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Massachusetts Water Resources Authority


March 27, 2024


Sean Navin
(617) 788-1112,

MWRA Releases Annual Deer Island Outfall Monitoring Report
The Massachusetts Bay ecosystem remains healthy

MWRA consultant (Battelle) staff measure water quality in Massachusetts Bay.

The Massachusetts Water Resources Authority (MWRA) is pleased to report that environmental monitoring results for 2022 show that the Deer Island Treatment Plant continues to operate as designed, and that discharges had no unexpected effects on the ecosystems of Massachusetts and Cape Cod Bays. Water quality is good and ocean communities remain healthy.

The “Boston Harbor Project” was an environmental success

Created in 1984, MWRA helped a polluted Boston Harbor by rebuilding an aging sewage treatment system and moving the effluent discharge from the Harbor to Massachusetts Bay.

The Latest Monitoring Results from the Harbor and Bay

A National Pollutant Discharge Elimination System (NPDES) permit for the Deer Island Treatment Plant requires MWRA to test the effluent before it is discharged into the Bay, and also to monitor the bay’s ecosystem. The 2022 Outfall Monitoring Overview details results from this monitoring program. Some are highlighted below.

  • Sewage solids discharged into the ocean are 10 times lower than in 1990.
  • Annual solids in treatment plant discharges, 1990-2022

  • Toxic pollutants like lead and copper that attach to solids have also decreased.
  • Annual toxic metal discharges, 1990-2022

Ocean communities near the Massachusetts Bay outfall are healthy.

Marine life thrives on an outfall diffuser in Massachusetts Bay

There are more than 100 kinds of animal living near the outfall, which is an indicator of a healthy community. Phytoplankton, tiny algae that float in ocean waters, are at healthy levels.

In the 1990s, some flounder had tumors associated with pollutants from sewage. No tumors have been seen in any fish for many years and flounder health has improved near the outfall.

Dissolved oxygen (DO) in the Bays system has fluctuated because of regional changes.

MWRA monitors to ensure that nutrients in the discharge do not fuel the overgrowth of algae and plants, which can deplete oxygen. We often detect localized, temporary increases in the nutrient ammonium near the outfall and occasionally farther away. In summer of 2022, bottom DO declined to low levels throughout the region, resulting from unusually warm waters and physical factors rather than treatment plant discharge.

Stellwagen Bank National Marine Sanctuary, 12 miles west of the MWRA outfall, supports fisheries and is a major feeding ground for whales. We did not detect nutrients from the discharge in these waters, and the bottom communities have remained unchanged. Bottom-water DO reached unusually low concentrations in Stellwagen Basin (on the western side), but rapidly increased after early winter mixing from storms.

MWRA has been documenting the recovery of Boston Harbor since 1991.

Boston's thriving urban harbor

A generation ago, Boston Harbor was polluted by a failing sewage treatment from a growing population. By the time MWRA was created in 1984, the Harbor was known as one of the dirtiest in the nation; but it has greatly improved from MWRA improvements, including moving the discharge of treated sewage from the Harbor to Massachusetts Bay.

A thriving eelgrass bed near the old discharge location (R08) off Deer Island. This sediment profile image shows healthy eelgrass, well oxygenated sediment, and good water clarity.

Contaminants of Emerging Concern

MWRA is working with other groups to research regional contaminants of emerging concerns, particularly per- and polyfluoroalkyl substances (PFAS), microplastics and pharmaceutical and personal care products (PPCPs). At no cost to ratepayers, MWRA participated in the planning of a pilot study these substances in the effluent and Massachusetts Bay. We are also supporting a research project by Woods Hole Oceanographic Institution, funded by Sea Grant, to investigate the distribution of microplastics throughout the Bay.

More Information

For more information about water quality in Boston Harbor, its tributary rivers and Massachusetts Bay, visit our Harbor and Bay Water Quality Report page.

2022 Outfall Monitoring Overview report


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