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On a recent drizzly Saturday, the rain wasn’t constant, but the flow into the canal was. Just below a grated metal bridge that connects Park Slope with Carroll Gardens, an almost-clear liquid could be seen streaming out of a cobblestone storm drain and into the Gowanus Canal.

The liquid was rainwater mixed with raw sewage – paint thinner, last night’s dinner scraps and human excrement, among many other possibilities. About 377 million gallons of sewage flow into the canal each year, according to a 2008 report by city officials.

The Gowanus Canal, one of two federal Environmental Protection Agency “superfund” cleanup sites in New York City, has been the ever-present industrial scourge in the middle of an otherwise desirable, brownstone-filled section of Brooklyn. And though city, state and federal government cleanups have started, they are expected to take decades to complete, and cost hundreds of millions of dollars.

For a dedicated group of two dozen or so local residents, it is not worth the wait. They view the Gowanus as their backyard, and the cleanup as their responsibility. They have begun using the canal as a testing ground for their own brand of environmental activism.

Eymund Diegel, 50, an urban planner, spent a recent Saturday running around an empty lot next to the carcinogen-filled Gowanus. In his hands, and then above his head, were a big yellow kite and a tiny digital camera strapped on with his daughter’s Hello Kitty hairbands.

This is his idea of a fun Saturday, but the kite-flying also served a purpose.

When the camera and kite come back down to earth, Mr. Diegel uses the photos to create high-resolution maps of the canal. By analyzing the images, he could spot where street runoff streams into the canal when it rains. He then shares that information with residents who plant gardens meant to capture the runoff before it hits the canal.

“You should be able to take pride in your backyard,” Mr. Diegel said. “This should be able to reflect the standards that we’re capable of.”

But there are hundreds of millions of gallons of obstacles in the way.

The Gowanus is one of several combined sewer overflows throughout the city. When it rains, the sewer system is inundated with water. Instead of letting it overflow into the streets, or overload treatment plants, the city lets it pour into places like the Gowanus.

The New York City Department of Environmental Protection plans to reduce the sewage content of the canal by repairing a tunnel that flushes fresh water into the Gowanus by next year. But the repair won’t eliminate the raw sewage problem.

That will take many more millions of dollars and many more years.

“I’m trying not to focus on the fact that there’s still sewage going into the canal,” said Alison Croney, 29, who helps with a composting program in an empty lot next to the Gowanus. “I focus more on what I can do in the moment.”

Ms. Croney and the other volunteers acknowledge their efforts pale in comparison to the large-scale government cleanup. But it doesn’t seem to matter.

“Once you begin to take control of your own environment and make changes on your own without asking other people’s permission, the canal became my backyard,” said Hans Hesselein, 30, an employee of the Gowanus Canal Conservancy, a nonprofit that educates residents and school groups about the canal.

Mr. Hesselein gets paid to help with the cleanup, and though he’s not required to be there on weekends, he often is, pulling out weeds and replanting gardens dirtied by the exhaust of idling buses and oil from industrial sites.

To him, and the others who spend their free time pitching in around the canal, their cleanup is less about realistic expectations, and more about claiming a former industrial wasteland as home.

“It’s incredibly empowering,” Mr. Hesselein said. “The canal gave me a purpose.”