2024 overview

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Massachusetts Water
Resources Authority
Environmental Quality
Department Report 2025-05 
2024 Outfall
Monitoring
Overview

Citation: 
Werme C, Libby PS, Carroll SR, Goodwin C. 2025. 2024 outfall monitoring overview. Boston:
Massachusetts Water Resources Authority. Report 2025-05. 63 p. 
Cover photos: 
Top: Researchers recover a rosette used to collect samples and measure parameters like
temperature, pH, and dissolved oxygen at station F23 off Deer Island on the September 2024
MWRA survey WN248. Photo credit P. Scott Libby, Battelle, Topsham, Maine.
Bottom: Blue mussels and sea anemones growing on an active outfall diffuser in 2023. Photo
credit Hecker Environmental, Falmouth, Massachusetts.

2024 
Outfall Monitoring Overview 
prepared by 
Christine Werme 
Independent Consultant 
P. Scott Libby 
Battelle 
72 Main Street 
Topsham, ME 04086 
Sally R. Carroll and Christopher Goodwin 
Massachusetts Water Resources Authority 
Environmental Quality Department 
Deer Island 
33 Tafts Avenue 
Boston, MA 02128 
November 11, 2025

2024 Outfall Monitoring Panel and Committees 
Outfall Monitoring Science Advisory Panel 
Judith Pederson, Massachusetts Institute of Technology Sea Grant (chair) 
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

2024 OUTFALL MONITORING OVERVIEW: EXECUTIVE SUMMARY 
i 
Executive Summary 
The Massachusetts Water Resources Authority (MWRA) provides water and
wastewater services to more than three million people and more than 5,500
businesses in metropolitan Boston. Since its beginnings in 1984, MWRA has strived
to minimize any adverse effects of wastewater discharges on the marine environment.
MWRA works with industrial dischargers to prevent pollutants from entering the
waste stream and provides primary and secondary sewage treatment to remove solid
material and ensure that pollutants in the remaining liquid effluent meet permitted
limits before discharge. Both the solid material, called sludge, and liquid effluent
were once discharged into Boston Harbor. Sludge is now processed into fertilizer for
use in gardening and landscaping. Treated effluent is discharged into Massachusetts
Bay. The discharge is at depth, below the sunlit portion of the water column where
plant growth occurs, preventing nutrients in the effluent from overstimulating algal
growth. 
MWRA’s permit to discharge treated effluent through its Massachusetts Bay outfall
requires monitoring the effluent before discharge and monitoring the water column,
sea floor, and living communities in Massachusetts Bay. In the more than two
decades that MWRA has discharged treated wastewater into Massachusetts Bay, there
have been no unanticipated effects or findings of environmental concern. 
Deer Island Treatment Plant effluent meets permit conditions 
Producing the cleanest possible sewage effluent is the most important way that
MWRA strives to protect the marine environment. In 2024, MWRA earned a
Platinum Peak Performance Award from the National Association of Clean Water
Agencies for 18 consecutive years with no permit-limit exceedances. 
One important measure of effluent quality is the suspended solids load, that is, the
total amount of solid material discharged each year. Treatment removes most of the
solids from sewage as sludge, but small particles remain suspended in the effluent.
The 2024 effluent solids discharges remained consistent over the past two decades,
measuring far lower than the amounts once discharged into Boston Harbor (Figure i,
top). Discharges of potentially toxic metals have followed similar patterns, remaining
low for more than 20 years (Figure i, bottom). Discharges of both solids and toxic
pollutants have been much less than originally had been anticipated during the
planning for the Massachusetts Bay discharge site.

2024 OUTFALL MONITORING OVERVIEW: EXECUTIVE SUMMARY 
ii 
Figure i. Annual discharges of sewage solids (top) and metals (bottom), 1990–2024. Source
reduction measures, including industrial pretreatment, ending sludge discharges, and secondary
effluent treatment dramatically decreased solids and metals discharges. Those decreases have been
sustained for more than 20 years. The effluent discharge was moved from Boston Harbor to
Massachusetts Bay in September 2000. 
Metals 
Solids

2024 OUTFALL MONITORING OVERVIEW: EXECUTIVE SUMMARY 
iii 
MWRA’s discharge has not led to eutrophication 
Sewage treatment removes some, but not all,
of the nutrients in wastewater, so MWRA
water-column monitoring focuses on
nutrient inputs and potential eutrophication,
the excessive growth and decay of
phytoplankton, the microscopic algae that
live in the water column (Figure ii). 
Eutrophication can lead to low levels of dissolved oxygen, harming all marine life. It
can also stimulate nuisance or toxic algal blooms, which can threaten marine mammal
and human health. In 2024, water-quality analyses, including measurements of
nutrient concentrations, phytoplankton biomass, and dissolved oxygen concentrations,
continued to demonstrate that MWRA’s treated effluent discharge has not led to
eutrophication in Massachusetts Bay. 
Figure ii. Typical phytoplankton from Massachusetts Bay. Phytoplankton include chains of
individual diatoms and larger dinoflagellates. Photo credit David Borkman, Pausacaco Plankton,
Saunderstown, Rhode Island. 
Diatom chains 
Dinoflagellate 
Eutrophication occurs when
nutrients overstimulate
phytoplankton growth, leading to
lower levels of oxygen when the
phytoplankton die and
decompose.

2024 OUTFALL MONITORING OVERVIEW: EXECUTIVE SUMMARY 
iv 
Seafloor communities are thriving 
MWRA’s monitoring of the sediments and animals at the sea floor (for example,
Figure iii) has shown that bottom-dwelling communities are influenced by sediment
types and water depth rather than by the effluent discharge. The total abundance of
animals, numbers of species, and the species makeup of bottom-dwelling
communities have remained within the ranges of natural variability. 
Figure iii. Polychaete worms dominate the soft-bottom habitats in Massachusetts Bay. The image
shows a polychaete that is about 2 millimeters long, taken from a former MWRA monitoring station.
Polychaetes can range in size from less than a millimeter to several meters; most are very small. This
polychaete is in a group sometimes called necklace worms, due to the constricted band depicted in the
lower middle part of the photograph. Necklace worms are found in a variety of soft- and hard-bottom
habitats. Photo credit Nancy J. Maciolek, Duxbury, Massachusetts.

2024 OUTFALL MONITORING OVERVIEW: EXECUTIVE SUMMARY 
v 
Pollutant levels in fish and shellfish remain low 
MWRA monitors the health of winter flounder, one of the region’s popular
commercial and recreational fish. Flounder live and feed at the sea floor, where they
are susceptible to contaminant-related physical abnormalities and health risks, but no
adverse effects have been detected in response to the Massachusetts Bay effluent
discharge. Instead, flounder health has improved both near the Massachusetts Bay
outfall and at the former sewage effluent outfall in Boston Harbor, a result of
pollution prevention and sewage treatment. Cancerous tumors have never been found
in fish caught near the Massachusetts Bay outfall. 
Every three years, including 2024, MWRA measures pollutant levels in winter
flounder, lobsters, and blue mussels. Results show continued low pollutant levels in
all three species (for example, Figure iv). 
Figure iv. Mercury concentrations in flounder and PCBs in lobsters caught near the
Massachusetts Bay outfall have remained low. MWRA has a Contingency Plan, which includes both
caution and warning thresholds, designed to alert the regulatory agencies of environmental changes.
Only the caution thresholds are shown. “Baseline” is 1992–2000, when effluent was discharged to
Boston Harbor; “outfall discharge” is years when the discharge was to Massachusetts Bay; ng/g wet =
nanograms per gram wet weight.

2024 OUTFALL MONITORING OVERVIEW: TABLE OF CONTENTS 
vi 
Table of Contents 
Executive Summary............................................................................................................. i 
1. Introduction ....................................................................................................................1 
2. Effluent ....................................................................................................................2 
2024 Effluent Characterization ................................................................................. 3 
Effluent Contingency Plan Thresholds ..................................................................... 9 
3. Water Column ...............................................................................................................10 
Physical Conditions ................................................................................................ 12 
Water Quality .......................................................................................................... 16 
Nutrients .............................................................................................................. 16 
Phytoplankton Biomass ...................................................................................... 19 
Dissolved Oxygen ............................................................................................... 20 
Plankton Communities ............................................................................................ 21 
Phytoplankton Communities ............................................................................... 22 
Zooplankton Communities.................................................................................. 24 
Stellwagen Bank National Marine Sanctuary ......................................................... 25 
Boston Harbor Water Quality ................................................................................. 26 
Water-Column Contingency Plan Thresholds ........................................................ 27 
4. Sea Floor ..................................................................................................................28 
Sediment Characteristics and Tracers ..................................................................... 30 
Soft-bottom Communities....................................................................................... 32 
Stellwagen Bank National Marine Sanctuary ......................................................... 38 
Boston Harbor Seafloor Monitoring ....................................................................... 38 
Seafloor Contingency Plan Thresholds ................................................................... 41 
5. Fish and Shellfish ..........................................................................................................42 
Flounder Condition and Health............................................................................... 44 
Fish and Shellfish Tissue Chemistry....................................................................... 46 
Fish and Shellfish Contingency Plan Thresholds ................................................... 49 
6. Special Topics ...............................................................................................................51 
Fish-and-Shellfish Monitoring Species................................................................... 51 
Winter Flounder .................................................................................................. 52 
American Lobster................................................................................................ 54 
Blue Mussel ........................................................................................................ 55 
Bays Eutrophication Model Update ........................................................................ 59 
References ..................................................................................................................59 
List of Acronyms ...............................................................................................................63

2024 OUTFALL MONITORING OVERVIEW: TABLE OF CONTENTS 
vii 
List of Figures 
Figure i. Annual discharges of sewage solids and metals, 1990–2024. ............................. ii 
Figure ii. Typical phytoplankton from Massachusetts Bay. .............................................. iii 
Figure iii. Polychaete worms dominate the soft-bottom habitats in Massachusetts Bay... iv 
Figure iv. Mercury concentrations in flounder and PCBs in lobsters caught near the
Massachusetts Bay outfall have remained low ...........................................v 
Figure 2-1. Annual and average rainfall in Boston, 1990–2024 .........................................2 
Figure 2-2. Annual full secondary-treated effluent flows and primary-blended flows,
1999–2024 ..................................................................................................3 
Figure 2-3. Monthly full secondary-treated and primary-blended flows and rainfall in
2024 ............................................................................................................4 
Figure 2-4. Average annual daily discharges of sewage solids, 1990–2024 .......................4 
Figure 2-5. Annual nitrogen discharges, 1996–2024. .........................................................6 
Figure 2-6. Annual metals discharges, 1990–2024. ............................................................7 
Figure 3-1. Water-column monitoring stations and instrumented buoys in Massachusetts
and Cape Cod Bays. .................................................................................11 
Figure 3-2. Flows of the Merrimack and Charles Rivers ..................................................13 
Figure 3-3. Surface- and bottom water temperature and salinity at Station N18 just to the
south of the outfall in 2024 compared to prior years................................14 
Figure 3-4. Upwelling and downwelling conditions at Station N18 in 2024, compared to
prior years. ................................................................................................15 
Figure 3-5. Ammonium concentrations at selected stations in 2024 compared to prior
years..........................................................................................................17 
Figure 3-6. Surface- and bottom-water ammonium concentrations during mixed and
stratified conditions ..................................................................................18 
Figure 3-7. Monthly areal chlorophyll concentrations at selected stations in 2024
compared to prior years ............................................................................19 
Figure 3-8. Near-bottom water dissolved oxygen concentrations in 2024 compared to
prior years .................................................................................................20 
Figure 3-9. Typical phytoplankton from Massachusetts Bay. ...........................................22 
Figure 3-10. Total phytoplankton abundance at selected stations in 2024 compared to
prior years .................................................................................................23 
Figure 3-11. A lexandrium catenella abundance in 2024 compared to 1992–2023...........24 
Figure 3-12. Total zooplankton abundance at selected stations in 2024 compared to prior
years..........................................................................................................25

2024 OUTFALL MONITORING OVERVIEW: TABLE OF CONTENTS 
viii 
Figure 3-13. Dissolved oxygen concentrations at the NERACOOS Buoy A01 and at
nearby deepwater MWRA survey Station F22 in 2024............................26 
Figure 4-1. Soft-bottom monitoring stations. ....................................................................29 
Figure 4-2. Concentrations of the effluent tracer Clostridium perfringens spores in 2024.
..................................................................................................................31 
Figure 4-3. Total organic carbon at selected stations, 1992–2024 ....................................31 
Figure 4-4. Total abundance of soft-bottom organisms by region, 1992–2024 ................32 
Figure 4-5. Total abundance of species by region, 1992–2024 .........................................33 
Figure 4-6. Polychaete worms dominate the soft-bottom habitats in Massachusetts Bay.34 
Figure 4-7. Ordination plot of 2024 Massachusetts Bay samples by location ..................36 
Figure 4-8. Percent fine sediments and water depth superimposed on the ordination plot
of the 2024 samples ..................................................................................37 
Figure 4-9. Total abundance of soft-bottom animals and number of species per sample
from Boston Harbor, 1991–2024 ..............................................................39 
Figure 4-10. Example of a sediment-profile image taken in 2024 south of Deer Island and
the north channel into the harbor ..............................................................40 
Figure 5-1. Fish-and-shellfish monitoring stations. ..........................................................43 
Figure 5-2. Centrotubular hydropic vacuolation in flounder liver, 2024 ..........................45 
Figure 5-3. Annual prevalence of the tumor precursor CHV in winter flounder, 1991–
2024 ..........................................................................................................45 
Figure 5-4. Annual prevalence of liver tumors in winter flounder, 1991–2024. ...............46 
Figure 5-5. Chlordane in flounder fillets, 1993–2024 .......................................................47 
Figure 5-6. PCBs in lobster meat, 1994–2024. .................................................................48 
Figure 5-7. Chlordane in mussel tissues, 1993–2024 ........................................................48 
Figure 5-8. Mercury concentrations in flounder and PCBs in lobster meat compared to
Contingency Plan thresholds over time. ...................................................50 
Figure 6-1. The Gulf of Maine has been warming faster than the global ocean ...............52 
Figure 6-2. Lost lobster traps and other “ghost” fishing gear interfere with flounder
collection ..................................................................................................53 
Figure 6-3. Commercial and recreational winter flounder landings throughout its range,
1981–2023. ...............................................................................................54 
Figure 6-4. Pre- and post-deployment mussel cages .........................................................56 
Figure 6-5. Mussel collection sites, 1998–2024 ................................................................57 
Figure 6-6. Blue mussels and sea anemones growing on an active outfall diffuser in 2023
..................................................................................................................58

2024 OUTFALL MONITORING OVERVIEW: TABLE OF CONTENTS 
ix 
List of Tables 
Table 2-1. Contingency Plan threshold values and 2024 results for effluent monitoring. ..9 
Table 3-1. Contingency Plan threshold values and 2024 results for water-column
monitoring. ...............................................................................................27 
Table 4-1. Contingency Plan threshold values and 2024 results for seafloor monitoring. 41 
Table 5-1. Contingency Plan threshold values and 2024 results for fish-and-shellfish
monitoring. ...............................................................................................49

2024 OUTFALL MONITORING OVERVIEW: INTRODUCTION 
1 
1. Introduction 
For more than 40 years, since its creation in 1984, the Massachusetts Water
Resources Authority (MWRA) has strived to minimize adverse effects of municipal
wastewater on the marine environment. A vigorous pretreatment program, a carefully
maintained treatment plant, and vigilant effluent and environmental monitoring have
ensured that marine life in Boston Harbor and Massachusetts Bay remain healthy and
vibrant. In September 2000, MWRA diverted its Deer Island Treatment Plant
wastewater effluent discharge from the shallow, enclosed waters of Boston Harbor to
a deeper, more open site in Massachusetts Bay. This milestone was one of the most
important components of the “Boston Harbor Project.” 
MWRA operates the Massachusetts Bay outfall 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 discharge. 
The NPDES permit requires MWRA to measure effluent quality before discharge and
to monitor the water, sea floor, and marine life in Massachusetts Bay. A Contingency
Plan attached to the permit requires MWRA to compare measured conditions against
“caution” and “warning” thresholds, developed from state standards and baseline
monitoring data. If a caution threshold is exceeded, MWRA must investigate the
issue. A warning threshold is a higher level of concern and could require both
investigation and a responsive action. Threshold exceedances do not imply
environmental harm, but rather provide notice of changes from past conditions.
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
Environmental Quality Reports list at www.mwra.com/our-environment/water- 
quality-reports/technical-reports . 
This 2024 Outfall Monitoring Overview fulfills a requirement of the NPDES permit,
which stipulates that MWRA produce an annual summary of monitoring results for
Massachusetts Bay, including results relevant to Stellwagen Bank National Marine
Sanctuary. The sanctuary’s closest boundary is about 20 kilometers to the east of the
MWRA outfall. 
Annual overviews also present special topics selected in response to permit
conditions and environmental concerns, often including research completed in
cooperation with other agencies and research organizations. Special topics for 2024
focus on key fish-and-shellfish monitoring species and provide an update on the
permit-required Bays Eutrophication Model.

2024 OUTFALL MONITORING OVERVIEW: EFFLUENT 
2 
2. Effluent 
MWRA’s NPDES permit and Contingency Plan set limits on effluent parameters and
pollutants. Contaminants of concern include suspended solids, organic material,
pathogens, nutrients, and toxic contaminants . Because wastewater entering Deer
Island Treatment Plant includes not only municipal sewage but also groundwater
infiltration and stormwater inflow, rainfall is an important factor in determining
annual wastewater flows and contaminant concentrations in the treated effluent. The
Boston area received 45 inches of rain in 2024, just above the 1990–2023 average of
44 inches (Figure 2-1). Most of the rainfall occurred in the early part of the year; the
summer and early fall were unusually dry. 
Figure 2-1. Annual and average rainfall in Boston, 1990–2024. Data are from the National Weather
Service rain gauge at Boston Logan International Airport. 
Effective wastewater treatment protects Massachusetts Bay. 
MWRA’s industrial pretreatment program and its primary and secondary sewage
treatment minimize contaminants discharged into Massachusetts Bay. In 2024,
the effluent met all permit limits. The total nitrogen discharge exceeded the
Contingency Plan caution threshold, as originally had been predicted from
projected population data for 2020. That exceedance continues to be reviewed
but is not considered an immediate environmental risk. 
Deer Island Treatment Plant marked 18 consecutive years of 100% compliance
with permit limits in 2024, earning a National Association of Clean Water
Agencies Platinum Peak Performance Award.

2024 OUTFALL MONITORING OVERVIEW: EFFLUENT 
3 
2024 Effluent Characterization 
Effluent characterization includes measurements
of flow, as well as contaminants of concern. In
conjunction with contaminant concentration
data, flow provides a measure of total
discharges to the environment. In 2024, flow
averaged 312 million gallons per day, lower
than the 327 million gallons per day average for
1999–2023 (Figure 2-2). Almost all effluent in
2024 received full secondary treatment. The
1.6% of the total flow receiving only primary
treatment was blended into fully treated effluent prior to disinfection and discharge
(Figure 2-3). 
Primary treatment uses physical processes to remove solid material through settling.
Most organic material and toxic pollutants are removed from the raw sewage through
primary treatment. Secondary treatment includes clarifiers to remove particles that do
not settle and biological processes to break down organic matter, further reducing
pollutants in the effluent. Blending occurs during intense rainfall, when flow to the
treatment plant exceeds what the secondary treatment process is designed to handle.
Diverting some flow around the secondary treatment process prevents street flooding
and sewage backups into homes and businesses. It also protects the secondary
treatment system, preventing washout of the beneficial bacteria that provide a critical
part of the process. Massachusetts regulations require MWRA to notify regulators and
the public whenever a blending event occurs. Historically, blended flows have fully
met permit limits, and they continued to meet those limits in 2024. 
Figure 2-2. Annual full secondary-treated effluent flows and primary-blended flows, 1999–2024 .
Average flow of 1999–2023 was 327 million gallons per day. 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. Historically, blended flows have
been minimal and met all permit limits. 
Flow refers to the rate at which
wastewater travels through the
treatment plant. It may refer to
flow entering the plant (influent),
but often refers to outflow from
the plant (effluent). Once
effluent is released to the
environment, it is often called
the discharge .

2024 OUTFALL MONITORING OVERVIEW: EFFLUENT 
4 
Figure 2-3. Monthly full secondary-treated and primary-blended flows (bars) and rainfall
(dashed line) in 2024. Months with primary-blended flows corresponded with months of greater
rainfall and snow melt. 
Most solid material is removed from sewage influent as sludge, and the remaining
particles are called total suspended solids. Solids discharge in 2024 remained
consistently low at 14 metric tons per day (Figure 2-4). Over the past two decades,
the total solids discharges have been only about 10% of what had been discharged in
1990–1991, when sludge as well as effluent was disposed of in Boston Harbor. 
Figure 2-4. Average annual daily discharges of sewage solids, 1990–2024. Solids discharges have
remained low.

2024 OUTFALL MONITORING OVERVIEW: EFFLUENT 
5 
Organic material in effluent consumes
oxygen as it decays. Biochemical oxygen
demand (BOD), a measure of the amount of
oxygen needed to decompose organic
material, remained well below levels that
would be expected to affect dissolved oxygen in the environment in 2024. MWRA
has consistently met its weekly and monthly BOD permit limits. 
MWRA monitors the effluent for fecal
coliform bacteria, one indicator of a variety of
pathogens that can make waters unsafe for
swimming or shellfish unhealthy for human
consumption. Fecal coliform bacteria levels in
the effluent have always been well below
permit limits. Under a separate program,
MWRA monitors fecal coliform bacteria and another bacteria indicator, 
Enterococcus , in Massachusetts Bay. Results at all locations, even those closest to the
outfall, meet the stringent shellfishing standards set by the state. 
MWRA’s permit has not set limits on
effluent nutrients, potentially toxic metals,
and organic compounds but requires
measurements and reporting. Nutrient
measurements include individual
components of total nitrogen (ammonium, nitrate, and nitrite). Potentially toxic
metals with reporting requirements include zinc, copper, lead, nickel, silver, and
mercury. Organic compounds include selected chlorinated pesticides and
polychlorinated biphenyls (PCBs), known as legacy pollutants. These contaminants
have been banned for use for decades, but break down slowly and persist in low
amounts in wastewater and stormwater runoff. 
The Contingency Plan attached to MWRA’s permit includes thresholds for nitrogen
discharges. The total nitrogen load for 2024 was 12,755 metric tons, above the
Contingency Plan caution threshold of 12,500 metric tons per year (Figure 2-5). This
threshold was set conservatively at about 90% of the warning threshold, 14,000
metric tons, which was the amount expected to be discharged annually by 2020,
based on population growth projections for the MWRA service area made in the
1990s. Actual population growth has been somewhat higher than projected, but
nitrogen loads have remained below the anticipated discharge, a result of water
conservation, better controls on groundwater infiltration, and good removal
efficiencies during treatment. The nitrogen caution threshold was exceeded once
before, in 2019, when the population size of the MWRA service area was about 6%
greater than had been predicted for 2020. 
Organic material , a major
constituent of untreated sewage
that consumes oxygen as it
decays, remains at safe levels. 
Nutrients control algal growth
and, in excess, can lead to lower
oxygen concentrations or
stimulate nuisance algal blooms. 
Pathogens include disease-
causing bacteria, viruses, and
protozoans, and are found in
human and animal waste.
Effluent bacteria levels continue
to meet permit limits.

2024 OUTFALL MONITORING OVERVIEW: EFFLUENT 
6 
Figure 2-5. Annual nitrogen discharges, 1996–2024. 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. 
Modeling has suggested that even higher nitrogen loads than the more stringent
warning threshold would not harm the environment (Deltares 2022). As required by
the permit, MWRA updates a report on potential nitrogen-removal strategies annually
(most recently, Davis et al. 2025). 
Ammonium is the form of nitrogen most readily taken up by phytoplankton and a
good effluent tracer. During the late 1990s and early 2000s, ammonium discharges
became a larger component of the total nitrogen discharge, a result of the treatment
processes for both sewage effluent and sludge. When sludge, for example, is
processed into fertilizer pellets, some ammonium-rich water is returned to the
treatment plant. Those increases have plateaued in recent years.

2024 OUTFALL MONITORING OVERVIEW: EFFLUENT 
7 
Metals discharges remained low in 2024
(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 hair salons, automobile-repair
shops, hospitals, residential household
products, and street runoff. 
Figure 2-6. Annual metals discharges, 1990–2024. Zinc and copper from water pipes and fixtures
remain the most abundant metals. Besides ending sludge discharge in 1991, 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 (summarized in Werme et al.
2021). Zinc discharges were about one quarter of the amounts predicted for 2020.
Copper discharges remained less than 20%, and all other metals discharges were less
than 10% of the predictions made in the 1990s. Except for copper, all metals meet
water quality standards even prior to discharge. 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, a result of drinking-water corrosion
control, which decreases leaching from water pipes. Removing lead from pipes and
service lines has significantly reduced lead levels in the effluent discharge. 
Toxic contaminants in the effluent
include heavy metals, such as
copper and lead, and organic
compounds, including some now-
banned pesticides and
polychlorinated biphenyls (PCBs).

2024 OUTFALL MONITORING OVERVIEW: EFFLUENT 
8 
Mercury discharges were only 1% of what had been anticipated during planning for
the outfall, largely a result of source-reduction efforts in dental and medical facilities.
Once considered an effluent tracer, silver is now often at levels below detection
limits, due to the decline in film-based photography, as well as efficient removal by
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) discharges, which include compounds produced by
combustion processes and are ubiquitous in the environment, now meet water quality
standards at discharge. Effluent data for some other organic compounds, such as 4,4’-
DDE, one of the most prevalent breakdown products of the chlorinated pesticide
DDT, show evidence of the very slow declines anticipated in the 1970s, when
chlorinated pesticides were banned. Discharges of PCBs, which were once used in
industrial and consumer electrical equipment, also remain well below the historic
loads discharged into Boston Harbor.

2024 OUTFALL MONITORING OVERVIEW: EFFLUENT 
9 
Effluent Contingency Plan Thresholds 
There were no effluent Contingency Plan exceedances based on permit limits in 2024
(Table 2-1). The total nitrogen threshold, set in the Contingency Plan attached to the
permit, was exceeded in 2024. MWRA continues to review this exceedance in the
context of the historical effluent data, population growth, and sampling patterns, but it
is not considered an immediate environmental risk. When planning for the outfall,
scientists and regulators projected that the more stringent warning level would be
exceeded by 2020. No increased phytoplankton biomass or decreased dissolved
oxygen concentrations or saturation resulted from the nitrogen discharge (see Section
3, Water Column). 
Table 2-1. Contingency Plan threshold values and 2024 results for effluent monitoring. 
Parameter Baseline Caution Level Warning Level 2024 Results 
Permit Limits and Contingency Plan Thresholds 
pH NA NA <6 or >9 Not exceeded 
Fecal coliform NA NA >14,000 fecal 
coliforms/100 mL Not exceeded 
Chlorine, residual NA NA >631 μg/L daily, 
>456 μg/L monthly Not exceeded 
Suspended solids NA NA >45 mg/L weekly 
>30 mg/L monthly Not exceeded 
cBOD NA NA >40 mg/L weekly, 
>25 mg/L monthly Not exceeded 
Acute toxicity NA NA LC50 <50% Not exceeded 
Chronic toxicity NA NA NOEC <1.5% 
effluent Not exceeded 
PCBs NA Aroclor>0.045 ng/L NA Not exceeded 
Plant performance NA 5 violations/year Compliance <95% 
of the time 100% compliance 
Flow NA NA >436 MGD 
average dry days Not exceeded 
Oil and grease NA NA >15 mg/L weekly Not exceeded 
Contingency Plan Thresholds 
Total nitrogen load NA >12,500 
mtons/year 
>14,000 
mtons/year 12,755 mtons 
NA = not applicable 
cBOD = carbonaceous biological oxygen demand 
LC50 = 50% mortality concentration 
NOEC = no observable effect concentration 
PCB = polychlorinated biphenyl 
Aroclor = total for a specified suite of PCB compounds (called Aroclors) 
Plant performance = compliance with permit conditions 
mL = milliliter 
μg/L = micrograms per liter 
mg/L = milligrams per liter 
ng/L = nanograms per liter 
MGD = million gallons per day 
mtons = metric tons

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
10 
3. Water Column 
MWRA’s water-column monitoring program measures and evaluates physical
oceanographic conditions, water quality, and phytoplankton and zooplankton
communities at stations in Massachusetts Bay, at the mouth of Boston Harbor, and in
Cape Cod Bay. Ship-based field surveys are augmented by data from instrumented
buoys and satellite imagery. 
Organic material, pathogens, and toxic contaminants are largely removed in
wastewater treatment and are unlikely to have any harmful effects on water quality or
plankton communities. Therefore, water-column monitoring focuses mainly on
potential effects of excess nutrients, which are less completely removed by treatment. 
Excess nutrients can cause eutrophication or promote growth of nuisance or toxic
algal blooms. Potentially toxic species include Alexandrium catenella , which occurs
regularly in Massachusetts Bay, and Pseudo-nitzschia , a species group that is often
present, but usually in low numbers and in its nontoxic forms. One nuisance, but not
toxic, phytoplankton species, Phaeocystis pouchetii , was a focus in past years, but its
presence and abundance have been shown to vary in response to physical
oceanographic conditions rather than the outfall, and it is no longer a focus of the
monitoring program. 
Each monitoring year includes nine ship-based surveys, completed monthly between
February and October. Five stations are in the “nearfield,” a 12×10-kilometer area
centered on the outfall, where some effects of the effluent were expected and have
been observed. Nine stations are in the more distant “farfield,” including the mouth of
Boston Harbor, Cape Cod Bay, and near Stellwagen Bank National Marine Sanctuary
(Figure 3-1). Nearfield Station N18, located just to the south of the outfall, is often
considered representative of the nearfield, as it has been consistently monitored over
time and would be likely to show any effects of the outfall, should they occur. 
Water-column monitoring focuses on eutrophication. 
Excess nutrient inputs can lead to low levels of dissolved oxygen or stimulate
nuisance or toxic algal blooms, so water-column monitoring evaluates nutrient
concentrations, water-quality parameters, and plankton. For most parameters,
2024 was a typical year in the water column, following well-established
seasonal patterns. There were no Contingency Plan threshold exceedances for
chlorophyll, dissolved oxygen, or nuisance phytoplankton species, confirming
that the outfall discharge has not led to eutrophication in Massachusetts Bay.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
11 
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.
Nearfield station labels begin with N, while farfield stations begin with F. NERACOOS = Northeastern
Regional Association of Coastal Ocean Observing Systems; NDBC = National Data Buoy Center.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
12 
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 catenella abundance at up to 19
stations. No additional surveys were necessary in 2024. 
The monitoring program benefits from collaboration with the Center for Coastal
Studies in Provincetown, Massachusetts, which samples the three stations in Cape
Cod Bay and near Stellwagen Bank National Marine Sanctuary. Due to rough sea
conditions, the August and October 2024 sampling in Cape Cod Bay occurred outside
a 48-hour target window for completing all sampling in both bays. 
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 (NOAA) 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 
Circulation and physical properties in Massachusetts Bay are driven by the larger
patterns of water flow in the Gulf of Maine. Water typically enters Massachusetts Bay
from the north, circulates in a counter-clockwise direction, and exits by
Provincetown, at the tip of Cape Cod. Consequently, Station F22, to the north and
east of the outfall, is considered a reference station for water entering the bay, while
stations to the south of the outfall are more likely to have detectable outfall
influences. 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. 
As in other temperate regions, there are
seasonal patterns to the physical structure
of the water column, extending from the
surface to the sea floor. In the winter, the
water column is well mixed, and dissolved
oxygen levels are uniformly high. Warm
water is less dense than cold water, so as
surface waters warm, the water column stratifies, effectively separating surface from
bottom waters. Bottom waters become isolated from the atmosphere and oxygen
production by phytoplankton. This isolation, combined with respiration and
decomposition of organic material, leads to lowered bottom-water dissolved oxygen
levels in late summer and fall. In the fall, cooling surface waters and strong winds
promote mixing, ending the stratified season and re-oxygenating deeper waters. 
Stratification separates warm,
surface water from colder, denser
bottom water. The boundary
between layers is called the
thermocline or pycnocline.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
13 
Stratification is also affected by rainfall and river flow to Massachusetts Bay. Inputs
of freshwater, which is lighter than seawater, contribute to stronger stratification. The
Merrimack River, which enters the Gulf of Maine just to the north of Massachusetts
Bay, is the largest river in the region and plays an important role in freshwater
delivery to Massachusetts Bay. Discharge from the smaller Charles River, which
flows into Boston Harbor, also influences conditions in the bay. 
Physical conditions in 2024 were largely typical of past years (Libby et al. 2025).
Over the course of the year, Massachusetts received average rainfall (see Figure 2-1
on page 2), but most of the rain fell in the early months of the year, while later
months were unusually dry. Flows from both the Merrimack and Charles Rivers
reflected that pattern, with record high river discharges in the early part of the year
and near-record low discharges through the summer and fall (Figure 3-2). 
Figure 3-2. Flows of the Merrimack (top) and Charles (bottom) Rivers. The much larger
Merrimack River flows into the Gulf of Maine just south of the New Hampshire border and greatly
influences the physical conditions in Massachusetts Bay. The Charles River flows into Boston Harbor.
Note differences in scale. The quarterly percentiles represent the 2024 discharges in comparison to the
entire record. m 3 /s = cubic meters per second. 
Air temperatures, measured at both the NERACOOS and NDBC buoys, were mostly
typical, with slightly higher spring and fall temperatures and lower summertime
temperatures than average. Surface- and bottom-water temperatures at Station N18
just south of the outfall showed similar patterns (Figure 3-3, top two panels). 
Surface- and bottom-water salinities at Station N18 reflected the high river discharges
in early 2024 (Figure 3-3, bottom panels), with record-low salinities recorded during
the first survey of the season. The low salinity measurements continued through May.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
14 
Figure 3-3. Surface- and bottom water temperature and salinity at Station N18 just to the south
of the outfall in 2024 compared to prior years. PSU = practical salinity units or parts per thousand.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
15 
Wind speeds and directions in 2024 were mostly typical of past years. One large
storm with winds from the northeast occurred in early April, producing waves large
enough to resuspend and transport bottom sediments. Similar but much smaller
storms occurred in May and June. In the past, spring and early summer storms with
winds from the northeast have transported cells of the potentially toxic phytoplankton
species Alexandrium catenella from the Gulf of Maine into Massachusetts Bay. 
As is typical, wind directions promoted downwelling conditions in the spring and fall
and upwelling in the summer months (Figure 3-4). Downwelling occurs when winds
come from the north, bringing water masses in from the Gulf of Maine. Upwelling
occurs when wind directions are from the south. Upwelling brings bottom waters to
the surface, promoting cooling throughout the water column and alleviating the
natural declines in dissolved oxygen concentrations that occur throughout the
summer. 
Figure 3-4. Upwelling and downwelling conditions at Station N18 in 2024, compared to prior
years. Pa = Pascal, a unit used to quantify wind stress and other pressure-related measurements.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
16 
Water Quality 
Water quality measurements include quantification of nutrients (nitrogen,
phosphorus, and silica), phytoplankton biomass (chlorophyll and particulate organic
carbon), and dissolved oxygen (concentration and percent saturation). Results for
2024 continued to confirm measurable outfall influence on some nutrient
concentrations at stations near the outfall, but no increases in other nutrients or
phytoplankton biomass and no decreases in dissolved oxygen parameters (Libby et al.
2025). 
Nutrients 
The most important nutrients that fuel phytoplankton growth in Massachusetts Bay
are two forms of nitrogen (nitrate and ammonium), phosphate, and silicate. Typical of
temperate, coastal waters, nutrient concentrations follow seasonal patterns, naturally
varying with phytoplankton uptake, seasonal stratification, river flow, and water
exchange with the Gulf of Maine. As anticipated by baseline field studies and
modeling, increased concentrations of some nutrients were detected in the immediate
vicinity of the outfall in 2024. These increases are usually measured within 10
kilometers of the outfall. Intermittent signals can be detected up to 20 kilometers from
the outfall. 
Nitrate concentrations remained close to long-term means throughout the survey
season, with some variations due to the high river discharges in the spring. MWRA
has not detected changes in nitrate concentrations in response to the effluent
discharge. Phosphate concentrations were also typical, except for one unusually high
measurement at Station N18 near the outfall in June. The high value may have
resulted from the sample being taken right in the plume, before it was completely
mixed into the surrounding water. Elevated concentrations may also reflect upwelling
conditions, which bring deep nutrient-rich water to the surface. Silicate
concentrations remained typical, without any detectable outfall signal. 
Ammonium concentrations have varied from any typical seasonal pattern at stations
nearest the outfall. Ammonium is the largest fraction of the total nitrogen in
wastewater (see Figure 2-5, page 6), and ambient concentrations are relatively low for
most of the year, making it a good effluent tracer. Since the discharge was relocated
from the harbor to the bay in late 2000, MWRA monitoring has detected variable and
elevated ammonium concentrations at stations nearest and immediately to the south
of the outfall, in the direction of the predominant currents (Figure 3-5).

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
17 
Figure 3-5. Ammonium concentrations at selected stations in 2024 compared to prior years. 
Increased and more variable concentrations of the sewage tracer ammonium are found at stations
closest to the outfall, while stations farther away have maintained concentrations similar to pre-outfall
levels. Ammonium concentrations at Station F23 at the mouth of Boston Harbor decreased abruptly
when discharge to Boston Harbor ended and have remained low. μM = micromoles. 
Especially high ammonium measurements at stations closest to the outfall are
sometimes a result of a sample being taken directly in the effluent plume. Similar to
phosphate, ammonium concentrations were especially elevated at Station N18 near
the outfall during the June survey. As in past years, ammonium levels remained lower
than the baseline concentrations at Station F23, at the mouth of Boston Harbor.
Ammonium concentrations dropped precipitously at Station F23 and at all stations
sampled as part of MWRA’s separate Boston Harbor monitoring program as soon as
effluent discharge to Boston Harbor ended and the Massachusetts Bay outfall
discharge began. 
As was anticipated during outfall siting and has occurred every year, the effluent
plume’s ammonium signature was detected in the surface waters of stations closest to
the outfall during the winter surveys, when the water column was relatively well-
mixed (Figure 3-6). During the summer, when the water column was stratified, the
plume was confined beneath the pycnocline, below depths where maximum
phytoplankton growth occurs.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
18 
Figure 3-6. Surface- and bottom-water ammonium concentrations during mixed and stratified
conditions. During winter, well-mixed conditions, the plume was evident in surface waters. During the
summer, the plume was confined below the pycnocline (yellow line) and below the depth of most
phytoplankton growth. Station N21 is directly over the outfall; m = meters; NH4 = ammonium; μM =
micromoles. 
February 2024: Mixed conditions August 2024: Stratified conditions
Depth (m) 
N S
W E
NH4 Concentration (μM)

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
19 
Phytoplankton Biomass 
More than two decades of monitoring have shown that the increases in ammonium
concentrations near the outfall have not stimulated phytoplankton growth or resulted
in increased phytoplankton biomass at any station. Phytoplankton biomass in the
water column is determined from measurements of chlorophyll concentrations and
fluorescence and from particulate organic carbon concentrations. 
Chlorophyll levels in 2024 remained close to historic means across all stations and
throughout the year (Figure 3-7). There was no repeat of the large phytoplankton
bloom of 2023, which occurred throughout a broad region, including much of New
England’s coastal waters. 
Figure 3-7. Monthly areal chlorophyll concentrations at selected stations in 2024 compared to
prior years. Survey phytoplankton biomass is reported as vertically summed (areal) chlorophyll from
measurements throughout the water column, from the surface to the sea floor, rather than as an amount
within a fixed volume. mg/m 2 = milligrams per square meter summed over depth. 
2024
1992-2023
Median
25-75%
Range 
Areal Chlorophyll (mg/m 2 )

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
20 
Dissolved Oxygen 
Typically, bottom-water dissolved oxygen concentrations and percent saturation
begin the year relatively high and steadily decline throughout the summer.
Stratification prevents exchange between the atmosphere and bottom waters, so
dissolved oxygen levels naturally decline at depth, as animals and microbes respire.
That general pattern persisted in 2024 (Figure 3-8). 
In May 2024, near-bottom dissolved-oxygen concentrations at most stations in
Massachusetts and Cape Cod Bays were unexpectedly lower than historic spring
ranges. The unusually early decline was most likely due to regional weather patterns.
Storms with winds from the northeast likely transported lower-oxygenated water from
the Gulf of Maine to Massachusetts Bay. Upwelling in June and July alleviated those
low-oxygen levels, and all measurements remained more typical throughout the rest
of the survey season. Annual minimum concentrations were greater than 6.25
milligrams per liter throughout Massachusetts Bay. 
Figure 3-8. Near-bottom water dissolved oxygen concentrations in 2024 compared to prior years. 
DO = dissolved oxygen; mg/L = milligrams per liter . 
2024
1992-2023
Median
25-75%
Range 
Bottom Water DO (mg/L)

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
21 
Plankton Communities 
Marine plankton includes plants and animals that move with winds, tides, and
currents, having limited or no ability to move on their own. 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 addresses concerns for
marine mammal 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 (Figure 3-9). Microflagellates are microscopically 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. Dinoflagellates have two perpendicular whip-like flagella, which propel
them up and down in the water column. Dinoflagellates are classified as
phytoplankton, because many species photosynthesize like plants. 
One common dinoflagellate, Alexandrium catenella , often blooms in Massachusetts
Bay and the wider Gulf of Maine region. It is the species known in New England for
causing “red tide.” Other geographic areas designate different species, usually other
dinoflagellates, as the cause for their local red tides. Many types of phytoplankton
can, when abundant, color the water column red or brown. 
Zooplankton communities in Massachusetts Bay include animal species that live their
entire lives as plankton, including abundant shrimp-like copepods, and species that
live in the plankton for only a part of their life cycles, such as barnacle and clam
larvae. Most species are small, but larger animals, including jellyfish, are also part of
the zooplankton community. Zooplankton provide food for many animals, including
other zooplankton, fish, sea turtles, and the endangered North Atlantic right whale.
Zooplankton abundance and species composition are highly variable through the
seasons and between years.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
22 
Figure 3-9. Typical phytoplankton from Massachusetts Bay. The large, hooked organism is the
dinoflagellate Tripos muelleri . The box-like chains are centric diatoms in the genus Skeletonema , and
the very thin chain is a species of the pennate diatom genus Pseudo-nitzschia . Tripos muelle ri typically
ranges from 200 to 400 micrometers in length. Photo credit David Borkman, Pausacaco Plankton,
Saunderstown, Rhode Island. 
Phytoplankton Communities 
Total phytoplankton abundance was at or near the long-term means at most stations
and during most surveys in 2024 (Figure 3-10; Libby et al. 2025). Average overall
abundance was about 1.1 million cells per liter, and there were no large blooms of
any single species. 
Microflagellates, which have consistently remained the most numerically abundant
plankton group, were particularly abundant in May and June. Numerically,
microflagellates made up 69% of the total phytoplankton community in 2024. Centric
diatoms were more abundant than in recent years, particularly in June and July. In
June, nearfield centric diatom abundance was three times greater than the historic
mean. Pennate diatoms were most abundant in May. Dinoflagellate abundance was
relatively low throughout 2024, a contrast to 2023, when a large bloom of the
dinoflagellate Tripos muelleri dominated the region from April through July.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
23 
Figure 3-10. Total phytoplankton abundance at selected stations in 2024 compared to prior
years . cells/L = cells per liter 
A small regional bloom of the pennate diatom genus Pseudo-nitzschia occurred in
May, with maximum abundance of more than 230,000 cells per liter. Some Pseudo-
nitzschia species produce domoic acid, a neurotoxin that causes amnesic shellfish
poisoning (ASP), a life-threatening condition in seabirds, marine mammals, and
humans that consume contaminated shellfish. Most of the cells observed were small,
indicative of the low-toxicity species Pseudo-nitzschia delicatissima . No toxin was
detected in state shellfish monitoring. Historically, ASP has not caused shellfish
closures in Massachusetts or Cape Cod Bays. 
Abundance of the potentially toxic dinoflagellate Alexandrium catenella remained
low in 2024, peaking later in the year than usual (Figure 3-11). A maximum
abundance of 96 cells per liter was observed at Station N01, northwest of the outfall,
in July. Abundances remained just below the Contingency Plan threshold and also
below the threshold for conducting Alexandrium Rapid-Response Study surveys. The
toxin produced by Alexandrium catenella causes paralytic shellfish poisoning (PSP),
a potentially deadly condition for people and marine mammals that consume
shellfish. No PSP toxicity was detected in Massachusetts Bay in 2024. 
2024
1992-2023
Median
25-75%
Range 
Abundance (million cells/L)

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
24 
Figure 3-11. A lexandrium catenella abundance in 2024 compared to 1992–2023. The highest
abundance in 2024 was 96 cells per liter, just below the caution threshold of 100 cells per liter. 
Alexandrium catenella typically ranges from 20 to 80 micrometers in diameter. Photo credit Donald M.
Anderson, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts. Cells/L + 1 = cells per
liter plus 1, a convention that allows abundances of zero to be displayed on this scale. 
Zooplankton Communities 
Total annual zooplankton abundance was largely typical in 2024, although there were
some atypical seasonal patterns (Figure 3-12). Increased abundance from usual winter
lows through the spring and summer were only modest, and at some stations, there
were substantial increases in abundance in October. 
As in past years, copepod adults and younger life stages dominated throughout the
year. Numerically, the small copepod species Oithona similis continued to be the
most abundant species in the zooplankton community, as it has been since the
beginning of the monitoring program. Early life stages of barnacles occurred
sporadically, particularly in February and March. The planktonic tunicate Oikopleura
dioica , which can be found in surface waters around the world, was abundant in May. 
The unusual October peaks observed at some stations reflected high numbers of
radiolarians, single-celled protozoans that build ornate silica-rich exoskeletons.
Radiolarians are more typically found in offshore waters, and their presence each year
since 2020 may reflect annual influxes of warm water masses from offshore.

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
25 
Figure 3-12. Total zooplankton abundance at selected stations in 2024 compared to prior years. 
The high historical values in July occurred in 2015, when sampling took place just after a major
bivalve spawning event. The increases at some stations in October 2024 compared to other years were
due to early life stages of copepods and to radiolarians, which are usually found in more offshore
waters. individuals/m 3 = individuals per cubic meter. 
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 in the northwest corner of the
sanctuary. Ammonium concentrations have not increased at Station F22 in the years
since the outfall began to discharge (see Figure 3-5, page 17), and there have been no
observed effects on phytoplankton biomass or dissolved oxygen levels. Dissolved
oxygen concentrations remained above state standards in 2024 (Figure 3-13). 
2024
1992-2023
Median
25-75%
Range 
Abundance (10,000 individuals/m 3 )

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
26 
Figure 3-13. Dissolved oxygen concentrations at the NERACOOS Buoy A01 and at nearby
deepwater MWRA survey Station F22 in 2024. Buoy values are daily means. The state water quality
standard for dissolved oxygen is a minimum of 6 milligrams per liter. mg/L = milligrams per liter. 
The sanctuary’s most recent condition report rated water quality within its borders as
“good,” and its most recent management plan agreed with MWRA’s assessment that
the outfall is not adversely affecting its waters (NOAA Office of Marine Sanctuaries
2020, 2023). 
Boston Harbor Water Quality 
Water quality in Boston Harbor greatly improved during the Boston Harbor Project,
and those advances remained evident in 2024. Perhaps the most dramatic
improvement in Boston Harbor was the decrease in ammonium concentrations, which
have remained low after dropping precipitously in 2000, when the effluent discharge
was diverted from the harbor to Massachusetts Bay. The reduction in nutrient inputs
has been accompanied by decreases in previously over-stimulated primary production
and phytoplankton biomass, an abatement of the harbor’s historically high level of
eutrophication (Werme et al. 2018, Taylor et al. 2020).

2024 OUTFALL MONITORING OVERVIEW: WATER COLUMN 
27 
Water-Column Contingency Plan Thresholds 
There were no exceedances of water-column thresholds in 2024 (Table 3-1). Overall,
the year was typical, with water-quality parameters close to the long-term means,
moderate dissolved-oxygen levels, and no nuisance algal blooms. 
Table 3-1. Contingency Plan threshold values and 2024 results for water-column monitoring. 
Parameter Baseline Caution 
Level 
Warning 
Level 2024 Results 
Chlorophyll 
Annual 72 mg/m 2 >108 mg/m 2 >144 mg/m 2 61 mg/m 2 
Winter/spring 50 mg/m 2 >199 mg/m 2 None 40 mg/m 2 
Summer 51 mg/m 2 >89 mg/m 2 None 56 mg/m 2 
Autumn 90 mg/m 2 >239 mg/m 2 None 102 mg/m 2 
Dissolved oxygen* 
Nearfield 
concentration 6.05 mg/L <6.5 mg/L <6.0 mg/L 6.24 mg/L 
Nearfield percent 
saturation 65.3% <80% <75% 69.2% 
Stellwagen 
concentration 6.23 mg/L <6.5 mg/L <6.0 mg/L 6.45 mg/L 
Stellwagen 
percent saturation 67.2% <80% <75% 68.7 % 
Nearfield 
depletion rate 0.024 mg/L/d >0.037 mg/L/d >0.049 mg/L/d 0.023 mg/L/d 
Nuisance algae nearfield Pseudo-nitzschia 
Winter/spring 6,735 cells/L >17,900 cells/L None 73 cells/L 
Summer 14,635 cells/L >43,100 cells/L None 28,200 cells/L 
Autumn 10,050 cells/L >27,500 cells/L None 727 cells/L 
Nuisance algae nearfield Alexandrium catenella 
Any nearfield 
sample 
Baseline 
maximum 
163 cells/L 
>100 cells/L None 96 cells/L 
PSP toxin extent NA New incidence None No new 
incidence 
“Nearfield” refers to the five stations closest to the outfall, where effects might be detected, if they
were to occur. Red shading shows a warning-level exceedance; orange shading shows a caution-level
exceedance. For dissolved oxygen parameters, “exceedances” refer to values less than the threshold. 
*Dissolved oxygen caution and warning levels represent numerical criteria, with the caveat “unless
background conditions are lower.” Results are therefore compared to the baseline as well as the caution
and warning levels. 
PSP = paralytic shellfish poisoning 
NA = not applicable 
mg/m 2 = milligrams per square meter 
mg/L = milligrams per liter 
mg/L/d = milligrams per liter per day 
cells/L = cells per liter

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
28 
4. Sea Floor 
The sea floor of Massachusetts Bay was shaped by glaciers, which scoured the
bottom and created Stellwagen Basin to the east of the outfall, deposited rocky debris
in elongated hills called drumlins, and left the sand-and-gravel ridge that forms
Stellwagen Bank. Tides and currents continue to shape the bay, particularly during
major storms with waves large enough to resuspend and transport sediments. Seafloor
habitats range from mud in soft-bottom depositional areas to rocky cobbles and
boulders on the hard-bottom tops and flanks of the drumlins. 
Seafloor monitoring assesses potential effects of the outfall on the health of both the
soft-bottom areas made up of sand, silt, and clay, and the rocky hard-bottom
drumlins. Most seafloor-monitoring studies focus solely on soft-bottom sediments,
but such depositional sites are somewhat rare in the immediate vicinity of the outfall.
MWRA conducts annual soft-bottom sampling surveys, while photographic and video
assessments of the hard-bottom areas occur every three years, most recently in 2023.
A separate monitoring program in Boston Harbor includes annual soft-bottom
sampling and analysis, as well as photographic sediment-profile imagery. 
In August 2024, MWRA sampled 14 Massachusetts Bay soft-bottom stations for
analysis of sediment grain-size distribution, the wastewater tracer Clostridium
perfringens spores, percent total organic carbon, and animal abundance and species
composition (Figure 4-1). The stations included four “inner nearfield” stations located
within two kilometers of the outfall; six “outer nearfield” stations in western
Massachusetts Bay but 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. The
transition area is included in the calculations, because early modeling suggested that
it would continue to be a depositional site, where elevated concentrations of
particulate organic carbon might be detected (Kropp et al. 2000). Some scientific
analyses do not include the transition station when comparing results from the
nearfield to those from the farfield, as it is near to and remains affected by historic
and continued inputs from Boston Harbor and other coastal sources. 
Seafloor monitoring assesses the health of habitats and communities. 
MWRA measures sediment characteristics and assesses animal communities living
in soft-bottom and rocky, hard-bottom habitats. No effects of the outfall have
been detected. Seafloor animal communities vary within typical natural cycles,
and are mostly influenced by sediment types and water depths.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
29 
Figure 4-1. Soft-bottom monitoring stations. Also shown are the instrumented buoys, the MWRA
outfall diffuser, and Stellwagen Bank National Marine Sanctuary. Nearfield station labels begin with
NF, while farfield stations and the transition station begin with FF. The transition station FF12 is
included with the nearfield stations to calculate Contingency Plan threshold parameters.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
30 
Sediment Characteristics and Tracers 
As in past years, sediment grain-size distributions varied broadly in 2024, ranging
from silt and clay at some stations (for example, Station NF12 just northwest of the
outfall and Station FF04 within Stellwagen Basin) to almost entirely sand at others,
including several stations near the outfall (Nestler et al. 2025). 
Within individual stations, sediment textures have remained stable over the years of
monitoring, with occasional changes following large storms. An April storm with
eight-meter waves, capable of resuspending and transporting sediments in
Massachusetts Bay, may have caused some changes in 2024 sediment samples.
Increased percentages of fine silts and clays, were found at several stations, including
Station NF17, a generally sandy station near the outfall, and farfield reference Station
FF01A. 
Clostridium perfringens is a species of anaerobic bacteria, found in the digestive
tracts of humans and other mammals and not completely removed by wastewater
treatment. It forms resting spores, which provide a sensitive tracer of sewage effluent
in sediments. Since spores can persist for decades, they do not reflect immediate
deposition onto the sea floor; instead, they can provide evidence of wastewater
discharges over years. 
In 2024, as in most other years, MWRA monitoring detected elevated abundance of 
Clostridium spores at some stations closest to the outfall (Figure 4-2). Highest spore
counts were found at Stations NF24 and NF14, located within two kilometers of the
outfall, and at Station NF22, just to the southwest of the outfall in the direction of
most frequent transport of the effluent plume. 
Percent total organic carbon content in the sediments was consistent with past results,
with no increased organic carbon concentrations at stations near the outfall (Figure
4-3) or at the transition station. In general, there are higher concentrations of total
organic carbon at stations with finer sediments, such as Station FF04 in Stellwagen
Basin, and lower concentrations at stations with coarser sediments such as NF17 near
the outfall. Overall, the total organic carbon data continued to show no signs of
organic enrichment from wastewater effluent, consistent with predictions made before
the outfall began to discharge and also consistent with an MWRA special study of
sediment metabolism, which was completed in 2010 (Tucker et al. 2010, Tucker et al.
2014).

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
31 
Figure 4-2. Concentrations of the effluent tracer Clostridium perfringens spores in 2024. 
Concentrations represent spore counts normalized to the percent of the sediment made up of fine silts
and clays, removing any effect of grain size from the analysis. As anticipated during outfall planning, 
Clostridium spore concentrations are elevated at stations closest and to the southwest the outfall, in the
most prevalent current direction. cfu = colony forming units, the measure used in spore counts. 
Figure 4-3. T otal organic carbon at selected stations, 1992–2024. No effects of the discharge have
been observed at any station. NF17 is in closer proximity and southwest of the outfall, where the
effects of the discharge might be expected but have not been found. FF01A is the northern reference
station, and FF04 is in Stellwagen Basin; both farfield stations are farther than 20 kilometers from the
outfall.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
32 
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, such as decreased
biodiversity or increased presence of the opportunistic species typically found in
disturbed habitats. 
The 14 soft-bottom samples collected and analyzed in 2024 contained 19,606
organisms, classified into 154 species and 21 other discrete taxonomic groups of
animals that could not be identified to the species level (Nestler et al. 2025). Both the
total number of organisms and the number of species were somewhat lower in 2024
than in 2023 across the regional groups, but both tallies remained within historic
ranges (Figures 4-4, 4-5). The several community diversity measures calculated for
nearfield samples remained at levels typical of past years. 
Figure 4-4. Total abundance of soft-bottom organisms by region, 1992–2024 . 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.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
33 
Figure 4-5. Total abundance of species by region, 1992–2024 . 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 (Figure 4-6), dominated
the samples 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 is found in many places around the world,
was the numerically dominant species in 1997–2005. It has declined in abundance
over recent years, but remains among a group of dominant species. Other common
polychaetes included Aricidea catherinae and Mediomastus californiensis , which also
have wide geographic distributions, and were present in numbers comparable to other
recent years.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
34 
Figure 4-6. Polychaete worms dominate the soft-bottom habitats in Massachusetts Bay. The
image shows the anterior end of a 2-millimeter-long syllid polychaete taken from a former MWRA
farfield station. Polychaetes can range in size from less than a millimeter to several meters; most are
very small. Syllid polychaetes are sometimes called necklace worms, due to the constricted band
depicted in the lower middle part of the photograph. They are found in a variety of soft- and hard-
bottom habitats. Photo credit Nancy J. Maciolek, Duxbury, Massachusetts.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
35 
Analyses of the seafloor communities by location continued to show no effects of the
outfall on total abundance of animals, relative abundance of species, 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. 
One type of community analysis, called cluster analysis, groups stations by similarity
in species composition and abundance. Cluster analysis of the 2024 samples
identified two main groups (I and II). Group I was dominated by crustaceans and
polychaete worms. Group II included a greater abundance of polychaetes and
mollusks than Group I. 
Some of the stations within Group II could be separated into more specific subgroups,
based on slight differences in species composition and abundance. Group II also
included two outlier stations, FF09 and FF04. Offshore Station FF09, located to the
southeast of the outfall had the largest number of mollusks of any station, including
small bivalves and a small snail. Frigidoalvania carinata . Station FF04, the offshore
station located within Stellwagen Basin, has consistently supported a unique
polychaete community, typical of the other deepwater, offshore stations that were
sampled in the past. Chaetozone anasimus , a moderate-sized polychaete first
identified in MWRA samples, dominated in 2024. In some past years, Station FF04
has been placed into a separate Group III. 
Further analysis, called ordination analysis, provides a picture of how closely the
groups identified in the cluster analysis relate to each other. The 2024 ordination
analysis continued to show no indication of any relation between community
composition and proximity to the outfall (Figure 4-7). Both main Groups I and II
included stations located at various distances and directions from the outfall. Group II
included stations from both nearfield station groups, the transition station between the
harbor and the outfall, and farfield stations to the northeast and southeast of the
outfall. These results were consistent with prior years.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
36 
Figure 4-7. Ordination plot of 2024 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. The closer two points are to each other in the figure, 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 transition station between Boston Harbor and the
outfall; FF = stations far from the outfall. (The 2D Stress value of 0.05 noted in the upper right is a
measure of good confidence in the analysis.)
As in past years, further assessment of the ordination analysis demonstrated that
variations in species distributions largely followed differences in sediment grain size
and water depth (Figure 4-8). 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 seafloor communities are driven by physical
factors rather than by the effluent discharge. The strong physical forces in the bay, as
well as the low pollutant levels in the discharge, have helped to maintain healthy soft-
bottom habitats throughout Massachusetts Bay. 
NF21,
NF22

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
37 
Figure 4-8. Percent fine sediments (top) and water depth (bottom) superimposed on the ordination
plot of the 2024 samples. Combining the ordination analysis with sediment grain-size and water-depth
data shows that sediment texture and depth are likely strong drivers of community composition. (The
2D Stress values noted in the upper right of each figure are a measure of good confidence in the analysis.) 
NF21 
NF21
NF22
NF22

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
38 
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 seafloor reference Station FF04 lies
within the depositional part of the sanctuary, Stellwagen Basin, where long-term
accumulation of pollutants could feasibly occur. 
Station FF04 is typical of deepwater, offshore habitats and is representative of a
larger group of stations that MWRA monitored in earlier years of the program. 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 in shallower areas, probably due to the distance
from shore, as well as greater depth and fine-grained sediments (Figure 4-8, above).
Analysis of data from 1992–2024 has demonstrated the temporal consistency of the
seafloor community at that station. 
The most recent management plan for the sanctuary (NOAA Office of Marine
Sanctuaries 2023) includes an action plan to assess all the major habitats within its
borders, including sand, boulder, gravel, mud, rocky areas, and shipwrecks. Their
most recent environmental assessment (NOAA Office of Marine Sanctuaries 2020)
found that changes to seafloor habitats and animal communities came mostly from
fishing. Fishing gear physically scrapes the sea floor, and fishing pressures can
change the species makeup of both seafloor and fish communities. 
Boston Harbor Seafloor Monitoring 
Seafloor conditions are much better in Boston Harbor than they were in the 1970s and
1980s, when both sewage sludge and effluent were discharged into its shallow,
confined waters. MWRA has conducted ongoing seafloor monitoring in Boston
Harbor since 1991, the year that sludge discharges ended. Each year, sediment
samples are taken from nine stations, and sediment-profile images are made at 61
locations throughout the harbor (Madray et al. 2025). 
Sediment textures vary throughout the harbor, ranging from mostly sand at some
stations, particularly in the outer harbor, to silt and clay at others. Percent total
organic carbon in harbor sediments has declined over time, reflecting the end of
sludge and effluent discharges. 
Over the past decade, the total abundances of organisms and number of species per
sample have stabilized (Figure 4-9), a sign of sustained improvement in seafloor
habitat conditions. In 2024, samples contained 29,013 specimens, classified into 122
species and 13 other discrete taxonomic groups. The predominant species have also
changed since the early years of monitoring, when samples were dominated by the
opportunistic species often found in disturbed habitats.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
39 
As in past years, the communities in 2024 varied along an inner-to-outer 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 confirmed
the gradient and clearly documented sustained improvements to habitat conditions
throughout the harbor over time. Sediment-profile images from 2024 (Figure 4-10)
depicted the healthy conditions, including animals and physical structures. 
Figure 4-9. Total abundance of soft-bottom animals (top) and number of species (bottom) per
sample from Boston Harbor, 1991–2024. The numbers of organisms and species have stabilized as
the harbor has recovered from past sludge and effluent discharges. 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. Mean +/- 1StdErr = the
mean of all measurements, plus or minus one standard error, a measure of variation among individual
samples.

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
40 
Figure 4-10. Example of a sediment-profile image taken in 2024 south of Deer Island and the
north channel into the harbor. The change from light to dark color shows the transition from
oxygenated to unoxygenated sediment; this transition zone has deepened in response to improved
conditions. Presence of organisms and feeding structures indicate a healthy environment. Scales at the
sides of the image are in centimeters. Photo credit RJ Diaz and Daughters, Ware Neck, Virginia. 
Stick 
Amphipod 
Worm 
Worm 
Worm Burrow 
Feeding Void

2024 OUTFALL MONITORING OVERVIEW: SEA FLOOR 
41 
Seafloor Contingency Plan Thresholds 
There were no exceedances of the seafloor Contingency Plan thresholds in 2024
(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
healthy seafloor habitats, unaffected by the wastewater discharge. Number of species
per sample and the several diversity and evenness indices used to evaluate community
structure remained higher than Contingency Plan limits, while the percent
opportunistic species, an indicator of disturbed or polluted habitats, was less than 3%,
far lower than the caution and warning thresholds of 10% and 25%. 
Table 4-1. Contingency Plan threshold values and 2024 results for seafloor monitoring.* 
Parameter Baseline Caution 
Level 
Warning 
Level 2024 Results 
Species per 
sample NA <42.99 NA 50.7 
Fisher’s log-series 
alpha NA <9.42 NA 11.0 
Shannon diversity NA <3.37 NA 3.80 
Pielou’s evenness NA <0.57 NA 0.67 
% opportunists NA >10% >25% 2.17% 
NA = not applicable 
* Number of species per sample is often referred to as species richness. Fisher’s log-series alpha and
Shannon diversity are indicators of diversity, calculated from both numbers of species and numbers of
individuals per species. Pielou’s evenness assesses how evenly numbers of individuals are distributed
among the species in the sample. Percent opportunists is the percentage of the total number of animals
that are indicators of disturbed habitat. The percent opportunist threshold is a maximum; all other
thresholds are minimum values.

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
42 
5. Fish and Shellfish 
Fish and shellfish are important to the regional identity and economy of
Massachusetts and can be good indicators of overall environmental health. The
monitoring program focuses on winter flounder, lobster, and cage-deployed blue
mussels. Flounder and lobster are important commercial and recreational species that
live on the sea floor, where they come into direct contact with sediments and feed on
bottom-dwelling animals, both likely sources for contaminant exposure. The blue
mussel is commercially and recreationally harvested and is a frequent biomonitoring
organism. Mussels filter large volumes of water and can concentrate metals and
organic compounds in their tissues. 
Sampling and deployment locations vary by species (Figure 5-1). Sampling locations
for winter flounder and lobster were selected to be far enough away from each other
to maximize the likelihood of sampling separate populations. Winter flounder are
unlikely to range farther than about two kilometers in areas with ample food. Lobster
migration is less well understood, although they are known to move from shallower
Boston Harbor to deeper water in Massachusetts Bay in the fall. Timing and
parameters measured vary by species. MWRA monitors winter flounder health each
year and conducts chemical contaminant analyses of flounder, lobster, and mussel
tissues every three years, including 2024. 
In 2024, flounder were collected two locations, at the Massachusetts Bay outfall site
and at Deer Island Flats, near the location where wastewater effluent was once
discharged into Boston Harbor. Lobsters were sampled near the outfall, at Deer Island
Flats, and at eastern Cape Cod Bay, a relatively pristine area that was once also
sampled for winter flounder. Commercially farmed mussels were deployed in cages at
several locations near the diffusers of the Massachusetts Bay outfall, within Boston
Inner Harbor, and at Deer Island Light at the mouth of Boston Harbor. 
Flounder sampling occurred over two days in April and May, and lobsters were
collected in July. Mussels were deployed for 40 and 60 days during the summer
months. Neither flounder nor lobster catches met their full target numbers at every
sampling site. A number of factors have affected catch success over time, and wild
mussel populations have also changed over the course of the monitoring program.
These changes are discussed in Section 6, Special Topics. 
Fish and shellfish are healthy, and pollutant levels remain low. 
MWRA monitors winter flounder health and quantifies chemical pollutants that
occur in wastewater effluent in flounder, lobster, and mussel tissues. Over time,
flounder health has improved, and contaminant concentrations have declined or
remained low in fish and shellfish from Boston Harbor and Massachusetts Bay.

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
43 
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 Stellwagen Bank Marine Sanctuary.

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
44 
Flounder Condition and Health 
Flounder sampling included trawls by a commercial fishing boat in April and a
backup catch at Deer Island Flats by the Massachusetts Division of Marine Fisheries
in May (Moore et al. 2024). The first survey yielded only 34 fish from near the outfall
and 20 from Deer Island Flats. The backup collection brought the Deer Island catch to
the full complement of 50 fish. Challenges with meeting catch targets are discussed
further and given geographic and historic context in Section 6, Special Topics. 
Mean ages, lengths, and weights were somewhat lower than in 2024 than in 2023 but
remained within the ranges for the monitoring program. The percentage of the catch
made up of females also declined slightly, but remained female-dominated, as is
common throughout northeast coastal populations (Moore et al. 2016). 
Measures of external condition, such as bent fin rays and occurrence and severity of
fin erosion, remained variable, but continued to reflect 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 observed on the bottom or “blind side”
of the fish in some years, were found on one fish from Deer Island Flats. 
Incidence of centrotubular hydropic vacuolation (CHV), a key tumor precursor and
mild liver condition (Figure 5-2) associated with exposure to contaminants, was only
8.8% at the Massachusetts Bay outfall and 10% at Deer Island Flats (Figure 5-3),
continuing to document a major environmental success story. More than 75% of
Boston Harbor flounder livers in the late 1980s showed at least some level of CHV.
Incidence has decreased across all age groups in both the harbor and the bay,
alleviating the concern prior to the Massachusetts Bay outfall startup that there could
be increased liver disease in the bay. In the few fish where CHV has been found,
severity of the condition has also declined over time. 
Cancerous liver tumors, also called 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 a fish taken from near the Massachusetts Bay
outfall site (Figure 5-4; see also Moore et al. 2018).

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
45 
Figure 5-2. Centrotubular hydropic vacuolation (CHV) in flounder liver, 2024. In the 1980s and
1990s, Boston Harbor winter flounder were riddled with similar nodules. Now they are rare. Photo
credit Michael Moore, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts. 
Figure 5-3. Annual prevalence of the tumor precursor CHV in winter flounder, 1991–2024. 
Nantasket Beach and East Cape Cod Bay are no longer sampled. CHV incidence and severity have
declined throughout the region. In 2024, sampling was limited to the outfall site and Deer Island Flats.

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
46 
Figure 5-4. Annual prevalence of liver tumors in winter flounder, 1991–2024 . No tumors have
ever been found in fish taken from near the Massachusetts Bay outfall. In 2024, sampling was limited
to the outfall site and Deer Island Flats. 
Fish and Shellfish Tissue Chemistry 
Flounder, lobster, and mussel tissues were analyzed for metals and organic
compounds known for their potential toxicity to marine life and human health.
Organic compounds included known carcinogens, such the banned organochlorine
pesticides DDT and chlordane; PCBs, which were used in electrical equipment and
other products; and PAHs, which derive from natural, domestic, and industrial
sources, such as coal, oil, and forest fires. PAHs enter the marine environment
through road runoff, sewage, and atmospheric deposition. 
Flounder tissues were taken from the same collections made for health assessments at
the Massachusetts Bay outfall and Deer Island Flats. Samples of winter flounder fillet
were analyzed for PCBs, pesticides, and mercury. Liver samples from the same fish
were analyzed for PCBs, pesticides, PAHs, and selected metals. 
Samples of lobster meat collected in July from those same two locations plus eastern
Cape Cod Bay were analyzed for PCBs, pesticides, and mercury. Digestive gland (the
hepatopancreas, often called tomalley) samples from the same animals were analyzed
for PCBs, pesticides, PAHs, and selected metals. 
Mussels, obtained from commercial grower Hollander and De Konig Mussel Farms
beds in Mount Desert Narrows, Maine, were deployed in cages in Boston Inner
Harbor, Deer Island Flats, and near the outfall, were analyzed for PCBs, pesticides,
PAHs, lipids, mercury, and lead. Concentrations following the 40- and 60-day
deployments were compared to concentrations in the reference mussels prior to
deployment.

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
47 
Concentrations of all contaminants remained low in flounder, lobster and mussel
tissues. Slow declines in concentrations of banned pesticides and PCBs continued
across all flounder and lobster tissue types and locations. These declines have been
particularly evident in samples from the fish caught near Deer Island. 
For example, despite small average increases in 2024, slow declines in concentrations
of the pesticide chlordane in flounder fillets have persisted throughout the monitoring
program (Figure 5-5). Smaller long-term declines in concentrations of PCBs have
been observed in lobster meat, including those taken from the relatively pristine
sampling area in eastern Cape Cod Bay (Figure 5-6). 
Figure 5-5. Chlordane in flounder fillets, 1993–2024 . Since 2003, sampling has occurred at three-
year intervals. Chlordane was widely used in agriculture and homes before its ban in the US in 1988. 
Pesticide concentrations in deployed mussels have also declined over time, a result
not only of pesticide bans, but also due to improved treatment and the outfall
relocation to Massachusetts Bay. Results from 2024 showed that contamination
remained greatest in Boston Inner Harbor, which has a legacy of inputs from sources
other than wastewater discharges. For example, accumulation of the banned pesticide
chlordane and its breakdown products was greatest in mussels deployed in Boston
Inner Harbor, intermediate in mussels near Deer Island, and lowest in mussels
deployed at locations near the Massachusetts Bay outfall (Figure 5-7). Other
contaminants showed similar patterns.

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
48 
Figure 5-6. PCBs in lobster meat, 1994–2024. Since 2003, sampling has occurred at three-year
intervals. PCB use was banned in 1979. 
Figure 5-7. Chlordane in mussel tissues, 1993–2024. Since 2003, sampling has occurred at three-
year intervals . Buoy B is the large navigation buoy located just south of the outfall.

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
49 
Fish and Shellfish Contingency Plan Thresholds 
There were no Contingency Plan threshold exceedances for fish-and-shellfish
parameters in 2024 (Table 5-1). The only flounder-health parameter, CHV, occurred
in 6.8% of the fish collected near the outfall, compared to the 24.4% baseline and a
caution threshold of 44.9%. 
Table 5-1. Contingency Plan threshold values and 2024 results for fish-and-shellfish
monitoring.* 
Parameter Baseline Caution Level Warning Level 2024 Results 
Flounder disease 
Liver disease 
(CHV) 24.4% 44.9% None 8.8% 
Flounder fillet 
PCB 0.033 ppm 1 ppm 
wet weight 
1.6 ppm 
wet weight 0.008 ppm 
Mercury 0.074 ppm 0.5 ppm 
wet weight 
0.8 ppm 
wet weight 0.101 ppm 
Flounder fillet, lipid normalized 
Chlordane 242 ppb 484 ppb None 71 ppb 
Dieldrin 63.7 ppb 127 ppb None 0 ppb 
DDT 775.9 ppb 1552 ppb None 257 ppb 
Lobster meat 
PCB 0.015 ppm 1 ppm 
wet weight 
1.6 ppm 
wet weight 0.008 ppm 
Mercury 0.148 ppm 0.5 ppm 
wet weight 
0.8 ppm 
wet weight 0.152 ppm 
Lobster meat, lipid normalized 
Chlordane 75 ppb 150 ppb None 5 ppb 
Dieldrin 161 ppb 322 ppb None 0 ppb 
DDT 341.3 ppb 683 ppb None 39 ppb 
Mussel tissue 
PCB 0.011 ppm 1 ppm 
wet weight 
1.6 ppm 
wet weight 0.007 ppm 
Lead 0.415 ppm 2 ppm 
wet weight 
3 ppm 
wet weight 0.208 ppm 
Mercury 0.019 ppm 0.5 ppm 
wet weight 
0.8 ppm 
wet weight 0.012 ppm 
Mussel tissue, lipid normalized 
Chlordane 102.3 ppb 205 ppb None 19.9 ppb 
Dieldrin 25 ppb 50 ppb None 0 ppb 
DDT 241.7 ppb 483 ppb None 16.8 ppb 
PAH 1080 ppb 2160 ppb None 632 ppb 
* Exceedances are all values greater than caution or warning thresholds. There were no exceedances in 2024 
CHV = centrotubular hydropic vacuolation

2024 OUTFALL MONITORING OVERVIEW: FISH AND SHELLFISH 
50 
Tissue sample results remained well below baseline measurements, as well as below
caution and warning thresholds. For example, concentrations of mercury in flounder
meat and PCBs in lobster meat remained at small fractions of the thresholds (Figure
5-8). Most caution-level exceedances were set at 50% of U.S. Food and Drug
Administration action levels for seafood consumption; most warning levels were set
at 80% of those standards. 
Figure 5-8. Mercury concentrations in flounder and PCBs in lobster meat compared to
Contingency Plan thresholds over time. baseline = 1992–2000, when effluent was discharged to
Boston Harbor; outfall discharge = years when the discharge was to Massachusetts Bay; ng/g wet =
nanograms per gram wet weight. Note that warning thresholds are not shown.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
51 
6. Special Topics 
In addition to the permit-required monitoring of the effluent and the water column,
sea floor, and fish and shellfish in Massachusetts Bay, MWRA also initiates or
collaborates on studies of Massachusetts Bay and the wider Gulf of Maine. These
studies meet specific permit conditions or better inform interpretations of MWRA
monitoring results. This year’s report focuses on the three fish-and-shellfish species
that have been key to the monitoring program: winter flounder ( Pseudopleuronectes
americanus ), American lobster ( Homarus americanus ), and blue mussel ( Mytilus 
edulis ). It also includes an update on the Bays Eutrophication Model. MWRA’s
NPDES permit to discharge effluent to Massachusetts Bay requires annual model
runs, and this year’s report reviews the model run for 2023. 
Fish-and-Shellfish Monitoring Species 
When MWRA began monitoring in the early 1990s, winter flounder supported a
popular recreational and commercial fishery in the region. They lived and fed on the
sea floor where they could be exposed to pollutants. Flounder caught in Boston
Harbor were known to have a variety of pollutant-related conditions, such as fin
erosion and tumors. Lobsters were the region’s most valuable fishery and were also
bottom-dwellers. Mussels, harvested from relatively pristine areas and deployed in
cages, were a frequent biomonitoring tool, as they filter large volumes of suspended
material in seawater and can concentrate pollutants in their tissues. 
The major concerns for fish-and-shellfish monitoring were that contaminants
discharged to Massachusetts Bay would compromise animal health or render them
unfit to eat. Some scientists and members of the interested public agreed that ending
sludge discharges and moving the effluent outfall farther offshore would improve
fish-and-shellfish health in the harbor but were concerned that population sizes in the
harbor might decline. They speculated that the high nutrient inputs from sewage
supported large, if unhealthy, prey populations. An opinion piece in the New York
Times described finding cracks in sewage pipes by following fishing boats (Wald
1987), and unpublished data from MWRA’s predecessor agency, the Metropolitan
District Commission, suggested that flounder and lobster were more numerous at sites
near to their outfalls than farther away. 
However, flounder and mussel populations have declined, and there is concern that
the lobster population may decrease in the future. The changes are region-wide, so are
likely driven by changing environmental conditions, such as the increasing
temperature in the Gulf of Maine (Figure 6-1), rather than by changes in sewage
inputs. On the other hand, the decades of MWRA monitoring (see Section 5, above)
have shown that there has been no adverse effect of the outfall on flounder health, and
that instead, fish from both Boston Harbor and Massachusetts Bay are healthier than
in the past. There has also been no increase in contaminant levels in any of the three
species at any location.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
52 
Figure 6-1. The Gulf of Maine has been warming faster than the global ocean. Figure courtesy of
the Gulf of Maine Research Institute, Portland, Maine. 2024 Gulf of Maine Warming Update. 
https://gmri.org/stories/warming-24 
Winter Flounder 
Prior to the Boston Harbor Project, winter flounder supported popular recreational
and commercial fisheries in Boston Harbor and the surrounding region. Tagging
studies showed that, while the fish move on- and off-shore, populations north of Cape
Cod had very small home ranges (Howe and Coates 1975), making them an ideal tool
for comparing the water quality of Boston Harbor, the Massachusetts Bay outfall site,
and other nearshore coastal areas. 
In the early days of monitoring, flounder were abundant throughout Boston Harbor
Massachusetts Bay, and Cape Cod Bay, and the monitoring program easily reached
the target catches of 50 fish per location within a prescribed trawling time. In recent
years, it has proved more difficult to catch fish, and 2018 was the last year that all 50
fish were taken from each site sampled. In 2023 and 2024, MWRA supplemented
samples with fish caught by the Massachusetts Division of Marine Fisheries. Declines
in recreational and commercial landings (the number of fish brought to shore and
recorded or sold) throughout New England have also made it more difficult to engage
the commercial fishing vessels that have been a part of the program.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
53 
One reason that sampling has been more difficult is the large amount of entangled
lobster traps and other fishing gear resting on the sea floor (Figure 6-2). However,
even accounting for entanglements and torn nets due to this “ghost gear,” winter
flounder landings have been declining throughout southern New England and the
Mid-Atlantic since the 1980s. At first, these declines were blamed on overfishing
rather than warmer waters or pollution (Frisk et al. 2018). However, subtle changes in
temperature regimes may also be a factor (Jeffries and Johnson 1974, Jefferies and
Terceiro 1985). 
Figure 6-2. Lost lobster traps and other “ghost” fishing gear interfere with flounder collection. 
Photo credit Michael Moore, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts. 
A recent assessment (Atlantic States Marine Fisheries Commission 2025) showed
region-wide declines in commercial and recreational landings across the Gulf of
Maine (north of Cape Cod, including Boston Harbor and Massachusetts Bay),
Southern New England/Mid-Atlantic, and Georges Bank stocks through 2023 (Figure
6-3). Massachusetts had by far the largest proportion of landings in 2023, making up
97% of the commercial landings and 77% of the reported recreational catch.
Commercial and recreational landings peaked in the early 1980s and then declined
steadily though the 1990s and 2000s. Declines were partially due to management
actions, particularly catch limits, which are designed to decrease landings. The
commission has not been able to complete a population-size assessment for fish from
the Gulf of Maine stock, which would provide better information than landings data.
However, management actions alone could not have caused the large declines in
landings, and both commercial fishermen and recreational anglers have noted
increased difficulty in locating and catching fish.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
54 
Figure 6-3. Commercial and recreational winter flounder landings throughout its range, 1981–
2023. Data compiled by the Atlantic States Marine Fisheries Commission from state compliance
reports, the Atlantic Coastal Cooperative Statistics Program, and the Marine Recreational Information
Program. 
American Lobster 
The lobster is the most iconic and valuable of New England seafood. Landings in
Massachusetts are second only to Maine, and Boston is considered the major location
for processing and shipping lobster to regions outside New England. Some scientists
were reluctant to include lobsters as part of the fish-and-shellfish monitoring
program, because they move on- and off-shore, and their home ranges are poorly
understood. Further, an MWRA review of risks to lobster larvae and juveniles found
that dilution and plume dynamics within the water column would protect sensitive life
stages from acute or chronic effects (Mitchell et al. 1998). However, lobsters were
included in the program in recognition of their importance to the identity and
economy of the region.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
55 
One complication in assessing lobster catches in Massachusetts is that, for
management purposes, catches from north of Cape Cod are included as Gulf of Maine
stock, while those south of the Cape are included within the Southern New England
stock. The Southern New England population has declined over time, with
temperature thought to be the major factor (Le Bris et al. 2018). There are conflicting
opinions about current or near-future declines in the Gulf of Maine stock. However,
Massachusetts has included all its catches, both north and south of Cape Cod, in new
minimum-size rules set forth in guidelines from the Atlantic States marine Fisheries
Commission to protect stocks. New Hampshire and Maine have rejected adopting
new rules. In its 2024 sampling program, MWRA trapped only 20 lobsters near the
Massachusetts Bay outfall, one short of a full complement of 21 lobsters. Full
complements were taken near Deer Island and in eastern Cape Cod Bay. 
Blue Mussel 
MWRA uses cage-deployed blue mussels to assess bioaccumulation of toxic
pollutants at sites in Boston Harbor and Massachusetts Bay (Figure 6-4). Mussels
harvested from relatively pristine sites are deployed at several sites near the outfall
and in Boston Harbor, and MWRA compares pollutant levels in the deployed mussels
to levels in a “reference” or “control” group of mussels taken from the same
collections but not deployed. In the early days of the monitoring program, natural
populations in relatively pristine habitats were widely available for collection and
deployment. Over time wild mussel beds declined, and MWRA now purchases
mussels for deployment from commercial growers. 
Mussel deployments occurred annually from 1991 through 2003 and have continued
every three years through 2024 (Figure 6-5). In the first years of monitoring, 1991–
1994, all the reference mussels were harvested from Hodkins Cove in Gloucester,
Massachusetts. In 1995, concern for possible mercury and lead pollution in
Gloucester prompted MWRA to use a second source, in Sandwich, Massachusetts, as
the reference site for metals measurements. In 2000, MWRA identified a new source,
in Rockport, Massachusetts, in with low levels of metals and organic pollutants, and
all collections were made from that site. Over time, the mussel populations decreased
in all the previously used sites, and in 2002, the collections moved northeast to
Stover’s Point, just northeast of Portland, Maine. In 2012, the collection site was
moved further northeast again, to Pemaquid Point, near Boothbay Harbor, Maine.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
56 
Figure 6-4. Pre- and post-deployment mussel cages. Mussels are collected from relatively pristine
sites and deployed for periods of 40 and 60 days. Photo credit Battelle, Norwell, Massachusetts.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
57 
Figure 6-5. Mussel collection sites, 1998–2024 . Mussel collection sites have steadily moved to the
northeast, with the exception of 2021, when they came from Salem State University’s deepwater
Northeastern Massachusetts Aquaculture Center site. 
The winter of 2014-2015 began late, but was unusually long and harsh. Boston had
almost no snow before January 1, 2015, but quickly exceeded 110 inches for the
season, thanks to almost 95 inches in just four storms in January and February. When
no wild mussel beds could be found for the 2015 cage deployments, some MWRA
team members speculated that the ice that scraped the mussels from intertidal zones
that year. Similar ice scouring events have been documented in the Gulf of Maine and
in Europe. 
The natural mussel beds did not recover, and since then, all mussels used in the
monitoring program have been farmed. In 2015, the mussels obtained from Tightrope
SeaFarms in Blue Hill Bay, Maine, were cultured on ropes and had thin shells,
resulting in great mortality during deployment. In 2021, mussels were obtained from
Salem State University’s offshore mussel farm. In 2018 and 2024, MWRA purchased
mussels from Hollander and de Koning Mussel Farms beds in Mount Desert Narrows,
Frenchman Bay, near Bar Harbor, Maine (Hollander and de Koning, personal
communication). The mussels in these beds are grown directly on the sea floor rather
than on ropes, and survival rates have been good.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
58 
Reasons for the declines in natural mussel beds in the region are not well understood.
Predation by non-native green crabs may be one factor. Warming temperatures are
almost certainly a factor, as MWRA’s mussel sources have moved steadily northward
throughout the monitoring period. Warming temperatures may also be leading to blue
mussels being found at increasing depths. Once thought of as an intertidal and
shallow-water species, they were first detected on an MWRA outfall diffuser head in
2005 and appear to be increasing in numbers both on the diffuser head monitored by
MWRA and on nearby large boulders (Figure 6-6; Goode et al. 2024). 
Figure 6-6. Blue mussels and sea anemones growing on an active outfall diffuser in 2023. Photo
credit Hecker Environmental, Falmouth, Massachusetts. 
All three fish-and-shellfish species monitored by MWRA have provided valuable
information, confirming predictions that the Massachusetts Bay discharge would not
impair fish health or render fish and shellfish unfit for consumption. However,
regional environmental changes may affect the species now or in the future.
MWRA’s long-term monitoring and studies by other agencies and scientists have
documented increased temperatures and salinities and decreased dissolved oxygen
concentrations over time (Codiga et al. 2025). Ongoing changes in physical
conditions can be expected to continue to affect the region’s biological populations
and communities.

2024 OUTFALL MONITORING OVERVIEW: SPECIAL TOPICS 
59 
Bays Eutrophication Model Update 
The Deer Island NPDES permit to discharge treated wastewater effluent to
Massachusetts Bay requires MWRA to maintain and annually run the Bays
Eutrophication Model. This combined circulation and water-quality model simulates
the parameters and processes that could lead to eutrophication and subsequent low
dissolved oxygen levels in Massachusetts Bay, Boston Harbor, and Cape Cod Bay.
Parameters include water temperature, salinity, circulation, nutrient concentrations,
and phytoplankton biomass. Annual model runs complement MWRA’s monitoring
program. Annual simulations have continually confirmed the early predictions that
nutrient inputs from the outfall will not lead to eutrophication. 
The most recent model simulation, for the year 2023, successfully reproduced many,
but not all, of the seasonal and geographic patterns measured during the shipboard
surveys (Deltares 2025). The model simulation successfully captured a stratification
peak during July and August, which was consistent with observed conditions. The
model also continued to show a signature of the effluent discharge, including an
expected decrease in total dissolved inorganic nitrogen concentrations with increased
distance from the outfall. 
Modeled chlorophyll concentrations continued to support the conclusion that the
discharge does not significantly influence nearshore phytoplankton biomass.
However, the model was unable to replicate the unusual regional bloom of the
dinoflagellate Tripos muelleri , which occurred across the Gulf of Maine from April
through July 2023. The model does not simulate individual species and therefore
predicted a more typical spring and summer pattern for generic dinoflagellates.
Consequently, the model simulation underestimated actual chlorophyll and particulate
organic carbon concentrations during the bloom period. 
The model captured the overall seasonal pattern of dissolved oxygen for 2023, with
higher concentrations in winter and declines through summer and fall. However, it
overestimated bottom water dissolved oxygen concentrations during summer and fall,
especially at stations where observed levels dropped below historic lows. This
overprediction is consistent with past model runs and may have been affected by
underestimation of oxygen consumption and failure to capture the effects of decay of 
Tripos muelleri at the end of the bloom. 
Overall, the 2023 model run continued to confirm that nutrient inputs from the outfall
did not cause eutrophication in Massachusetts Bay.

2024 OUTFALL MONITORING OVERVIEW: REFERENCES 
60 
References 
Atlantic States Marine Fisheries Commission. 2025. Review of the interstate
management plan for winter flounder ( Pseudopleuronectes americanus ), 2023 fishing
year. Prepared by the Plan Review Team, approved April 2025. 23 p. 
Cibik SJ, Lemieux KB, Davis CS, Anderson DM. 1998. Massachusetts Bay plankton
communities: characterization and discussion of issues relative to MWRA’s outfall
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140 p. 
Codiga DL, Keay KE, Libby PS. 2025. Thirty-one years of warming and oxygen
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Deltares. 2024. Simulations of 2023 hydrodynamics and water quality in the
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2023 OUTFALL MONITORING OVERVIEW: LIST OF ACRONYMS 
63 
List of Acronyms 
ASP Amnesic shellfish poisoning 
BOD Biochemical oxygen demand 
cBOD Carbonaceous biochemical oxygen demand 
cells/L Cells per liter 
cfu Colony forming units 
CHV Centrotubular hydropic vacuolation 
DDT Dichlorodiphenyltrichloroethane 
DIF Deer Island Flats 
DO Dissolved oxygen 
ECCB Eastern Cape Cod Bay 
EPA U.S. Environmental Protection Agency 
kg/m 3 Kilograms per cubic meter 
LC50 50% mortality concentration 
LNB Large navigation buoy 
m Meters 
m 3 /s Cubic meters per second 
mg/L Milligrams per liter 
mg/L/d Milligrams per liter per day 
mg/m 2 Milligrams per square meter 
mg/m 3 Milligrams per cubic meter 
mL Milliliters 
MGD Million gallons per day 
mtons Metric tons 
MWRA Massachusetts Water Resources Authority 
μg/L Micrograms per liter 
μm Micrometer 
μM Micromoles 
ng/g wet Nanograms per gram wet weight 
NA Not analyzed/not applicable 
NDBC National Data Buoy Center 
NERACOOS Northeastern Regional Association of Coastal and Ocean Observing Systems 
ng/L Nanograms per liter 
NOAA National Oceanic and Atmospheric Administration 
NOEC No observed effects concentration 
NPDES National Pollutant Discharge Elimination System 
OS Outfall site 
Pa Pascal 
PAH Polycyclic aromatic hydrocarbon 
PCB Polychlorinated biphenyl 
PSP Paralytic shellfish poisoning 
PSU Practical salinity units 
TN Total nitrogen 
TSS Total suspended solids

Massachusetts Water Resources Authority 
Deer Island 
33 Tafts Avenue • Boston, MA 02128 
617-242-6000