Something stinks down at the C&O Canal. A bouquet that brings to mind dirty socks, rotten eggs and broken toilets hangs in notorious stench zones, waiting to ambush park visitors.

For years, the slow-flowing canal water took the blame for the reek. Maybe the flow was stagnating and turning funky. "I figured it was a necessary evil if you were going to have a nice canal next door," said Burr Gray, president of the citizens association in the occasionally malodorous neighborhood of Cabin John.

But in fact, the canal water is innocent. The aroma is actually Essence of Loudoun. Fragrance of Fairfax. Montgomery No. 5.

A huge underground sewer pipe runs 43 miles from Dulles International Airport to the Blue Plains treatment plant in Southwest Washington, collecting sewage along the way. In the C&O Canal National Historical Park, more than two dozen vents in the pipe belch pungent hydrogen sulfide gas. Some of the vents are a few yards from the canal towpath and the Capital Crescent Trail.

Flush it in Dulles, inhale it in D.C.

The Potomac interceptor--as the pipe is known--and its vents (most of which are along the Potomac River) are the responsibility of the D.C. Water and Sewer Authority, which operates Blue Plains. Because all the sewage in the pipe originates in bathrooms in Loudoun, Fairfax and Montgomery counties, the authority charges those users to maintain the pipe, and they will have to pay to fix the smell, according to Mike Marcotte, the authority's chief engineer.

But critics say the authority is moving too slowly, and yesterday a lawsuit to force it to correct the situation was filed in U.S. District Court by the American Canoe Association, the Potomac Conservancy and the Canoe Cruisers Association of Greater Washington.

The stink has been evident for more than three decades, almost since the pipe was built in the early '60s, according to longtime park users. In 1962, the District signed an agreement with the National Park Service, promising to install and maintain charcoal filters in the vents to control the odor. But the filters either were not installed or not maintained, Marcotte said.

"If you could just imagine for a minute openly venting a huge sewer in Yellowstone, or the Grand Canyon or one of the other well-visited national parks," said Matthew Logan, executive director of the Potomac Conservancy. "It's horrendous."

The Park Service, the steward of the canal park, has been meeting with the sewer authority to try to reach an agreement on what should be done.

Some particularly egregious vents are near the Chain Bridge and Lock 10, according to Logan. Another bad one is down a path in the woods near Fletcher's Boat House, where people come to rent canoes.

Marcotte, who would not comment on the lawsuit because the authority had not received copies, said many complaints came in this past summer, when the drought may have caused a more concentrated--i.e., yuckier--mixture to move through the pipe.

After years of suburban development, the six- to eight-foot-wide interceptor is nearing capacity. The vents keep air flowing through the sewer to prevent backups and slow corrosion of the pipe.

Marcotte said the authority hopes to have recommendations on how to control the smell by the end of the year.

On a recent afternoon, an ill wind blew through the grate near Fletcher's Boat House, bending nearby vegetation.

This is nowhere near as bad as it gets, said Ray Fletcher, who runs the boat concession. He said the smell spikes in mornings and evenings and is particularly bad on hot days.

Until they speak to Fletcher, people assume the smell is from the canal.

"That's too bad," he said. "The canal is wonderful, and it gets this reputation as this stinking, smelling thing."

In parts of Cabin John, the summertime odor sometimes cancels outdoor barbecues, said resident Missy Cochrane. Ascribing it to the canal was a pleasant form of denial.

"As long as you have the romantic notion that it's somehow the canal, it's okay," Cochrane said. "It's a totally different thing to realize you're hanging around an open toilet."

CAPTION: Park visitors often blame the C&O Canal for the odor coming from this sewer vent near Fletcher's Boat House.