In the murky waters of an abandoned Prince William County quarry, Virginia environmental officials have discovered the region's first colony of zebra mussels, setting off a race to stop the aggressive creatures from reaching the Chesapeake Bay.

The dime-size mussels, originally from the waters of Russia and Ukraine, have spread like a plague through the Great Lakes, killing off marine life, clogging drinking water intakes, clinging to anything solid. It costs more that $5 billion a year to control them.

"It's an extremely aggressive filter feeder and can overfilter the water to the extent that it can lead to almost sterility," said Ann Swanson, executive director of the Chesapeake Bay Commission. "They are difficult or impossible to eliminate once established."

Swanson said zebra mussels live in fresh water and probably couldn't survive in the saltier parts of the bay. But because the creatures have found their way into the Occoquan River watershed, the more immediate concern is that they could affect Lake Manassas and the Occoquan Reservoir, which together provide drinking water for 600,000 area residents.

Virginia's struggle with its clingy new visitors mirrors Maryland's epic battle last year with the snakehead, an aggressive, invasive fish that became established in an Anne Arundel County pond.

So far, state officials say repeated inspections of Lake Manassas and the Broad Run tributary have found no evidence that the mussels have migrated out of the quarry. But that could change once the weather grows warmer.

"Everything that could be done should be done to prevent them from coming into Virginia's drinking water sources," said Jeanne Bailey, spokeswoman for the Fairfax County Water Authority, which operates the Occoquan Reservoir and has stepped up monitoring efforts.

The mussels were discovered last fall by a diver in the Millbrook quarry near Haymarket. The spot is popular with divers and scuba students and features a sunken boat, airplane and school bus.

A dispute with the quarry's owner and the Dive Shop, the Fairfax County business that controls access to it, has prevented state scientists from conducting more tests and moving to eradicate the mussels, according to state officials. An emergency bill passed by the General Assembly that would give state officials access to the quarry and the right to do what is necessary to eradicate the mussels is on its way to the desk of Gov. Mark R. Warner (D). In the meantime, County Board Chairman Sean T. Connaughton (R-At Large) directed the county staff yesterday to try to speed up the investigation.

"This is very serious stuff," Connaughton said. "If they are reluctant to let us on the property, we may need to use county authority and resources."

Ray Fernald, an official with the Virginia Department of Game and Inland Fisheries, said, "We're just not able to do everything we need to do without more free access into the quarry as soon as possible."

Maryland officials eliminated the snakehead infestation by dumping poison in the Crofton pond. Virginia officials are investigating how to deal with the mussels before they spread. The mussels can be killed with chlorine or other poisons, and they cannot tolerate very hot or very cold water, Fernald said.

"However, all of these treatments have their limits. We don't know whether they would be effective or safe in a quarry," he said.

The zebra mussels have spread throughout the Great Lakes since they were first spotted in the mid-1980s. Juvenile mussels are the size of a grain of sand and can be easily transported on the propeller of a boat, the feet of a duck or in the equipment used by scuba divers.

Beyond the Great Lakes, they have spread quickly through the Mississippi River drainage area and other waterways east of the Rockies.

They also have been spotted in a lake in New York State and in a quarry in Pennsylvania. Both of those sites are within the 64,000-square-mile Chesapeake watershed.

Along with the Virginia discovery, that makes "three isolated situations. And there is great hope that they will remain isolated. But we don't know that," Swanson said, adding that there are fears the mussels could be transported by boats and other equipment into the upper reaches of the bay where there is more fresh water. "It is certainly cause for concern and something that can be carefully monitored."

John Wall, manager of the Dive Shop, said state officials have acted in bad faith by sneaking into the quarry as civilian divers. He said state officials are welcome at the quarry as long as they announce themselves and sign a liability waiver like anybody else.