Prince William County's supervisors, under threat of massive fines by the state, face a decision today on giving up their fight against joining the region's expensive and controversial new sewage treatment plant.

Virginia's State Water Control Board, meeting Sunday night in Richmond, adopted a resolution "to insist" that affected jurisdictions connect with the $82 million Upper Occoquan Sewage Authority plant and called for legal action in the event of "unnecessary delay."

Meanwhile, sewage authority workmen yesterday began knocking out bulkheads in sewer mains in Fairfax County, sending the first of millions of gallons of sewage flowing toward the plant on the banks of Bull Run.

Prince William, which had been asked by sewage authority officials to begin hooking up yesterday, has refused to do so and has been seeking an extension on the June 30 expiration of its permit for its own Greater Manassas Sanitary District sewer plant.

The state's position virtually ruled out any extension and Prince William County, which would face fines of up to $10,000 a day for discharging treated sewage without a permit, is expected to decide at its meeting today what step to take next.

Donald L. White, chairman of the board of supervisors and an outspoken foe of the Upper Occoquan authority, said yesterday, "We can't go beyond June 30. We'll wait for advice from legal counsel tomorrow. Then we are going to have to take some action." The supervisors are scheduled to meet today.

White, who testified before the state board, said he was generally pleased by a series of eight resolutions adopted by the board on the Upper Occoquan authority.

He said that a further look at rates in the Manassas sanitary district would be required due to "a lot of unknowns right now."

Statements by White and other officials that sewer and water rates would triple and quadriple because of the Upper Occoquam hookup had resulted in a public uproar by residents over the escalation in rates. Those estimated rates for an average Greater Manassas user were scaled back from $140 a quarter to $194 a quarter and are now at $77 a quarter.

White's statement appeared to hold open the possibility that the board would agree to hook up, counting on the state board's other actions to help hold rates within reason.

Those actions included having the Upper Occoquan authority document the estimated $15 million in additional cost added to the construction of the plant by the Federal Environmental Protection Agency's "operable unit concept." This EPA regulation since eliminated by Congress, required that each stage of construction result in a unit that could function alone. That helped escalate plant costs from an entimated $42 million to an actual $32 million. The state board proposed that Congress be asked to reimburse Upper Occoquan the costs resulting from the EPA policy.

The state board also asked that the state provide $3.5 million. Virginia had agreed to pay 10 percent of the construction costs but refused to pay on the amount beyond $42 million. And the state board asked for grants to improve sewer mains in the Upper Occoquan member jurisdictions of Fairfax, Prince William, Manassas and Manassas Park.

In a potentially major action, the state board also called for an outside consultant to reevaluate the Occoquan watershed policy which includes stringent requirements aimed at protecting the Occoquan Reservoir, the source of drinking water for 600,000 Northern Virginians. Prince William County oficials have called for a relaxation of the standards.

Norman Cole, former chairman of the state board and the "father" of the Upper Occoquan plant and policy, said yesterday that the restudy had the potential for "great mischief" if requirements were loosened.