High bacteria levels from storm runoff over the past week that closed 12 beaches on the South Shore have subsided, leaving only Wollaston Beach closed.

All five ocean beaches in Marshfield, Lighthouse and Humarock beaches in Scituate and Town Beach in Hingham passed followup testing in water samples collected on Thursday.

White Horse Beach in Plymouth, originally listed by the state as closed, is open.

All four sections of Wollaston Beach remain posted, although followup tests showed negligible bacteria levels.

The other 61 salt-water beaches on the South Shore passed bacteria tests this week and are open for swimming.

See water quality test results for each community and for Cape Cod, the South Coast and North Shore.

For more on Quincy beaches, call 617-376-1288, or visit tpl-beaches. For more on Wollaston Beach, call 617-626-4972.

HOW BEACHES ARE TESTED

Sixy-five beaches on the South Shore are tested for intestinal bacteria found in humans and animals.

High levels indicate the possible presence of disease-causing microbes that are present in sewage but are more difficult to detect. Bacterial colonies are filtered from three ounces of water and placed on a gel infused with nutrients and chemicals designed to promote growth.

Left in an incubator, the single cells isolated on the filter grow explosively, forming colonies visible to the naked eye. After one day, the colonies are counted and if they exceed 104 colonies, the beach is closed to swimming.

If the past five samples have a mean exceeding 35 colonies, the beach must also be closed to swimming.

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Marshfield, Scituate, Hingham beaches reopen, Wollaston closed - Wicked Local Braintree