MWRA online - home
Home
Water System
Sewer System
Harbor and Bay
School Program
About MWRA
Doing Business with MWRA
Contact MWRA


Boston Harbor and Massachusetts Bay
MWRA Environmental Quality Department

Archive

Contingency Plan Ambient Thresholds

MWRA developed a Contingency Plan to ensure that effluent discharge from the Massachusetts Bay effluent outfall does not result in adverse impacts to water quality in that area of the ocean or nearer the shore. The Plan identifies receiving water or "ambient" thresholds that can indicate whether conditions in the Bay may be changing. When monitoring shows that "Caution" or "Warning" threshold levels for certain environmental indicators are exceeded, the Exceedance Reports are posted on this website.

Nuisance 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. Because there was public concern that effluent nutrients could feed a red tide bloom in the vicinity of the Massachusetts Bay outfall, or otherwise increase the abundance of nuisance algae, the Contingency Plan includes thresholds for unusually high levels of certains species of nuisance algae.

Alexandrium catenella (formerly referred to as Alexandrium fundyense) is a species of nuisance algae that 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 (A. catenella plus unidentified Alexandrium spp.) seen in any nearfield sample.

Pseudo-nitzschia 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 Pseudo-nitzschia multiseries, the closely related Pseudo-nitzschia pungens, and any unidentified Pseudo-nitzschia species.

Phaeocystis pouchetii blooms usually occur during February to May. As of 2017 seasonal Phaeocystis thresholds are no longer a part of the Contingency Plan. This species is not toxic, but individual cells can aggregate in gelatinous colonies that may be poor food for zooplankton and cause nuisance foam on the sea-surface. A series of threshold exceedances occurred between 2001 and 2016. In each case, MWRA and its monitoring team evaluated the exceedances and determined the blooms were natural occurrences with no evident connection to outfall discharge. MWRA's Science Advisory Panel, and state and federal regulators agreed with this conclusion, and approved deleting the Phaeocystis threshold.

Benthic Diversity Threshold

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 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).

A series of diversity threshold exceedances occurred between 2010 and 2014, in each case triggered by high species diversity as measured by Pielou's J' and Shannon-Wiener H'. In each case, MWRA and its monitoring team evaluated the exceedances and determined the high species diversity represented natural fluctuations in sea-floor communities with no evident connection to outfall discharge.  State and federal regulators, and their Outfall Monitoring Science Advisory Panel agreed with this conclusion. In 2017 MWRA proposed deleting the upper range species diversity thresholds, and regulators agreed. 

Benthic Opportunist Thresholds

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.

Chlorophyll Thresholds

Chlorophyll 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.

There are annual and seasonal chlorophyll thresholds for the "nearfield," the group of stations within about three miles from the outfall that are most likely to be affected by nutrient-rich effluent. Because the levels of chlorophyll in the water naturally vary over the year, there are separate thresholds for different seasons. In most years, Massachusetts Bay experiences a "spring bloom" characterized by high chlorophyll levels as lengthening days provide enough sunlight for algae to grow quickly. Chlorophyll typically drops in summer, as the nutrients in well-lit surface waters are used up. When the weather cools, the surface and bottom waters mix, which usually gives rise to a "fall bloom" as nutrient-rich bottom waters are mixed up into the well-lit surface layers. As the days become short, chlorophyll levels drop again since there is not enough light for algae to grow.


Dissolved Oxygen (DO) Thresholds

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 life.

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 Depletion Rate: Even 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.

Fish and shellfish Thresholds

Threshold parameters for fish and shellfish include levels of toxic contaminants in flounder, lobster, and mussels and liver disease (measured as CHV) in flounder.  Some thresholds are based on U.S. Food and Drug Administration limits for maximum concentrations of specific contaminants in edible portions of food.  Others were developed from the baseline-monitoring results.

Fish and Shellfish Monitoring

The monitoring program focuses on three indicator species: winter flounder (Pseudopleuronectes americanus), lobster (Homarus americanus), and blue mussels (Mytilus edulis).  Winter flounder and lobster are important resource species.  They live and feed on the sea-floor, and are exposed to contaminants through their gills and their feeding.  Mussels are also resource species, but are included in the program because, like other filter feeders, mussels process large volumes of water and can concentrate toxic metals and organic compounds in their tissues.  They can be readily maintained in fixed cages, so they are convenient monitoring tools. 

Flounder and lobster are sampled from Deer Island Flats, near the outfall site, and Cape Cod Bay, Flounder are also taken near Nantasket Beach.  Mussels are deployed at the outfall, at Deer Island Light, and in Boston’s inner Harbor.

Winter Flounder

Flounder are collected annually.  Whole fish are examined for external lesions or other abnormalities, and flounder livers are examined to quantify disease, including a condition  known as centrotubular hydropic vacuolation (CHV) that is a precursor to liver tumors, and tumors themselves, also known as neoplasia.   CHV and neoplasia are associated with long-term exposure to contaminants in the environment. 

Analyses of selected toxic contaminants in the filet s and livers of flounder are conducted every third year, including 20015 and 20018, to determine concentrations and to evaluate whether contaminant burdens approach human health consumption limits.  Chemical analyses fillets and livers include PCBs, pesticides, mercury, and lipids. 

Lobster

Commercial lobstermen collect lobsters for the monitoring program every third year, including 2015 and 2018.  Lobsters are examined for external conditions, and contaminant analyses are carried out on meat and hepatopancreas (tomalley) samples.  Samples are analyzed for lipids, PCBs, pesticides, and mercury. 

Blue Mussel

Mussels are collected from a clean reference site every third year, including 2015 and 2018.  They are placed in cages which are set out as specific depths at the survey stations.   

After a minimum deployment of 40 days or a preferred deployment of 60 days, chemical analyses are performed on mussel tissues samples.  Samples are analyzed for PCBs, pesticides, PAHs, lipids, mercury, and lead.

Sediment Oxygen Thresholds

The redox potential discontinuity (RPD) threshold measures the depth of the oxygenated layer in marine sediment as a measure of ecosystem health. A diverse bottom-dwelling community includes organisms that mix water and oxygen down into the sediment. In an overenriched environment, organic material deposited on the sediment surface can use up the available oxygen and smother the bottom-dwelling community. Such areas, including some areas of Boston Harbor, have a thin or nonexistent oxygenated layer. The thickness of the oxygenated layer is called the redox potential discontinuity (RPD) depth. In MWRA's monitoring program, the RPD depth is estimated from sediment-profile images, cross-sections of the upper several centimeters of the sediment taken with a special mud-penetrating prism and camera. The threshold for RPD is half the mean measured in the baseline period (that is, if the thickness of the oxygenated layer fell to less than half the thickness measured pre-discharge, a caution threshold would be exceeded.) Sediment profile imaging for MWRA monitoring is conducted in August.

Return to the top