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September 5 , 2001

MWRA Outfall Tunnel Working Well at One Year Anniversary

After one year of operation, the 9.5 mile-long outfall tunnel discharging Metro Boston’s treated wastewater to Massachusetts Bay is receiving positive reviews from scientists. Recent investigations have found that the outfall’s multi-headed diffuser field in 100-foot deep water is providing for optimal mixing and dilution. Scientists also found that marine life around the outfall area is thriving. In addition, there has been significant improvements in Boston Harbor’s water quality.

On September 6, 2000, MWRA started the outfall tunnel and ceased all discharges to old pipes off Deer Island, a practice which degraded water quality in shallow Boston Harbor.

MWRA’s new Deer Island Wastewater Treatment Plant has performed well and is meeting the stringent federal/state discharge permit. The third and final battery of the secondary treatment process was brought on-line in March. MWRA receives on average about 350 million gallons per day from 43 communities.

"This is great news for the environment" said Frederick Laskey, MWRA Executive Director. "The outfall tunnel system is working well and water quality in Boston Harbor has improved dramatically" said Laskey. "Massachusetts Bay is one of the most intensely monitored areas of water in the world and so far everything seems to be fine."

MWRA’s Boston Harbor Project consists of the 1.3-billion-gallon capacity Deer Island plant, outfall tunnel, and a sludge-to-fertilizer plant in Quincy. The $3.8 billion project is essentially complete, with final site landscaping underway on Deer Island.

Massachusetts Bay

MWRA conducted a full-scale dilution study of the outfall tunnel system in July, a month when the least amount of mixing would be expected due to thermal stratification of the ocean. An environmentally friendly, rose-colored dye called rhodamine was added to the wastewater discharge from the Deer Island plant and its dilution at the outfall diffuser field was tracked by shipboard instruments and sampling over a three day period. The survey found that the diffusers provide a minimum of a 100 to 1 dilution within 50 meters of the discharge, indicating that the outfall is working as designed.

"There is no physical or water quality signature of the outfall discharge outside of a very small area around the 54 diffusers spread over a 1 mile area" said Dr. Michael Mickelson, MWRA Program Manager for Outfall Monitoring. MWRA monitors water quality and chemistry, seafloor conditions, and fish/shellfish life at dozens of locations throughout the bay.

Using a remotely operated underwater vehicle in July, scientists carried out a video and photographic survey of the plant and animal communities that live on rocks near the outfall. Abundant life was found on and near the active diffuser heads, including densely growing sea anemones, sea squirts, flounder, cod, sponges and other animals. An April fish survey found large numbers of flounder near the outfall area. "The diffuser caps have blended right into the ocean floor and its active marine life ecosystem" said Dr. Mickelson.

MWRA surveys for floatable objects of human origin near the outfall field, using fine-meshed nets towed by boats, have turned up nothing.

Boston Harbor

The elimination of MWRA wastewater discharges to Boston Harbor has significantly helped improve water quality across the harbor, especially around the old outfall location off Deer Island. "Bacteria, algae and nutrients have diminished across the harbor, while water clarity has risen" said Dr. David Taylor, MWRA Project Manager for Harbor Monitoring. Based on nine months of data, Dr. Taylor found that sewage-related bacteria levels have declined 80%, algae levels have dropped about 50%, and the nutrient ammonium is 80% lower.

Water clarity has doubled from 6.6 feet to 12.5 feet near the old outfalls off Deer Island and increased from 10.5 feet to 12.1 feet on a harbor-wide basis. Increased visibility has been reported by fishermen, recreational boaters, and people flying in and out of Logan airport.

"The rapid improvements in Boston Harbor are tremendous for recreational users, fisherman and marine life" said Bob Durand, Secretary of Environmental Affairs. "This couldn’t come at a better time with the startup of the Boston Harbor Islands National Park" said Durand.

Independent Scientific Oversight

MWRA’s extensive water quality monitoring program and permit compliance is closely observed by the Outfall Monitoring Science Advisory Panel (OMSAP), a group of independent scientists that advise EPA and the Massachusetts Department of Environmental Protection. OMSAP will be hosting two public workshops in October, one on Cape Cod and one in Boston, to review the outfall monitoring results after the first year of operation.

Further details on Massachusetts Bay, Boston Harbor and the Deer Island plant can be found at:

New Outfall Site
Boston Harbor

OMSAP’s web address is