The trouble started when Alexandria officials asked William J. Blanton to dredge part of its Potomac waterfront so boats could dock at the new Old Town marina, scheduled to open in April.

Blanton said fine, gladly accepting the $425,000 contract. Just after the New Year, Blanton's company, Marine Structural Applications Inc. of Dumfries, Va., built a makeshift pier on the Occoquan River near Lorton. The soggy Alexandria silt would be barged to the pier, unloaded onto trucks and dumped in Occoquan Regional Park at Lorton, operated by the Northern Virginia Regional Park Authority.

At least that was the plan.

But Occoquan residents learned about the proposed dumping and starting complaining.

"Everybody knows how much trouble Alexandria has with hydrilla," said Laurie Dahl, an Occoquan resident, speaking about the fast-growing weed that in recent years has clogged some of Alexandria's marinas. "We keep a boat here and we are worried that the hydrilla will spread here."

Hydrilla, almost unheard of a few years ago, now covers about 1,900 acres of the Potomac. Although lauded by some as a sign that a once-polluted Potomac is clean enough for green aquatic weeds to thrive, hydrilla is proving an expensive pest.

Over the next decade, the Army Corps of Engineers estimates it will cost local, state and federal governments about $3 million to cut through only 1 percent of the thick mat-like hydrilla that clogs river channels and marinas.

Occoquan residents telephoned everybody from the engineers to the Environmental Protection Agency, complaining that they feared hydrilla roots would drop off the barges and start multiplying in their waters.

Apparently listening to the residents' fears, the Northern Virginia Regional Park Authority halted Blanton's efforts Jan. 16, a day before he started scooping.

"I've been tearing my hair out," Blanton said last week. "I don't think there was much logic in the Park Authority's decision. There were safeguards built into the (Army Corps of Engineers) permit."

The Army engineers, aware of how quickly hydrilla spreads, required Blanton to use watertight buckets, load barges only to a minimum level, and offload the dirt in watertight sealed trucks, according to Bruce F. Williams, chief of the permit section at the engineers' Norfolk office.

"But there were some people concerned that this wouldn't be enough," Williams said. The engineers decided Friday to grant a modified permit allowing Blanton to barge the soil to Jones Point below Alexandria where it will be loaded onto trucks and taken to a landfill near Lorton.

David Hobson, the Park Authority's associate capital program director, said he had no comment on why they stopped Blanton. Another park authority employe said she was told to say "no comment" because of a "possible legal matter."

Apparently the authority is concerned that Blanton, who said he spent $40,000 to $50,000 constructing the pier, clearing the roads to the landfill, and installing a retaining wall at the Occoquan riverfront, will sue. He hasn't so far.

"There are just so many regulations, I can't move," Blanton said. Now he says he's grappling with another problem. If he moves the silt quickly as the EPA wants, in air-tight buckets and trucks, it will be soaked when it arrives at the landfill. But the Virginia Health Department forbids the dumping of soil that contains more than 20 percent moisture.

And there's one other rule, the engineers say. Blanton must be finished with the dredging before March 15. That's when fish start spawning, not to mention the beginning of the boating season.