2022 overview

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Massachusetts Water
Resources Authority
Environmental Quality
Department Report 2023-09 
2022 Outfall
Monitoring
Overview

Citation: 
Werme C, Wu D, Libby PS, Carroll SR, Charlestra L, Ellis-Hibbett D, Goodwin C. 2023. 2022
outfall monitoring overview. Boston: Massachusetts Water Resources Authority
Report 2023-09. 61 p. 
Cover photos: 
Photos courtesy of Battelle
Top: Battelle technicians deploying sampling rosette, February 2023.
Bottom: Stern view from February 2023 survey.

2022
Outfall Monitoring Overview 
prepared by 
Christine Werme 
Independent Consultant 
P. Scott Libby 
Battelle
72 Main Street
Topsham, ME 04086 
David Wu, Sally R. Carroll, Lucner Charlestra, Denise Ellis-Hibbett,
and Christopher Goodwin 
Massachusetts Water Resources Authority
Environmental Quality Department
Deer Island
33 Tafts Avenue
Boston, MA 02128 
November 8, 2023

2022 Outfall Monitoring Panel and Committees 
Outfall Monitoring Science Advisory Panel 
Judith Pederson, Massachusetts Institute of Technology Sea Grant (chair)
Robert Beardsley, Woods Hole Oceanographic Institution
Peter Burn, Suffolk University
Virginia P. Edgcomb, Woods Hole Oceanographic Institution
Loretta Fernandez, Northeastern University
Robert Kenney, University of Rhode Island
Mark Patterson, Northeastern University
Jeff Rosen, Corona Environmental Consulting
Juliet Simpson, Massachusetts Institute of Technology Sea Grant
Juanita Urban-Rich, University of Massachusetts, Boston 
Inter-Agency Advisory Committee 
Massachusetts Bays National Estuary Partnership
Massachusetts Coastal Zone Management
Massachusetts Department of Environmental Protection
Massachusetts Division of Marine Fisheries
NOAA Fisheries
Stellwagen Bank National Marine Sanctuary
US Army Corps of Engineers
US Environmental Protection Agency
US Geological Survey 
Public Interest Advisory Committee 
Association to Preserve Cape Cod
Boston Harbor Now
Cape Cod Commission
Center for Coastal Studies
Conservation Law Foundation
Massachusetts Audubon Society
Massachusetts Bays National Estuary Partnership
MWRA Advisory Board
MWRA Wastewater Advisory Committee
Safer Waters in Massachusetts
Save the Harbor/Save the Bay

i 
Plain Language Summary 
This overview is provided in plain language to make the technical information clearer
to more readers. 
Every home uses water for washing dishes, flushing the toilet, taking showers, or
watering the garden. Once the water goes down the drain of our sink, shower, or
toilet, it is called wastewater or sewage . The wastewater from Boston and 42 other
cities and towns flows through underground pipes that lead to the Deer Island
Wastewater Treatment Plant. The treatment plant removes pollutants and kills
disease-causing bacteria that are present in sewage.
The Massachusetts Water Resources Authority (MWRA) runs the Deer Island plant,
which is located in Boston, near the town of Winthrop. The first step of sewage
treatment separates solid material, known as sludge , from the liquid effluent . Sludge
is turned into fertilizer for gardening and landscaping. Treated effluent is sent out into
Massachusetts Bay through a nine-mile pipe and outfall .
The Environmental Protection Agency, the federal governmental agency responsible
for protecting air and water, allows the ocean outfall as long as MWRA tests the
water, sediment, plants, fish, and shellfish to make sure they are not being harmed.
For example, like people, marine animals need enough oxygen to live, and MWRA
checks to make sure that there is enough oxygen in the water. MWRA also looks for
harmful bacteria and pollutants that remain in the sewage effluent.

ii 
What MWRA found out in 2022: 
Ten times fewer sewage solids go into the ocean than in 1990 
While most solid material in sewage is removed as sludge, small particles remain in
the effluent. Many pollutants that are toxic to marine life, such as lead and copper, are
attached to these particles. The line in the graph below shows the decrease in the
amounts of solids discharged from 1990 through 2022. Discharges of other pollutants
have similarly declined. 
Ocean communities near the outfall are healthy 
Nutrients in sewage effluent have not caused too much phytoplankton to grow. 
Phytoplankton are small algae that float in ocean waters. If they get too many
nutrients from sewage, they can grow too much and use up oxygen in the water when
they die and decompose. As in other years, there was more of the nutrient called
ammonium near the ocean outfall than before the discharge, but it did not increase
phytoplankton growth or hurt marine life. 
Less solid material is discharged
into the ocean

iii 
Many types of animals continue to live on the sea floor near the outfall. MWRA
found more than 100 kinds of animals living on the ocean bottom near the outfall, one
indicator of a healthy community. Tiny worms called bristle worms or polychaetes
were the most common creatures. These animals live on the ocean floor and are
affected more by the types of sand or sediments they live in and how deep the water
is than by the effluent from the outfall. 
Fish and shellfish are healthier. MWRA has been checking flounder, lobsters, and
mussels in Massachusetts Bay and Boston Harbor since 1992. In the early years,
some flounder had tumors, likely caused by pollutants in poorly treated sewage. No
tumors have been found in any fish for nearly 20 years. Sometimes, fish show other
signs of disease, but fish in 2022 were among the healthiest ever seen. 
Boston Harbor is a thriving urban harbor 
After MWRA stopped sending effluent and sludge into Boston Harbor more than 20
years ago, the water and harbor sea floor got cleaner. MWRA still checks Boston
Harbor to see how much better it has become. 
MWRA tests the water at many locations in Boston Harbor.

iv 
Report Highlights 
This 2022 Outfall Monitoring Overview summarizes results from the MWRA
monitoring program in Massachusetts Bay and presents highlights from its
companion program in Boston Harbor. MWRA began to monitor the harbor in 1991,
when aging treatment plants were discharging wastewater effluent and sludge to the
harbor. Monitoring in the bay began in 1992, in anticipation of September 2000,
when effluent discharges were diverted from the shallow harbor to the deeper and
more open waters of Massachusetts Bay. 
The Boston Harbor Project has been an environmental success 
Looking at Boston Harbor today, it is hard to imagine how polluted it was a
generation ago. When the Massachusetts Water Resources Authority (MWRA)
was created by the state legislature in 1984, the harbor was known as one of
the dirtiest in the nation. Sewage from two failing treatment plants was
separated into its liquid and solid components for treatment and disinfection,
but then both the liquid effluent and solid sludge were discharged into the
harbor. The Boston Harbor Project ended all discharge of sludge, converting it
instead to fertilizer, and took steps to minimize the effects of effluent
discharge. 
Source reduction prevents pollutants from entering the waste
stream. MWRA has championed projects to lessen household hazardous
waste disposal and minimize mercury discharges from hospitals and
dentists. The Toxic Reduction and Control department works to ensure
that many pollutants never reach the treatment plant. 
Improved treatment removes pollutants before discharge. A series of
upgrades to MWRA’s Deer Island Treatment Plant started with a new
primary treatment plant, which separates the solid material from the
liquid effluent. Batteries of secondary treatment, which greatly
improves pollutant removal, began operation in 1997–2001. All effluent
is disinfected before discharge. Most pollutants now meet federal and
state standards when they are discharged, even before dilution. 
Better dilution further protects the environment. In September 2000,
the MWRA effluent discharge was diverted from Boston Harbor, through
a 9.5-mile tunnel and diffuser system to Massachusetts Bay. The bay
outfall’s location and distance from shore ensure that sensitive
resources are protected. Initial effluent dilution in the bay is about five
times greater than that of the harbor outfall it replaced, and the
greater depth ensures that summertime discharges are beneath the
sunlit range where plant growth occurs, preventing the possible
overstimulation of algal growth by effluent nutrients.

v
A National Pollutant Discharge Elimination System (NPDES) permit regulates
MWRA’s Deer Island Treatment Plant discharge. The permit requires MWRA to
monitor the effluent before discharge, as well as the receiving water, sea floor, and
fish and shellfish in the bay to detect any unexpected or adverse effects. A
Contingency Plan attached to the permit mandates responses by MWRA and the
regulatory agencies in the event that monitoring detects a potential environmental
concern.
It has been more than two decades since the Deer Island Treatment Plant first
discharged treated wastewater into Massachusetts Bay. In 2022, MWRA earned a
Platinum 16 award from the National Association of Clean Water Agencies for 16
consecutive years with no exceedances of effluent permit limits. The health of Boston
Harbor has improved considerably over those past decades, without harming
Massachusetts Bay. Monitoring has shown no unanticipated or meaningful effects of
the treated wastewater on the water, the sea floor, or the fish and shellfish. 
Deer Island Treatment Plant produces cleaner effluent 
Preventing pollutants from entering the wastewater system is the first step for
ensuring “clean” effluent. MWRA has championed projects to lessen household
hazardous waste disposal and minimize mercury discharges from hospitals and
dentists. An industrial pretreatment/pollution prevention program works to ensure that
many toxic contaminants never reach the treatment plant.
One of the most important measures of effluent quality is the suspended solids load,
the total amount of solid material discharged over each year. Primary treatment
removes most of the solid material (“solids”) from sewage as sludge, but small
particles remain in the liquid effluent. The 2022 effluent solids load was consistent
with other recent years and far lower than amounts once discharged into the harbor
(Figure i, top). Discharges of toxic metals have also greatly decreased, as most
pollutants attach to the solids and are removed with them (Figure i, bottom).

vi 
Figure i. Annual effluent solids (top) and toxic metals (bottom) discharges, 1990–2022. Source
reduction, industrial pretreatment, and secondary treatment have dramatically decreased solids and
metals discharges. 
MWRA’s discharge has not affected oxygen in the bay 
Primary and secondary treatment remove some, but not all, of the important nutrients
in wastewater (nitrogen, phosphorus, and silica), so MWRA water-column
monitoring focuses on potential nutrient effects. One concern for the bay discharge is
that nutrient inputs could lead to eutrophication, a condition that results from the
excessive growth and decay of phytoplankton, microscopic algae that live in the
water column. Phytoplankton blooms are natural and necessary, but when excessive,
their eventual die-off and decomposition depletes oxygen. Oxygen levels naturally
decline in deep water during the late summer and fall, but in eutrophic waters,
depleted oxygen can impair or even kill animal life near the sea floor.

vii
Nitrogen is the main nutrient controlling phytoplankton growth in marine waters. The
most abundant form of nitrogen in the MWRA discharge, ammonium, is a good
effluent tracer. Increased ammonium concentrations have been frequently detected at
stations closest to the outfall since the offshore discharge began (Figure ii).
Variability has also increased at stations closest to the outfall, as sampling at those
stations occurs before the discharge is fully mixed into surrounding water.
Intermittent increases are sometimes also detected at greater distances. These
increases were anticipated during planning for the outfall. 
Figure ii. Station-average ammonium concentrations (μM) at selected stations in 2022 compared
to prior years. Ammonium is a good sewage tracer, and elevated levels are frequently detected at
stations closest to the outfall. Farther from the outfall, concentrations are similar to those before
discharge began. The red lines show average results from pre-2000. The blue lines show results from
after discharge was moved from the harbor to the bay. The “data range” spans all measurements
through 2021. 
The nutrients in the MWRA discharge have not increased phytoplankton growth or
contributed to reduced dissolved oxygen levels in Massachusetts Bay, partially
because the discharge is in deep water. During the summer months, warmer surface
waters keep the discharge plume below the depth where most phytoplankton grow,
and there has been no increase in phytoplankton biomass, even at the stations closest
to the outfall.

viii
In the fall of 2022, late summer, deepwater oxygen did reach low levels throughout
the region, lower than some MWRA Contingency Plan thresholds. Because these
declines in dissolved oxygen were also detected in areas outside the area of influence
of the MWRA discharge, we know that those conditions resulted from physical
factors, particularly unusually warm waters, rather than from the effluent discharge
and eutrophication. 
Seafloor communities remain healthy and diverse 
MWRA’s studies of the sediments and animals at the sea floor have also shown little
change related to the outfall. Bottom-dwelling animal communities are influenced by
the seafloor sediment types and water depths rather than by the discharge. Concerns
that the effluent might lead to low oxygen conditions, smother bottom-dwelling life,
or increase sediment contaminants have been dispelled. The numbers of animals,
numbers of species (Figure iii), and the species makeup of the communities have
remained relatively constant, within the ranges of natural variability. 
Figure iii. Total number of seafloor species at Massachusetts Bay stations near the outfall, 1992–
2022. The number of species is one measure of community health, and it has been relatively stable
since the outfall discharge began in 2000, remaining within the expected ranges. Abundance of
invasive species, which are signs of degraded habitat, has declined. Falling below the “threshold limit”
would indicate a decrease in the number of species that warranted further examination. 
Fish and shellfish are healthier and safer than ever 
MWRA monitors winter flounder, a popular commercial and recreational fish that
lives and feeds on the bottom in contact with the sediment. Measurements include
physical abnormalities and potential health risks, such as fin disease, ulcers, tumor
precursors, and cancerous liver tumors. Flounder health has not declined in
Massachusetts Bay, as some had feared before the outfall began to discharge. Rather,
it has even improved near the Massachusetts Bay outfall and has greatly improved
near the former outfall in Boston Harbor (Figure iv).

ix 
Figure iv. Annual prevalence of the tumor precursor centrotubular hydropic vacuolation (CHV)
in winter flounder near the Massachusetts Bay outfall site and at Deer Island Flats, 1991–2022. 
Tumor precursors have declined over time at both locations. OS = Outfall Site, DIF = Deer Island Flats 
In past years, most recently in 2021, MWRA also tested flounder, lobsters, and
mussels for toxic metals and pesticides. Levels of toxic contaminants have declined in
Boston Harbor and remained low or even declined in fish and shellfish sampled near
the Massachusetts Bay outfall. Because there has been no evidence of effects of the
discharge, the regulatory agencies have approved several reductions in the timing and
geographic extent of monitoring. 
Effluent does not affect Stellwagen Bank Marine Sanctuary 
There have been no measurable effects on the Stellwagen Bank National Marine
Sanctuary, located offshore from the MWRA outfall. Stellwagen Bank supports rich
recreational and commercial fisheries and is a major feeding ground for whales.
Nutrients in the discharge are not detected in offshore waters, and the bottom
communities have remained the same as before the discharge began.
Bottom-water dissolved oxygen concentrations in Stellwagen Basin, on the western
side of the sanctuary, did reach unusually low levels in 2022. Oxygen concentrations
naturally decrease through the summer before fall mixing restores the entire water
column to well-oxygenated conditions. The low late-summer oxygen levels
throughout the region in 2022 were the result of physical conditions, particularly
warm waters, rather than an effect of the outfall.

x 
Figure v. Dissolved oxygen concentrations at the NERACOOS Buoy A01 (lines) and at nearby
deepwater MWRA survey Station F22 (symbols) in 2022 . Dissolved oxygen concentrations reached
unusually low levels in 2022 but rapidly increased following early winter mixing events such as
storms, after the final MWRA survey of the year. 
Boston Harbor is cleaner and still improving 
While monitoring in Massachusetts Bay has shown that the outfall does not degrade
the environment, Boston Harbor continues to improve since the effluent discharge
was diverted from the harbor to the bay. Boston Harbor monitoring stations that had
been subject to effluent discharge before September 2000 now exhibit healthy
habitats, including thriving eelgrass, a sign of good seafloor conditions (Figure vi).

xi 
Figure vi. An eelgrass bed continues to thrive near the former outfall location near Deer Island. 
This sediment-profile image taken at Station R08 (red dot on the map) shows healthy eelgrass, well
oxygenated sediment, and excellent water clarity. 
.

xii 
Table of Contents 
Plain Language Summary ..................................................................................................... i 
Report Highlights ................................................................................................................ iv 
1. Introduction ....................................................................................................................1 
2. Effluent ....................................................................................................................2 
2022 Effluent Characterization ..................................................................................... 2 
Effluent Contingency Plan Thresholds .......................................................................... 8 
3. Water Column ...................................................................................................................9 
Physical Conditions ................................................................................................... 11 
Water Quality ............................................................................................................ 14 
Nutrients ............................................................................................................... 14 
Phytoplankton Biomass (Chlorophyll) ..................................................................... 18 
Dissolved Oxygen ................................................................................................. 20 
Plankton Communities ............................................................................................... 21 
Phytoplankton Communities ................................................................................... 21 
Zooplankton Communities ..................................................................................... 25 
Stellwagen Bank National Marine Sanctuary ............................................................... 27 
Boston Harbor Water Quality ..................................................................................... 28 
Water-Column Contingency Plan Thresholds .............................................................. 29 
Dissolved Oxygen Exceedances .............................................................................. 30 
Alexandrium catenella Abundance Exceedance ....................................................... 33 
4. Sea Floor ..................................................................................................................34 
Sediment Characteristics and Tracers .......................................................................... 36 
Soft-bottom Communities .......................................................................................... 37 
Stellwagen Bank National Marine Sanctuary ............................................................... 42 
Boston Harbor Seafloor Monitoring ............................................................................ 43 
Seafloor Contingency Plan Thresholds ........................................................................ 46 
5. Fish and Shellfish ............................................................................................................47 
Flounder Characteristics ............................................................................................. 50 
Flounder Health ......................................................................................................... 50 
Winter Flounder Contingency Plan Thresholds ............................................................ 52 
6. Special Studies ................................................................................................................53 
Notification of Sewage Discharges ............................................................................. 53 
Data Management ...................................................................................................... 54 
Bays Eutrophication Model Update ............................................................................ 56 
7. Update on the NPDES Permit ..........................................................................................57 
References ..................................................................................................................58 
List of Acronyms ................................................................................................................61

xiii 
List of Figures 
Figure i. Annual effluent solids and toxic metals discharges, 1990–2022. ....................... vi 
Figure ii. Station-average ammonium concentrations at selected stations in 2022
compared to prior years. .......................................................................... vii 
Figure iii. Total number of seafloor species at Massachusetts Bay stations near the
outfall, 1992–2022.................................................................................. viii 
Figure iv. Annual prevalence of the tumor precursor centrotubular hydropic vacuolation
in winter flounder near the Massachusetts Bay outfall site and at Deer
Island Flats, 1991–2022............................................................................ ix 
Figure v. Dissolved oxygen concentrations at the NERACOOS Buoy A01 and at nearby
deepwater MWRA survey Station F22 in 2022. ........................................x 
Figure vi. An eelgrass bed continues to thrive near the former outfall location near Deer
Island. ....................................................................................................... xi 
Figure 2-1. Annual and average rainfall in Boston, 2000–2022 .........................................2 
Figure 2-2. Annual full secondary-treated effluent flows and primary-blended flows,
2000–2022. .................................................................................................3 
Figure 2-3. Monthly full secondary-treated and primary-blended flows and rainfall during
2022 ............................................................................................................3 
Figure 2-4. Annual discharges of sewage solids, 1990–2022. ............................................4 
Figure 2-5. Annual nitrogen discharges, 2000–2022. ......................................................... 6 
Figure 2-6. Annual metals discharges, 1990–2022. ............................................................7 
Figure 3-1. Water-column monitoring stations and instrumented buoys in Massachusetts
and Cape Cod Bays. .................................................................................10 
Figure 3-2. Flows of the Merrimack and Charles Rivers ..................................................12 
Figure 3-3. Surface- and bottom water temperature and salinity at Station N18 just to the
south of the outfall in 2022 compared to prior years ................................13 
Figure 3-4. Ammonium concentrations at selected stations in 2022 compared to prior
years ..........................................................................................................15 
Figure 3-5. (Left) Surface- and bottom-water ammonium on February 10, 2022, during
mixed conditions. (Right) Cross-sections of concentrations through the
water column along transects connecting selected stations ......................16 
Figure 3-6. (Left) Surface- and bottom-water ammonium on June 28 and July 26, 2022
during stratified conditions. (Right) Cross-sections of concentrations
throughout the water column along transects connecting selected stations
..................................................................................................................17 
Figure 3-7. Areal chlorophyll concentrations at selected stations in 2022 compared to
prior years .................................................................................................18

xiv 
Figure 3-8. Moderate-Resolution Imaging Spectroradiometer satellite imagery of surface
chlorophyll concentrations in 2022 ..........................................................19 
Figure 3-9. Near-bottom water dissolved oxygen concentrations in 2022 compared to
prior years .................................................................................................20 
Figure 3-10. Total phytoplankton abundance at selected stations in 2022 compared to
prior years. ................................................................................................22 
Figure 3-11. Phytoplankton species typical of Massachusetts Bay ...................................22 
Figure 3-12. Total abundance of centric diatoms, 1992–2022 ..........................................23 
Figure 3-13. Maximum nearfield Alexandrium abundance for 2022 surveys compared to
prior years .................................................................................................25 
Figure 3-14. Total zooplankton abundance at selected stations in 2022 compared to prior
years ..........................................................................................................26 
Figure 3-15. Dissolved oxygen concentrations at the NERACOOS Buoy A01 and at
nearby deepwater MWRA survey Station F22 in 2022 ............................27 
Figure 3-16. Enterococcus in Boston Harbor, 1994–2022 ................................................28 
Figure 3-16. Bottom dissolved oxygen concentrations and percent saturation in the
nearfield and Stellwagen Basin, 2022. .....................................................31 
Figure 3-17. Results of a simple regression model using temperature and salinity data to
predict dissolved oxygen minima in the nearfield. ...................................32 
Figure 3-18. Alexandrium catenella abundance, 1992–2022 ............................................33 
Figure 4-1. Soft-bottom monitoring stations .....................................................................35 
Figure 4-2. Concentrations of the effluent tracer Clostridium perfringens spores in 2022.
..................................................................................................................36 
Figure 4-3. Total organic carbon at selected stations, 1992–2022. ...................................37 
Figure 4-4. Total abundance of soft-bottom organisms by region, 1992–2022 ................38 
Figure 4-5. Polychaete worms dominate the soft-bottom habitats in Massachusetts Bay.39 
Figure 4-6. Ordination plot of 2022 Massachusetts Bay samples by location ..................40 
Figure 4-7. Percent fine sediments superimposed on the ordination plot of the 2022
samples .....................................................................................................41 
Figure 4-8. Ordination plot of soft-bottom communities at selected stations over time,
1992–2022 ................................................................................................42 
Figure 4-9. Total abundance of soft-bottom animals and number of species per sample in
Boston Harbor samples, 1991–2022. ........................................................44 
Figure 4-10. Sediment profiles depicting biologically, physically, and mixed dominated
sediment surfaces at three stations in 2021 and 2022...............................45 
Figure 5-1. Fish-and-shellfish monitoring stations............................................................48 
Figure 5-2. External assessments are made at sea, and livers are removed for laboratory
analysis .....................................................................................................49

xv 
Figure 5-3. Annual prevalence of the tumor precursor centrotubular hydropic vacuolation
in winter flounder near the Massachusetts Bay outfall site and at Deer
Island Flats, 1991–2022............................................................................51 
Figure 5-4. Annual prevalence of neoplasia in winter flounder, 1991–2022 ....................51 
Figure 6-1. Monitoring data are collected by MWRA and its consultants under stringent
quality assurance and quality control, following the Plan-Do-Check-Act
cycle for continuous quality improvement ...............................................55 
Figure 6-2. Example of a presentation used to identify suspicious data. ..........................55 
Figure 7-1. Deer Island staff in front of the sludge digesters ............................................57 
List of Tables 
Table 2-1. Contingency Plan threshold values and 2022 results for effluent monitoring. ..8 
Table 3-1. Contingency Plan threshold values and 2022 results for water-column
monitoring ................................................................................................29 
Table 4-1. Contingency Plan threshold values and 2022 results for seafloor monitoring. 46 
Table 5-1. Contingency Plan threshold values and 2022 results for winter flounder........52

1 
1. Introduction 
Since 1984, the Massachusetts Water Resources Authority (MWRA) has worked to
minimize adverse effects of municipal wastewater discharges on the marine
environment through a vigorous pretreatment program, a carefully maintained
treatment plant, and vigilant effluent and environmental monitoring. One of the most
important steps in what is known as The Boston Harbor Project came in September
2000, when the regional effluent discharge was diverted from the shallow, enclosed
waters of Boston Harbor to deeper, more open waters in Massachusetts Bay. Since
then, the Massachusetts Bay outfall has operated under a National Pollutant
Discharge Elimination System (NPDES) permit, jointly issued by the U.S.
Environmental Protection Agency (EPA) and the Massachusetts Department of
Environmental Protection. An independent Outfall Monitoring Science Advisory
Panel has assisted EPA in addressing scientific questions about potential
environmental effects of the outfall on Massachusetts Bay.
The NPDES permit requires MWRA to measure effluent quality before discharge and
to monitor the water, sea floor, and marine life that could be affected in
Massachusetts Bay. A Contingency Plan attached to the permit requires comparing
measured conditions against “caution” and “warning” thresholds, developed from
state standards and pre-2001 baseline monitoring data. If a caution threshold is
exceeded, MWRA must investigate the issue, while a warning threshold is a higher
level of concern that could require both investigation and responsive action.
Background information about the monitoring program (Werme et al. 2012), the most
current monitoring plan (MWRA 2021) and Contingency Plan (MWRA 2001), past
plans and overviews, and study-specific technical reports are available on MWRA’s
technical report list at MWRA Environmental Quality Department Technical Reports.
This annual Outfall Monitoring Overview report fulfills a requirement of the NPDES
permit that MWRA produce an annual summary of monitoring results for
Massachusetts Bay. Annual overviews also report on special studies, conducted in
response to permit conditions and environmental concerns, often in cooperation with
other agencies. This year’s report on special studies focuses on MWRA efforts under
a new Massachusetts law requiring notifications to government agencies and the
public of unanticipated or inadequately treated sewage discharges, MWRA’s
environmental data management system, and an update of the Bays Eutrophication
Model, which simulates the processes affecting water quality throughout
Massachusetts Bay and the surrounding waters.
This annual overview also reports on results relevant to the Stellwagen Bank National
Marine Sanctuary, located offshore from the MWRA outfall, and presents some
findings from MWRA’s in-house Boston Harbor monitoring program. Harbor
monitoring documents the success of the Boston Harbor Project, while Massachusetts
Bay monitoring ensures that no new harm occurs offshore.

2 
2. Effluent 
Deer Island Treatment Plant continued to operate as designed through 2022, earning
MWRA the National Association of Clean Water Agencies Platinum 16 Peak
Performance Award for facilities with 100% compliance with effluent permit limits
over 16 consecutive years. 
2022 Effluent Characterization 
Wastewater entering Deer Island Treatment Plant includes not only municipal sewage
but also groundwater infiltration and stormwater inflow. Consequently, rainfall is an
important factor in determining annual wastewater flow and contaminant
concentrations in the treated effluent. The Boston area received only 31 inches of rain
in 2022, almost 20 inches less than 2021 and well below the 2000–2022 average
(Figure 2-1). The summer was especially dry, with the driest July ever recorded in
Boston. 
Figure 2-1. Annual and average rainfall in Boston, 2000–2022. Rain data are from the National
Weather Service rain gauge at Boston Logan International Airport. 
Treated effluent is the best way to protect Massachusetts Bay 
MWRA’s vigorous pretreatment program and its secondary sewage treatment
minimize the contaminants discharged into Massachusetts Bay. Effluent
monitoring ensures that limits on pathogens, organic material, nutrients, and
toxic contaminants are met.
.

3
With low rainfall, annual total effluent flow in 2022 was substantially lower than the
flows in 2021 and other wet years (Figure 2-2). Flow averaged 276 million gallons
per day, less than 85% of the average 330 million gallons per day since 2000. Almost
all the effluent in 2022 received full secondary treatment, with less than 1% of the
total flow consisting of primary-only treated effluent blended into secondary-treated
effluent prior to discharge (Figure 2-3). (Primary treatment is a physical process,
which involves removal of solid material through settling; secondary treatment
includes bacterial decomposition, which removes many more pollutants. All effluent
is disinfected before discharge.) 
Figure 2-2. Annual full secondary-treated effluent flows and primary-blended flows, 2000–2022 .
During large storms, flow exceeding the secondary capacity of the plant is diverted around the
secondary process; this primary-treated flow is blended with the fully treated flow before disinfection.
and discharge. Blended flows meet all permit limits. 
Figure 2-3. Monthly full secondary-treated and primary-blended flows and rainfall during 2022. 
In 2022, more than 99% of total flows received complete primary and secondary treatment.

4
Blending primary-treated effluent occurs when inflow to the treatment plant exceeds
what the secondary treatment process is designed to handle. Blending occurs only
during heavy rain, and blended flows fully meet permit limits. In 2022, blending
events were so rare and amounts of blended flow so low that it is difficult to discern
them in Figures 2-2 and 2-3. Massachusetts regulations require MWRA to notify
regulators and the public whenever blending occurs. (See Section 6, Special Studies,
for more information about notifications.)
In 2022, the total suspended solids load, those particles remaining after most of the
solid material is removed as sludge, was about 14 tons per day, well below the
amounts discharged into the harbor in the 1990s (Figure 2-4). In recent years, the total
solids load has averaged only about 10% of what had been discharged to the harbor in
1990–1991. 
Figure 2-4. Annual discharges of sewage solids, 1990–2022. Solids discharges decreased with each
accomplishment of the Boston Harbor Project and have remained low. 
Biochemical oxygen demand (BOD),
a measure of the amount of oxygen
needed for decomposing organic
material in effluent, remained well
below levels that would be expected
to affect dissolved oxygen in the
environment. MWRA has consistently
met its BOD permit limit. 
Organic material , a major constituent
of untreated sewage, consumes oxygen
as it decays and can affect growth and
productivity of phytoplankton.
Treatment removes more than 80% of
the biochemical oxygen demand, a
measure of the amount of oxygen
consumed by microorganisms as they
break down the organic material.

5
MWRA monitors the effluent for fecal
coliform bacteria, one indicator of a variety of
pathogens that can make shellfish unhealthy
for human consumption. Levels of fecal
coliform bacteria in the effluent have always
been well below permit limits. MWRA also
monitors fecal coliform bacteria in
Massachusetts Bay, and results at all
locations, even those closest to the outfall,
meet the stringent shellfishing limits set by the
state.
MWRA’s permit and monitoring plan
do not have limits for nutrients, toxic
metals and organic compounds, but do
require reporting. Nutrient
measurements include individual
components of total nitrogen
(ammonium, nitrate, and nitrite) and similar forms of other nutrients (such as total
phosphorus and phosphate). Toxic metals with reporting requirements include lead,
mercury, and other metals. Organic compounds include selected pesticides,
polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs).
The total nitrogen load for 2022 was 11,888 metric tons, higher than in 2021 but
remaining below the Contingency Plan caution threshold of 12,500 metric tons
(Figure 2-5). This threshold was set conservatively at about 90% of the warning
threshold, 14,000 metric tons, which was the level expected to be reached by 2020,
based on population growth projections for the MWRA service area. Nitrogen loads
have remained well below that anticipated amount. The caution threshold was
exceeded once, in 2019; however, modeling has suggested that even loads much
higher than the more stringent warning threshold would not harm the environment
(Deltares 2022; see Section 6, Special Studies for further discussion). MWRA
updates its report on potential nitrogen-removal strategies annually (most recently in
Ellis-Hibbett et al. 2023). 
Nutrients control algal growth and, at
too high concentrations, can lead to
eutrophic conditions, which can cause
or exacerbate low levels of dissolved
oxygen, excess turbidity, and nuisance
algal blooms. 
Pathogens, including bacteria,
viruses, and protozoa, are found
in human and animal waste and
some can cause disease. Human
exposure to pathogens is
unlikely in effluent discharged
into Massachusetts Bay, as the
outfall was sited to be far from
shellfishing and swimming areas.

6 
Figure 2-5. Annual nitrogen discharges, 2000–2022. The warning threshold, 14,000 metric tons per
year, was set as the load anticipated for 2020; the caution threshold was set at 90% of the warning
threshold. Actual loads have remained lower than anticipated. A model run simulating inputs that were
150% of the actual 2020 load predicted no adverse effects on the environment. 
The portion of the effluent total nitrogen load composed of ammonium, a form of
nitrogen readily taken up by phytoplankton (microscopic algae and other plant-like
organisms) and a good effluent tracer, increased in comparison to 2021 after two
years of decreases. Steady increases in ammonium discharges through most years of
monitoring had been a result of the treatment processes for both sewage sludge and
effluent; reasons for the recent decreases are unclear and may simply reflect sampling
variability.
Metals loads remained low in 2022 (Figure
2-6). Zinc continued to be the most
abundant metal, followed by copper. Both
are present in water pipes and fixtures.
Other notable sources of zinc to wastewater
include beauty shops, automobile-repair
shops, and hospitals; residential household
products, such as shampoos, ointments, and
laundry detergent; and street runoff
(MWRA, unpublished data). 
Toxic contaminants include heavy
metals, such as copper and lead,
and organic compounds, including
now-banned pesticides and
polychlorinated biphenyls. Elevated
levels of toxic contaminants could
directly affect marine communities
or could bioaccumulate, eventually
affecting human health.

7 
Figure 2-6. Annual metals discharges, 1990–2022. Source reduction, industrial pretreatment, and
secondary treatment have dramatically decreased metals loads in the effluent. 
Total loads of all metals remained small percentages of what had been anticipated
during planning for the Massachusetts Bay outfall. Copper discharges were less than
20% of what had been predicted, and all other metals discharges were less than 10%
of the predictions (Werme et al. 2021). Except for copper, all metals meet water
quality standards prior to discharge. In the receiving water initial dilution (the amount
of mixing within a strictly defined area near the outfall) further reduces
concentrations. Copper discharges meet the standard after initial dilution and have
declined over time, a result of drinking-water corrosion control, which decreases
leaching from water pipes, and other source-reduction efforts. Corrosion control, as
well as secondary treatment, have also significantly reduced lead levels in the effluent
discharge.
Mercury discharges were only 1% of what had been anticipated during planning for
the outfall. Silver, once considered a good effluent tracer, is now often at levels below
detection limits, a result of the decline in film-based photography, as well as good
removal efficiencies with secondary treatment.
Discharges of organic contaminants have varied slightly from year to year but have
been well below loads historically discharged into Boston Harbor. Annual polycyclic
aromatic hydrocarbon (PAH) loads, for example, were only 74 kilograms in 2022,
compared to the more than 3,000 kilograms of PAHs estimated to have been
discharged in wastewater effluent into Boston Harbor in 1991 (Rex et al. 1992). PAH
concentrations now meet water quality standards at discharge. For some other organic
compounds, such as 4,4’-DDE, one of the most prevalent breakdown products of the
pesticide DDT, the effluent data show evidence of the very slow declines anticipated
in the 1970s when it was banned along with other pesticides and toxic chemicals.

8 
Effluent Contingency Plan Thresholds 
There were no effluent Contingency Plan exceedances in 2022 (Table 2-1).
Exceedances of effluent permit limits have been rare, and none have occurred since
2006. The total nitrogen caution level, which is not a permit limit but set in the
Contingency Plan, was not exceeded in 2022. 
Table 2-1. Contingency Plan threshold values and 2022 results for effluent monitoring. 
Parameter Baseline Caution Level Warning Level 2022 Results 
Permit Condition and Contingency Plan Thresholds 
pH NA None <6 or >9 Not exceeded
Fecal coliform NA None >14,000 fecal
coliforms/100 mL Not exceeded
Chlorine, residual NA None >631 μg/L daily,
>456 μg/L monthly Not exceeded
Suspended solids NA None >45 mg/L weekly
>30 mg/L monthly Not exceeded
cBOD NA None >40 mg/L weekly,
>25 mg/L monthly Not exceeded
Acute toxicity NA None LC50 <50% Not exceeded
Chronic toxicity NA None NOEC <1.5%
effluent Not exceeded
PCBs NA Aroclor>0.045 ng/L None Not exceeded
Plant performance NA 5 violations/year Compliance <95%
of the time 100% compliance
Flow NA None >436 MGD
average dry days Not exceeded
Oil and grease NA None >15 mg/L weekly Not exceeded 
Contingency Plan Thresholds 
Total nitrogen load NA >12,500
mtons/year
>14,000
mtons/year 11,888 mtons 
NA = not applicable
cBOD = carbonaceous biological oxygen demand
LC50 = 50% mortality concentration
NOEC = no observable effect concentration
PCB = polychlorinated biphenyl
Plant performance = compliance with permit conditions

9 
3. Water Column 
MWRA’s water-column monitoring program evaluates physical oceanographic
processes, water quality, and phytoplankton and zooplankton communities at stations
in Massachusetts Bay, at the mouth of Boston Harbor, and in Cape Cod Bay (Figure
3-1). Ship-based field surveys are augmented by instrumented buoys and satellite
imagery. Water-column monitoring is primarily focused on nutrients, as inputs of
other pollutants present in the effluent, such as organic material, pathogens, and toxic
contaminants, are more completely removed in wastewater treatment and are unlikely
to have any harmful effects.
Excess nutrient levels can cause eutrophication, which occurs when nutrients
overstimulate phytoplankton growth, leading to lower levels of dissolved oxygen or
promoting nuisance or toxic algal blooms. One nuisance phytoplankton species, 
Phaeocystis pouchetii , was a focus in past years but has been shown to vary in
response to physical conditions rather than the outfall and is no longer considered a
concern. Potentially toxic species include Alexandrium catenella , which occurs
regularly in Massachusetts Bay, and Pseudo-nitzschia , a species group that occurs in
Massachusetts Bay, but usually in low numbers and in its nontoxic forms.
Nine ship-based surveys are completed between February and October of each year.
Five stations are in the “nearfield,” a 12- by 10-kilometer area centered on the outfall,
where some effects of the effluent were expected and have been observed. Nine are in
the more distant “farfield,” including the mouth of Boston Harbor, Cape Cod Bay,
and near Stellwagen Bank National Marine Sanctuary offshore from the outfall.
Additional surveys may be triggered by elevated abundance of the potentially toxic
phytoplankton species Alexandrium catenella . These Alexandrium Rapid Response
Study surveys (Libby et al. 2013) provide in situ hydrographic data and water
samples for measuring nutrients and Alexandrium abundance at up to 19 stations.
Three of these additional surveys were conducted in July 2022. 
Water-column monitoring looks for signs of eutrophication in the bay 
MWRA’s effluent contains nitrogen and other nutrients, which at high
concentrations could overstimulate phytoplankton growth and lead to low
levels of dissolved oxygen or increased severity of nuisance algal blooms.
Monitoring assesses physical conditions, water quality (nutrients,
phytoplankton biomass measured as chlorophyll, and dissolved oxygen), and
phytoplankton and zooplankton communities to detect signs of eutrophication.

10 
Figure 3-1. Water-column monitoring stations and instrumented buoys in Massachusetts and
Cape Cod Bays. Data are obtained from surveys, the NERACOOS A01 and NDBC 44013 buoys, and
satellite imagery. Also shown are the outfall and the Stellwagen Bank National Marine Sanctuary.

11
The monitoring program benefits from collaboration with the Center for Coastal
Studies in Provincetown, Massachusetts, which samples the three water-column
stations in Cape Cod Bay and near Stellwagen Bank National Marine Sanctuary.
Regulators have set a target that, whenever possible, sampling in Cape Cod Bay
should occur within 48 hours of the Massachusetts Bay survey. In 2022, the October
Cape Cod Bay sampling occurred three days after the Massachusetts Bay survey; all
other surveys were within the 48-hour window.
Surveys are supplemented by measurements at two instrumented buoys: the
Northeastern Regional Association of Coastal and Ocean Observing Systems
(NERACOOS) Buoy A01 off Cape Ann at the northern limit of Massachusetts Bay,
and the National Oceanic and Atmospheric Administration’s National Data Buoy
Center (NDBC) Buoy 44013 in central Massachusetts Bay. The National Aeronautics
and Space Administration provides Moderate Resolution Imaging Spectroradiometer
satellite imagery of chlorophyll fluorescence, a measure of phytoplankton biomass. 
Physical Conditions 
Following a relatively rainy 2021, with the highest summer river flows measured in
the past 30 years, 2022 was dry. River flows from the Merrimack and Charles Rivers
were high in the winter and early spring, but later spring, summer, and early fall flows
were among the lowest of the monitoring program (Figure 3-2; Libby et al. 2023).
While there has been no discernable long-term trend in river flow, year-to-year
variability in rainfall and the influx of freshwater in river discharges can exert large
effects on conditions in Massachusetts Bay, such as increasing stratification, which
separates surface from bottom waters during summer months. 
Circulation and physical properties in Massachusetts Bay are driven by
the larger pattern of water flow in the Gulf of Maine. Water enters
Massachusetts Bay from the north, circulates in a counter-clockwise direction,
and exits by Race Point, at the tip of Cape Cod. Water flows vary from week-
to-week and depend on the strength of the current and on wind speed and
direction. The coastal current is strongest during spring runoff from rivers and
streams.
In the winter, the water column is well mixed, and nutrient levels are high. As
surface waters warm, the water column stratifies, separating surface from
bottom waters and leading to lowered bottom-water dissolved oxygen levels in
late summer and fall. Inputs of freshwater, which is lighter than seawater, can
contribute to stronger stratification. Cooling surface waters and strong winds
in the fall promote mixing, ending the stratified season and re-oxygenating
bottom waters.

12 
Figure 3-2. Flows of the Merrimack (top) and Charles (bottom) Rivers. The Charles River flows
into Boston Harbor. The much larger Merrimack River flows into the Gulf of Maine just south of the
New Hampshire border, but greatly influences Massachusetts Bay. Low river flow, which occurred in
2022, leads to high salinity in Massachusetts Bay. Red lines are 2022 data. Results from 1992–2021
are in light blue. The quarterly percentiles represent the 2022 flows in comparison to the entire record. 
Surface- and bottom-water temperatures were warm during most survey months, for
example, at Station N18 just south of the outfall (Figure 3-3, top two panels).
Temperatures dipped during the July Alexandrium surveys, when air temperatures
remained high, but upwelling brought cooler waters to the surface. Water
temperatures rose quickly after the upwelling subsided.
Surface- and bottom-salinities at Station N18 were very high throughout most of the
year, a result of the low river inflows (Figure 3-3, bottom panels). Warm waters and
high salinities correlate with low late-summer dissolved oxygen conditions in
Massachusetts Bay.

13 
Figure 3-3. Surface- and bottom water temperature and salinity at Station N18 just to the south
of the outfall in 2022 compared to prior years. The black points and lines are results from 2022; the
blue lines and shading are results from 1992–2021. PSU = practical salinity units or parts per thousand.

14
Wind speeds and directions, which can exert large effects on conditions in the bay,
were typical through 2022, with no storm producing large waves capable of
resuspending and transporting bottom sediments, as occurs in some years. Two wind
events in April were conducive to bringing cells of the nuisance algal species 
Alexandrium catenella into Massachusetts Bay from the Gulf of Maine. Such wind
events have frequently triggered Alexandrium blooms in past years; however, in
2022, a bloom did not begin until later in the season. 
Water Quality 
Water quality measurements include quantification of nutrients (nitrogen,
phosphorus, and silica), phytoplankton biomass (measured as chlorophyll), and
dissolved oxygen. These measurements address the concern that excess nutrients
could stimulate phytoplankton growth (increase biomass) and exacerbate low bottom-
water oxygen conditions. Results for 2022 continued to confirm measurable outfall
influence on some nutrient concentrations at stations near the outfall, but no resulting
increase in phytoplankton biomass (Libby et al. 2023). Bottom-water dissolved
oxygen levels, which typically begin the year relatively high and steadily decline
during the summer, stratified season, reached lower levels than in past years. Those
conditions were a result of high temperature and salinity rather than a response to
nutrients in the outfall discharge. 
Nutrients 
Concentrations of most forms of dissolved inorganic nutrients near the outfall stayed
within the ranges measured in previous years, with many measurements at the historic
medians. Prevalent forms of the nutrients, including nitrate, phosphate, and silicate,
continued to show the typical seasonal patterns that were present before the bay
outfall began to discharge. Nutrient levels naturally vary with phytoplankton uptake,
exchange with the Gulf of Maine, and river flow.
Nitrogen, including its dissolved forms of nitrate and ammonium, is the most
important nutrient for phytoplankton growth in marine waters. Ammonium is also the
largest fraction of the total nitrogen in wastewater (see Figure 2-5, page 6), making it
a good effluent tracer. Since the discharge was relocated from the harbor to the bay in
late 2000, variable and elevated levels of ammonium have been detected at stations
near the outfall (Figure 3-4).

15
Especially high ammonium concentrations at stations closest to the outfall are
sometimes a result of a sample being taken directly in the effluent plume before it is
completely mixed into the surrounding water. For example, for most of the year,
ammonium levels were particularly high at Station N21, located right at the outfall.
However, during one July survey, ammonium levels were lower at Station N21 and
higher at Station N18, immediately to the south. These results indicate a possible shift
in water flows, pushing the plume to the south as it mixed into the water column. 
Figure 3-4. Ammonium concentrations (μM) at selected stations in 2022 compared to prior years. 
Increased concentrations of the sewage tracer ammonium are found at stations closest to the outfall,
while stations further away have maintained concentrations simitar to pre-outfall levels. The “data
range” spans all measurements.

16
As is typical, the effluent plume’s ammonium signature was evident in surface waters
at stations closest to the outfall during the winter surveys, when the water column was
relatively well-mixed (Figure 3-5). During the summer stratified season, the plume
was confined beneath the pycnocline, below depths where maximum phytoplankton
growth occurs (Figure 3-6). During the June survey, the ammonium signature was
detected at greater distance from the outfall than sometimes occurs. Similar to past
years, this extended plume signal is intermittent and was absent during the July
survey. 
Figure 3-5. (Left) Surface- and bottom-water ammonium on February 10, 2022, during mixed
(non-stratified) conditions. (Right) Cross-sections of concentrations through the water column
along transects connecting selected stations . Station N21 is directly over the outfall. During winter,
well-mixed conditions, the plume is evident in surface waters. Detectable elevated ammonium
concentrations were found along west to east as north to south transects. Dots include results from
MWRA’s in-house Boston Harbor monitoring program.

17 
Figure 3-6. (Left) Surface- and bottom-water ammonium on June 28 and July 26, 2022 during
stratified conditions. (Right) Cross-sections of concentrations throughout the water column
along transects connecting selected stations . On both dates, the plume was confined below the
pycnocline, below the depth of most phytoplankton growth. The distance that the plume could be
detected was greater in June than in July, reflecting natural variability in oceanographic conditions.
The yellow lines denote the pycnocline. Dots include results from MWRA’s in-house Boston Harbor
monitoring program .

18 
Phytoplankton Biomass (Chlorophyll) 
The localized ammonium concentration increases have not stimulated phytoplankton
growth or resulted in increased phytoplankton biomass at any station, even those
closest to the outfall (Figure 3-7). Survey phytoplankton biomass is reported as
vertically summed (areal) chlorophyll from measurements collected throughout the
water column from the surface to the sea floor. Seasonal peaks often occur in the
winter or spring and again in the fall, corresponding to typical spring and fall
phytoplankton blooms, although timing and magnitude vary from year to year.
During the 2022 survey season, areal chlorophyll levels peaked during March at
many stations, particularly those in shallow water and to the south of the outfall.
There was no indication of any increase in chlorophyll levels compared to the years
before the Massachusetts Bay outfall began to discharge. 
Figure 3-7. Areal chlorophyll concentrations (milligrams per square meter) at selected stations in
2022 compared to prior years. For some stations, historic data (shaded areas) extend later in the year
than the current survey schedule. 
Satellite imagery, which helps fill in the gaps between survey dates, showed moderate
chlorophyll concentrations in January and early February, prior to the start of the
survey season (Figure 3-8). Similar to the survey data, chlorophyll levels increased in
March, likely the result of a spring phytoplankton bloom, typical of healthy coastal
waters. Increased phytoplankton biomass in May often signals a bloom of a nuisance
species, Phaeocystis pouchetii , but survey measurements did not confirm a bloom in
2022. The satellite imagery also showed elevated levels in the fall, reflecting another
bloom, also typical of coastal waters and consistent with past observations.

19 
Figure 3-8. Moderate-Resolution Imaging Spectroradiometer satellite imagery of surface chlorophyll
concentrations in 2022 . The dates (marked in each frame) are highly weather-dependent and do not represent
consistent intervals of time. The numbers 1–9 show the timing of the routine MWRA surveys; and the letters
A–C show additional surveys conducted in response to elevated abundance of the potentially toxic
dinoflagellate Alexandrium catenella . The imagery helps fill the temporal gaps between surveys.

20 
Dissolved Oxygen 
In general, dissolved oxygen levels (concentration and percent saturation) in bottom
water begin the year relatively high and steadily decline throughout the summer,
when warm temperatures stratify the water column into distinct layers. (The boundary
between layers is called the thermocline, based on temperature measurements, or the
pycnocline, based on density which reflects changes in both temperature and salinity.)
Stratification prevents exchange between the atmosphere and bottom waters, so
dissolved oxygen levels naturally decline at depth, as animals and microbes respire.
Dissolved oxygen concentrations in Massachusetts Bay bottom water declined
steadily from March through the end of the survey season, with some rebounds during
the notable July upwelling event, which temporarily disrupted stratification (Figure 3-
9). The steady decline spanned the monitoring area, reaching the lowest levels
observed by the monitoring program at some stations, and also detected at the
NERACOOS Buoy A01. Percent oxygen saturation, a measure of how much oxygen
is dissolved in the water compared to how much oxygen the water could hold at a
given temperature and salinity, also reached low levels throughout the region. Low
concentrations and percent saturation resulted in Contingency Plan caution and
warning threshold exceedances in the nearfield and in Stellwagen Basin. These
exceedances are discussed on page 29. 
Figure 3-9. Near-bottom water dissolved oxygen concentrations (milligrams per liter) in 2022
compared to prior years. For some stations, historic data (shaded ranges) extend later in the year than
the current survey schedule.

21
Low dissolved oxygen concentrations also occurred in Cape Cod Bay, but unlike
some recent years, there was no hypoxia or anoxia (extremely low- or no-oxygen
conditions that can kill fish, lobsters, and other marine life). Researchers in Cape Cod
Bay had noted a number of factors contributing to the hypoxia in those years,
including upwelling-unfavorable winds, a strong and deep thermocline, and
abundance of a phytoplankton species Karenia mikimotoi , which is well-suited to
shallow waters with deep thermoclines (Scully et al. 2022). In 2022, winds more
typically favored upwelling, and although Karenia was present, there was no large
bloom. 
Plankton Communities 
MWRA monitors phytoplankton and zooplankton abundance and species composition
to detect any changes that might be associated with the outfall discharge (Cibik et al.
1998). A focus on potentially toxic or nuisance phytoplankton species further ensures
environmental and human health.
Massachusetts Bay phytoplankton includes small algae and other plant-like
organisms, such as microflagellates, centric (rounded) and pennate (elongated)
diatoms, and dinoflagellates. Microflagellates are extremely small and are typically
the most abundant phytoplankton type in Massachusetts Bay. Centric and pennate
diatoms have silica-rich cell walls and often dominate spring blooms. The potentially
toxic dinoflagellate Alexandrium catenella often blooms in Massachusetts Bay and
the wider Gulf of Maine region and is the species known in New England as causing
“red tide.” Other geographic areas designate different species, usually other
dinoflagellates, as red tides, and many types of phytoplankton can, when abundant,
color the water column.
Zooplankton communities in Massachusetts Bay include animal species that live their
entire lives as plankton, such as copepods, and those that live in the plankton only a
part of their life cycles, such as barnacle and other larvae. Most species are small, but
larger animals, such as jellyfish, are also part of the zooplankton community.
Zooplankton abundance and species composition are highly variable through the
seasons and between years. 
Phytoplankton Communities 
Total phytoplankton abundance increased in 2022 after a period of comparatively low
numbers (Figures 3-10 and 3-11, Libby et al. 2023). The 2022 annual abundance in
the nearfield (the five stations closest to the outfall) was 1,127,147 organisms per
cubic meter, ranking 18 th out of 31 years of monitoring, about the middle of the long-
term range, and more than one and a half times the 2019–2021 average, when
numbers were relatively low.

22 
Figure 3-10. Total phytoplankton abundance (million cells per liter) at selected stations in 2022
compared to prior years . For some stations, historic data (shaded ranges) extend later in the year than
the current survey schedule. 
Figure 3-11. Phytoplankton species typical of Massachusetts Bay. Pseudo-nitzschia and 
Asterionellopsis are groups of pennate diatoms, Thalassionema are centric diatoms, and Phaeocystis is
a flagellate. Some species of Pseudo-nitzschia produce toxins, and Phaeocystis can be a nuisance if it
grows into large, chain-forming colonies. Photo credits David Borkman, Rhode Island Department of
Environmental Management, Providence, Rhode Island 
Phaeocystis
Pseudo-nitzschia Asterionellopsis 
Thalassionema

23
One reason for the overall increase in abundance was a centric diatom bloom in
March. Winter and spring diatom blooms are common in coastal waters, but the
March 2022 bloom was the largest measured by the monitoring program since 1999
(Figure 3-12). This bloom was dominated by Skeletonema species, a worldwide genus
found across a broad range of physical conditions. Although Skeletonema has been
present throughout the monitoring program, its abundance in 2022 far exceeded any
prior year. Other species groups, such as Thalassiosira , have dominated in the past.
None of the dominant centric diatom species is potentially toxic, and the results pose
no environmental concern. The unprecedented early Skeletonema bloom may,
however, signal a shift in physical conditions in Massachusetts Bay. 
Figure 3-12. Total abundance of centric diatoms, 1992–2022. Blue bars are abundances near the
surface; orange bars art abundances at the depth of maximum chlorophyll concentrations (Cmax). The
large peak in 1999 was primarily Thalassiosira ; the peak in 2022 was largely Skeletonema . Photo
credit David Borkman, Rhode Island Department of Environmental Management, Providence, Rhode
Island. 
Pennate diatoms also bloomed in March, including the potentially toxic genus 
Pseudo-nitzschia . Some Pseudo-nitzschia species produce domoic acid, a neurotoxin
that causes amnesic shellfish poisoning (ASP), a life-threatening illness in seabirds,
marine mammals, and humans that consume contaminated shellfish. Historically,
ASP has not caused shellfish closures in Massachusetts or Cape Cod Bays, and there
were no closures in 2022. Pseudo-nitzschia was particularly numerous at coastal
stations, reaching a maximum abundance of 680,000 cells per liter at Station F13 off
Scituate, Massachusetts, but it was not found at the mouth of Boston Harbor or at
Cape Cod Bay stations. Maximum abundance at some coastal and nearfield stations
was the highest measured by the monitoring program. However, the species found
was Pseudo-nitzschia delicatissima , which is not known to be a major toxin-
producing species, and the Massachusetts Division of Marine Fisheries, which
monitors shellfish, detected no ASP toxin.

24
Dinoflagellates were also more abundant in 2022 than in recent years, dominated by
the genus Prorocentrum , the genus Tripos (formerly Ceratium) , and Karenia
mikimotoi . Throughout the monitoring program, dinoflagellate abundance has been
cyclic, increasing and decreasing on approximately 11-year natural cycles. The small
species Prorocentrum minimum bloomed in May . This species has been known to
produce toxins but has not been considered a nuisance species in Massachusetts Bay. 
Tripos species were present in the summer and fall. Karenia mikimotoi was present
for a sixth year, after first being detected in 2017, but at lower abundance than in past
years. The drivers for the cycles in dinoflagellate abundance are not yet understood.
Abundance of the potentially toxic dinoflagellate Alexandrium catenella was higher
than in many years during the spring. Its toxin causes paralytic shellfish poisoning
(PSP), a potentially deadly poison to people who consume shellfish. In 2022, the
Massachusetts Department of Marine Fisheries detected PSP toxicity in
Massachusetts Bay mussels on June 22. Northern Massachusetts shellfishing had
already been closed by PCP toxicity, so the closure area was extended south to
Plymouth, Massachusetts. The presence of toxicity also triggered Alexandrium Rapid
Response Study surveys. Two stations were added to the already-scheduled regular
June survey, which was completed on June 28, and three additional surveys of 19
stations were conducted in July. 
Alexandrium abundance exceeded the Contingency Plan caution threshold (Figure 3-
13). The highest Alexandrium cell count for 2022 was found at one of the additional
stations added to the June survey, 10,180 cells per liter. Overall abundances during
that survey were comparable to previous large blooms and decreased during the
subsequent July surveys. Additional information on the Contingency Plan exceedance
is presented on page 33.

25 
Figure 3-13. Maximum nearfield Alexandrium abundance for 2022 surveys compared to prior
years. Photo credit Don Anderson, Woods Hole Oceanographic Institution, Woods Hole,
Massachusetts. 
Zooplankton Communities 
Total zooplankton abundance and seasonal patterns were mostly typical in 2022
(Figure 3-14, Libby et al. 2023). As in past years, copepod adults and younger life
stages dominated in every survey. The small copepod species Oithona similis 
continued to dominate the zooplankton community. Early life stages of copepods
were particularly abundant at offshore stations in March and July.

26 
Figure 3-14. Total zooplankton abundance (10,000 individuals per cubic meter) at selected
stations in 2022 compared to prior years. For some stations, historic data (shaded regions) extend
later in the year than the current survey schedule. The high historic values in July, including those that
extend beyond the y-axis, occurred in July 2015 when larval bivalves were especially abundant, as
sampling took place after a major spawning event. Abundances in 2022 were close to average. 
As in 2020 and 2021, July 2022 samples included high numbers of radiolarians,
single-celled protozoans that build ornate silica exoskeletons. Radiolarians are more
typically found in offshore waters, and their presence since 2020 appears to reflect an
influx of water masses from offshore. Another group, called doliolids, which are
planktonic tunicates (often called sea squirts), occurred for the first time, providing
further evidence of intrusion of water from offshore.

27 
Stellwagen Bank National Marine Sanctuary 
The NPDES permit requires MWRA to report on results relevant to Stellwagen Bank
National Marine Sanctuary. Water-column Station F22 is just north of Stellwagen
Basin, to the west of the sanctuary, and is representative of offshore conditions. The
instrumented NERACOOS Buoy A01 is located within the sanctuary.
Ammonium levels have not increased at Station F22 since the outfall began to
discharge (see Figure 3-4, page 15). Phytoplankton biomass has also not increased.
There were no unusual chlorophyll levels in offshore water samples, measurements at
the NERACOOS Buoy A01, or satellite imagery in 2022. No effects on chlorophyll
levels in the offshore areas of Massachusetts Bay, including the sanctuary, were
predicted as a result of moving the discharge from Boston Harbor to the bay, and
none have been measured.
Dissolved oxygen concentrations and percent saturation in the deep water at Station
F22 in Stellwagen Basin did reach low levels in the fall, failing to meet the
Contingency Plan warning thresholds. These results have not been attributed to the
outfall and are discussed further on page 30, below. Data from the NERACOOS
Buoy A01 (which are not used to calculate Contingency Plan threshold parameters)
showed the decline during the stratified season and also documented the rapid return
to oxygenated conditions following fall mixing events (Figure 3-15). 
Figure 3-15. Dissolved oxygen concentrations at the NERACOOS Buoy A01 (lines) and at nearby
deepwater MWRA survey Station F22 (symbols) in 2022 . Dissolved oxygen concentrations reached
low levels in 2023 but rapidly increased following the late fall and early winter mixing events, after the
final MWRA survey of the year. The state water quality standard for dissolved oxygen is 6.0 mg/L.

28 
Boston Harbor Water Quality 
Water quality in Boston Harbor greatly improved during the decades of the Boston
Harbor Project, and those advances remained evident in 2022. Perhaps the most
dramatic improvement in Boston Harbor was the immediate decrease in ammonium
levels, which dropped precipitously in 2000 after the effluent discharge was diverted
from the harbor to the bay, and have remained low. The decreases in nutrient inputs
have been accompanied by decreases in primary production and phytoplankton
biomass, an abatement of the harbor’s historic high level of eutrophication (Werme et
al. 2018, Taylor et al. 2020).
Sewage-related bacterial contamination has also declined, no longer spiking during
years with heavy rain (Figure 3-16). Concentrations of the sewage-indicator bacteria 
Enterococcus began to decline when sludge discharges to the harbor ended in 1991.
Subsequent upgrades to Deer Island Treatment Plant and the stormwater system have
continued improvements. While bacteria concentrations remain routinely higher in
the western part of the harbor, which is closer to river and stormwater runoff than the
western harbor, all parts of the harbor meet state standards for swimming. 
Figure 3-16. Enterococcus in Boston Harbor, 1994–2022. Western Harbor stations are close to river
mouths and urban stormwater. High Enterococcus concentrations in 1996 and 1998 correlated with
high rainfall. Subsequent high-rainfall years have not had similar spikes, and Boston Harbor meets
state standards for swimming.

29 
Water-Column Contingency Plan Thresholds 
There were exceedances of four water-column thresholds in 2022, including
exceedances for dissolved oxygen parameters in September and October and
exceedance of the abundance threshold for the nuisance algal species Alexandrium
catenella in June and July (Table 3-1). Oxygen threshold exceedances included
nearfield dissolved oxygen concentration and Stellwagen Basin concentration and
percent saturation. Alexandrium presence was first noted by measurements of PSP
toxicity by the Massachusetts Department of Marine Fisheries on June 22, and the
threshold exceedance was determined by cell counts in eight of ten nearfield samples
on June 28. 
Table 3-1. Contingency Plan threshold values and 2022 results for water-column monitoring. 
“Nearfield” refers to the five stations closest to the outfall, where effects might be detected, if they
were to occur. 
Parameter Baseline Caution
Level
Warning
Level 2022 Results 
Dissolved oxygen* 
Nearfield
concentration 6.05 mg/L <6.5 mg/L <6.0 mg/L 5.85 mg/L
Nearfield percent
saturation 65.3% <80% <75% 66.6%
Stellwagen
concentration 6.23 mg/L <6.5 mg/L <6.0 mg/L 6.08 mg/L
Stellwagen
percent saturation 67.2% <80% <75% 65.4 %
Nearfield
depletion rate 0.024 mg/L/d >0.037 mg/L/d >0.049 mg/L/d 0.027 mg/L/d 
Chlorophyll 
Annual 72 mg/m 2 >108 mg/m 2 >144 mg/m 2 52.5 mg/m 2 
Winter/spring 50 mg/m 2 >199 mg/m 2 None 623.3 mg/m 2 
Summer 51 mg/m 2 >89 mg/m 2 None 42 mg/m 2 
Autumn 90 mg/m 2 >239 mg/m 2 None 50.6 mg/m 2 
Nuisance algae nearfield Pseudo-nitzschia 
Winter/spring 6,735 cells/L >17,900 cells/L None 502 cells/L
Summer 14,635 cells/L >43,100 cells/L None 5,770 cells/L
Autumn 10,050 cells/L >27,500 cells/L None 1,280 cells/L 
Nuisance algae nearfield Alexandrium catenella 
Any nearfield
sample
Baseline
maximum
163 cells/L
>100 cells/L None 10,180 cells/L
PSP toxin extent NA New incidence None No new
incidence 
*Dissolved oxygen caution and warning levels represent numerical criteria, with the caveat “unless
background conditions are lower.” Results are therefore compared to the baseline rather than to the
caution and warning levels.
PSP = paralytic shellfish poisoning
NA = not applicable

30 
Dissolved Oxygen Exceedances 
The Contingency Plan includes caution and warning thresholds for concentrations,
percent saturation, and the rate of oxygen depletion during the summer months at the
nearfield stations close to the outfall and in Stellwagen Basin. Those stations close to
the outfall would be most likely to show an effect of the outfall, while the thresholds
in Stellwagen Basin ensure that there are no adverse effects on the Stellwagen Bank
National Marine Sanctuary.
The thresholds were developed in the mid-1990s and were based on the established
Massachusetts regulatory standards of the time. For example, the warning threshold
for concentration was set at the state standard, 6.0 milligrams per liter, and the
caution threshold somewhat higher, 6.5 milligrams per liter. However, for some
parameters, baseline levels were lower than the standards, so in those cases, the
Contingency Plan directs that monitoring results be compared to the baseline rather
than the standards. Baseline oxygen concentrations in both the nearfield and
Stellwagen Basin were lower than the 6.5 milligram per liter caution threshold.
In the intervening years, regulators have recognized that marine organisms remain
fully protected at lower oxygen levels than the stringent standards of the 1990s. Some
New England states (but not Massachusetts) and New York have relaxed their
regulatory standards, and standards for oxygen concentrations now range from 4.8 to
5.0 milligrams per liter.
Percent saturation levels were also based on the Massachusetts standard of the 1990s,
but measurements were lower than the caution levels in seven of eight baseline years,
1992–1999, and also in 2000, the year that the Massachusetts Bay outfall began to
discharge. No further excursions below the caution thresholds occurred until 2021
(Figure 3-16). Massachusetts dropped percent oxygen saturation from its state
standards in 2008, after research showed that percent saturation was not strongly
associated with ecosystem health.
The 2022 exceedances have not been attributed to the outfall, but instead appear to be
the result of widely recognized regional physical conditions, particularly warming
waters, which have been broadly documented in the Gulf of Maine, and high salinity,
which is also correlated with lower dissolved oxygen levels. There is no evidence that
increased nutrient inputs, either from the outfall or other sources, have fueled algal
blooms that could result in lower dissolved oxygen measurements.

31 
Figure 3-16. Bottom dissolved oxygen concentrations (top) and percent saturation (bottom) in
the nearfield and Stellwagen Basin, 2022 . Nearfield = the five stations closest to the outfall where
effects might be expected to occur.

32
A simple regression model developed during baseline monitoring shows the
relationship between temperature, salinity, and dissolved oxygen minima in nearfield
bottom waters (Geyer et al. 2002). For 2022, the model predicted even lower
dissolved oxygen concentrations than were measured, based on both the high
temperatures and high salinity (Figure 3-17). The model had good agreement with
observed minima during the baseline and the first years after the Massachusetts Bay
outfall began to discharge. Discrepancies between the observed and modeled results
in 2015–2018 were likely due to changes in wind patterns. The upwelling-favorable
winds in 2022 were more similar to those earlier monitoring years, and the agreement
with the model was comparable to those earlier years. 
Figure 3-17. Results of a simple regression model using temperature and salinity data to predict
dissolved oxygen minima in the nearfield. Because both temperature and salinity were high in 2022,
the model predicted an even lower dissolved oxygen minimum than was observed. 
Many scientists both within and outside the MWRA monitoring program are studying
increasing water temperatures in Massachusetts Bay and have found an apparent shift
to earlier warming in April and May. No long-term trends in river inflow and
resulting variations in salinity have been detected. MWRA is paying close attention to
the ongoing research into understanding effects of climate changes on ocean water
quality in the region and will continue to evaluate results in that context.

33 
Alexandrium catenella Abundance Exceedance 
Alexandrium catenella occurs regularly in Massachusetts Bay, typically peaking in
abundance in May and June. Alexandrium blooms were first documented in Maine in
the 1950s and in Massachusetts Bay in the early 1970s. Blooms were common in the
1970s and 1980s, followed by a lull in the 1990s and early 2000s.
When the water-sample threshold for Alexandrium abundance was set at 100 cells per
liter, the Outfall Monitoring Science Advisory Panel recognized that blooms were
patchy and unpredictable, so the panel also endorsed an alternative threshold based on
new geographic extent of PSP toxin in shellfish. The cell-count caution threshold
(there is no warning threshold) was not exceeded in the first five years after the
outfall began to discharge to Massachusetts Bay, but it has been exceeded ten times
since 2005, including in 2022 (Figure 3-18). The threshold for new incidence of PSP
toxin, has never been exceeded.
Massachusetts Bay blooms have typically originated from cyst beds found to the
north of Massachusetts Bay, along the coast of Maine (Anderson et al. 2014). Cysts
are the dormant overwintering life stage of Alexandrium . Blooms in Massachusetts
Bay have occurred when wind patterns are suitable for bringing water and
phytoplankton into the bay from the Gulf of Maine. This pattern persisted in 2022.
Cysts have, however, been detected in Massachusetts Bay as well as in coastal Maine,
and in one prior year, 2021, wind patterns suggest that the bloom could have
originated from a more local source. This hypothesis and its implications are being
investigated by MWRA and its consultants. 
Figure 3-18. Alexandrium catenella abundance, 1992–2022. Alexandrium blooms were common in
the 1970s and 1980s, less frequent in the 1990s and early 2000s, and have exceeded the caution
threshold ten times since 2005. 
1
10
100
1000
10000
100000 
Alexandrium per sample
(CELLS/L +1 ) 
baseline outfall discharge caution threshold

34 
4. Sea Floor 
Seafloor monitoring assesses potential outfall effects on the health of the sandy and
finer-grained soft-bottom and rocky hard-bottom habitats of Massachusetts Bay. Soft-
bottom surveys are conducted annually, while photographic and video assessments of
the hard-bottom areas occur at three-year intervals, most recently in 2023. (Those
2023 results are currently being analyzed and will be presented in next year’s outfall
monitoring overview.) A separate program in Boston Harbor includes annual soft-
bottom surveys and photographic sediment-profile imagery.
In August 2022, MWRA sampled 14 soft-bottom stations for the analysis of grain-
size distribution, the effluent tracer Clostridium perfringens spores, total organic
carbon, and infauna (animal) abundance and species composition (Figure 4-1). The
stations included four “inner nearfield” stations located within two kilometers of the
outfall; a second set of six “outer nearfield” stations in western Massachusetts Bay
somewhat farther from the outfall; one station in the “transition” area between Boston
Harbor and the nearfield; and three “farfield” reference stations in Massachusetts and
Cape Cod Bays. For the purposes of testing Contingency Plan thresholds for the sea
floor, “nearfield” includes both nearfield groups and the transition station, for a total
of 11 stations. (Note that water-column and seafloor monitoring sample different
stations and have somewhat different definitions of near- and farfield.) 
Seafloor monitoring assesses the health of habitats and communities 
MWRA’s continued seafloor monitoring includes assessments of sediment
characteristics and animals living in soft- and hard-bottom communities. The
initial concerns for the sea floor ecosystem, such as potential eutrophication or
accumulations of toxic contaminants in the sediments have been disproven by
years of monitoring. There have not been any changes to the seafloor animal
communities outside natural and typical cycles.

35 
Figure 4-1. Soft-bottom monitoring stations. The inset shows the three farfield stations, located at
distance from the outfall. Also shown are the instrumented buoys, the MWRA outfall diffuser, and
Stellwagen Bank National Marine Sanctuary.

36 
Sediment Characteristics and Tracers 
As in past years, sediment grain-size distributions in 2022 varied broadly, ranging
from silt and clay at some stations (particularly Station FF04 within Stellwagen Basin
offshore from the outfall) to almost entirely sand at others (Nestler et al. 2023).
Within individual stations, sediment textures have remained generally stable over the
years of monitoring, with occasional changes following large storms. 
Clostridium perfringens is an anaerobic bacteria species found in the digestive tracts
of humans and other mammals. In the presence of oxygen, it forms persistent resting
spores, and those spores provide a sensitive tracer of sewage effluent. In 2022, as in
other years, it was possible to detect elevated abundance of Clostridium spores at
some stations located closest to the outfall (Figure 4-2). In most years, Clostridium 
spore concentrations have been elevated at stations located within two kilometers of
the outfall. Unusually, spore concentrations were also higher at two of the reference
stations quite far from and out of range of potential outfall influence, as much as four
times higher than past measurements. A review of field and laboratory procedures
found no anomalies in sample handling or analysis, and these unexpectedly higher
concentrations remain under investigation. 
Figure 4-2. Concentrations of the effluent tracer Clostridium perfringens spores in 2022. 
Concentrations represent spore counts (cfu, colony forming units) normalized to the percent of the
sediment made up of fine silts and clays. As anticipated during outfall planning, Clostridium spore
concentrations are elevated at stations closest to the outfall.

37
Percent total organic carbon content was consistent with past results, with no
increased organic carbon from sewage effluent at stations near the outfall (Figure
4-3). In general, stations with finer sediments, such as Station FF04, offshore from
the outfall in Stellwagen Basin, have higher mean total organic carbon
concentrations, while stations with coarser sediments, such as Station NF17 just to the
south of the outfall have lower concentrations. Overall, the total organic carbon data
continue to show no signs of organic enrichment from the discharge, consistent with
predictions made before the outfall began to discharge and also consistent with a
detailed special study of sediment metabolism, which was completed in 2010 (Tucker
et al. 2010, Tucker et al. 2014). 
Figure 4-3. Total organic carbon at selected stations, 1992–2022. Station NF12 is more than two
kilometers northwest of the outfall; and NF17 is in close proximity (less than two kilometers) and
southwest of the outfall, where the effects of the discharge might be expected but have not been found.
Station FF01A is the northern reference station and FF04 is in Stellwagen Basin; both are farther than
20 kilometers from the outfall. No effects of the discharge have been observed at any station; total
organic carbon content is correlated with sediment type rather than distance from the outfall. 
Soft-bottom Communities 
A variety of small animals live in the sediments, including segmented polychaete
worms, mollusks, and crustaceans. MWRA monitors abundance and species
composition of animals living within soft-bottom communities to document any
changes that might be associated with the outfall discharge. Effluent discharges have
been known to change species makeup and decrease biodiversity.

38
The 14 soft-bottom samples collected and analyzed in 2022 yielded 22,538
organisms, classified into 173 species and 22 other discrete taxonomic groups of
animals that could not be identified to the species level (Nestler et al. 2023). Total
abundance of organisms was slightly lower in 2022 than in 2021 in both nearfield
station groups and at the transition station between Boston Harbor and the outfall
(Figure 4-4). Most of the several diversity measures calculated for the samples were
also slightly lower than in 2021, but all measurements remained within the ranges
found throughout the monitoring program. Minor changes and cycles are
characteristic of seafloor communities in the region. 
Figure 4-4. Total abundance of soft-bottom organisms by region, 1992–2022 . Regions include the
nearfield within two kilometers of the outfall, the outer nearfield farther than two kilometers from the
outfall, the transition area between Boston Harbor and the outfall, and the farfield. 
Small polychaete worms (sometimes known as bristle worms; for example, Figure
4-5), dominated at most stations, as they have for the duration of the monitoring
program. These segmented worms are among the most common marine organisms.
The polychaete Prionospio steenstrupi , which was the numerically dominant species
in 1997–2005, was slightly less abundant in 2022 than in the past several years, but
remained among a group of dominant species. Other common polychaetes, including 
Aricidea catherinae and Mediomastus californiensis , were present in numbers
comparable to those of other recent years.

39 
Figure 4-5. Polychaete worms dominate the soft-bottom habitats in Massachusetts Bay. 
Polychaetes can range in size from less than a millimeter to several meters; most are very small. This
species, Kirkegaardia baptistae , is about 14 millimeters long and frequently found in fine-grained,
offshore sediment. MWRA samples were used in its original description. Photo credits James A.
Blake, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts. 
Community analyses continued to show no effects of the outfall on relative
abundance of animals or community composition. A series of analyses assessed
patterns in the soft-bottom communities over space and time and found no particular
species or type of community specifically associated with the outfall.

40
One type of community analysis, called cluster analysis, groups stations by similarity
in species composition and abundance, designating groups by numbers, which are
used in further analyses and shown in the figures below. Similar to prior years, cluster
analysis of the 2022 samples identified two main groups (I and II) and one “outlier”
group (III). Group I was dominated by crustaceans and polychaete worms. Group II
included a greater abundance of polychaetes and also mollusks; it could be further
subdivided into three subgroups (IIA, IIB, and IIC). As in past years, the outlier group
occurred only the offshore station located within Stellwagen Basin, which has
consistently supported a unique polychaete community, typical of a larger number of
deepwater, offshore stations sampled in the past.
Further analysis, called ordination analysis, provides a picture of how closely the
bottom communities identified in the cluster analysis relate to each other. The 2022
ordination analysis continued to show no indication of any relation of community
composition to proximity to the outfall (Figure 4-6). Both main groups I and II
included stations located at various distances and directions from the outfall. Group II
included stations in both nearfield station groups, the transition station between the
harbor and the outfall, and farfield stations to the northeast and southeast of the
outfall. 
Figure 4-6. Ordination plot of 2022 Massachusetts Bay samples by location. The ordination
analysis shows that the distance from the outfall does not correspond with bottom community
composition. Each point on the plot represents one of 14 stations, and varying colors and shapes depict
locations. Placement of the points indicates similarity of the communities—the nearer two points are to
each other, the more similar their seafloor communities. NF<2k = stations within two kilometers of the
outfall; NF >2k = stations near but more than two kilometers from the outfall; Tran = the one station
between Boston Harbor and the outfall; FF = stations far from the outfall. (The 2D Stress value noted
in the upper right is a measure of good confidence in the analysis.)

41
As in past years, further assessment of the ordination analysis demonstrated that
variations in species distributions largely followed differences in sediment grain size
(Figure 4-7). Water depth was another important factor in determining community
structure. That is, seafloor animal communities at stations with similar sediment
textures or water depths tended to be more closely related, as shown by closer
proximity on the ordination plots. These results continued to validate past findings
that the Massachusetts Bay sea floor is dominated by physical factors rather than by
the discharge. The strong physical forces in the bay, as well as the low pollutant
levels in the effluent discharge, have helped to maintain healthy soft-bottom habitats
throughout Massachusetts Bay. 
Figure 4-7. Percent fine sediments superimposed on the ordination plot of the 2022 samples. 
Combining the ordination analysis with sediment grain-size data shows that sediment texture is likely a
strong driver of community composition. Data are shown as in Figure 4-6, with the size of each station
marker indicating the percentage of fine sediments (percent fines). Stations with similar “percent fines”
tend to be represented in the same groups. Group I included only stations with coarser sediments, and
Group II subgroups reflected a range of finer-grained sediments. (The 2D Stress value noted in the
upper right is a measure of good confidence in the analysis.) 
Examining data over the past 31 years of monitoring, another ordination analysis
demonstrated the temporal stability of the seafloor communities in Massachusetts
Bay. Samples collected from selected stations in 1992–2022 showed relatively high
levels of similarity within each station over the 31-year period, with individual
stations remaining grouped over time (Figure 4-8).

42 
Figure 4-8. Ordination plot of soft-bottom communities at selected stations over time, 1992–2022. 
Closer proximity of the data points indicates greater similarity of the communities; each station
grouped relatively closely over the 31-year period, demonstrating the stability of Massachusetts Bay
sediments over time. NF17 is near (less than two kilometers) and southwest of the outfall, where the
effects of the discharge might be expected but have not been found. Station NF24 is just to the south of
the outfall. Station FF01A is the northern reference station. FF04 is in Stellwagen Basin. (The 2D
Stress value noted in the upper right is a measure of good confidence in the analysis.) 
Stellwagen Bank National Marine Sanctuary 
The NPDES permit to discharge from Deer Island Treatment Plant into
Massachusetts Bay requires annual reporting on results relevant to Stellwagen Bank
National Marine Sanctuary. MWRA’s deepwater reference Station FF04 lies within
the depositional part of the sanctuary, Stellwagen Basin, where long-term
accumulation of pollutants and their effects would be expected if they were to occur.
Station FF04 is typical of the deepwater sites offshore from the outfall, representative
of a larger group of stations that were monitored in earlier years of the program, and
it continues to support a soft-bottom community typical of what had been found at the
larger suite of deepwater stations. Communities at those stations have always shown
distinct differences from those found at shallower areas, probably due to their
distance from shore, as well as their depth and their fine-grained sediments (Figures
4-6 through 4-8, above). Superimposing percent grain size on the ordination plot for
2022 samples continued to show those natural differences, and the ordination plot
using data from 1992–2023 further demonstrated the temporal consistency of the
seafloor community at that station. 
Station
NF17
NF24
FF01A
FF04
2D Stress: 0.1

43 
Boston Harbor Seafloor Monitoring 
While the chemistry and biology of the Massachusetts Bay sea floor have not been
harmed by the outfall, conditions are much better and continue to improve in Boston
Harbor, which received sewage sludge discharge until 1991 and effluent discharge
until September 2000. MWRA has conducted ongoing seafloor monitoring in Boston
Harbor since 1991. Each year, sediment samples are taken from nine stations, and
sediment-profile images are taken at 61 stations throughout the harbor.
Sediment textures within individual harbor stations have been generally stable over
time, but have ranged from mostly sand at some stations to silt and clay at others.
Concentrations of total organic carbon in harbor sediments have declined over time,
reflecting the lowered inputs associated with each milestone of the Boston Harbor
Project.
Over the past decade, the total abundance of organisms and number of species per
sample have stabilized (Figure 4-9), responses to continued and sustained
improvement in seafloor habitat conditions. In 2022, samples yielded 34,325
specimens, classified into 139 species and 14 other discrete taxonomic groups. When
the harbor received wastewater inputs, the bottom fauna was dominated by
opportunistic species, animals that respond to inputs of nutrients and organic matter
and are typical of degraded habitats. Abundances varied greatly from year to year,
another characteristic of opportunistic species.
As in past years, the communities in 2022 varied along an outer-to-inner harbor
gradient, reflecting the greater stream and river inputs to the inner harbor and greater
tidal flushing at the outer harbor stations. Sediment-profile imaging has also
confirmed the gradient and clearly documented improvements to habitat conditions
throughout the harbor, showing sustained responses to each phase of the Boston
Harbor Project.

44 
Figure 4-9. Total abundance of soft-bottom animals (top) and number of species (bottom) per
sample in Boston Harbor samples, 1991–2022. Both numbers of organisms and numbers of species
have stabilized as the harbor has recovered. Data are from eight harbor stations that have been
consistently monitored since 1991. “Outfall startup” refers to the beginning of discharge through the
Massachusetts Bay outfall and the end of discharge to the harbor. 
10
20
30
40
50
60 Outfall Startup
1990 1994 1998 2002 2006 2010 2014 2018 2022
Mean +/- 1StdErr

45
Sediment-profile images depicted the continuing good conditions in 2022. There were
few storms in 2021 and 2022, but physical processes continued to dominate over
biological processes in shaping the shallow sediments (Figure 4-10). Prior to the mid-
2000s, biological processes, such as burrows and tube mats, had a greater role, likely
due to the past inputs of sewage-derived organic matter, which fueled growth of the
animals that built the burrows and mats. In 2022, none of the 61 sediment-profile
stations were dominated by biological processes. Sediments continued to be well-
oxidized, with no evidence of excess organic matter. Eelgrass, a sign of a healthy sea
floor, continued to be present at Station R08 in Deer Island Flats, near the location of
the former effluent outfall. 
Figure 4-10. Sediment profiles depicting biologically (BIO), physically (PHY), and mixed
(BIO/PHY) dominated sediment surfaces at three stations in 2021 and 2022. No examples of
biologically dominated sediments were found in 2022. The transition from light to darker sediments in
the left-most pictures reflects the transition from oxygenated sediments with depth. The right-most
picture, showing sediments fully dominated by physical conditions was oxygenated throughout its
depth. The red markings on the photo edges are centimeters of depth. Station T03 is south of Deer
Island in President Roads; Station R10 is in the Inner Harbor; and Station R23 is in Nantasket Roads,
offshore from Quincy, Hingham, and Hull Bays.

46 
Seafloor Contingency Plan Thresholds 
There were no threshold exceedances for the seafloor Contingency Plan thresholds in
2022 (Table 4-1). For the purposes of threshold testing, “nearfield” includes stations
from both nearfield groups (both nearer and farther than two kilometers from the
outfall) and the station in the transition area between Boston Harbor and the
Massachusetts Bay outfall.
All threshold parameters continued to confirm that Massachusetts Bay maintains
persistently healthy seafloor habitats, unaffected by the outfall discharge. Number of
species per sample and the diversity and evenness indices remained higher than
Contingency Plan limits, while the percent opportunistic species remained far below
any level of concern, only 0.02% compared to caution and warning thresholds of 10%
and 25%. 
Table 4-1. Contingency Plan threshold values and 2022 results for seafloor monitoring. 
Parameter Baseline Caution
Level
Warning
Level 2022 Results 
Species per
sample NA <42.99 None 58.5
Fisher’s log-series
alpha NA <9.42 None 12.2
Shannon diversity NA <3.37 None 4.05
Pielou’s evenness NA <0.57 None 0.69
% opportunists NA >10% >25% 0.02% 
NA = not applicable

47 
5. Fish and Shellfish 
Fish and shellfish are important to the regional identity and economy of
Massachusetts. Fish and shellfish can also be good indicators of overall
environmental health. Monitoring protects fish health and ensures safe seafood.
Monitoring focuses on winter flounder, lobster, and cage-deployed blue mussels.
Flounder and lobster are two important commercial and recreational species that live
on the sea floor and feed on bottom-dwelling animals, likely sources for contaminant
exposure. Mussels filter large volumes of water and can concentrate toxic metals and
organic compounds in their tissues. Sampling locations vary by species (Figure 5-1).
MWRA monitors winter flounder health each year and conducts chemical
contaminant analyses of flounder, lobster, and mussel tissues every three years, most
recently in 2021. Flounder health assessment was the only monitoring activity in
2022. It measured external conditions of the fish and the presence of liver disease,
including precancerous conditions and tumors. 
Fish and shellfish monitoring highlights animal health and safe seafood 
In planning for the Massachusetts Bay effluent discharge, the main concern for
fish and shellfish was that chemical contaminants could directly affect bottom
feeders like flounder and lobster or filter feeders such as mussels.
Winter flounder health has improved not only in Boston Harbor, but also near the
Massachusetts Bay effluent outfall. Contaminant levels in flounder liver and fillet,
lobster meat and digestive gland, and mussels have not increased since the
outfall began to discharge. In fact, levels of many contaminants have declined,
particularly near the former harbor outfalls. The good results have allowed for
reductions in fish and shellfish monitoring.

48 
Figure 5-1. Fish-and-shellfish monitoring stations. IH = Inner Harbor; DIF = Deer Island Flats; DIL
= Deer Island Light; OS = Outfall Site; OS-M = Outfall Site, Mussels; LNB = Large Navigation Buoy
(buoy south of the outfall); ECCB = Eastern Cape Cod Bay. Also shown are the outfall, the
instrumented buoys, and the Stellwagen Bank Marine Sanctuary.

49
In 2022, for the first time, flounder monitoring was conducted at only two sites, at the
Massachusetts Bay outfall and at Deer Island Flats, near the former Boston Harbor
outfall. Through 2021, fish were also collected from eastern Cape Cod Bay, which
was considered a relatively pristine control site. In past years, other locations
provided a greater geographic representation of the area. That past monitoring
concluded that there have been no adverse effects of the relocated outfall and that
instead, conditions have improved throughout the region. Consequently, a formal
study with a control site was deemed no longer necessary. However, the regulatory
agencies asserted a continued interest in documenting flounder health in
Massachusetts Bay and sustained improvements in Boston Harbor, so MWRA
continues to sample and evaluate sites at the bay outfall and Deer Island Flats.
All sampling was completed on one day in April 2022. As in past years, scientists
completed external health assessments at sea and preserved livers for laboratory
analysis (Moore et al. 2023; Figure 5-2). 
Figure 5-2. External assessments are made at sea, and livers are removed for laboratory analysis. 
Photo credit Eric Rydbeck, Normandeau Associates, Bedford, New Hampshire.

50 
Flounder Characteristics 
A full complement of 50 fish was collected from near the Massachusetts Bay outfall
site, where the catch per unit effort was among the highest of the monitoring program.
Catches at Deer Island Flats in Boston Harbor were somewhat higher in 2022 than in
2021, but despite more than three hours of trawling, the time limit designated in the
work plan, only 29 fish were taken. Catches have been relatively low throughout the
region for the past several years. Southern New England winter flounder populations
have declined since the 1980s, possibly a result of overfishing (for example, Frisk et
al. 2018). MWRA survey trawls at Deer Island Flats have also been compromised by
abandoned lobstering gear and algal growth clogging the trawl net, and both those
conditions hampered the 2022 catch.
Mean age of fish in 2022 ranged from 4.7 to 5.6 years, and standard length (length
from the tip of the head to the base of the tail fin) ranged from 271 to 280 millimeters,
within the values observed in past years. Weights were also within the typical range
for the program. As is common throughout northeast coastal populations (Moore et al.
2016), the catches were dominated by females. 
Flounder Health 
Measures of external condition, such as bent fin
rays and occurrence and severity of fin erosion,
remained variable but continued to suggest
improved conditions since the 1980s and 1990s.
Incidence of fin erosion, which has been
associated with excess ammonium and other
pollutants, remained low at both the
Massachusetts Bay outfall site and Deer Island
Flats. Ulcers, which are poorly understood but are
observed on the bottom or “blind side” of the fish
in some years, were not present in 2022.
The incidence of centrotubular hydropic vacuolation (CHV), a key tumor precursor
and mild liver condition associated with exposure to contaminants, was the lowest
ever measured at both sites, only 4% at the Massachusetts Bay outfall (similar to
2021) and only 3.4% at Deer Island Flats (Figure 5-3). The decrease in CHV
incidence at Deer Island Flats is especially impressive, as more than 75% of Boston
Harbor flounder had CHV in the late 1980s. CHV incidence has been lower across all
age groups in both the harbor and the bay, alleviating the concern prior to the outfall
relocation that there could be increased liver disease in Massachusetts Bay. In the few
fish where CHV has been found, severity of the condition has also declined. 
Winter flounder disease was
a main indicator of harbor
pollution in the 1990s .
Ensuring that there was no
increase in disease in
Massachusetts Bay was a major
goal of the Boston Harbor
Project, and that has been
successful.

51 
Figure 5-3. Annual prevalence of the tumor precursor centrotubular hydropic vacuolation
(CHV) in winter flounder near the Massachusetts Bay outfall site and at Deer Island Flats, 1991–
2022. CHV incidence has declined throughout the region. OS = Outfall Site, DIF = Deer Island Flats 
Liver tumors or neoplasia, which were present in up to 10% of flounder from Boston
Harbor in the 1980s, remained absent, as they have since 2004. Tumors have never
been found in fish taken from near the Massachusetts Bay outfall site (Figure 5-4; see
also Moore et al. 2018). 
Figure 5-4. Annual prevalence of neoplasia (liver tumors) in winter flounder, 1991–2022. OS =
Massachusetts Bay Outfall Site; DIF = Deer Island Flats. No tumors have ever been recorded in fish
taken from near the Massachusetts Bay outfall.

52 
Winter Flounder Contingency Plan Thresholds 
There was no Contingency Plan threshold exceedance for the only parameter
measured in 2022, flounder liver disease, measured as prevalence of CHV, in fish
caught near the Massachusetts Bay outfall (Table 5-1). CHV incidence remained less
than 10% of the caution threshold and also far less than levels found during the
baseline period before the Massachusetts Bay outfall began to discharge. 
Table 5-1. Contingency Plan threshold values and 2022 results for winter flounder 
Parameter Baseline Caution Level Warning Level 2022 Results 
Flounder disease 
Liver disease
(CHV) 24.4% >44.9% None 4% 
CHV = centrotubular hydropic vacuolation, a liver tumor precursor associated with degraded habitats

53 
6. Special Studies 
Besides monitoring the effluent and the water column, sea floor, and fish and
shellfish in Massachusetts Bay, MWRA conducts special studies in response to
specific permit requirements, scientific questions, and public concerns. This year’s
report focuses on both specific studies and other ongoing activities, including MWRA
notifications of unanticipated or partially treated sewage discharges, data quality
control and management, and the most recent update of the Bays Eutrophication
Model, a permit-required water quality and hydrodynamics model that simulates the
processes affecting water quality throughout Massachusetts Bay and the surrounding
waters. 
Notification of Sewage Discharges 
In January 2021, Massachusetts passed “an Act promoting awareness of sewage
pollution in public waters, ensuring public notification of unanticipated or
inadequately treated sewage discharges.” Under this law and regulations issued by the
Massachusetts Department of Environmental Protection in 2022, MWRA must make
public notifications of “blending events,” combined sewer overflows (CSOs), and
sanitary sewer overflows (SSOs).
Blending occurs when partially treated effluent (primary-treated flow) is blended into
fully treated effluent (primary- and secondary-treated flow) before rejoining at the
disinfection basin, where all effluent is disinfected and dechlorinated before
discharge. Blending occurs only during heavy rainfall, when inflow to the treatment
plant exceeds what the secondary treatment process is designed to handle. Diverting
some flow around secondary treatment prevents street flooding and backups into
homes and businesses. Blending also protects the secondary treatment system,
preventing washout of the beneficial bacteria that are part of that process.
CSOs occur in sewer systems where storm sewers connect to sanitary sewers. In the
MWRA service area, these combined systems are found in Boston, Cambridge,
Chelsea, and Somerville. During especially large rainstorms, the stormwater and
sewage flow that exceeds the sewer capacity is discharged through designated outfalls
in the stormwater system. These local municipal and MWRA CSO discharges prevent
overloading the treatment plant and avoid backups to streets, homes, and businesses.
Controlling and eliminating CSO discharges has been a major part of MWRA’s
Boston Harbor Project.

54
SSOs include any unintended or unauthorized discharge from the sewage system.
SSOs can occur from a blocked pipe, a break in a sewer line, or from excess rainwater
leaking into the sewers. Common dry-weather causes of SSOs are buildups of grease
and bathroom wipes. MWRA has worked to inform the public about improper
disposal of grease, wipes, and other items that can cause pipe blockages, and created
a website page, It's a Toilet, not a Trash Can! Household Waste Disposal.
MWRA began providing website notifications of SSOs in late 2015 and of treated
CSOs in 2016, in advance of the sewage notification law. Email and text notifications
of untreated CSOs and blending events were added in 2022. Within two hours of
confirming a blending event or discharge, MWRA issues notifications to relevant
local, state, and federal government agencies, as well as interested members of the
public. News agencies that report on local issues and organizations that serve affected
environmental justice communities also receive the notifications. In 2023, MWRA
began working with local communities to post warning signs at public access points
located near CSOs.
More information about MWRA’s notifications and how to sign up for them can be
found at the Rapid Notifications page on the MWRA website. The page also has links
to historical blending events, CSOs and SSOs. 
Data Management 
Data management is as important to the success of marine monitoring programs as
the collection of data (National Research Council 1990). Since its inception, MWRA
has prioritized attention to data quality and management (Figure 6-1). Automated
systems, including web-based applications for sending data to MWRA, make it easy
to gather and check data at the collection point, where it is easiest to correct errors.
Further automated checking occurs when data are loaded into the MWRA database,
and MWRA data management staff work with data providers to resolve issues
flagged by those checks. Data checks include a variety of tools, such as statistical
summaries, checks for station locations and sampling times, and visual presentations
to identify suspicious results (for example, Figure 6-2).
Each portion of the monitoring program is governed by a Quality Assurance Project
Plan (QAPP). QAPPs ensure that all project participants understand the goals, the
work to be done, and the data quality standards that apply to their individual tasks.
Specific field and laboratory work elements follow approved standard operating
procedures. Quality assurance officers at MWRA’s Department of Laboratory
Services and within the MWRA contractor organizations ensure that QAPPs and
standard operating procedures are followed.

55 
Figure 6-1. Monitoring data are collected by MWRA and its consultants under stringent quality
assurance and quality control, following the Plan-Do-Check-Act cycle for continuous quality
improvement. 
Figure 6-2. Example of a presentation used to identify suspicious data. This plot depicts particulate
organic carbon (POC) and particulate organic nitrogen (PON) at one station compared to historic data.
The suspect data point is flagged for review.

56
MWRA follows a four-step corrections process to address data errors: (1) verify with
the data collector that a correction is necessary; (2) correct the error in the database;
(3) implement automated procedures to ensure that similar errors will be detected in
the future, and (4) look for and correct similar errors in the historical data. Seasonal
and annual data are not considered “final” until they have been reviewed by the
monitoring team.
All MWRA technical reports, including this annual overview, draw on data from the
central database, which is also used for compliance with the NPDES permit and court
orders. MWRA also provides data to academic, scientific, and community members
interested in the environmental health and physical characteristics of Boston Harbor
and Massachusetts Bay. All QAPPs and study-specific technical reports are available
on the MWRA Environmental Quality Department Technical Reports List. 
Bays Eutrophication Model Update 
MWRA’s permit to discharge wastewater effluent to Massachusetts Bay requires
maintenance and annual runs of the Bays Eutrophication Model (BEM), a combined
circulation and water quality model, which simulates the parameters and processes
affecting potential eutrophication and subsequent low dissolved oxygen levels in
Massachusetts Bay, Boston Harbor, and Cape Cod Bay. Parameters include water
temperature, salinity, circulation, nutrients, and phytoplankton biomass. A precursor
to the BEM was instrumental in siting the outfall, and annual simulations have
continually confirmed the early predictions that the nutrient inputs from the outfall
would not lead to eutrophication in Massachusetts or Cape Cod Bays. Annual runs
complement MWRA’s field program.
The most recent BEM simulation, for the year 2021, successfully captured many of
the seasonal and geographic conditions measured by the survey team (Deltares 2023).
For example, the model accurately simulated late spring nutrient depletion as
phytoplankton began to bloom and the typical drop in dissolved oxygen levels in the
late summer. Results were consistent with previous years’ model runs, continuing to
confirm field monitoring observations that nutrient inputs from the outfall discharge
did not influence phytoplankton biomass or dissolved oxygen.
MWRA has run a scenario in which effluent nitrogen loads were set to 50% higher
than the actual 2020 load (equivalent to 24% higher than the warning threshold).
Even with those additional nutrients, the outfall discharge did not adversely affect the
environment (Deltares 2022). At the simulated high load, the model predicted
increases in dissolved inorganic nitrogen concentrations throughout Massachusetts
Bay. Significant increases in ammonium levels (larger than one micromolar, a
concentration unit based on molecular weight) were limited to within ten kilometers
of the outfall. The model simulation also predicted minor localized increases in
chlorophyll and decreases in dissolved oxygen concentrations, but no effects beyond
the immediate vicinity of the outfall.

57 
7. Update on the NPDES Permit 
MWRA’s permit to discharge treated sewage effluent into Massachusetts Bay became
effective in August 2000. While EPA issues permits with an effective period of five
years, the permit has been administratively continued since its first approval.
EPA issued a new draft permit, NPDES Permit No. MA0103284, on May 31, 2023,
with public comments due by November 28, 2023. After the close of the public
comment period, EPA will review, respond, and issue a final permit. This period of
moving from draft to final permit issuance can take months to years. The
Massachusetts Department of Environmental Protection has also issued a companion
Surface Water Discharge permit which incorporates by reference the EPA NPDES
permit requirements. 
Figure 7-1. Deer Island staff in front of the sludge digesters.

58 
References 
Anderson DM., Keafer BA, Kleindinst JL, McGillicuddy DJ, Martin JL, Norton K,
Pilskaln CH, Smith JL, Sherwood CR, Butman B. 2014. Alexandrium fundyense cysts
in the Gulf of Maine: long-term time series of abundance and distribution, and
linkages to past and future blooms. Deep Sea Research, Part II, Topical Studies in
Oceanography 103:626.
Cibik SJ, Lemieux KB, Davis CS, Anderson DM. 1998. Massachusetts Bay plankton
communities: characterization and discussion of issues relative to MWRA’s outfall
relocation. Boston: Massachusetts Water Resources Authority. Report 1998-08.
140 p.
Deltares. 2022. Massachusetts Bay outfall treated effluent discharge plume
characteristics from the EPA-supported near-field mixing model. Boston:
Massachusetts Water Resources Authority. Report 2022-03. 24 p.
Deltares. 2023. Simulations of 2021 hydrodynamics and water quality in the
Massachusetts Bay system using the Bays Eutrophication Model. Boston:
Massachusetts Water Resources Authority. Report 2023-07. 83 p.
Ellis-Hibbett D, Davis S, Hunt C, Wladkowski J. 2023. 2022 technical survey of
nitrogen removal alternatives for the Deer Island Treatment Plant. Boston:
Massachusetts Water Resources Authority. Report 2023-01. 20 p.
Frisk MG, Dolan TE, McElroy AE, Zacharias JP, Xu H, Hice LA. 2018. Assessing
the drivers of the collapse of winter flounder: implications of management and
recovery. Journal of Sea Research 141:1–13.
Geyer WR, Libby PS, Giblin, A. 2002. Influence of physical controls on dissolved
oxygen variation at the outfall site. Boston: Massachusetts Water Resources
Authority. Letter Report. 20 p.
Libby PS, Rex AC, Keay KE, Mickelson MJ. 2013. Alexandrium Rapid Response
Study survey plan. Revision 1. Boston: Massachusetts Water Resources Authority.
Report 2013-06. 13 p.
Libby PS, Borkman DG, Geyer WR, Turner JT, Costa AS, Goodwin C, Wang J.
2023. 2022 water column monitoring results. Boston: Massachusetts Water Resources
Authority. Report 2023-DRAFT.
MWRA. 2001. Massachusetts Water Resources Authority Contingency Plan,
Revision 1. Boston: Massachusetts Water Resources Authority. Report 2001-ms-71.
47 p.

59
MWRA. 2019. Ambient monitoring plan and contingency plan for Massachusetts Bay
outfall: Monitoring questions statue and 2000–2018 threshold test results. Boston:
Massachusetts Water Resources Authority. Report 2019-03. 36 p.
MWRA. 2021. Ambient monitoring plan for the Massachusetts Water Resources
Authority effluent outfall, Revision 2.1, August 2021. Boston: Massachusetts Water
Resources Authority. Report 2021-08. 107 p.
Moore MJ, McElroy AE, Geoghegan P, Siskey MR, Pembroke A. 2016. Flounder
monitoring report: 2015 results. Boston: Massachusetts Water Resources Authority.
Report 2016-05. 40 p.
Moore, M, Pembroke A, Nestler E, Hall M, Lefkovitz L, Lambert M, Keay K. 2018.
Toxics source reduction and sewage upgrades eliminated winter flounder liver
neoplasia (1984–2017) from Boston Harbor, MA, USA. Diseases of Aquatic
Organisms. 131:239-24.
Moore MJ, Madray ME, Rutecki DA. 2023. Flounder monitoring report: 2022 results.
Boston: Massachusetts Water Resources Authority. Report 2022-15. 20 p.
Natural Research Council. 1990. Managing troubled waters: The role of marine
environmental monitoring. Washington, DC: The National Academies Press. 125 p.
Nestler EC, Madray ME. 2023. Outfall benthic monitoring report: 2022 results.
Boston: Massachusetts Water Resources Authority. Report 2023-DRAFT.
Rex AC, Keay KE, Smith WM, Cura JJ, Menzie CA, Steinhauer MS, Connor MS.
1992. The state of Boston Harbor: 1991. Boston: Massachusetts Water Resources
Authority. Report 1992-03. 106 p.
Rex AC, Connor MS. 1997. The state of Boston Harbor: questions and answers about
the new outfall. Boston: Massachusetts Water Resources Authority. Report 1997–05.
32 p.
Scully ME, Geyer WR, Borkman D, Pugh TL, Costa A, Nichols OC. 2022.
Unprecedented summer hypoxia in southern Cape Cod Bay: an ecological response to
regional climate change? Biogeosciences 19:3523–3536.
Taylor DI, Oviatt CA, Giblin AE, Tucker J, Diaz RJ, Keay K. 2020. Wastewater
input reductions reverse historic hypereutrophication of Boston Harbor, USA. Ambio
49:187–196.
Tucker J, Kelsey S, and Giblin A. 2010. 2009 Benthic nutrient flux annual report.
Boston: Massachusetts Water Resources Authority. Report 2010-10. 27 p.

60
Tucker J, Giblin AE, Hopkinson CS, Kelsey SW, Howes BL. 2014. Response of
benthic metabolism and nutrient cycling to reductions in wastewater loading to
Boston Harbor, USA, Estuarine, Coastal and Shelf Science 151:54–68.
Werme C, Rex AC, Hunt, C. 2012. Outfall monitoring overview background: 2012
update. Boston: Massachusetts Water Resources Authority. Report 2012-02. 59 p.
Werme C, Keay KE, Libby PS, Codiga DL, Taylor DI, Charlestra L, Carroll SR.
2018. 2017 outfall monitoring overview. Boston: Massachusetts Water Resources
Authority Report 2018-07. 53 p.
Werme C, Codiga D, Libby PS, Carroll SR. 2021. 2020 outfall monitoring overview.
Boston: Massachusetts Water Resources Authority Report 2021-10. 55 p.

61 
List of Acronyms 
ASP Amnesic shellfish poisoning
BOD Biochemical oxygen demand
cBOD Carbonaceous biochemical oxygen demand
cfu Colony forming units
CHV Centrotubular hydropic vacuolation
CSO Combined sewer overflow
DDT Dichlorodiphenyltrichloroethane
DIF Deer Island Flats
ECCB Eastern Cape Cod Bay
EPA U.S. Environmental Protection Agency
IAAC Inter-Agency Advisory Committee
LC50 50% mortality concentration
LNB Large navigation buoy
MGD Million gallons per day
MWRA Massachusetts Water Resources Authority
NA Not analyzed/not applicable
NDBC National Data Buoy Center
NERACOOS Northeastern Regional Association of Coastal and Ocean Observing Systems
NOEC No observed effects concentration
NPDES National Pollutant Discharge Elimination System
OS Outfall site
OS-M Outfall site mussels
PAH Polycyclic aromatic hydrocarbon
PCB Polychlorinated biphenyl
PIAC Public Interest Advisory Committee
POC Particulate organic carbon
PON Particulate organic nitrogen
PSP Paralytic shellfish poisoning
PSU Practical salinity units
QAPP Quality assurance project plan
SSO Sanitary sewer overflow

Massachusetts Water Resources Authority 
Deer Island
33 Tafts Avenue • Boston, MA 0212 8 
www.mwra.com
617-242-6000