The findings came as something of a shock to sun worshipers in South Boston on Friday. Even on hot days, many assumed it was better to bake on the sand with a paperback than risk their health by plunging into the frigid, green-brown waves lapping the shores.

But Boston’s beaches, once synonymous with sewage and sludge, boast some of the cleanest waters of any urban beach in America — cleaner even than the iconic sun-splashed tourist meccas of Waikiki Beach in Honolulu and South Beach in Miami, according to a new report by a local environmental group.


“I just didn’t think the beaches around here were clean,” said Maureen Sullivan, a 49-year-old South Boston resident who was sunning herself at M Street Beach. “A lot of times there’s this brown film on the shore — it almost looks like dirty suds.”

The report, by Save the Harbor/Save the Bay, a Boston-based environmental advocacy group, analyzed thousands of bacteria samples from 15 public beaches in nine communities, from Nahant and Lynn on the North Shore to Quincy and Hull on the South Shore; the tests were taken weekly or daily by state officials, depending on the location.

The results showed that, on average, the beaches were safe for swimming 96 percent of the time last summer.

This is the fourth year that Save the Harbor/Save the Bay has produced its “water quality report card,” but the first time the group has compared Boston’s beaches to popular beaches around the country.

The report found that, as in past years, the beaches in South Boston and Revere were the cleanest in the Boston area, acing 100 percent of their bacteria tests last summer. The beaches in Dorchester and Lynn recorded the lowest scores, as they have in the past, but were still safe for swimming about 88 percent of the time in 2014.


On the national stage, the report found South Boston’s beaches had cleaner water than the beaches in Virginia Beach, Va., Coney Island, N.Y., Santa Monica Beach, Calif., and, yes, Waikiki and South Beach. The finding was based on comparable water quality testing data taken between 2012 and 2014 by local officials in those states and then reported to the Environmental Protection Agency.

“These beaches [in the Boston-area], from best to worst, are significantly better than they were 20 years ago, and they’re significantly better than most of the urban beaches in the country,” said Bruce Berman, director of strategy, communications and programs at Save the Harbor/Save the Bay. “We should be really proud.”

Berman attributed results to the cleanup of Boston Harbor, a decades-long project that Save the Harbor/Save the Bay strongly supported. The harbor was for years a national punchline, branded “the filthiest harbor in America” by Vice President George H.W. Bush during his 1988 campaign against Governor Michael S. Dukakis.

Acting on a judge’s orders, the government spent more than $4 billion modernizing the Deer Island sewage treatment plant in the 1990s and building a 9.5-mile tunnel that carries treated sewage away from the shore and discharges it into the deep waters of Massachusetts Bay.

The Deer Island sewage water treatment plant, in the background, is a major reason that Boston Harbor has become clean over the past years. David L. Ryan/Globe Staff

In 2011, facing another court order to clean up the harbor, the state opened a separate 2-mile-long, $225 million tunnel under Day Boulevard in South Boston to carry sewage and stormwater away from city beaches and to a new pumping station at Conley Terminal in South Boston.


Despite the massive pricetags, the projects had a significant impact, said Frederick Laskey, executive director of the Massachusetts Water Resources Authority.

“It’s great for the folks who live in the city and may not have the means to go to the Cape or the Maine shore to find a clean beach,” he said. “Well, they can find it right here.”

Still, even clean beaches can be forced to close when it rains, flushing bacteria and contaminants from sewers and drains into the ocean, Berman said. Last year, 51 “red flags” were flown on Boston-area beaches, signifying dangerously high levels of bacteria, compared to 109 in 2013, the report said.

The drop was due to a decrease in rainfall in the Boston area, from 13 inches on average in previous summers to just 7 inches last summer, according to the report.

“The big lesson is that you can swim on most of these beaches in any weather, but you should be cautious,” on the beaches that had lower scores in the report, such as King’s Beach on the Lynn-Swampscott Line (88 percent) and Savin Hill Beach in Dorchester (87 percent).

As for the brown, soapy film that Sullivan, the South Boston beach-goer, complained about, it’s probably a harmless bloom of algae, Berman said. “It smells sweet,” he said, “and, trust me, sewage doesn’t smell sweet.”


But even clean water may not be enough to persuade some residents to dive into ocean. It’s just too cold, said Michael Guarnieri, 55, as he gazed at the water from a beach chair in South Boston, jazz on the radio. “I swim at the YMCA,” he said.

And then there was Alex Barbolla, 26, who said she would never take a dip, no matter how clean the ocean. “I don’t like open water,” she said, as she relaxed on a towel on Carson Beach, chatting with two friends. “I get scared that’s something going to bite me.”

Michael Levenson can be reached at mlevenson@globe.com. Follow him on Twitter @mlevenson.