Alexandrium: 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)Aquifer: A geologic formation that holds groundwater. Many non-MWRA communities rely on wells drilled into aquifers for their drinking water supplies.
Benthic 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.
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 Boston’s 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 MWRA’s 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.
Chlorination: 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.
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.
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: 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.
Nearfield: The nearfield sampling stations are within about 7 kilometers (4 miles) of the outfall diffuser, to monitor any changes occurring near the discharge.
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. 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 high.
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
Outfall: Pipe releasing wastewater at a fixed location.
Often outfalls are placed away from shorelines in areas where the
wastewater will be rapidly diluted.
Pathogens: 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.
Pycnocline: A horizontal layer or depth range of changing density in the water. The word "pycnocline" stems from the Greek word "puknos" , meaning "thick" , and "-cline" means "gradual change" or "gradient". Because density varies with temperature and salinity, a pycnocline encompasses a layer of changing temperature (thermocline) and changing salinity (halocline). When surface water has higher temperature its density is lower, creating stratification, or layering of the water (see graphic).
Adapted from a graphic from the website of the University of Hawaii at Manoa
Understanding stratification is important in measuring the effects of wastewater discharges into Massachusetts Bay. A pycnocline is present in Massachusetts Bay from about May through October, when winds are weaker and surface waters heat up due to more plentiful sunshine; typically the pycnocline is up to 10 meters (33 feet) thick, and occurs at about 10-30 meters below the surface. During the unstratified winter months, there is no pycnocline and effluent from the MWRA outfall on the seafloor tends to mix upward to the water surface, while during stratified conditions the rest of the year it tends to remain deeper than the pycnocline.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/.
Runoff: 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.
Sediment 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.
Sediment Enrichment: 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.
Sediment Oxygen Demand:
Surfactants: 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.
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.
Watershed: 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.