The Gilmore administration, in a rare rebuke of a local government's environmental efforts, has urged Fairfax County to crack down on home construction that has sent silt streaming into a tiny lake in recent years.

Although Lake Martin in Oakton covers less than eight acres, the dispute reflects one of the largest problems complicating the health of the Chesapeake Bay: Development increases the amount of impermeable surface in the bay's huge watershed, boosting the runoff of harmful nutrients and sediments into its tributaries.

In a letter last week, the Virginia Soil and Water Conservation Board asked county officials to immediately halt all regulated construction projects in the 324-acre Lake Martin watershed "until all damages to the lake and contributing tributaries have been abated." Streambeds should be restored and the lake's sediment deposits dredged by March 31, the board said.

"We realize that's a very ambitious schedule, but this problem has been lingering for over two years," David G. Brickley, director of the state Department of Conservation and Recreation, said yesterday.

Brickley described the intervention of Gov. James S. Gilmore's administration as "not usual." He said the conservation board merely requested corrective action--instead of seizing control of remediation efforts, as it could have done--because "I'm a strong believer in cooperation." He added, "I've got a lot of confidence in Fairfax County."

However, in a letter he wrote to the county last month, Brickley had said that although state officials were "initially confident" that aggressive steps to protect the lake would be taken, "Unfortunately, actions by the county to date have not resulted in cessation of impacts."

About 30 homes, many averaging $700,000 in price, have been built recently or are under construction near several streams that feed Lake Martin, which was created in 1972 and is privately owned. Work continued yesterday as Fairfax County sought an opinion from the state attorney general about whether it has the power to order builders to stop.

"If we have the authority to comply with this order, of course we're going to do it," said Katherine K. Hanley, chairman of the Fairfax County Board of Supervisors. "Certainly, we don't want to be in violation of any sediment and erosion-control laws, and if we can enforce the state's request, I believe we will."

Yesterday, county officials detailed the hundreds of inspections of the Lake Martin sites since 1997--and their issuing of 13 violation notices and two stop-work orders--to bolster their contention that Fairfax has been vigilant in upholding environmental quality. "This commitment by the county far exceeds that required by the state," County Executive Robert J. O'Neill Jr. wrote in a letter to Brickley.

Between last December and September, sediment in Lake Martin increased by 2,500 cubic yards, a county survey found, the equivalent of 350 dump-truck loads. "Where I could canoe three years ago, I can walk now with no trouble," said Chris Koerner of the Fox Heritage Homeowners Association, whose members live in the area immediately surrounding the lake.

But the principle upstream developer, George S. Webb of the Airston Group, called the estimate of 2,500 cubic yards so huge it was "literally impossible" and said the homeowners association was merely trying to find a way to get outsiders to pay for the dredging of silt that accumulated long before recent development.

Last week, the Chesapeake Executive Council, the bay's principle policymaking body, agreed to try to reduce the transformation of open space as a way to slow the buildup of nutrients and sediment. Of the six states and agencies represented on the council, only Virginia opposed setting a goal of cutting the loss by 30 percent, arguing that land-use controls were a local responsibility.