WASHINGTON -- Hydrogen sulfide gas in a sewer line is the source of a foul odor along the C&O Canal.

The smell has hot spots. From Cabin John and Glen Echo south to the Chain Bridge and into Georgetown, hikers, bikers and motorists notice the odor.

The sewer line is called the Potomac interceptor, and it carries sewage from Fairfax and Loudoun counties in Virginia and Montgomery County to the Blue Plains treatment facility in southeast Washington.

Cabin John and Glen Echo residents' complaints about the smell are on record. News4's Chris Gordon reported that there is an odor control plan dating to 2000.

The National Park Service, which controls the C&O Canal, said it wants the problem solved.

"The Park Service and D.C. water authority, D.C. WASA, have had numerous conversations and meetings and planning events in order to try to ameliorate the smell," said Park Service spokesman Bill Line. "The bottom line is it's there, it's obnoxious, we don't like it, no one likes it."

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The D.C. Water and Sewer Authority said they will build facilities along the canal but first they need building permits from several jurisdictions.

"These facilities will vent the sewer gases that cause the objectionable odors," said WASA spokesman David McLaughlin. "It will treat those vented gases through a carbon absorption system and essentially eliminate the odor problems."

Permits approvals and construction of the facilities could take a couple of years. In the meantime, WASA will take temporary steps to control the smell by injecting chemicals into the sewer to try to eliminate the odor.