Water quality at Boston area beaches declined in 2021, report says

Early morning sun highlighted the sand on Pleasure Bay in South Boston.David L. Ryan/Globe Staff

The overall water quality safety rating for beaches in Greater Boston declined in 2021, according to a new report by environmental group Save the Harbor/Save the Bay, which gave the region’s beaches a rating of 86 percent — a 7 percentage point decrease from 2020 and a six-year low.

The decrease in water quality safety, determined by the amount of bacteria in a sample, is a result of “far more” rainfall events, which have a “significant impact on water quality on many beaches,” occurring in 2021 than in 2020, the organization said. In all, 19 storms exceeded a half-inch of rainfall, while 12 exceeded one inch of rainfall, the report said.

As a result, the report cautioned against using yearly figures to draw conclusions about water quality on the whole.

“These seasonal variations are why Save the Harbor/Save the Bay is reluctant to draw conclusions from results for individual years, preferring to rely on multi-year averages in evaluating overall water quality on the region’s public beaches,” the statement said.

The 2021 rating represents the lowest figure since at least 2016, when area beaches were rated at 96 percent. The average rating for all beaches during the time span is 92 percent.

Fifteen beaches are graded in the annual report, which covers the “Boston Harbor region” — spanning from Nahant Beach to Nantasket Beach. Beaches’ scores are calculated as “the percent of samples that comply with the Massachusetts Department of Public Health single sample limit for bacteria,” according to the report. Weekly water testing for beaches began in late May 2021 and continued through Labor Day.

Despite the heavy rainfall, four beaches received perfect scores — Pleasure Bay and City Point Beach in South Boston, Nantasket Beach in Hull, and Winthrop Beach. Six others — M Street Beach and Carson Beach in South Boston, Constitution Beach in East Boston, Revere Beach, Short Beach, and Wollaston Beach in Quincy — all scored between 82 percent and 94 percent.

“Considering the wet weather, most of the region’s beaches scored quite well, earning A’s and B’s,” said Save the Harbor/Save the Bay’s executive director Chris Mancini in a statement.

However, King’s Beach in Lynn and Swampscott, and Tenean Beach in Dorchester scored well below the year’s average safety rating — each receiving scores in the 60s, according to the report. King’s scored a meager 68 percent while Tenean lagged further behind at 63 percent.

The report also assesses the Public Health Department’s beach posting and flagging protocols, which Mancini said “failed to make the grade again this year.”

“We believe that there are better ways to provide timely and accurate information about beach water quality to those who need it most and would like to work with DPH and other stakeholders to get it right,” Mancini said in the statement.

The group found that 75 percent of red flags posted on Constitution Beach in East Boston in 2021 were wrong and an “astonishing” 100 percent of red flags there were incorrect in 2020. The problem of incorrect flagging, Mancini said, is exacerbated by the fact that the Bureau of Environmental Health does not offer its beach water quality locater in languages aside from English even though many beachgoers speak a language other than English at home.

Working with state and city agencies, “we can provide more accessible, timely and accurate water quality data to beachgoers, improving public access to the beach and better protecting the public’s health,” said Save the Harbor’s director of strategy & communications Bruce Berman in the statement.

Berman said the organization intends to apply the data review process it used to evaluate posting and flagging at Constitution Beach to other beaches in the region going forward.

“On many beaches, simply installing an accurate and accessible rain gauge and making the information available online in real time with a QR code would provide better information. We can — and should — do better than the current system, which relies on yesterday’s results which are a terrible predictor of today’s water quality,” he said.