2017 overview

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Massachusetts Water
Resources Authority
Environmental Quality
Department Report 2018-07

Citation:
Werme C, Keay KE, Libby PS, Codiga DL, Taylor DI, Charlestra L, Carroll SR. 2018.
2017 outfall monitoring overview. Boston: Massachusetts Water Resources Authority.
Report 2018-07. 53 p.
Cover photo credits clockwise from top left:
Sea anemones and other organisms on an outfall riser in Massachusetts Bay: 
Barbara Hecker and Normandeau Associates 
Sediment sampling in Boston’s Inner Harbor:
Winter flounder in eelgrass bed: 
Chris Baker, Normandeau Associates
Chris Pickerell, (NOAA Fisheries file photo)

2017
Outfall Monitoring Overview 
prepared by 
Christine Werme 
Independent Consultant
Oakland, CA 94612 
Kenneth E. Keay 
Massachusetts Water Resources Authority
Environmental Quality Department
100 First Avenue
Charlestown Navy Yard
Boston, MA 02129 
P. Scott Libby 
Battelle
72 Main Street
Topsham, ME 04086 
Daniel L. Codiga, David I. Taylor, Lucner Charlestra,
Sally R. Carroll 
Massachusetts Water Resources Authority
Environmental Quality Department
100 First Avenue
Charlestown Navy Yard
Boston, MA 02129 
November 5, 2018

2017 Outfall Monitoring Panel and Committees 
Outfall Monitoring Science Advisory Panel (OMSAP) 
Robert Beardsley, Woods Hole Oceanographic Institution
Robert Kenney, University of Rhode Island
Judy Pederson, Massachusetts Institute of Technology Sea Grant
Michael Shiaris, University of Massachusetts, Boston
James Shine, Harvard School of Public Health
Geoffrey Trussell, Northeastern University Marine Science Center
Juanita Urban-Rich, University of Massachusetts, Boston 
Inter-Agency Advisory Committee (IAAC) 
US Geological Survey
Massachusetts Coastal Zone Management
US Army Corps of Engineers
Stellwagen Bank National Marine Sanctuary
US Environmental Protection Agency
Massachusetts Division of Marine Fisheries
Massachusetts Department of Environmental Protection 
Public Interest Advisory Committee (PIAC) 
Save the Harbor/Save the Bay
MWRA Wastewater Advisory Committee
Conservation Law Foundation
Massachusetts Audubon Society
MWRA Advisory Board
Association to Preserve Cape Cod
Safer Waters in Massachusetts
The Boston Harbor Association

i 
Table of Contents 
List of Figures ............................................................................................................... ii 
List of Tables ............................................................................................................... iv
Executive Summary ...................................................................................................... v
1. Introduction ............................................................................................................... 1 
2. Effluent ..................................................................................................................... 2 
2017 Effluent Characterization ................................................................................. 2 
Contingency Plan Thresholds ................................................................................... 7 
3. Water Column ........................................................................................................... 8 
Physical Conditions ................................................................................................ 10 
Water Quality .......................................................................................................... 12 
Phytoplankton Communities ................................................................................... 18 
Zooplankton Communities...................................................................................... 22 
Stellwagen Bank National Marine Sanctuary ......................................................... 23 
Boston Harbor ......................................................................................................... 24 
Contingency Plan Thresholds ................................................................................. 25 
4. Sea Floor ................................................................................................................. 26 
Sediment Characteristics and Tracers ..................................................................... 29 
Sediment Contaminants .......................................................................................... 31 
Soft-bottom Communities....................................................................................... 32 
Sediment-profile Imaging ....................................................................................... 34 
Hard-bottom Communities ..................................................................................... 35 
Stellwagen Bank National Marine Sanctuary ......................................................... 36 
Boston Harbor ......................................................................................................... 37 
Contingency Plan Thresholds ................................................................................. 39 
5. Fish and Shellfish .................................................................................................... 41 
Flounder Health ...................................................................................................... 42 
Contingency Plan Thresholds ................................................................................. 43 
6. Special Studies ........................................................................................................ 44 
Spotlight on Eutrophication .................................................................................... 44 
Nitrogen Inputs ................................................................................................... 45 
Nitrogen Concentrations in the Water Column................................................... 45 
Phytoplankton Biomass ...................................................................................... 46 
Dissolved Oxygen ............................................................................................... 47 
Sea Floor Communities....................................................................................... 48 
References ................................................................................................................... 52 
List of Acronyms ........................................................................................................ 53

ii 
List of Figures 
Figure i. Annual solids loads, 1990–2017 .................................................................... v 
Figure ii. Average ammonium concentrations at selected stations during harbor and
bay discharge years ...................................................................................................... vi 
Figure iii. Lush growth of sea anemones and barnacles on an active diffuser head in
2017............................................................................................................................. vii 
Figure 2-1. Annual rainfall in Boston, 1990–2017 ....................................................... 2 
Figure 2-2. Annual primary-blended and full secondary effluent flows, 1999–2017 .. 3 
Figure 2-3. Monthly primary-blended and full secondary flows and rainfall during
2017............................................................................................................................... 3 
Figure 2-4. Annual solids loads, 1990–2017 ................................................................ 4 
Figure 2-5. Annual nitrogen loads, 1996–2017 ............................................................ 5 
Figure 2-6. Annual metals loads, 1990–2017. .............................................................. 6 
Figure 3-1. Water-column monitoring stations in Massachusetts and Cape Cod bays. 8 
Figure 3-2. Flows of the Merrimack and Charles rivers ............................................. 10 
Figure 3-3. Surface-water temperature at NDBC Buoy in 2017 compared to prior
years. ........................................................................................................................... 11 
Figure 3-4. Stratification at nearfield Station N18 in 2017 compared to prior years. 11 
Figure 3-5. Average ammonium concentrations at selected stations in 2017 compared
to prior years ............................................................................................................... 13 
Figure 3-6. (Left) Surface- and bottom-water ammonium on March 25, 2017, during
mixed conditions. (Right) Cross-sections of concentrations throughout the water
column along transects connecting selected stations. ................................................. 14 
Figure 3-7. (Left) Surface- and bottom-water ammonium on July 26, 2017, during
stratified conditions. (Right) Cross-sections of concentrations throughout the water
column along transects connecting selected stations .................................................. 15 
Figure 3-8. Moderate Resolution Imaging Spectroradiometer satellite imagery of
surface chlorophyll concentrations in 2017 ................................................................ 16 
Figure 3-9. Areal chlorophyll by station in Massachusetts Bay in 2017 .................... 17 
Figure 3-10. Near bottom water dissolved oxygen concentrations at selected stations
in 2017 compared to prior years ................................................................................. 18 
Figure 3-11. Total phytoplankton abundance at selected stations in 2017 compared to
prior years ................................................................................................................... 19 
Figure 3-12. Mean nearfield abundance of Phaeocystis pouchetii in 1992–2017. ..... 20 
Figure 3-13. Nearfield abundance of Alexandrium catenella, 1992–2017. ................ 21 
Figure 3-14. Total zooplankton abundance at selected stations in 2017 compared to
prior years ................................................................................................................... 22 
Figure 3-15. Dissolved oxygen at the NERACOOS Buoy A01 and the deepwater
measurements at Station F22 in Stellwagen Basin, and at Station N18 in the
nearfield.. .................................................................................................................... 23 
Figure 4-1. Soft-bottom monitoring stations .............................................................. 26

iii
Figure 4-2. Sediment-profile imaging stations ........................................................... 27 
Figure 4-3. Hard-bottom stations ................................................................................ 28 
Figure 4-4. Concentrations of Clostridium perfringens spores, corrected for sediment
grain size, in 2017 ....................................................................................................... 30 
Figure 4-5. Total organic carbon at selected stations, 1992–2017 ............................. 30 
Figure 4-6. Mean concentrations of total PCBs at four areas in Massachusetts Bay,
1992–2017................................................................................................................... 31 
Figure 4-7. Ordination plot of 2017 Massachusetts Bay samples by location............ 32 
Figure 4-8. Percent fine sediments and depth superimposed on the ordination plot of
the 2017 infauna samples ............................................................................................ 33 
Figure 4-9. Annual apparent color RPD depth for data from nearfield stations. ........ 34 
Figure 4-10. Lush growth of sea anemones and barnacles on an active diffuser head in
2017............................................................................................................................. 35 
Figure 4-11. Overturned boulder at a northern reference station, where deep-draft
tankers have been observed to anchor ........................................................................ 36 
Figure 4-12. Soft-bottom sampling stations in Boston Harbor. .................................. 37 
Figure 4-13. Mean concentrations of Clostridium perfringens spores at selected
harbor stations, 1991–2017 ......................................................................................... 38 
Figure 4-14. Ordination plot of 2017 Boston Harbor infauna samples ...................... 38 
Figure 5-1. Fish-and-shellfish monitoring stations ..................................................... 41 
Figure 5-2. “Ghost” lobstering gear brought up in MWRA flounder trawls can make
collection difficult, especially at Deer Island Flats and near the outfall site. ............. 42 
Figure 5-3. Annual prevalence of centrotubular hydropic vacuolation, 1991–2017 .. 43 
Figure 6-1. Ammonium concentrations at selected stations in Boston Harbor and
Massachusetts Bay during harbor discharge and bay discharge years ....................... 46 
Figure 6-2. Chlorophyll concentrations at selected stations in Boston Harbor and
Massachusetts Bay during harbor discharge and bay discharge years ....................... 47 
Figure 6-3. Annual minimum dissolved oxygen concentrations at selected stations in
Boston Harbor and Massachusetts Bay during harbor discharge and bay discharge
years ............................................................................................................................ 48 
Figure 6-4. Total organic carbon concentrations at muddy stations in Boston Harbor
and Massachusetts Bay during harbor discharge and bay discharge years................. 49 
Figure 6-5. Species richness at muddy stations in Boston Harbor and Massachusetts
Bay during harbor discharge and bay discharge years................................................ 50 
Figure 6-6. Eelgrass in Boston Harbor is thriving in areas where it was not previously
seen ............................................................................................................................. 51

iv 
List of Tables 
Table 2-1. Projected and actual loads of metals and organic contaminants in MWRA
effluent. ......................................................................................................................... 6 
Table 2-2. Contingency Plan threshold values and 2017 results for effluent
monitoring ..................................................................................................................... 7 
Table 3-1. Contingency Plan threshold values and 2017 results for water-column
monitoring ................................................................................................................... 25 
Table 4-1. Contingency Plan threshold values and 2017 results for sea-floor
monitoring ................................................................................................................... 40 
Table 5-1. Contingency Plan threshold value and 2017 results for fish-and-shellfish
monitoring ................................................................................................................... 43

v 
Executive Summary 
This outfall monitoring overview for 2017 marks the twenty-sixth year of the
Massachusetts Water Resources Authority (MWRA) Massachusetts Bay monitoring
program and more than seventeen years since effluent discharge was diverted from
the shallow, confined waters of Boston Harbor to the deeper, more open waters of
Massachusetts Bay.
Deer Island Treatment Plant continued to operate as designed, earning MWRA the
National Association of Clean Water Agencies Platinum 11 Peak Performance Award
for facilities with 100% compliance with permit conditions over eleven consecutive
years. Solids discharges to Massachusetts Bay remained low, with only a fraction of
the discharges that had been made to Boston Harbor in the 1990s (Figure i).
Discharges of toxic metals and organic compounds were well below what had been
anticipated when discharge to Massachusetts Bay was planned. 
Figure i. Annual solids loads, 1990–2017. Before December 1991, biosolids (sludge) removed during
treatment were disinfected and discharged into Boston Harbor. Ending biosolids disposal, ending
effluent discharge to the southern portion of the harbor from Nut Island, implementing secondary
treatment, and ending all discharges to the harbor in September 2000 were important steps in the
Boston Harbor Project. Since 2006, variability in solids discharges to Massachusetts Bay can be mostly
attributed to variation in flow.

vi
Water-column monitoring in Massachusetts Bay continued to confirm predictions that
limited effects of the discharge would be detectable for some parameters but only at
stations located in close proximity to the outfall and not at levels that would cause
environmental concern. Elevated concentrations of the nutrient ammonium, for
example, can be detected close to the outfall, but those increases are limited in
comparison to the large decreases in ammonium observed in Boston Harbor (Figure
ii). The Massachusetts Bay outfall has not increased harmful phytoplankton blooms
or lowered oxygen levels in the bay. 
Figure ii. Average ammonium concentrations at selected stations during harbor and bay
discharge years. 
106
141
F22
N18
F15
F06 
September 2000
Harbor
Discharge
Bay
Discharge 
September 2000
Harbor
Discharge
Bay
Discharge 
September 2000
Harbor
Discharge
Bay
Discharge 
September 2000
Harbor
Discharge
Bay
Discharge 
September 2000
Harbor
Discharge
Bay
Discharge 
September 2000
Harbor
Discharge
Bay
Discharge

vii
Healthy benthic communities continue to thrive on the Massachusetts Bay sea floor,
where habitats continue to be controlled more by physical factors, such as storm-
induced waves and currents, than by outfall discharges. Lush communities of sea
anemones and barnacles cover the active outfall diffusers, which are also frequented
by fish (Figure iii). 
Figure iii. Lush growth of sea anemones and barnacles on an active diffuser head in 2017. Photo
credit Barbara Hecker/Normandeau Associates.
Winter flounder remain healthy, without the widespread fin erosion and tumors of the
1980s and 1990s. No tumors have ever been detected in fish taken from near the
Massachusetts Bay outfall.
MWRA remains committed to the mission it was given in 1984, to improve
conditions in Boston Harbor without damage to Massachusetts Bay. The twenty-six
years of Massachusetts Bay monitoring have confirmed predictions made before
discharge began and verified continued good effluent treatment and operations at
Deer Island Treatment Plant.

1 
1. Introduction 
Since its creation by the Massachusetts state legislature in 1984, the Massachusetts
Water Resources Authority (MWRA) has worked to minimize the effects of its
municipal wastewater discharge on the marine environment. The mission of what
became known as MWRA’s Boston Harbor Project included reducing inflow of
contaminants into the waste stream, ending biosolids (sludge) discharge, improving
wastewater-treatment facilities, and providing better dilution of the effluent
discharge. Throughout most of its history, MWRA has conducted long-term
environmental monitoring in Boston Harbor, where both biosolids and effluent were
once discharged, and in Massachusetts Bay, where highly treated wastewater effluent
is currently discharged through a deepwater tunnel and diffuser system.
Biosolids discharge to Boston Harbor ended in December 1991, all discharges to the
southern portion of the harbor ended in 1998, and upgrades to Deer Island Treatment
Plant were completed in 1995–2001. A major milestone was reached in September
2000, when all effluent discharge was diverted from Boston Harbor to the deeper, less
confined waters of Massachusetts Bay. The relocated outfall operates 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 (MassDEP). An independent Outfall Monitoring
Science Advisory Panel (OMSAP) provides technical review to the regulatory
agencies.
The NPDES permit requires monitoring of effluent and receiving waters, sediments,
and biota to assess compliance with permit conditions and with a permit-required
Contingency Plan. Results are presented in annual outfall monitoring overviews,
such as this report. Background information about the monitoring program (Werme
et al. 2012), the monitoring plan (MWRA 2010), the Contingency Plan (MWRA
2001), past plans and overviews, and study-specific technical reports are available at
http://www.mwra.state.ma.us/harbor/enquad/trlist.html, MWRA’s technical report
list.
This year’s outfall monitoring overview presents results from 2017, marking the
twenty-sixth year of Massachusetts Bay monitoring and more than seventeen years of
discharge from the deepwater outfall. The report presents information relevant to
permit and Contingency Plan conditions in the effluent, water column, sea floor, and
winter flounder, as well as special studies conducted in response to permit conditions
and environmental concerns. This year’s special studies section focuses on
eutrophication. Data relevant to the Stellwagen Bank National Marine Sanctuary,
located offshore from the outfall, are presented in sections covering the water column
and the sea floor. The report also includes pertinent information derived from
MWRA’s separate monitoring efforts in Boston Harbor.

2 
2. Effluent
2017 Effluent Characterization 
Deer Island Treatment Plant continued to operate as designed through 2017, earning
MWRA the National Association of Clean Water Agencies (NACWA) Platinum 11
Peak Performance Award. This NACWA award recognizes facilities with 100%
compliance with effluent permit limits over eleven consecutive years.
Wastewater influent to Deer Island Treatment Plant includes not only municipal
sewage but also groundwater infiltration and stormwater inflow. Consequently,
rainfall is an important factor determining wastewater flows and contaminant loads in
the MWRA effluent. After two years of drought, the Boston area received 43.3
inches of rain in 2017, equal to the average rainfall for 1990–2017 (Figure 2-1), and
suggesting that flows and contaminant loads should be somewhat higher in 2017 than
in 2015 or 2016 and about average for the period since the Boston Harbor Project. 
Figure 2-1. Annual rainfall in Boston, 1990–2017. 
Effluent flows in 2017 were actually somewhat lower than the 1999–2017 average,
about 328 million gallons per day (MGD; Figure 2-2). Almost all the flow, 99%,
received full primary and secondary treatment, with only trace discharges of primary-
treated effluent blended with fully treated effluent prior to discharge in any month
(Figure 2-3). (During large storms, flow exceeding the 700-MGD secondary capacity
of the plant is diverted around the secondary process to prevent washing out the
essential microbes that carry out secondary treatment. These diverted flows receive
primary treatment and then are combined or “blended” with full secondary-treated
flows before disinfection and discharge.)

3 
Figure 2-2. Annual primary-blended and full secondary effluent flows, 1999–2017. During large
storms, flow exceeding the secondary capacity of the plant is diverted around the secondary process.
Primary-treated flows are blended into secondary-treated flows before discharge. 
Figure 2-3. Monthly primary-blended and full secondary flows and rainfall during 2017. Highest
flows occurred in April, corresponding to heavy rainfall.

4
The total suspended solids load to Massachusetts Bay was higher than in 2015 or
2016, but also low, well below the loads discharged to Boston Harbor before the
relocated outfall began operation in 2000 (Figure 2-4). Carbonaceous biological
oxygen demand, a measure of the amount of oxygen consumed by microorganisms,
also remained low, well below levels that would be expected to affect dissolved
oxygen concentrations at the discharge (not shown). 
Figure 2-4. Annual solids loads, 1990–2017. Before December 1991, biosolids (sludge) removed
during treatment were disinfected and discharged into Boston Harbor. Ending biosolids disposal,
ending effluent discharge to the southern portion of the harbor from Nut Island, implementing
secondary treatment, and ending all discharges to the harbor in September 2000 were important steps
in the Boston Harbor Project. Since 2006, variability in solids discharges to Massachusetts Bay can be
mostly attributed to variation in flow.
The total nitrogen load was lower than in 2016, remaining just under its caution-level
Contingency Plan threshold (Figure 2-5) and well below the warning-level threshold,
14,000 metric tons per year. The caution level was somewhat arbitrarily set as 90%
of the of the warning level; the warning level was the projected total nitrogen load for
the year 2020. Since discharges to the harbor ended, total nitrogen loads have been
lower than were predicted at the start of the Boston Harbor Project.
The portion of the nitrogen load made up of ammonium declined after a record high
in 2016. Increased ammonium loads are a consequence of the biological treatment
process and addition of ammonium-rich liquids from the biosolids pelletizing plant.
Total effluent nitrogen loads reflect variability in nitrogen levels in the influent
reaching Deer Island, with about 30% of the nitrogen removed during treatment.

5
There have been no observed adverse environmental effects due to nitrogen discharge
into Massachusetts Bay, and water-quality modeling has suggested that even large
increases to the nitrogen discharge would have no adverse effect on the environment.
The relatively high nitrogen load measured in 2016 was evaluated during permit-
required computer modeling of water quality in Massachusetts Bay. In addition to the
model run with actual 2016 effluent nutrient measurements, modelers conducted a
projection run, artificially introducing a 20% increase in effluent nitrogen above the
2016 load. Model results indicated that even the projection of 20% increased
nitrogen loads over actual 2016 loads would have only negligible effects on the water
column (Zhao et al. 2017). 
Figure 2-5. Annual nitrogen loads, 1996–2017. During outfall planning, managers and regulators
assumed that discharges would total 14,000 tons per year by 2020. TN = total nitrogen, 1.2X scenario
= a 20% increase over the actual 2016 load
Metals loads also remained low in 2017, with zinc and copper continuing to comprise
most of the annual discharges (Figure 2-6). Metals loads are now mostly a factor of
rainfall and flow rates rather than variations in inputs to the waste stream. Except for
copper, metals meet water quality standards prior to discharge, while copper meets
the standard after initial dilution at the Massachusetts Bay outfall. Once considered a
sewage tracer, silver is no longer detected in the effluent in appreciable amounts, a
result of high removal efficiencies associated with secondary treatment and the
change from film to digital photography.

6 
Figure 2-6. Annual metals loads, 1990–2017. 
Polycyclic aromatic hydrocarbon and other organic contaminant loads (not shown)
were low, as they have been throughout the monitoring program. Discharges of
organic contaminants have varied slightly from year to year but have been well below
levels historically discharged into Boston Harbor.
Total discharges of both metals and organic compounds remained well below
predictions made during the planning and permitting process for the Massachusetts
Bay outfall (Table 2-1). Mercury discharges were about 1% of the projected load,
while copper discharges were just over one quarter of what had been projected.
Loads of total PCBs, and 4-4’ DDE (a breakdown product of the banned pesticide
DDT) were only about 1% of the loads anticipated in the Supplemental
Environmental Impact Statement (SEIS) prepared during planning (EPA 1988). 
Table 2-1. Projected and actual loads of metals and organic contaminants in MWRA effluent. 
Loads of metals and organic contaminants are far below those projected during planning and
permitting of the Massachusetts Bay outfall. 
Parameter SEIS Projected Load
(kg/year)
2017 Load
(kg/year)
Percent
Projected Load 
Chromium 3,517 501 14%
Copper 11,945 3,062 26%
Lead 4,961 456 9.2%
Mercury 216 2.7 1.3%
Nickel 8,926 1,208 14%
Silver 299 27 9.0%
Total PCBs 50 0.52 1.0%
4-4’ DDE 28 0.34 1.2% 
SEIS = Supplemental Environmental Impact Statement

7 
Contingency Plan Thresholds 
There were no permit violations in 2017 and no exceedances of the Contingency Plan
effluent thresholds (Table 2-2). Effluent threshold exceedances have been rare
throughout the duration of the monitoring program, and none have occurred over the
past eleven years. 
Table 2-2. Contingency Plan threshold values and 2017 results for effluent monitoring. 
Parameter Baseline Caution Level Warning Level 2017 Results 
pH NA None <6 or >9 Not exceeded
Fecal coliform NA None >14,000 fecal
coliforms/100 mL Not exceeded
Chlorine, residual NA None >631 μg/L daily,
>456 μg/L monthly Not exceeded
Suspended solids NA None >45 mg/L weekly
>30 mg/L monthly Not exceeded
cBOD NA None >40 mg/L weekly,
>25 mg/L monthly Not exceeded
Acute toxicity NA None LC50 <50% Not exceeded
Chronic toxicity NA None NOEC <1.5%
effluent Not exceeded
PCBs NA Aroclor>0.045 ng/L None Not exceeded
Plant performance NA 5 violations/year Compliance <95%
of the time Not exceeded
Flow NA None >436 MGD
average dry days Not exceeded
Total nitrogen load NA >12,500
mtons/year
>14,000
mtons/year Not exceeded
Oil and grease NA None >15 mg/L weekly Not exceeded 
NA = not applicable
cBOD = carbonaceous biological oxygen demand
LC50 = 50% mortality concentration
NOEC = no observable effect concentration
PCB = polychlorinated biphenyl
Plant performance = compliance with permit conditions

8 
3. Water Column 
Water-column monitoring evaluates relevant oceanographic processes, water quality,
and phytoplankton and zooplankton communities at stations in Massachusetts Bay, at
the mouth of Boston Harbor, and in Cape Cod Bay (Figure 3-1). 
Figure 3-1. Water-column monitoring stations in Massachusetts and Cape Cod bays.

9
Sampling during nine surveys of fourteen stations in 2017 included vertical profiles
of physical, chemical, and biological parameters in the nearfield (the 10 by 12-
kilometer area around the outfall where some effects of the effluent were expected
and have been observed), and at farfield reference stations, including a station
northwest of the outfall and stations in Cape Cod Bay and near the Stellwagen Bank
National Marine Sanctuary. Analyses included data from ten additional stations,
sampled as part of MWRA’s Boston Harbor water-quality monitoring program, when
sampling dates were within a few days of the outfall-monitoring surveys. During the
June 13 survey, abundance of the toxic dinoflagellate Alexandrium catenella 
(formerly known as Alexandrium fundyense and responsible for red tide in New
England) exceeded 100 cells per liter (see Figure 3-13, page 21), triggering four 
Alexandrium special surveys, conducted in June and July.
The program continued to benefit from collaboration with the Center for Coastal
Studies at Provincetown, Massachusetts, which conducts a monitoring program in
Cape Cod Bay and, as part of the MWRA monitoring program, samples the water-
column stations in Cape Cod Bay and near the Stellwagen Bank National Marine
Sanctuary. Regulators have set a target that, whenever possible, sampling in Cape
Cod Bay should occur on the same day as Massachusetts Bay sampling. Surveys for
which sampling cannot occur within 48 hours should be reported in this annual outfall
monitoring overview. In 2017, the April Massachusetts Bay survey was delayed by
instrument repairs and subsequent engine problems, delaying the survey to ten days
after the Cape Cod Bay survey. All other surveys were conducted within the 48-hour
window.
As in past years, the field monitoring program was augmented by measurements on
two instrumented buoys: the Northeastern Regional Association of Coastal and Ocean
Observing Systems (NERACOOS) Buoy A01 and the National Oceanic and
Atmospheric Administration’s National Data Buoy Center (NDBC) Buoy 44013.
The National Aeronautics and Space Administration provided Moderate Resolution
Imaging Spectroradiometer (MODIS) satellite imagery of chlorophyll and sea-surface
temperature.

10 
Physical Conditions 
Monitoring has shown that the water quality in the vicinity of the outfall and
throughout Massachusetts and Cape Cod bays is heavily influenced by weather, river
inflows, and other physical factors. Information about physical conditions has proven
key to interpreting the annual water-column monitoring data.
Corresponding to the higher rainfall in the Boston area in 2017 compared with 2015
and 2016 (see Figure 2-1), discharges from the Merrimack and Charles rivers were
also higher than in recent years, and, on an annual basis, close to average for the
monitoring program (Figure 3-2). Seasonally, discharge from the Merrimack River
was particularly higher than average during the spring, and discharge from the
Charles River was especially low in the winter. 
Figure 3-2. Flows of the Merrimack (top) and Charles (bottom) rivers. Dark red lines are 2017
data. Results from 1992–2016 are in light blue. The quarterly percentiles represent the 2017 flows in
comparison to the entire 26-year record.
Surface-water temperatures, measured at the NDBC Buoy 44013 (Figure 3-3) and at
nearby Station N18 (not shown) were warmer than average at the beginning and end
of the year, setting records as the long-term maxima for the monitoring program for
January and February (Libby et al. 2018). Surface waters were relatively cool during
June, corresponding to a period of strong and consistent upwelling of cool water from
depth.

11 
Figure 3-3. Surface-water temperature at NDBC Buoy in 2017 compared to prior years. Vertical
dashed lines denote timing of MWRA surveys.
Stratification followed a typical annual pattern in 2017 but was particularly strong in
early June, corresponding to freshwater inflow in the surface water, and weaker than
usual in late June and July (Figure 3-4). Surface-water salinity measurements (not
shown) were also low in June, a result of the high discharge from the Merrimack
River during the spring (see Figure 3-2). 
Figure 3-4. Stratification at nearfield Station N18 in 2017 compared to prior years. Black points
are results from individual surveys in 2017. Results from 1992–2016 are in blue: line is the 50 th
percentile, dark shading spans the 25 th to 75 th percentile, and light shading spans the range. For Station
N18, historic data extend later in the year than the current survey schedule.

12 
Water Quality 
As in past years, water quality measurements for 2017 included quantification of
nutrients, phytoplankton biomass (measured as chlorophyll and particulate organic
carbon), and dissolved oxygen. Results continued to confirm predictions of
measurable outfall influence in some parameters, but only at stations very near the
outfall, and no unexpected adverse effects (Libby et al. 2018).
During 2017, dissolved inorganic nutrient concentrations in the nearfield fell mostly
within the ranges measured in previous years. As in past years, concentrations of
ammonium at nearfield stations very close to the outfall varied greatly and did not
show a seasonal pattern. Ammonium is the largest fraction of the total nitrogen in
wastewater and provides a tracer that could identify possible adverse effects of the
outfall, if they were to occur.
Elevated ammonium concentrations have been detected at Stations N18 and N21,
both in the immediate vicinity of the outfall, since the discharge began in 2000. As
noted in Section 2, Effluent, ammonium makes up a large majority of the total
nitrogen in the outfall discharge. In 2017, the signature was especially apparent
during some surveys but not others, particularly the last survey of the year, when
concentrations were low (Figure 3-5). During the late July survey, elevated
ammonium concentrations were also detected further south, at Station F10, to the
southwest of the outfall. Overall, the ammonium signature of the outfall was typical
of the years since the discharge was relocated from Boston Harbor to Massachusetts
Bay.

13 
Figure 3-5. Average ammonium concentrations (micromolar) at selected stations in 2017
compared to prior years. Black points and line are results from individual surveys in 2017. Red lines
and shading show data from Boston Harbor discharge years. Results from September 2000–2016 are in
blue. Line is the 50 th percentile, dark shading spans the 25 th to 75 th percentile, and light shading spans
the range. For Stations N04, N18, and N21, historic data extend later in the year than the current
survey schedule.
As in previous years, the plume’s ammonium signature was evident in surface waters
during the winter and spring when the water column was relatively well-mixed
(Figure 3-6) but was largely confined beneath the pycnocline during the summer,
stratified season (Figure 3-7). The ammonium signature could be detected only
within 10 to 20 kilometers of the outfall in both well-mixed and stratified seasons,
consistent with predictions made during planning for the outfall.
No increase in phytoplankton biomass, measured as chlorophyll or particulate organic
carbon, has been evident at the outfall since the discharge began, and none was
measured in 2017 (see further discussion in Section 6, Special Studies). Satellite
imagery (Figure 3-8) showed elevated levels of chlorophyll in January and February,
evidence of a winter-spring diatom bloom in process when the sampling season
began. Both satellite and survey data (Figure 3-9) showed elevated chlorophyll levels
across much of Massachusetts Bay in May and June, during the period that 
Alexandrium cell counts triggered special surveys. Summer chlorophyll levels
peaked during the Alexandrium special surveys in July, probably due to prolonged
upwelling during that time. High chlorophyll levels observed by satellite imagery in
November were coincident with a late fall bloom of centric diatoms Skeletonema spp.

14 
Figure 3-6. (Left) Surface- and bottom-water ammonium on March 25, 2017, during mixed
conditions. (Right) Cross-sections of concentrations throughout the water column along transects
connecting selected stations. Station N21 is directly over the outfall.

15 
Figure 3-7. (Left) Surface- and bottom-water ammonium on July 26, 2017, during stratified
conditions. (Right) Cross-sections of concentrations throughout the water column along transects
connecting selected stations . Station N21 is directly over the outfall.

16 
Figure 3-8. Moderate Resolution Imaging Spectroradiometer satellite imagery of surface
chlorophyll concentrations in 2017. These images are heavily weather dependent and do not
represent consistent intervals of time. The numbers and letters show the timing of the nine regular and
four special MWRA surveys.

17 
Figure 3-9. Areal chlorophyll by station in Massachusetts Bay in 2017. 
Levels of dissolved oxygen in surface and bottom waters were lower than typical
during the early months of monitoring in 2017. Typically, concentrations of
dissolved oxygen in bottom waters of Massachusetts Bay fall steadily from highest
concentrations in the spring to lowest in the fall, with recovery after the breakdown of
stratification in the fall. In 2017, as in 2016, that steady decline was interrupted by a
mixing event in June, which returned concentrations to levels about average for the
monitoring program. Typical mixing in the fall, which re-aerates the water column,
was delayed until November in 2017, resulting in relatively low dissolved oxygen
minima for the year. No measurements fell below the 6 mg per liter water quality
standard (Figure 3-10). 
Feb 18 Mar 25 Apr 24 May 16 Jun 13
Jun 21 Jun 28 Jul 08 Jul 16 Jul 26
Aug 23 Sep 06 Nov 01 
0 50 100 150 200 250
mg/m 2

18 
Figure 3-10. Near bottom water dissolved oxygen concentrations (milligrams per liter) at selected
stations in 2017 compared to prior years. Black points and line are results from individual surveys in
2017. Results from 1992–2016 are in blue: line is the 50 th percentile, dark shading spans the 25 th to 75 th
percentile, and light shading spans the range. For Stations N04, N18, and N21, historic data extend
later in the year than the current survey schedule. 
Phytoplankton Communities 
As in recent years, total observed phytoplankton abundances were somewhat low
throughout the year and throughout Massachusetts Bay (Figure 3-11, Libby et al.
2018). In the nearfield, overall abundance was about 65% of the long-term mean.
The results can be partially explained by a lack of a winter-spring diatom bloom
during the survey season and by absence of a bloom of the nuisance species 
Phaeocystis pouchetii . There were abundance peaks in the late spring and again in
late summer or fall throughout Massachusetts Bay. (The July Alexandrium special
surveys, which measured high levels of phytoplankton biomass, did not include
identification and enumeration of other phytoplankton groups.)

19 
Figure 3-11. Total phytoplankton abundance (million cells per liter) at selected stations in 2017
compared to prior years . Black points and line are results from individual surveys in 2017. Results
from 1992–2016 are in blue: line is the 50 th percentile, dark shading spans the 25 th to 75 th percentile,
and light shading spans the range. The green line on the map indicates the outfall. For Stations N04
and N18, historic data extend later in the year than the current survey schedule.
During the course of the monitoring program, there have been frequent spring 
Phaeocystis blooms, and after 2000, regional blooms occurred every year. Since
2013, Phaeocystis abundances have been relatively low, both in the nearfield and
throughout Massachusetts Bay, and this trend continued in 2017 (Figure 3-12). Peak
abundances occurred in Cape Cod Bay in April, at levels much lower than in a typical
bloom. Phaeocystis cells were found only in April and May and in just half the
Massachusetts Bay samples.

20 
Figure 3-12. Mean nearfield abundance of Phaeocystis pouchetii in 1992–2017 . Note log scale.
A bloom of the potentially toxic genus of pennate diatoms, Pseudo-nitzschia , closed
shellfish beds in Maine and Rhode Island in 2017. At abundance, Pseudo-nitzschia 
can cause amnesic shellfish poisoning (ASP). Pseudo-nitzschia was detected in
Massachusetts Bay in 2017, but at levels well below those that could cause ASP.
As noted above, abundance of the toxic dinoflagellate Alexandrium catenella 
exceeded 100 cells per liter during the June survey and triggered four special surveys.
On June 13, Alexandrium was detected at all ten stations, in abundances ranging from
one to 494 cells per liter (Figure 3-13). During the first special survey, elevated
abundances were detected at three northern stations near Cape Ann, with the highest
cell counts near the NERACOOS Buoy A01. Alexandrium continued to be present in
late June and early July, and the bloom ended by late July. The spatial and temporal
pattern of the bloom conformed to past understandings of Alexandrium bloom
dynamics, that is, that cells from established populations off the coast of Maine were
carried into Massachusetts Bay. No established Alexandrium populations have been
found within Massachusetts Bay.

21 
Figure 3-13. Nearfield abundance of Alexandrium catenella, 1992–2017. Note log scale. 
The 2017 Alexandrium bloom caused elevated levels of paralytic shellfish poisoning
(PSP) toxicity outside Massachusetts Bay, resulting in closure of shellfish beds in
Maine, New Hampshire, and into Massachusetts, as far south as Gloucester. No PSP
toxicity was detected in Massachusetts Bay. Winds and water circulation patterns at
the time of the bloom were not conducive for transporting Alexandrium cells from
offshore Massachusetts Bay to coastal shellfish beds. 
An unusual bloom of the dinoflagellate Karenia mikimotoi was detected in September
2017, with a maximum abundance of 337,800 cells per liter at Station N07, west of
the outfall. Overall, abundance was greatest at offshore stations and at the depths of
the chlorophyll maxima. Karenia had never before been detected by the monitoring
program and is not recorded as a member of the regional Gulf of Maine
phytoplankton community. A similar species has been found in Woods Hole,
Massachusetts, and in coastal Rhode Island, but that species is generally confined to
less saline waters. Its presence in 2017 was noted along the coast from Boston north
to Portland, Maine. While Karenia mikimotoi has been classified as potentially
harmful, its toxins have not been well-characterized, and there are no known threats
to human health.

22 
Zooplankton Communities 
Zooplankton communities continued to be dominated by the typical mix of copepod
nauplii, copepodites, and adults and by meroplankton, those animals that live only a
portion of their lives in the plankton. The small copepod Oithona similis continued to
dominate most samples, with Acartia spp. dominating in Boston Harbor and the much
larger Calanus finmarchicus sometimes dominating in offshore samples. Annual
peak abundances of total zooplankton were somewhat higher than those in 2016 but
did not reach the record highs observed in 2015 (Figure 3-14, Libby et al. 2018).
Those 2015 abundances were driven largely by the presence of meroplankton,
specifically bivalve veliger larvae, in July and August. Generally, zooplankton
abundance peaks in the summer, but somewhat unusually, in 2017, there were two
peaks, the first in May and the second in the late summer to early fall. The early
abundance peaks may have occurred in response to warm water temperatures.
Trend analysis has shown a sustained increase in total zooplankton abundance from
2006 through 2017, following a steady increase during 1995–2000 and a decrease
during 2000–2006. Opposite trends in phytoplankton abundance suggest zooplankton
grazing may be an important factor for both phytoplankton and zooplankton
communities. 
Figure 3-14. Total zooplankton abundance (10,000 animals per square meter) at selected stations
in 2017 compared to prior years. Black points and line are results from individual surveys in 2017.
Results from 1992–2016 are in blue: line is the 50 th percentile, dark shading spans the 25 th to 75 th
percentile, and light shading spans the range. The green line on the map indicates the outfall. For
Stations N04 and N18, historic data extend later in the year than the current survey schedule.

23 
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 the Stellwagen
Bank National Marine Sanctuary. Water column Station F22 is in Stellwagen Basin,
to the northwest of the sanctuary, and is considered to be representative of northern,
offshore conditions.
Ammonium levels have remained low at Station F22 since the outfall began to
discharge (see, for example, Figure 3-5, above). Levels have also remained low at
Station F06, located to the south of the outfall and offshore. In contrast, increased
ammonium levels have been detected in the nearfield, while decreases have been
detected at representative harbor and coastal stations.
Sampling at Station F22, as well as data from NERACOOS Buoy A01 and satellite
imagery, detected no unusual chlorophyll levels in offshore regions in 2017. No
effects on chlorophyll levels in the offshore were predicted, and none have been
measured.
Bottom-water dissolved oxygen concentrations at Station F22 were healthy
throughout 2017, and both survey observations and data from NERACOOS Buoy
A01, located within the sanctuary, showed the typical decline during the stratified
season (Figure 3-15). Data from the buoy also documented the return to oxygenated
conditions as a result of fall mixing events. 
Figure 3-15. Dissolved oxygen at the NERACOOS Buoy A01 and the deepwater measurements at
Station F22 in Stellwagen Basin, and at Station N18 in the nearfield. The buoy values are daily
means.

24 
Boston Harbor 
Water quality in Boston Harbor has improved during the past 20 years, and those
improvements were sustained in 2017. MWRA’s in-house Boston Harbor monitoring
program confirmed that harbor-wide concentrations of total nitrogen and phosphorus
remained low, as they have since effluent discharges to the harbor ended.
Perhaps the most dramatic improvement in Boston Harbor has been the decrease in
ammonium levels (see Figure 3-6, above, and Section 6, Special Studies).
Ammonium concentrations dropped precipitously when the effluent discharge was
diverted from the harbor to Massachusetts Bay in 2000 and have remained low. In
contrast, ammonium levels have increased only at stations closest to the
Massachusetts Bay outfall, such as Station N18. However, because of increased
dilution at the offshore outfall, those increases in Massachusetts Bay have been
substantially less than the concurrent decreases in ammonium concentrations in the
harbor. Levels remain unchanged at stations farther from the outfall.
Peak abundance of Acartia spp., the dominant zooplankton species in Boston Harbor,
occurred earlier in 2017 than had been observed in the 1990s. Since the diversion of
sewage effluent from the harbor, Acartia abundances have peaked earlier, and warm
temperatures also may have contributed to the earlier growth in 2017.

25 
Contingency Plan Thresholds 
All water-quality parameters were within normal ranges during 2017. There was one
Contingency Plan caution-level threshold exceedance for a nuisance algae measure, 
Alexandrium catenella abundance (Table 3-1), which has been reviewed and is not
thought to have been caused or exacerbated by the outfall. In 2017, EPA and
MassDEP approved MWRA’s proposal, endorsed by OMSAP, to discontinue the
seasonal thresholds for Phaeocystis pouchetii . 
Table 3-1. Contingency Plan threshold values and 2017 results for water-column monitoring. 
Parameter Baseline Caution Level Warning Level 2017 Results
Dissolved oxygen* 
Nearfield
concentration 6.05 mg/L <6.5 mg/L <6.0 mg/L 7.33 mg/L
Nearfield percent
saturation 65.3% <80% <75% 78.9%
Stellwagen
concentration 6.23 mg/L <6.5 mg/L <6.0 mg/L 7.36 mg/L
Stellwagen percent
saturation 67.2% <80 <75% 77.2%
Nearfield depletion
rate 0.024 mg/L/d >0.037 mg/L/d >0.049 mg/L/d 0.013 mg/L/d 
Chlorophyll 
Annual 72 mg/m 2 >108 mg/m 2 >144 mg/m 2 77 mg/m 2
Winter/spring 50 mg/m 2 >199 mg/m 2 None 88 mg/m 2
Summer 51 mg/m 2 >89 mg/m 2 None 58 mg/m 2
Autumn 90 mg/m 2 >239 mg/m 2 None 99 mg/m 2 
Nuisance algae nearfield Pseudo-nitzschia 
Winter/spring 6,735 cells/L >17,900 cells/L None 68 cells/L
Summer 14,635 cells/L >43,100 cells/L None 273 cells/L
Autumn 10,050 cells/L >27,500 cells/L None 1,780 cells/L 
Nuisance algae nearfield Alexandrium catenella 
Any nearfield
sample
Baseline maximum
163 cells/L >100 cells/L None
494 cells/L
caution-level
exceedance
PSP toxin extent NA New incidence None No new incidence 
*Dissolved oxygen caution and warning levels represent numerical criteria, with the caveat “unless
background conditions are lower.” Results are therefore compared to the baseline rather than to the
caution and warning levels. PSP = paralytic shellfish poisoning, NA = not applicable

26 
4. Sea Floor 
Seafloor monitoring in 2017 included sampling and analysis of soft-bottom sediment
conditions, effluent tracers, chemical contaminants, and infauna at 14 stations,
sediment-profile imaging at 23 stations, and video assessment of 23 hard-bottom
stations (Figures 4-1 to 4-3). Anthropogenic contaminants analyses and assessment
of hard-bottom habitats occur at three-year intervals, including 2017. 
Figure 4-1. Soft-bottom monitoring stations. Also shown are the instrumented buoys, the MWRA
outfall diffuser, and the Stellwagen Bank National Marine Sanctuary.

27 
Figure 4-2. Sediment-profile imaging stations. Also shown are the NDBC buoy and the MWRA
outfall diffuser. SPI = sediment-profile imaging

28 
Figure 4-3. Hard-bottom stations. Also shown are the instrumented buoys, the MWRA outfall
diffuser, and the Stellwagen Bank National Marine Sanctuary.

29
Soft-bottom sediment sampling was completed in August 2017, with samples
analyzed for grain-size distribution, total organic carbon, the effluent tracer 
Clostridium perfringens spores, metal and organic contaminants, and benthic infauna.
The 14 stations included one station in the “transition” area, located between Boston
Harbor and the nearfield stations (FF12); four nearfield stations located within two
kilometers of the outfall (NF13, NF14, NF17, NF24); six nearfield stations located
within Massachusetts Bay but farther than two kilometers from the outfall (NF04,
NF10, NF12, NF20, NF21, NF22); and three farfield reference stations located in
Massachusetts Bay and Cape Cod Bay (FF01A, FF04, and FF09). For the purposes
of threshold testing, “nearfield” includes the transition station, as well as both
nearfield groups, for a total of eleven stations.
Sediment-profile imaging was also completed in August 2017. Triplicate images
from 23 stations were used to measure the apparent reduction-oxidation (redox)
potential discontinuity (RPD) depth, an approximation of the depth of oxygen
penetration into the sediments.
Hard-bottom surveys were completed in late June 2017, including 480 minutes of
analog video and GoPro, Inc. HD video with simultaneous still photographs. The
remotely operated vehicle, a Benthos, Inc. Mini Rover, was somewhat noisier but
more reliable than equipment used in the past, conceivably affecting sightings of fish
and other mobile animals. 
Sediment Characteristics and Tracers 
As in past years, sediment grain-size distributions in 2017 varied broadly among
stations, ranging from silt and clay at some stations to almost entirely sand at others
(Nestler et al. 2018). Sediment grain-size distributions have remained generally
consistent at individual stations over the years of the monitoring program. Changes
have been mostly associated with large storms with wave-driven currents sufficient to
re-suspend bottom sediments.
As in past years since the offshore outfall began to discharge, it was possible to detect
elevated levels of effluent tracer Clostridium perfringens spores at the stations located
closest to the outfall (Figure 4-4). Clostridium are anaerobic bacteria found in
mammalian (including human) digestive tracts and that form persistent spores in
oxygen-rich conditions. Statistical analyses have shown that the increases close to
the discharge are consistent with predictions made during planning for the outfall.
Percent organic carbon content analyses were also consistent with past results (Figure
4-5). In general, stations with finer sediments, such as Station FF04 within
Stellwagen Basin, have higher mean total organic carbon concentrations, while
stations with coarser sediments, such as Station NF17 just to the south of the outfall,
have lower concentrations. Total organic carbon concentrations continue to show no
signs of organic enrichment from the effluent discharge, even at stations closest to the
outfall.

30 
Figure 4-4. Concentrations of Clostridium perfringens spores, corrected for sediment grain size,
in 2017. 
Figure 4-5. Total organic carbon at selected stations, 1992–2017. FF01A is the northern reference
station; FF04 is within Stellwagen Basin; NF12 is >2km northwest of the outfall; NF17 is in close
proximity and southwest of the outfall, where effects of the discharge might be expected. 
Total Organic Carbon (%)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Outfall Startup
1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 2017
FF01A
FF04
NF12
NF17

31 
Sediment Contaminants 
Sediment samples analyzed for toxic metals and organic contaminants continued to
provide no evidence of any effect of the Massachusetts Bay outfall. Higher levels of
contaminants were associated with fine-grained sediments and with proximity to
Boston Harbor, which is the historic contaminant source. No sample from any station
located within two kilometers of the outfall had contaminant levels exceeding
conservative federal benchmarks that define toxicity in marine sediments. Those
benchmarks, known as “effects range low” or ER-L, represent concentrations below
which toxicity has rarely been observed.
Concentrations of most sediment contaminants in 2017 were lower than those in
earlier years, and in 22 of the 26 sediment contaminants for which there are
Contingency Plan thresholds, the average 2017 nearfield concentrations were lower
than the lowest annual averages measured during baseline monitoring in 1992–2000.
For some contaminants, including the toxic metals chromium and lead (not shown),
concentrations in sediments from near the outfall were the lowest measured through
the duration of the monitoring program. Concentrations of organic contaminants also
remained low. For some organic contaminants, such as PCBs (Figure 4-6), the results
have shown continued long, slow declines, as was predicted when those products
were banned in the 1970s. 
Figure 4-6. Mean concentrations of total PCBs at four areas in Massachusetts Bay, 1992–2017. 
NF = nearfield, Tran = transition zone, FF = farfield 
Total PCB Concentration (ng/kg dry)
0
5
10
15
20
25
30
35 Outfall Startup
1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 2017
NF <2km
NF >2km
Tran
FF

32 
Soft-bottom Communities 
The 14 soft-bottom samples collected and analyzed in 2017 yielded 23,038
organisms, classified into 188 species and 26 other discrete taxonomic groups
(Nestler et al. 2018). Total abundances of organisms were higher in 2017 than in
2016 throughout the region, and as in recent years, total abundance was highest at the
only station within the transition area located between the harbor and the nearfield.
The mean numbers of species per sample were also higher in 2017 than in 2016. 
Community analyses showed no effects of the outfall on relative abundance or
community composition. A series of multivariate analyses assessed spatial and
temporal patterns in the soft-bottom benthic communities and found no particular
species or type of community specifically associated with the outfall.
As in past years, a cluster analysis identified two main infaunal assemblages, with an
outlier at Station FF04, which is offshore in the deeper waters of Stellwagen Basin
and has consistently had a distinct community. Ordination analysis continued to
show no indication of any relation of species composition to proximity to the outfall,
with nearfield and farfield stations represented in both major assemblages (Figure 4-
7). Analyses further continued to demonstrate that variations in species distributions
largely followed differences in sediment type and depth (Figure 4-8). 
Figure 4-7. Ordination plot of 2017 Massachusetts Bay samples by location. Each point on the plot
represents one of the 14 stations; similarity of species composition is indicated by proximity of points
on the plot. Faunal assemblages (Groups I-II, and sub-groups) identified by cluster analysis are circled
on the plot. NF = nearfield, Tran = transition zone, FF = farfield

33 
Figure 4-8. Percent fine sediments (top) and depth (bottom) superimposed on the ordination plot
of the 2017 infauna samples. Each point on the plot represents one of the 14 stations; similarity of
species composition is indicated by proximity of points on the plot. Faunal assemblages (Groups I-II,
and sub-groups) identified by cluster analysis are circled on the plot.

34 
Sediment-profile Imaging 
Sediment-profile images continued to show no adverse effects of the outfall (Nestler
et al. 2018). Rough topography and physical processes in the nearfield remain the
more important factors in determining benthic habitat quality.
As in past years, the average RPD depth (the depth to which oxygen penetrates into
sediments as determined by color changes) was deeper than average RPD depths
measured during the baseline period (Figure 4-9), with a mean depth of 4.4 cm for all
stations where RPDs could be measured. At nine of the 23 stations, the RPD was
deeper than the bottom of the images, either because of sandy sediments or sediment
compaction. The environmental concern before the outfall began to discharge was
that the RPD would become shallower, due to increases in sediment organic matter
causing stress on sensitive sediment-dwelling organisms. A deeper RPD continued to
indicate there have been no adverse effects from the discharge. 
Figure 4-9. Annual apparent color RPD depth for data from nearfield stations. The mean RPD
has been deeper than in baseline years, continuing to indicate that there has been no adverse effect
from the discharge.
Monitoring has shown that physical rather than biological processes, such as storms
and storm-induced sediment transport, can be primary stressors on the Massachusetts
Bay sea floor. Physical processes were particularly evident in the first years of
sediment-profile monitoring in Massachusetts Bay, as large storms occurred in
October 1991 and December 1992. Strong storms in the spring of 2001, just after the
Massachusetts Bay outfall startup, also diminished the role of biological processes in
structuring the sediments. While biological processes are evident in some years, the
active hydrodynamic conditions in Massachusetts Bay have promoted rapid effluent
dispersion and prevented the accumulation of organic matter, which could have led to
environmental degradation.

35 
Hard-bottom Communities 
As in past years, the video surveys of the hard-bottom habitats in the vicinity of the
outfall showed that the communities are spatially diverse, but have remained
relatively constant through time. Coraline algae, the most abundant taxonomic group,
has decreased in abundance, first in northern stations, and now throughout the region.
Abundance of upright algae, which were common but patchily distributed,
particularly at the northern reference stations, has also declined over time. During
2014 and 2017, wide areas of both living and dead barnacles covered some surfaces.
The active and unopened diffuser heads included in the monitoring program have
continued to support healthy and relatively stable communities (Nestler et al. 2018).
In 2017, the active diffuser was almost entirely covered by a dense stand of the frilled
anemone Metridium senile , barnacles, and mussels (Figure 4-10). The inactive
diffuser head was similarly covered by anemones, barnacles, and also the sea peach
tunicate Halocynthia pyriformis . 
Figure 4-10. Lush growth of sea anemones and barnacles on an active diffuser head in 2017. 
Photo credit Barbara Hecker/Normandeau Associates .

36
Since 2007, the hard-bottom surveys have documented severe disturbance caused by
deep-draft tankers anchoring at the northern reference site, Station T7-1, and evidence
of disturbance was found again in 2017 (Figure 4-11). The disturbance includes
overturned boulders and anchor scars. 
Figure 4-11. Overturned boulder at a northern reference station, where deep-draft tankers have
been observed to anchor. Photo credit Barbara Hecker/Normandeau Associates. 
Stellwagen Bank National Marine Sanctuary 
The NPDES permit to discharge from Deer Island Treatment Plant into
Massachusetts Bay requires annual reports on results relevant to the Stellwagen Bank
National Marine Sanctuary. MWRA’s deepwater reference Station FF04 lies within
the depositional part of the sanctuary, in Stellwagen Basin, where long-term
accumulation of pollutants and their effects could be detected if they were to occur.
Station FF04 is typical of the deep waters offshore from the outfall, representative of
a number of stations monitored in earlier years of the program, and it continues to
support an infaunal community typical of what had been found at the larger suite of
deepwater stations. The deepwater stations, including Station FF04, have always
shown distinct differences from those found at shallower stations, probably due to
their depth, their fine-grained sediments, and their distance from shore.
Superimposing percent grain size and depth on the ordination plot for 2017 infauna
samples continued to show these differences (see Figure 4-8, above).

37 
Boston Harbor 
While the chemistry and biology of the Massachusetts Bay sea floor have not been
affected by the relocated outfall, conditions have greatly improved and continue to
improve in Boston Harbor, a result of the Boston Harbor Project, more recent
enhancements to treatment, and remediation of combined sewer overflows. MWRA
has conducted ongoing seafloor monitoring in Boston Harbor since 1991. Annual
sediment and infauna samples are taken from nine stations (Figure 4-12). 
Figure 4-12. Soft-bottom sampling stations in Boston Harbor. Nine stations are sampled each year
for sediment characteristics and infauna analyses.
Concentrations of total organic carbon (not shown) and Clostridium perfringens 
spores (Figure 4-13) have declined over time. Infaunal diversity has increased,
reflecting continued improvement in habitat conditions. An ordination plot of Boston
Harbor infaunal samples shows a separate community in stations in the outer harbor
and a most unique fauna at Station T04 at the mouth of Savin Hill Cove, a sheltered
cove that tends to accumulate pollutants more than surrounding areas (Figure 4-14).
Sediment-profile imaging (not shown) has also confirmed an inner- to outer-harbor
gradient.

38 
Figure 4-13. Mean concentrations of Clostridium perfringens spores at selected harbor stations,
1991–2017. 
Figure 4-14. Ordination plot of 2017 Boston Harbor infauna samples. Two samples are collected
per station. Group I comprises stations from the outer harbor; Group II includes Station C019 in the
inner harbor and Station T07 in Quincy Bay. The assemblage most different from others was found at
Station T04, at the mouth of Savin Hill Cove, in Dorchester Bay. 
0
100
200
300
400
500
600
700
800
900
1000
1100
1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 2017
T01
T02
T04
T08
C019

39 
Contingency Plan Thresholds 
There were no seafloor Contingency Plan threshold exceedances in 2017 (Table 4-1).
Levels of PAHs, other organic contaminants, and metals remained well below
thresholds. Average RPD depth was deeper than the thresholds and also deeper than
baseline values.
Diversity and other benthic community parameters were also within Contingency
Plan limits. Until 2017, the community-parameter thresholds had upper as well as
lower bounds. In 2017, EPA and MassDEP approved MWRA’s proposal, endorsed
by OMSAP, to discontinue the upper bounds as Contingency Plan thresholds. In past
years, even when diversity levels exceeded the upper bounds of the caution-level
thresholds, there were no indications that environmental conditions had worsened or
that there had been any effect of the outfall. Increased diversity is typically
considered a good outcome in benthic habitats.

40 
Table 4-1. Contingency Plan threshold values and 2017 results for sea-floor monitoring. 
Parameter Baseline Caution Level Warning Level 2017 Results
Polycyclic aromatic hydrocarbons (PAHs) (ng/g dry weight) 
Acenaphthene 22.7 – 43.5 None 500 24.8
Acenaphylene 30.3 – 43.1 None 640 24.5
Anthracene 101 – 159 None 1,100 86.2
Benzo(a)anthracene 206 – 302 None 1,600 230
Benzo(a)pyrene 204 – 298 None 1,600 145
Chrysene 164 – 296 None 2,800 191
Dibenzo(a,h)anthracene 27.8 – 38.3 None 260 34.1
Fluoranthene 422 – 621 None 5,100 308
Fluorene 35.5 – 66.6 None 540 29.1
Naphthalene 53.6 – 103 None 2,100 36.4
Phenanthrene 273 – 431 None 1,500 220
Pyrene 412 – 579 None 2,600 281
Total HMW PAH 2,790 – 3,850 None 9,600 2,450
Total LMW PAH 1,390 – 1,630 None 3,160 870
Total PAHs 4,180 – 5,400 None 44,792 3,320 
Other organic contaminants (ng/g dry weight) 
p,p’-DDE 0.386 – 1.00 None 27 0.2
Total DDTs 2.51 – 5.69 None 46.1 0.3
Total PCBs 10.2 – 20.2 None 180 4.8 
Metals ( μ g/g dry weight) 
Cadmium 0.0727 – 0.185 None 9.6 0.1
Chromium 59.2 – 79.9 None 370 27
Copper 19.1 – 25.2 None 270 11
Lead 41.1 – 46.3 None 218 23
Mercury 0.159 – 0.353 None 0.71 0.2
Nickel 15.7 – 17.2 None 51.6 9.4
Silver 0.335 – 0.485 None 3.7 0.1
Zinc 49.5 – 57.5 None 410 33 
Sediment parameters 
RPD depth NA <1.18 cm None 4.12 cm 
Benthic community parameters 
Species per
sample NA <42.99 None 63.5
Fisher’s log-series
alpha NA <9.42 None 13.2
Shannon diversity NA <3.37 None 3.93
Pielou’s evenness NA <0.57 None 0.658
% opportunists NA >10% >25% 1.23% 
HMW = high molecular weight; LMW = low molecular weight
NA = not applicable; RPD = redox potential discontinuity

41 
5. Fish and Shellfish 
Each year, MWRA monitors the health of winter flounder from the Massachusetts
Bay outfall site, Deer Island Flats near the former Boston Harbor outfall, off
Nantasket Beach just outside the harbor, and eastern Cape Cod Bay (Figure 5-1).
Every three years, most recently in 2015, monitoring includes chemical analyses of
flounder fillets and liver, lobster meat and hepatopancreas, and cage-deployed blue
mussels. Sampling and analysis in 2017 were limited to winter flounder health
assessments. 
Figure 5-1. Fish-and-shellfish monitoring stations. Also shown are the two instrumented buoys, the
MWRA outfall diffuser, and the boundaries of the Stellwagen Bank National Marine Sanctuary.
Winter flounder stations are DIF = Deer Island Flats, ECCB = Eastern Cape Cod Bay, NB = Nantasket
Beach, OS = Outfall Site

42 
Flounder Health 
Annual flounder monitoring focuses on external condition and the presence of early
liver disease and tumors (neoplasia). In April 2017, 50 sexually mature flounder
were collected from each of the four sites: Deer Island Flats, off Nantasket Beach, the
outfall site, and eastern Cape Cod Bay (Moore et al. 2017). Catch per unit effort was
highest at eastern Cape Cod Bay. Abandoned fishing gear, sometimes referred to as
“ghost” gear, continued to interfere with catches, particularly in muddy depressions at
the outfall and at Deer Island Flats (Figure 5-2). 
Figure 5-2. “Ghost” lobstering gear brought up in MWRA flounder trawls can make collection
difficult, especially at Deer Island Flats and near the outfall site. 
Across the sites, mean age of fish ranged from 4.1 to 5.7 years, and standard length
ranged from 272 to 316 millimeters, within the ranges for the monitoring
program. As is common throughout northeast coastal populations, the catches were
dominated by females (see Moore et al. 2016 for analysis of sex ratios in MWRA and
other northeast flounder studies).
Measures of external condition continued to suggest improved conditions in
comparison to studies from the 1980s and 1990s, and there continued to be no
evidence of neoplasia. No fish had blind-side ulcers. Incidence of fin erosion, an
indicator of ammonium and other pollutants, was highest in fish from Deer Island
Flats and lowest in fish from near the outfall. Tumors have not been observed by the
monitoring program since 2004 and have never been observed in fish taken from the
outfall site.

43
The incidence of centrotubular hydropic vacuolation (CHV), a mild condition
associated with exposure to contaminants and a neoplasia precursor, remained lower
than the baseline observations (Figure 5-3). Incidence of CHV remained low in fish
from the outfall site and fell in fish from Deer Island Flats, after an increase in 2015
and 2106. Average severity of CHV (not shown) also remained lower than baseline
levels. Severity of CHV in fish from Deer Island Flats has greatly fallen since 1991;
levels of severity have been relatively constant throughout the program since 2005. 
Figure 5-3. Annual prevalence of centrotubular hydropic vacuolation (CHV), 1991–2017. 
DIF = Deer Island Flats, ECCB = Eastern Cape Cod Bay, NB = Nantasket Beach, OS = Outfall Site 
Contingency Plan Thresholds 
There was no Contingency Plan threshold exceedance for the only parameter
measured for 2017 (Table 5-1). Incidence of CHV, the most common indicator of
liver disease in winter flounder of the region, was 8% in fish taken from the vicinity
of the outfall, far lower than the 44.9% caution threshold or 24.4% baseline average. 
Table 5-1. Contingency Plan threshold value and 2017 results for fish-and-shellfish monitoring. 
Parameter Baseline Caution Level Warning Level 2017 Results
Flounder disease 
Liver disease
(CHV) 24.4% >44.9% None 8% 
CHV = centrotubular hydropic vacuolation 
CHV Prevalence (%)
0
10
20
30
40
50
60
70 Outfall Startup
1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 2017
DIF
ECCB
NB
OS

44 
6. Special Studies
Spotlight on Eutrophication 
Besides monitoring the effluent and the water column, sea floor, and fish and
shellfish in Massachusetts Bay, MWRA conducts special studies in response to
specific permit requirements, scientific questions, and public concerns. This report’s
special studies section focuses on eutrophication, which was one of the largest
concerns when the outfall was planned and sited.
Nutrients, especially nitrogen and phosphorus, are essential for the growth of all
plants, including marine phytoplankton and seagrasses. In excess, nutrients,
particularly nitrogen, can overstimulate marine phytoplankton growth, and when
those algal cells die, they sink to the bottom, using up oxygen as they decompose.
Excess phytoplankton growth and depressed levels of dissolved oxygen are the major
attributes of eutrophication.
Prior to the Boston Harbor Project, Boston Harbor suffered from very eutrophic
conditions. Alleviating eutrophication in the harbor without initiating eutrophic
conditions in Massachusetts Bay was a primary goal of the wastewater treatment
improvements and outfall relocation.
Scientists and modelers were confident conditions would improve in Boston Harbor
without causing harm to Massachusetts Bay. Along with the greater dilution at the
Massachusetts Bay outfall, discharging into deeper waters would trap the effluent
below the summer pycnocline, in darker waters where less phytoplankton growth can
occur. Managers and regulators, however, recognized there would be ongoing
questions and concerns about whether Massachusetts Bay could assimilate the
thousands of tons of nitrogen that would be discharged each year. Those concerns
were recognized in MWRA’s monitoring and contingency plans, and there are now
26 years of baseline and monitoring results to confirm the predictions made during
outfall siting and design. All results to date confirm that the Boston Harbor Project
dramatically decreased eutrophication in Boston Harbor without causing
eutrophication in Massachusetts Bay.

45 
Nitrogen Inputs 
One reason for concerns that excess nitrogen in MWRA effluent might cause
eutrophication in the bay was that nutrient inputs to the treatment plant were expected
to increase over time. Also, the dissolved forms of nutrients taken up by
phytoplankton were the only components of sewage entering the treatment plant that
were not expected to be reduced substantially by secondary treatment. In fact, inputs
to the plant have not increased as much as anticipated (see Figure 2-5), and the plant
has performed better than expected, removing approximately 30% of the nitrogen
from influent rather than the 10% that had been projected.
Monitoring has also shown that overall total nitrogen inputs to Massachusetts Bay
have not increased in response to the Boston Harbor Project. Monitoring in the
1990s, prior to the outfall relocation, showed almost all the effluent nitrogen entering
Boston Harbor eventually reached the bay. Studies have also found more than 90%
of the nitrogen entering Massachusetts Bay originates from the wider water mass of
the Gulf of Maine; only about 3% of the total nitrogen inputs to the bay could be
attributed to the outfall discharge. 
Nitrogen Concentrations in the Water Column 
At the onset of the Boston Harbor Project, concentrations of ammonium, the form of
nitrogen most available to phytoplankton, were expected to fall in Boston Harbor and
increase in the immediate vicinity of the outfall. Those predictions were correct—
ammonium concentrations in Boston Harbor plummeted when the outfall was
relocated to Massachusetts Bay, while increased concentrations were measured in the
immediate vicinity of the outfall (Figure 6-1). Those increases in ammonium
concentrations resulting from the Massachusetts Bay discharge have been limited in
both magnitude and geographic extent. Larger increases have been confined to areas
within a kilometer of the discharge, and detectable increases have been observed only
within 10 to 20 kilometers of the outfall.

46 
Figure 6-1. Ammonium concentrations at selected stations in Boston Harbor and Massachusetts
Bay during harbor discharge and bay discharge years. 
Phytoplankton Biomass 
Decreasing phytoplankton biomass in Boston Harbor is major evidence for decreased
eutrophication. Through the 1990s, when effluent discharge was to Boston Harbor,
one symptom of eutrophication was seen in strong summertime algal blooms, which
often discolored the water and persisted for weeks. Summertime concentrations of
chlorophyll, a measure of algal biomass, decreased in Boston Harbor after the
discharge moved offshore (Figure 6-2). No changes in summertime chlorophyll
levels have been seen in Massachusetts Bay, where summer chlorophyll levels are
naturally lower than in the shallower waters of Boston Harbor. There have also been
no indications of phytoplankton species changes in the bay, and no blooms of toxic or
noxious species have been attributed to or exacerbated by the offshore outfall.

47 
Figure 6-2. Chlorophyll concentrations at selected stations in Boston Harbor and Massachusetts
Bay during harbor discharge and bay discharge years. 
Dissolved Oxygen 
One of the biggest public concerns about the Massachusetts Bay outfall was that it
would result in harmfully low levels of dissolved oxygen in bay bottom waters in the
summer. Low levels of dissolved oxygen can be found even in relatively pristine
waterbodies. However, while dissolved oxygen levels have improved in Boston
Harbor, they have remained unchanged in Massachusetts Bay (Figure 6-3). Instead,
monitoring has shown the dissolved oxygen concentrations and saturation in
Massachusetts Bay are controlled by physical factors, such as temperature, salinity,
and the timing of the fall breakdown of stratification of the water column.

48 
Figure 6-3. Annual minimum dissolved oxygen concentrations at selected stations in Boston
Harbor and Massachusetts Bay during harbor discharge and bay discharge years. 
Sea Floor Communities 
Monitoring has also shown improvements to the bottom habitats of Boston Harbor
without any measures of harm to Massachusetts Bay. At muddy stations within
Boston Harbor, where changes would be most likely to be detected, total organic
carbon concentrations have decreased (Figure 6-4) and species richness (number of
species per sample) has increased (Figure 6-5). Because bottom-dwelling organisms
have responded to other factors besides eutrophic conditions, and some improvements
occurred prior to relocation of the outfall, increases in the harbor in species richness
since the beginning of the monitoring have been larger than shown in Figure 6-5. No
increases total organic carbon concentrations or decreases in species richness at
muddy stations in Massachusetts Bay have occurred, even at Station NF24, the station
closest to the Massachusetts Bay outfall. 
106
141
F22
N18
F15
F06 
Harbor
Discharge
Bay
Discharge 
Harbor
Discharge
Bay
Discharge 
Harbor
Discharge
Bay
Discharge 
Harbor
Discharge
Bay
Discharge 
Harbor
Discharge
Bay
Discharge 
Harbor
Discharge
Bay
Discharge

49 
Figure 6-4. Total organic carbon (TOC) concentrations at muddy stations in Boston Harbor and
Massachusetts Bay during harbor discharge and bay discharge years. Station labels have been
shortened for clarity on the map: 3 = T03, 4 = FF04, 6 = T06, 12 = NF12, 22 = NF22, 24 = NF24 
4
22
24
12
3
6 
0
1
2
3
4 
3 
September 2000
Harbor
Discharge
Bay
Discharge
Carbon [%]
0
1
2
3
4 6 
September 2000
Harbor
Discharge
Bay
Discharge
Carbon [%]
0
1
2
3
4
September 2000 12 
Carbon [%] 
Harbor
Discharge
Bay
Discharge
0
1
2
3
4
Harbor
Discharge
Bay
Discharge
Carbon [%] 22 
September 2000
0
1
2
3
4 
24 
Harbor
Discharge
Bay
Discharge
Carbon [%]
September 2000
0
1
2
3
4 
4 
Harbor
Discharge
Bay
Discharge
September 2000 
Carbon [%]

50 
Figure 6-5. Species richness at muddy stations in Boston Harbor and Massachusetts Bay during
harbor discharge and bay discharge years. Station labels have been shortened for clarity on the
map: 3 = T03, 4 = FF04, 6 = T06, 12 = NF12, 22 = NF22, 24 = NF24 
4
22
24
12
3
6 
0
20
40
60
80 12 
Number of Species
Harbor
Discharge
Bay
Discharge
September 2000 
0
20
40
60
80 3 
Harbor
Discharge
Bay
Discharge
Number of Species
September 2000
0
20
40
60
80 
24 
Number of Species
Harbor
Discharge
Bay
Discharge
September 2000
0
20
40
60
80 6 
Number of Species
Harbor
Discharge
Bay
Discharge
September 2000
0
20
40
60
80 22 
Number of Species
September 2000
Harbor
Discharge
Bay
Discharge
0
20
40
60
80 4 
Harbor
Discharge
Bay
Discharge
Number of Species
September 2000

51
Another indication of decreased eutrophication and habitat enrichment, eelgrass beds,
which provide a healthy habitat for fish and shellfish, have begun to grow in parts of
Boston Harbor where seagrasses had not previously been seen (Figure 6-6). No
concurrent habitat degradation has occurred in Massachusetts Bay, where monitoring
has shown that physical processes, such as storms and storm-induced sediment
transport, are the primary stressors on the environment. Bottom-dwelling animal
communities have remained healthy and diverse, with no indication of eutrophic
effects. 
Figure 6-6. Eelgrass in Boston Harbor is thriving in areas where it was not previously seen,
providing habitat for flounder and other fish and shellfish. Photo credit Phil Colarusso, EPA.

52 
References 
EPA. 1988. Boston Harbor wastewater conveyance system. Final Supplemental
Environmental Impact Statement (SEIS). Boston, Massachusetts. United States
Environmental Protection Agency Region I.
Libby PS, Borkman DG, Geyer WR, Turner JT, Costa AS, Wang J, Codiga D. 2018.
2017 water column monitoring results. Boston: Massachusetts Water Resources
Authority. Report 2018-04.
MWRA. 2001. Massachusetts Water Resources Authority Contingency Plan,
Revision 1. Boston: Massachusetts Water Resources Authority. Report 2001-ms-071.
47 p.
MWRA. 2010. Ambient monitoring plan for the Massachusetts Water Resources
Authority effluent outfall, Revision 2, July 2010. Boston: Massachusetts Water
Resources Authority. Report 2010-04. 107 p.
Moore MJ, McElroy A, Geoghegan P, Siskey M, Pembroke AE. 2016. Flounder
monitoring report: 2015 results. Boston: Massachusetts Water Resources Authority.
Report 2016-05. 40 p.
Moore M, Nestler EC, Rutecki DA. 2017. Flounder monitoring report: 2017 results.
Boston: Massachusetts Water Resources Authority. Report 2017-14. 19 p.
Nestler EC, Diaz RJ, Hecker B. 2018. Outfall benthic monitoring report: 2017 results.
Boston: Massachusetts Water Resources Authority. Report 2018-05.
Werme C, Rex AC, Hunt, C. 2012. Outfall monitoring overview background: 2012
update. Boston: Massachusetts Water Resources Authority. Report 2012-02. 59 p.
Zhao L, Beardsley RC, Chen C, Codiga DL, Wang L. 2017. Simulations of 2016
hydrodynamics and water quality in the Massachusetts Bay system using the Bays
Eutrophication Model. Boston: Massachusetts Water Resources Authority. Report
2017-13. 111 p.

53 
List of Acronyms 
ASP Amnesic shellfish poisoning
cBOD Carbonaceous biochemical oxygen demand
CHV Centrotubular hydropic vacuolation
DDT Dichlorodiphenyltrichloroethane
DIF Deer Island Flats
DO Dissolved oxygen
ECCB Eastern Cape Cod Bay
EPA U.S. Environmental Protection Agency
ER-L Effects range low
FF Farfield
HMW High molecular weight
IAAC Inter-Agency Advisory Committee
LC50 50% mortality concentration
LMW Low molecular weight
MassDEP Massachusetts Department of Environmental Protection
MGD Million gallons per day
MODIS Moderate Resolution Imaging Spectroradiometer
MWRA Massachusetts Water Resources Authority
NA Not analyzed/not applicable
NACWA National Association of Clean Water Agencies
NB Nantasket Beach
NERACOOS Northeastern Regional Association of Coastal Ocean Observing Systems
NF Nearfield
NDBC National Data Buoy Center
NOEC No observed effects concentration
NPDES National Pollutant Discharge Elimination System
OMSAP Outfall Monitoring Science Advisory Panel
OS Outfall site
PAH Polycyclic aromatic hydrocarbon
PCB Polychlorinated biphenyl
PIAC Public Interest Advisory Committee
PSP Paralytic shellfish poisoning
RPD Redox potential discontinuity
SD Standard deviation
SEIS Supplemental environmental impact statement
SPI Sediment-profile imagery

Massachusetts Water Resources Authority
100 First Avenue • Boston, MA 02129
www.mwra.com
617-242-6000