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Massachusetts Water Resources Authority


December 13, 2021


Ria Convery
(617) 788-11105,

MWRA Releases Annual Deer Island Outfall Monitoring Report


Lush growth, primarily sea anemones, on an active outfall diffuser in 2020

MWRA's Outfall Monitoring Overview is an annual report that summarizes the results of MWRA's monitoring of the environmental effects of the effluent discharges from the Deer Island Treatment Plant. MWRA monitors the effluent, as well as the water, sediment and the health of fish and shellfish in Massachusetts and Cape Cod Bays. 2020 marked the 29th consecutive year of monitoring and 20 years since the effluent discharge moved to Massachusetts Bay when the outfall came on-line.

The COVID-19 pandemic impacted environmental monitoring in 2020, as safe alternative sampling protocols were developed and then implemented between MWRA, it's regulators and sampling partners. Gaps in data, due to COVID-19, precluded calculation of three Contingency Plan thresholds, which was acceptable to regulators in the context of disruptions due to the pandemic. Regulators and members of the advisory panel expressed their appreciation for how proactive and resourceful MWRA and its consultants were, to keep the impacts of the pandemic so limited.

MWRA is pleased to report that test results for 2020 show that the Deer Island Treatment Plant continues to operate as designed, and that discharges through the MWRA Outfall had no unexpected effects on the waters or ecosystems of Massachusetts and Cape Cod Bays. Water quality remains good and both the plankton and seafloor animal communities remains healthy and diverse.


Summary of Effluent Quality Monitoring

  • Deer Island Treatment Plant earned a fourteenth consecutive Platinum Peak Performance Award from the National Association of Clean Water Agencies for 100% compliance with permit effluent limits;
  • Low rainfall of 38 inches (below the 44 inch average held from 1990-2020) this year meant effluent flow from Deer Island in 2020;
  • Virtually all flow received full secondary treatment;only trace amounts of primary-treated effluent were blended with fully treated effluent prior to discharge.
  • Total Suspended Solids loads from effluent were less than 12 tons/day, a fraction of the solids load discharged in the early 1990's;
  • Metal loads and organic contaminant loads (e.g., poly-chlorinated biphenyls or PCBs) in Deer Island effluent also remained low.
  • Flounder liver disease remains low.

MWRA is also playing an active role working with other agencies and institutions in researching regional CECs (contaminants of emerging concerns, e.g., per- and polyfluoroalkyl substances, microplastics and pharmaceutical and personal care products). At no cost to ratepayers, MWRA participated in the planning of a multi-agency pilot study investigating PFAS and PPCPs in Deer Island effluent and in Massachusetts Bay. Additionally, MWRA is supporting a research project by Woods Hole Oceanographic, funded by Sea Grant, to investigate and better understand the distribution of microplastics throughout Massachusetts Bay.

solids graph

Solids discharge levels have been reduced by over 90% since 1990

The amount of solids, or organic particles, in MWRA effluent remained low in 2020. Overall, solids discharge levels have been reduced by over 90% since 1990. Solids are a concern as they could make the receiving water cloudy and contribute to reduced dissolved oxygen concentrations in the water and bottom sediments. Also, most metals and other priority pollutants attach to solids, so removing solids also removes metals and other pollutants from the effluent discharge into Massachusetts Bay. Levels of solids and metals are even lower than predictions made before MWRA improved and upgraded sewage treatment.

solids graph

Actual pollutant loads in MWRA effluent compared with early predictions

Metals in effluent remained at low levels in 2020. Metals can be toxic to marine life, and their discharge has been greatly reduced by improved secondary treatment and MWRA’s enforcement of industrial pre-treatment programs.

solids graph

Marine life thrives on an outfall diffuser, July 2020

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.

2020 Outfall Monitoring Overview summary
2020 Outfall Monitoring Overview-full report
Printable pamphlet on Harbor and Bay bacteria monitoring


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