The Fairfax County Board of Supervisors voted unanimously yesterday to oppose a plan by a Maryland company to put sludge from a District sewage treatment plant on thousands of acres of farmland in Fauquier County.

Officials of Fairfax and Prince William counties have expressed concern that the sludge, from the Blue Plains Sewage Treatment Plant in the District, could pollute the Occoquan Reservoir, which supplies drinking water to 600,000 Northern Virginians.

Yesterday's action by Fairfax supervisors follows a similar move by the Upper Occoquan Sewage Authority, the regional planning group, which concluded that the sludge proposal "is in total contradiction" of state efforts to protect the Occoquan Reservoir.

The sludge proposal must be approved by the Virginia Water Control Board, which is expected to vote on the proposal soon. The State Health Department has already given the project its approval.

Farmers in Fauquier County, who asked the company to devise a sludge plan for them, favor the proposal, which could save them thousands of dollars on fertilizer, which can cost $100 to $200 an acre. But the Annapolis-based Bio Gro, which has a contract with the District to dispose of sludge, would give the sludge free to the farmers. The initial project would cover 4,000 acres.

Democratic Supervisor Audrey Moore, who led the opposition against the Bio Gro proposal, said, "Apparently the reason the company is looking at Virginia is that the folks in Maryland have taken a dim view of it."

"The reason that we are in Virginia," countered Bio Gro's Jane Forste, "is that we were asked by the farmers of Fauquier County to come." She noted that the company now applies about 350 tons of sludge a day on farms in Prince George's County.

Bio Gro, which began talking to farmers last summer, received approval from the Fauquier County Board of Supervisors in December. But Prince William and Fairfax officials did not learn of the project until March.

Fairfax officials say their objections are based on plans to place the sludge on top of the land, rather than inject it into the soil. The officials contend that there is a greater chance for runoff into the Occoquan if the sludge is placed on top of the soil, an assertion that Bio Gro officials dispute.

Moore complained that the sludge would be high in phosphorus, which could cause algae to grow in the reservoir. Bio Gro soil scientist Bill Dunlop agreed, but said that much of the phosphorus is not water-soluble and thus the sludge runoff would contain less phosphorus than fertilizer runoff.

Fairfax officials also complained that Bio Gro and state officials have rejected recommendations for regular, independent monitoring by the Occoquan Monitoring Laboratory. Forste said Bio Gro has no objections to that requirement.

On other matters, the board delayed action on a priority list for engineering work on road improvement projects to give the county staff more time to explain why certain projects were excluded or given low priority.

The board also asked the staff for more information on why the cost estimates of rehabilitating the old Fairfax County Courthouse have jumped from $3.3 million to $5.9 million.