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Boston Harbor and Massachusetts Bay
MWRA Environmental Quality Department

Bacterial Pollution in Boston Harbor


Untreated or poorly treated sewage can carry disease-causing microorganisms--bacteria, viruses, and parasites. These pathogens can threaten public health if sewage contaminates swimming areas or shellfish. Indicator bacteria like Enterococcus and fecal coliform are found in sewage. If high numbers of these indicator bacteria are present in the water, there is a risk of disease. Historically, poorly treated wastewater and discharges of sewage solids, or sludge, were significant sources of bacteria to the harbor, documented as far back as the 1930s.

Enterococcus is the measure of recreational marine water quality recommended by the U.S. Environmental Protection Agency (EPA). From July 1998 to August 2000, when all wastewater was receiving secondary treatment and updated disinfection at Deer Island and being discharged to the harbor, bacterial water quality improved so much that most of the harbor met EPA’s most stringent swimming criteria.

Sources of bacterial pollution to the harbor are now local: stormwater, CSOs, boats, animals, and birds all potentially contribute to the problem. These local sources of pollution primarily affect the harbor’s shoreline and beaches, the least well-flushed areas of the harbor. “Hot spots” of bacteria persist at the mouth of the Neponset River and southern Dorchester Bay; in the Inner Harbor, especially Fort Point Channel; the mouth of the Mystic River, and along Wollaston Beach.

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