and Massachusetts Bay
Alexandrium tamarense typically may
bloom during April to June and can cause paralytic shellfish poisoning,
known as PSP or red tide; it has been periodically found in Massachusetts
since the 1970s. Toxicity is generally not found in shellfish until
much higher cell counts are seen in the overlying waters. To calculate
the threshold we determine the maximum number of total Alexandrium
cells (Alexandrium tamarense plus unidentified Alexandrium
spp.) seen in any nearfield sample. (See also: Red Tide)
A geologic formation that holds groundwater.
Many non-MWRA communities rely on wells drilled into aquifers for
their drinking water supplies.
See Enterococcus and Fecal Coliform
Diversity Thresholds: One way
to track the status of a marine ecosystem is to measure the diversity
of the organisms in the communities that comprise the ecosystem,
such as the soft-sediment communities (benthic infauna) in the sediment.
The benthic diversity thresholds are intended to indicate whether
there is a change from baseline conditions (either toward more or
less diversity) now that the outfall is discharging. Of the dozens
of statistical measures of diversity that have been developed by
researchers over the past few decades, four are tracked within the
MWRA monitoring program to show possible changes in diversity.
Two of these indices, total number of species
per sample and Fisher's log-series alpha, are measures of species
richness (how many species are present). Both measures track species
richness while total species per sample is easy to describe to a
general audience, Fisher's Log-series alpha has a theoretical grounding
favored by some researchers. The other two diversity indices tracked
by MWRA's monitoring are among those most commonly used by ecologists
in many environments. Pielou's J' is a measure of how evenly individuals
are distributed among species in a community. Samples where most
species have about the same number of individuals have high evenness,
while samples where most of the individuals belong to one or a few
species have low evenness. Finally, Shannon-Wiener H' is a diversity
measure that is sensitive both to species richness and to species
evenness in a community.
The process by which toxic chemicals can
accumulate in animal tissues and become concentrated when they move
up the food chain.
The extreme winter storms of December 1992 caused 24-foot seas in
the vicinity of the outfall, moving sediments and burying some areas
under inches of sand, mud, or gravel even though the ocean is about
100 feet deep in the area. This physical disturbance was at least
partially the cause of the decline seen in the two richness indices
between 1992 and 1993. The communities recovered rapidly, and by
the late 1990s appeared to be showing a several-year cycle in species
richness (data from farfield stations also show this apparent trend).
Biochemical Oxygen Demand
(BOD): BOD is used as a measure of the quantity of oxygen consumed by microorganisms during the decomposition of organic matter. Bacteria and fungi use oxygen as they break down vegetation into carbon dioxide, water, phosphate and nitrate. As oxygen is consumed, the level of dissolved oxygen in the stream begins to decrease.
BOD is typically divided into two parts- carbonaceous oxygen demand and nitrogenous oxygen demand. Carbonaceous biochemical oxygen demand (CBOD) is the result of the breakdown of organic molecules such a cellulose and sugars into carbon dioxide and water. The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of biogradable pollutants plus the oxygen demand of non-biodegradable oxidizable pollutants.
Organisms such as plants and algae produce oxygen when there is a sufficient light source. During times of insufficient light, these organisms consume oxygen. When the dissolved oxygen is too low, aquatic life intolerant of low oxygen levels become stressed. Eventually, species sensitive to low dissolved oxygen levels are replaced by species that are more tolerant of adverse conditions, significantly reducing the diversity of aquatic life.
In 1985, a federal court order set an ambitious schedule
for the newly formed MWRA to plan and construct new sewage treatment facilities.
These facilities would end the discharge of untreated and partially treated
sewage to Boston Harbor. The so-called "Boston Harbor Project"
helped the water, sediments, and living natural communities in Boston
Harbor recover from centuries of receiving Greater Bostons sewage. This
undertaking included four major construction projects:
1. Facilities at the Fore River shipyard in Quincy to
process sewage sludge into commercial fertilizer pellets, ending the discharge
of sludge into the harbor.
2. A new secondary wastewater treatment facility, the
new Deer Island Treatment Plant (DITP), to replace the failing and undersized
primary treatment plants at Deer Island and Nut Island (NITP).
3. A tunnel from Nut Island to DITP to transport South
System sewage to DITP for secondary treatment, enabling flows from throughout
MWRAs service area to receive secondary treatment and greatly lessening
pollution to the harbor.
4. An outfall-diffuser system to discharge treated effluent
9.5 miles offshore into Massachusetts Bay, increasing dilution and minimizing
potential environmental impacts in the bay.
In addition to taking on these major construction projects, MWRA has addressed
the problem of combined sewer overflows (CSOs), which discharge a mixture
of stormwater runoff and sewage directly into the harbor during heavy
rainstorms. In the 1980s, 88 CSOs in the harbor and its tributary rivers
discharged an estimated 3.3 billion gallons of partially treated or raw
combined sewage annually. MWRA's CSO Control Plan includes the closing
of many CSOs and the addition of treatment to others.
Disinfection by adding the chemical chlorine,
the active ingredient in household bleach. Chlorine is very effective
at killing pathogens but is also poisonous to other organisms at
high enough concentrations.
is a measure of the amount of microscopic plants (phytoplankton
or algae) in the water. In Massachusetts Bay, production of algae
is the basis of the food web. However, excessive growth of algae
can lead to undesirable consequences, such as oxygen depletion at
depth due to decomposition of organic matter. Effluent from the
outfall is rich in nutrients, and therefore could potentially cause
excessive algal growth.
Water clarity in the harbor and the rivers is primarily affected by concentrations of algae and suspended solids. Secchi disks are a simple way to approximate the transparency of water. White or black-and-white disks are lowered into the water and the maximum depth at which they are visible is recorded. Large secchi disk depths indicate good water clarity. Secchi disk depths less than 1.8 meters indicate poor water clarity. An
antiquated sewer system in which storm runoff and sewage from homes
and businesses are carried by the same pipes.
Combined Sewer Overflow (CSO):
(1) A structure designed to provide relief for a combined sewer
system during wet weather. (2) An overflow event that occurs when
the volume of stormwater entering a combined sewer system overwhelms
the capacity of the system.
Sewer Overflows (CSOs)
Some communities have combined systems
that carry sewage and stormwater runoff in the same pipe.
During heavy rainstorms, the volume of flow is sometimes more
than the pipes can carry, causing mixed stormwater and sewage
discharges from outfall pipes into Boston Harbor and its tributary
Sanitary Sewer Overflows
Sanitary (not combined) sewer systems are not designed to
carry stormwater runoff. Heavy rains leaking into sewer pipes
can cause these systems to overflow into a stream or other
body of water.
Drainage systems collect rainwater runoff from streets and
channel it to a nearby river or harbor. Unfortunately, storm
drainage is frequently
contaminated with sewage from leaking pipes or illegal sewer
connections from buildings. Animal waste on the streets also
contaminates stormwater, as does car exhaust, street dirt,
Learn more about CSOs at http://www.mwra.state.ma.us/03sewer/html/sewcso.htm.
CSO Treatment Facility:
A treatment facility that operates in wet
weather to treat combined sewage before discharge. Includes disinfection;
may also include some removal of suspended solids or floatable pollutants.
The addition of a chemical (usually sodium
bisulfate or sodium thiosulfate) to neutralize the toxicity of chlorine
after it has been used for disinfection.
Disinfection: Untreated sewage carries large numbers of potentially disease-causing pathogens originating in human waste that would be a health hazard if discharged into recreational or shellfishing areas. Llike most wastewater treatment plants, Deer island uses a form of chlorine (sodium hypochlorite, the active ingreadient in bleach) to disinfect wastewater before discharging. Unfortunately, sodium hypochlorite is toxic not only to microbes but also, in high enough concentrations, to aquatic life.
Dissolved Oxygen (DO):
Fish and other aquatic animals breathe oxygen
dissolved in the water. Algae and other plants growing in the water
produce oxygen, and atmospheric oxygen also dissolves in water at
the surface. In polluted ecosystems dissolved oxygen can fall below
levels necessary to sustain
Oxygen Depletion Rate:
if dissolved oxygen concentrations remain healthy, an excessively
rapid rate of decrease could signal a future problem. A low rate
indicates DO dropped only slowly. The threshold for DO depletion
rate is based on a change from the baseline; the caution threshold
is a rate faster than 1.5 times the baseline mean rate, while the
warning threshold is twice the baseline mean rate.
Infiltration & Inflow (I/I):
Infiltration is the seeping of
groundwater into leaky pipes during wet weather. This is a large, unwanted source of water to sewers.
is the illegal discharge of stormwater to sanitary (uncombined) sewers,
sump pumps, roof drains, and basement and foundation drains from
homes directly to sewers instead of to drainage systems.
This inflow adds large volumes of stormwater to already overtaxed
Thresholds: The concentration of dissolved oxygen (DO)
in the water indicates the balance between production by algae and
consumption by aquatic organisms and the decomposition of organic
matter. Excessive organic matter may result in oxygen depletion,
which may in turn adversely affect the aquatic ecosystem. The amount
of oxygen that the water can hold is related to water temperature,
salinity, and pressure; thus, the percent saturation of dissolved
oxygen is a measure that takes these factors into account. Monitoring
locations for which there are DO thresholds include the "nearfield,"
the group of stations within about three miles from the outfall,
and "Stellwagen Basin," a deep area nine miles east of
the outfall. DO thresholds apply to the part of the year when the
water column is stratified, i.e. from June - October.
Dissolved oxygen concentration and percent saturation naturally
fell below 6 mg/l on occasion during the baseline period. The state
standard, on which the thresholds were based, allows an exception
to numerical thresholds if background conditions are lower, as is
the case here; thus, the threshold is not exceeded unless the value
falls below the threshold and below background.
wastewater, the final liquid by-product of the wastewater treatment
process flowing out of a treatment plant.
The nearfield sampling stations are within about 3 miles of the outfall diffuser, to monitor any changes occurring near the discharge.
Algae Thresholds: Nuisance algal blooms
are less predictable than the normal, beneficial algal blooms that
produce oxygen and food for marine life; some nuisance blooms did
occur during the baseline monitoring period. There is public concern
that effluent nutrients could feed a red tide bloom in the vicinity
of the new outfall, or otherwise increase the abundance of nuisance
algae. Therefore, the Contingency Plan has thresholds for abundance
of Alexandrium, Phaeocystis
pouchetii, and Pseudonitzschia,
which are triggered if the abundance of any of these becomes unusually
Nutrients such as nitrogen and phosphorous
are necessary for the growth
of plants, but excess nutrients can accelerate the growth of algae
or aquatic weeds. In marine waters the limiting nutrient is nitrogen, so that adding nitrogen increases algae. In
fresh water, the limiting nutrient is usually phosphorus. Many detergent
manufacturers have eliminated phosphates from their products in
order to avoid causing nutrients pollution of freshwater lakes and
Benthic Organisms: The presence
of pollution-tolerant or opportunistic species is another measure
of possible pollution impact on sediments in the vicinity of the
outfall. These are species that can build up to high population
levels in response to, for example, increased deposition of organic
matter. In their selection of an outfall location in 1988, EPA modeled
the deposition of organic matter and determined that with a secondary
discharge, impacts would be minimal.
Based on a review of the species found in Boston Harbor, Massachusetts
Bay, and Cape Cod Bay sediments during baseline sampling, several
species have been identified as opportunists: Capitella spp. and
Capitella capitata complex, Polydora cornuta, Streblospio benedicti,
Ampelisca abdita, Ampelisca vadorum, Ampelisca macrocephala, and
Mulinia lateralis. The Ampelisca species were included in the list
because they are tolerant to moderate levels of organic enrichment,
even though they cannot tolerate high levels. For example, the appearance
of large populations of Ampelisca in Harbor sediments in the mid-1990s
was one of the early signals of the Harbor's recovery.
The Contingency Plan thresholds for percent opportunists were set
well below levels seen in Boston Harbor throughout the 1990s.
Pipe releasing wastewater at a fixed location.
Often outfalls are placed away from shorelines in areas where the
wastewater will be rapidly diluted.
Organic Matter: If a large amount
of organic material--leaves, plants, wastewater or plant growth
spurred by excess nutrients--decomposes in a body of water, it depletes
the dissolved oxygen level necessary to support aquatic life.
Short for "pathogenic microorganisms,"
these disease-causing viruses and bacteria enter the harbor through
inadequately treated or raw sewage and from animal and bird waste
in storm runoff. They can cause stomach ailments, ear, eye, or skin
infections, and even serious diseases such as hepatitis.
PAHs -- polycyclic aromatic hydrocarbons. PAHs are a group of chemicals that are formed during the incomplete burning of coal, oil, gas, wood, garbage, or other organic substances, such as tobacco and charbroiled meat. There are more than 100 different PAHs. They can also be found in substances such as crude oil, coal, coal tar pitch, creosote, and roofing tar. They are found throughout the environment in the air, water, and soil. They can occur in the air, either attached to dust particles or as solids in soil or sediment. Studies of people show that individuals exposed by breathing or skin contact for long periods to mixtures that contain PAHs and other compounds can also develop cancer.
PCBs -- polychlorinated biphenyls are synthetic organic chemicals comprising 209 individual chlorinated biphenyl compounds (known as congeners). Exposure to each of these compounds is associated with different levels of risk for harmful effects. There are no known natural sources of PCBs. Although PCBs are no longer manufactured in the United States, there are ways people can still be exposed to them, including eating contaminated seafood.
pH: pH is a measure of the alkalinity or acidity of the effluent. Small
fluctuations in pH do not have an adverse effect on marine
environments, because seawater is well buffered. Secondary
treatment technology at Deer Island tends to produce effluent at the
low end of the range.
Phaeocystis: Phaeocystis pouchetii, one of several species of so-called
"nuisance algae," usually blooms during February to April but can bloom at
any time. The species is not toxic, but individual cells can aggregate
in gelatinous colonies that may be poor food for zooplankton. MWRA monitors Phaeocystis in its outfall ambient monitoring program. Reporting
on seasonal abundances of Phaeocystis near the outfall is
part of the Contingency
Plan. Adverse aesthetic or other impacts from Phaeocystis blooms could include persistent foam on the sea surface and/or unpleasant,
acrid odors reminiscent of burning plastic. For information on Phaeocystis
exceedances, see the Contingency
Plan Exceedances page.
Red Tide: The term "red tide" most often refers to Alexandrium tamarense, one of several harmful phytoplankton species. Alexandrium tamarense typically may bloom during April to June and can cause paralytic shellfish poisoning, known as PSP; it has been periodically found in Massachusetts since the 1970s.
For more information go to http://www.whoi.edu/redtide/.
Pseudonitzschia multiseries blooms
can occur during November to March and produce domoic acid, which
can cause a condition known as amnesic shellfish poisoning. To calculate
the threshold we calculate the nearfield average count of algae
in a group that includes the toxic species Pseudonitzschia multiseries, the closely related Pseudonitzschia pungens,
and any unidentified Pseudonitzschia species.
Rainfall or snowmelt moving over and through the ground. As the runoff moves, it carries away natural and human-made pollutants, depositing them into lakes, rivers, wetlands, coastal waters, and underground water sources. Runoff can carry bacteria and nutrients from livestock, pet wastes, and faulty septicsystems; fertilizers, herbicides, and pesticides from agricultural and residential areas;
and oil, grease, and toxic chemicals from automobiles and other sources.
A sewerage system that
carries wastewater only from the drains and toilets of homes and
Sanitary Sewer Overflows:
These overflows occur when the capacity of a sewer system is overwhelmed
and the sewer "backs up," spilling untreated wastewater
into nearby waterways.
Contamination: Sediments tell us the history of pollutant inputs
because, over time, sediments accumulate toxic contaminants. Contaminants
stick to particles, which settle to the bottom. Concentrations of
toxic organic compounds and most heavy metals in the water in Boston
Harbor meet state standards; there are very few measurements of
toxic chemicals in the freshwater portions of the harbor's tributaries,
however. Even if water quality standards are met, toxic chemicals
can accumulate in animal tissues or in bottom sediments to levels
that harm marine life.
Even if the water's dissolved oxygen does not fall
too low, a lot of organic matter falling on the bottom produces
an organic-rich sediment. Bacteria in the sediment use up oxygen
as they consume the organic matter. While healthy sediments have
an oxygenated layer above a layer that is anaerobic (without oxygen),
very organic-rich sediments have no oxygenated layer. In this environment,
anaerobic bacteria can thrive (as they use sulfur instead of oxygen),
and produce toxic hydrogen sulfide. Few animals can live in this
oxygen-poor, sulfide-rich environment; the resulting low diversity
and small number of animals indicate an unhealthy ecosystem.
Sludge: The solids which settle out during the sewage treatment process.
Sludge from the Deer Island Treatment Plant was discharged into Boston Harbor until 1991. More Info.
Chemical compounds, such as those found in detergents, that
break down grease and other kinds of dirt. They are toxic to aquatic
life in high concentrations.
Total Suspended Solids.
Water Quality Standards (see table):
Inland Water Class B: These waters are designated as a habitat for fish, other aquatic life, and wildlife, and for primary and secondary contact recreation. Where designated they shall be suitable as a source of water supply with appropriate treatment. They shall be suitable for irrigation and other agricultural uses and for compatible industrial cooling and process uses. These waters shall have consistently good aesthetic value.
Coastal and Marine Class SB: These waters are designated as a habitat for fish, other aquatic life, and wildlife, and for primary and secondary contact recreation. In approved areas they shall be suitable for shellfish harvesting with depuration (Restricted Shellfishing Areas). These waters shall have consistently good aesthetic value.
geographic area in which water, sediments, and dissolved
materials drain to a common outleta point on a river or lake,
an estuary (like Boston Harbor), or an ocean.