Officials at the federal Environmental Protection Agency on Wednesday released the results of its study of the canal, which was named a Superfund site last March over the objections of the Bloomberg administration.
The cleanup, which officials at the agency said would surely involve major dredging, is expected to start by 2015 and last 10 to 11 years, at an estimated cost of $300 million to $500 million, to be paid for by polluters.
Tests of water, sediment and tissue from fish like striped bass and white perch showed heavy contamination from a combination of industrial and sewage discharges, some of which continue, federal officials said. The canal, in Brooklyn, is polluted with more than a dozen contaminants, including suspected carcinogens like polychlorinated biphenyls, or PCBs; metals like mercury, lead and copper; and debris, including sunken vessels.
For more than a century after it was carved out of tidal wetlands and streams in the 1860s, the Gowanus served as a teeming route for oil refineries, chemical plants, tanneries, manufactured-gas plants and other heavy industry along its banks.
Most of the industrial traffic has faded since the 1960s, although waste still flows into the canal, and it is used by some businesses and recreational boaters from neighborhoods near it, including Park Slope, Cobble Hill, Carroll Gardens and Red Hook.
The most prevalent pollutant in the canal, officials said Wednesday, is a group of chemicals known as P.A.H.’s, for polycyclic aromatic hydrocarbons, another suspected carcinogen. They are formed during the incomplete burning of coal, oil, gas, wood, garbage and other organic substances, and were found mostly near former manufactured-gas plants along the canal.
In a telephone conference call with reporters, the environmental agency’s regional administrator in New York, Judith A. Enck, repeated warnings that people should not swim in the canal or eat fish from it. While boating should not be restricted, Ms. Enck said, recreational users should avoid coming in contact with the water.
“What we found is no surprise,” she said. “The report paints a pretty serious picture of the level of contamination.”
Ms. Enck said the contamination was so severe that she was not ready to say whether the Gowanus would ever be “swimmable and fishable.” On a positive note, she said air samples from around the canal did not reveal contamination above “acceptable” safety standards.
In opposing the designation of the Gowanus as a Superfund site, the Bloomberg administration had argued that the label could set off legal battles with polluters, prolong the dredging operation and scare off developers. It proposed its own cleanup plan.