Swimming in the Hudson River is unsafe 21 percent of the time because of pollution from sewage, according to a report issued Tuesday that sampled the water from New York City to Troy.
But the study, by the environmental group Riverkeeper, said most places are acceptably clean most of the time, and that illustrates progress.
"That would certainly not have been the case a few years ago," said Andrew Jule of Columbia University, one of two scientists who took part in the study.
The findings also show pollution levels varying widely, even on the same day and within a quarter-mile, which means contamination is "typically a local problem," the report said.
John Lipscomb, director of the project and captain of Riverkeeper's sampling boat, said at a news conference on a beach in Croton-on-Hudson that the variability is good news.
"If it was a universal problem, we'd have to solve it universally," he said. "As it is, we can work on it incrementally ... site by site, problem by problem."
Riverkeeper's president, Paul Gallay, said the 48-page report was titled "How is the Water?" because that's the question Riverkeeper is asked most often.
He urged federal, state and local governments to fund more testing and invest in infrastructure including state-of-the-art treatment plants. He also called for right-to-know laws to inform the public about pollution and for stricter enforcement of clean-water laws already on the books.
In an apparent allusion to tea party Republicans pushing for spending cutbacks, he said, "To make tea, you need clean water."
The Riverkeeper report is based on monthly samples taken from 2006 to 2010 that were tested for bacteria linked to sewage. The samples came from 75 sites along the 155-mile river.
The Hudson has only four designated swimming beaches — and the samples taken there were generally clean — but there are more than 100 other places where people go into the water "unofficially," the group said.
The report complains that residents are rarely informed about pollution in the river. State Sen. Adriano Espaillat of Manhattan said he plans to hold hearings in Albany with a goal of establishing a mandatory notification system.
Espaillat's district includes the North River Wastewater Treatment Plant, which was taken offline after an engine room fire last month, causing about 200 million gallons of raw sewage to be spewed into the Hudson.
"Why is it that I know when there's a rain delay at Yankee Stadium — I get a Twitter, a Facebook, it's on radio, it's on TV — but we cannot know when millions of gallons are dumped?" the senator asked.
When it was working, the treatment plant did fairly well. Samples taken at the point where it returns treated water to the river were unacceptable 11 percent of the time.
A treatment plant in Yonkers made the report's list of seven locations where the results were never unacceptable. The others were the beach where the news conference was held, Dyckman Street in Manhattan, Irvington Beach, Emeline Beach in Haverstraw, Fort Montgomery in Highlands and the Poughkeepsie drinking water intake.
Ten sampling locations had unacceptable readings at least half the time: the Gowanus Canal and Newtown Creek in Brooklyn, Sparkill Creek, the sewage treatment plant at Piermont, the Piermont Pier, the Tarrytown Marina, the Newburgh boat launch ramp, the Kingston treatment plant, Island Creek in Glenmont and Albany's Dunn Memorial Bridge.
Tributaries that flow into the Hudson, such as the Newtown and Sparkill creeks, did not fare well in general. Readings taken at the point where the tributaries meet the river found unacceptable levels of the bacteria 34 percent of the time. The problem could be illegal sewer hookups, leaking septic systems or leaking sewage pipes, the report said.
It said the tributaries may be the reason that the relatively rural area between Bear Mountain and Catskill had worse results than the suburban region of Westchester and Rockland.
The study also found that New York City did better than the Albany region, which has far fewer people. It said New York has been maintaining and upgrading its sewage treatment system, and Albany uses a much narrower and shallower section of the river.