Several times each week, Eymund Diegel does something unthinkable to most New Yorkers: He dips his bare hand in the Gowanus Canal.

Mr. Diegel, an environmental planner who may well be the foremost expert on the hydrology of the Gowanus watershed, is fully aware of the toxicity of the brackish, green-gray waters of the canal. The Gowanus, in Brooklyn, was declared a Superfund site in 2010, marking it as one of the three most polluted locations in New York City, along with Newtown Creek, on the Brooklyn-Queens border, and a former thorium plant in Ridgewood, Queens.

Yet there was Mr. Diegel on a pleasant Thursday morning, paddling along in a canoe, collecting samples from the 1.8-mile canal for analysis by the New York City Water Trail Association. It should be noted that each of his hands has five fingers, a normal hue and full function.

“The Gowanus Canal is a crime scene,” Mr. Diegel said. “If you know you’ve got a crime, you have to ask what caused it.”