A proposal to fertilize farm land in Fauquier County with free sludge from two sewage plants has some officials in nearby counties worried that storm water runoff from the farms could end up harming local water supplies.

The state Water Control Board has agreed to withhold permission from Bio Gro Systems Inc. of Annapolis, which wants to distribute the sludge, and to hold a public hearing on the issue April 19.

"We are concerned about the effect on the Occoquan reservoir," Prince William County Supervisor Kathleen K. Seefeldt said last week. "Portions of Fauquier are in the Occoquan watershed and drain right into the reservoir."

She said some areas of the country have sludge with high concentrations of heavy metals that make it dangerous to use as fertilizer.

However, Bio Grow spokesman D. Lyle Jarrett said last week the sludge, which comes from the Blue Plains Sewage Treatment Plant in the District and the Piscataway Sewage Plant in Prince George's County, is tested daily by an independent laboratory and is less toxic than most commercial fertilizers.

He said the Washington Suburban Sanitary District pays Bio Gro to remove the sludge and the company already distributes it free to farms in Prince George's County and Dickerson County, Va.

"Fauquier farmers came to us," he said. "We can save them $50 to $100 per acre in fertilizer costs and it's better quality as well."

Bio Gro applied for state permits in November but neighboring county officials said they were unaware of the plan until earlier this year.

"Fairfax County is not totally against the idea," Fairfax County Executive Jay Hamilton Lambert said. "The county just wants more information about the potential impact."

Sludge from such states as New York and New Jersey may be unsuitable for fertilizer, but sludge from the Washington area is much purer and safer, said Ed Miller of the Virginia Water Control Board.

Miller said heavy industry in the New Jersey area causes high concentrations of metals and toxics in the sewage system.

"That's not a problem at Blue Plains and Piscataway because there is very little heavy industry around here," he said.