MWRA Releases Annual Outfall Monitoring Results

The Massachusetts Water Resources Authority (MWRA) is pleased to report in the latest Outfall Monitoring Overview that data continue to show no unexpected or serious effects on Massachusetts and Cape Cod Bays from Deer Island Treatment Plant discharges. Water quality remains good and both the plankton and seafloor animal communities are healthy and diverse. A National Pollutant Discharge Elimination System (NPDES) permit requires MWRA to test the effluent before discharging it into Massachusetts Bay, and to also monitor multiple components of the bay’s ecosystem. 2023 marks the 17th year that Deer Island has been in full compliance with the requirements of the discharge permit, earning MWRA an award from the National Association of Clean Water Agencies. 

Summary of 2023 Results

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Total suspended solids (TSS) discharges from 1990-2023.
Total suspended solids (TSS) discharges from 1990-2023.
  • Concentrations of heavy metals like zinc, copper, and lead remain low in the Deer Island effluent discharge, as do organic contaminants like poly-chlorinated biphenyls (PCBs).
  • Massachusetts Bay water quality is good, and data show that the outfall continues to have minimal impact on measured parameters including nutrients, dissolved oxygen, and plankton. However, a large regional algal bloom combined with the overarching changing conditions regionally affected dissolved oxygen concentrations in 2023 (discussed later).
  • Seafloor communities around the outfall are healthy and diverse.  Analyses using data collected during seafloor surveys have shown that the discharge has not influenced the kinds of animals observed in the region.  Instead, the biggest influences of animal community composition are the seafloor sediment (rocky, sandy, etc.) and the water depth.
  • No flounder liver disease was found in flounder caught in Boston Harbor. In contrast, in the late 1980s more than 75% of Harbor flounder examined had early liver disease, a condition called centrotubular hydropic vacuolation (CHV).

Boston Harbor Water Quality

The Harbor’s history of receiving poorly treated, nutrient rich sewage had made it one of the most contaminated harbors in the country. This made the Harbor vulnerable to eutrophication, a condition of low dissolved oxygen resulting from an overgrowth of phytoplankton. After MWRA diverted the Deer Island discharge to Massachusetts Bay in 2000, Boston Harbor experienced a large decrease in ammonium, a nutrient that is present in sewage, a large decrease in pathogens, and increases in bottom water dissolved oxygen.  The Harbor is now an urban waterbody with good water quality and healthy seafloor communities and sediments, and these trends continued in 2023. 

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A mantis shrimp burrow at Deer Island flats, near the old Boston Harbor discharge location. The lighter areas show where oxygen has penetrated the sediments, a sign of good health.
A mantis shrimp burrow at Deer Island flats, near the old Boston Harbor discharge location. The lighter areas show where oxygen has penetrated the sediments, a sign of good health.

Environmental Conditions are Changing Throughout the Region

Summertime water temperatures are increasing in Massachusetts Bay and the wider Gulf of Maine. One impact of the rising temperatures was the low dissolved oxygen levels measured in Massachusetts Bay during the late summer 2023 (as well as in 2021 and 2022), as warmer water “holds” less oxygen.  Wind direction has also changed--summertime winds increasingly come from the southeast, which may bring warmer, less oxygenated water into the Bay. Spring and fall phytoplankton blooms are beginning earlier and lasting longer. 

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Summertime surface water temperature in Massachusetts Bay has increased over 1 degree F per decade.
Summertime surface water temperature in Massachusetts Bay has increased over 1°F per decade.


There was a large bloom of the plankton species Tripos muelleri in 2023, which is commonly seen at low levels in Massachusetts Bay.  As the bloom ended in the summer, the dead and dying Tripos consumed oxygen, and may have contributed to the area’s low dissolved oxygen levels. These dissolved oxygen levels were generally lower than historical levels, but not low enough to harm marine life. MWRA is participating in workshops and sharing data with organizations that are assessing this bloom to understand what drives its development and frequency. 

Stellwagen Bank National Marine Sanctuary

Stellwagen Bank supports fisheries and is a major feeding ground for whales. The Sanctuary’s western boundary is about 12 miles east of the MWRA outfall. Monitoring near the Sanctuary has found low oxygen levels, likely a result of these regional changes, but has not identified any effects from the outfall discharge.

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Marine life thrives on an outfall diffuser in Massachusetts Bay, 2023
Marine life thrives on an outfall diffuser in Massachusetts Bay, 2023