Beach report: Southie water still the cleanest around Boston Harbor

Beachgoers this summer may want to head to South Boston, which has some of the cleanest urban beaches around Boston Harbor, according to an annual water quality report card from the nonprofit Save the Harbor/Save the Bay.

Pleasure Bay and City Point beaches in South Boston earned perfect cleanliness scores — the only ones out of the 15 metropolitan beaches in the report. Other high-scoring spots included M Street Beach and Carson Beach.

Outside South Boston, Nantasket Beach in Hull and Revere Beach also earned strong marks in the report, which graded public beaches based on how often they were safe for swimming. Chris Mancini, executive director of Save the Harbor/Save the Bay, said Boston boasts some of the "best swimming beaches in the country."

"Almost all of them are on or within half a mile of public transportation. So take the blue line to Revere, take the red line down to South Boston," he said. "It's a great scene. It's great water quality."

The only low grades went to King’s Beach in Lynn and Tenean Beach in Dorchester. Outdated stormwater infrastructure has led to frequent contamination at both beaches, Mancini said.

While most locations scored similar grades to previous years, several saw decreases in water quality, including two historically clean beaches: Savin Hill Beach in Dorchester had a "dramatic" drop, and Nahant Beach dipped slightly, according to the report. Both are still considered safe for swimming.

Save the Harbor/Save the Bay is partnering with researchers and state officials to study why water quality dipped in those areas. Mancini said it could be due to variability in the timing of tests, warming ocean temperatures due to climate change or a new source of contamination from sewage overflows.

Intense storms that have become more common with climate change can trigger these overflows. They happen when storm runoff from heavy rainfall combines with sewage and flows into rivers and streams. Sewer overflows can worsen water quality along Boston's rivers, beaches and waterways.

The report used water quality testing data from the Massachusetts Department of Conservation and Recreation. State officials suggest that all beachgoers wait 24 hours after heavy rainfall before hitting the water, except in South Boston. The construction of the Deer Island Waste Treatment Plant has led to significant improvements in water quality for that area, even during stormy weather.

" You can go swimming in a downpour in South Boston and you're gonna be fine," Mancini said.

Brian Arrigo, commissioner of the state Department of Conservation and Recreation, said despite heavy rainfall so far this year, beaches around Boston Harbor are safe for swimming. Officials suggest monitoring the state's website for updated water quality information before hitting the beach this summer.