir on which to base predictions of the impact of population growth on water quality and supply.

Austan S. Librach, NVPDC director of regional resources, said the other jurisdictions participating in the program approved their shares contingent upon similar action by all participants.

Because of the Prince William action, he said that "others might pull out and it could kill the program entirely." At the least, the NVPDC will have to go back to Fairfax and Loudoun counties, Alexandria, Manassas, Manassas Park and the Fairfax Water Authority for renewed approval, he said.

"We hope to put together a management program that would allow us to analyze the impact of individual development," Librach said. "We're beginning to develop an understanding of what it would take to preserve and clean up" the reservoir that lies between Fairfax and Prince William counties and supplies water to Alexandria, Fairfax and eastern Prince William.

Prince William Board Chairman Alice E. Humphries who could supply the swing vote, said, "So much of our taxpayers money goes into preserving the Occoquan, yet we don't seem to be getting a fair break on the use of the Occoquan."

"We're paying to protect it and then paying for removal of water from it. If it's Fairfax County's reservoir they should be paying for the protection," Humphries said.