By Christopher Barnett
|Preparation of tunnel boring machine in Conley Shaft.|
Clambakes and clam chowder are synonymous with the Boston shore. But for many years, recreational fishermen risked fines up to $2,000 for clamming in and around Boston Harbor. The reason: the long-term pollution of the harbor by wastewater, sewage sludge, combined sewer overflows (CSOs) and stormwater runoff had raised bacterial, viral, and chemical contamination of shellfish to levels hazardous for human consumption.
|Construction of the tunnel began in August 2006.|
Clam bakers and chowder hounds are once again safe to pursue their favorite foods in local waters and the community can once again enjoy recreational activities on local beaches as a result of the completion of the Massachusetts Water Resources Authority's (MWRA) massive Boston Harbor cleanup, including the new North Dorchester Bay combined CSO storage tunnel — the last piece of the MWRA's plan to improve South Boston's beaches and shellfish beds. Conceived and implemented over a 20-year period, the plan was intended to provide comprehensive control of wet weather discharges to North Dorchester Bay, including the elimination of CSOs to meet the Bay's Class B (fishable/swimmable) water quality standard.
The story of the Boston Harbor cleanup goes back to 1985, when the Commonwealth of Massachusetts was sued for violating the federal Clean Water Act of 1972. In response to the federal district court action, which became known as the Boston Harbor case, the Massachusetts legislature created the MWRA to undertake a massive cleanup that became known as the Boston Harbor Project.
|Since coming on-line in May 2011, the North Dorchester Bay storage tunnel and related facilities have prevented 201 million gallons of CSO and stormwater from discharging into Massachusetts Bay.|
One of the earliest and most dramatic steps to address the degradation of Boston Harbor was the MWRA's action to halt the dumping of sewage sludge to the harbor in December 1991. Another turning point in the cleanup of the harbor was the decommissioning of the Nut Island sewage treatment plant in Quincy in July 1998 and the rerouting of its flow to Deer Island in Winthrop, where a new wastewater treatment plant was built to provide both primary and secondary treatment. Harbor water quality improved further in 2000 with the commissioning of the Deer Island effluent outfall tunnel. The 9.5-mile, 25-foot-diameter tunnel carries twice-treated effluent into Massachusetts Bay, where it is discharged through 55 diffuser risers spread over a mile and a half of seabed.
|The 17-foot-diameter, 2-mile-long, near-surface tunnel at North Dorchester Bay provides nearly 19 million gallons of storage capacity for CSO runoff and separate stormwater flows.|
After completion of the new Deer Island secondary treatment plant in 2001, the MWRA was able to turn its attention to the control of pollution from CSOs during rainy weather. The keystone of this effort was the South Boston CSO storage tunnel in North Dorchester, which included a larger-diameter tunnel along South Boston's beaches, sized to provide storage of CSO flows, as well as storm drains along Morrissey Boulevard to provide a five-year level of stormwater control for the beaches. Construction of the tunnel began in August 2006 and was completed in November 2009.
The Final Piece
The North Dorchester Bay CSO storage tunnel and the facilities connected to it form the final piece and represent the single largest component of the MWRA's $857 million long-term CSO control plan, approved by U.S. Environmental Protection Agency and accepted by the federal court in 2006. The 17-foot-diameter, 2-mile-long, near-surface tunnel at North Dorchester Bay provides nearly 19 million gallons of storage capacity for CSO runoff and separate stormwater flows. It is connected to a 15-million-gallon-per-day pump station located away from residential areas at the Massachusetts Port Authority's Conley container terminal. After the storm peak has passed, the pump station pumps the combined wastewater to the existing sewer system where it is conveyed to Deer Island for treatment. The tunnel system also includes a below-ground ventilation and odor-control facility at the tunnel's upstream end, and six drop shaft structures that connect the existing combined sewer outfalls to the storage tunnel.
In addition to capturing CSO and separate stormwater flows, the North Dorchester Bay CSO project includes a conduit that diverts stormwater flows from the Carson Beach area to Savin Hill Cove during storms greater than the one-year storm, which effectively increases the level of stormwater control for the South Boston beaches from a one-year storm to a five-year storm. Up to the one-year storm, all stormwater and CSO that formerly discharged to South Boston beaches is captured in the North Dorchester Bay tunnel. In the larger storms, the tunnel captures approximately one million gallons of the first flush before diverting the Carson Beach area stormwater to Savin Hill Cove. Gates within the CSO and stormwater diversion structures that direct flows into the tunnel are kept in the open position, except during extreme storms. This means dry weather infiltration flow is also captured by the tunnel and prevented from draining to the beaches.
Project engineers devised a unique control strategy that integrates the use of regional rainfall forecasts to meet capture objectives. Along with the collection system, runoff and tunnel hydraulic models, the weather prediction application maximizes the capture of stormwater while reserving adequate storage capacity for total capture of CSO up to the 25-year design storm.
Diversion structure gates, weirs, and near-surface piping were built at six outfalls to direct the flow to either the storage tunnel or the outfalls themselves. With one exception, all the tunnel drop shafts are able to receive both CSO and stormwater flow. At the outfalls, separate diversion structures for CSO and stormwater are provided.
The diversion chambers feature a weir that directs flow to the tunnel and a hydraulically operated gate designed to isolate flow from the tunnel. The CSO diversion structure weirs and associated piping are sized to allow peak flow from a 25-year storm to enter the associated piping and allow peak flow from a five-year storm to enter the tunnel without overtopping the weir. The structure's hydraulically operated gates are controlled to close should the tunnel reach its high level set point, a feature designed to prevent overfilling as well as surges. When the gates close, any remaining CSO is discharged to the outfalls over the diversion structure weirs.
|Thanks to MWRA's massive cleanup of Boston Harbor, South Boston beaches are once again safe and enjoyable — and among the cleanest urban beaches in the nation.|
Before completion of the North Dorchester Bay CSO storage tunnel, CSO discharge affected South Boston beaches approximately 20 times a year. Since coming on-line in May 2011, the North Dorchester Bay storage tunnel and related facilities have prevented 201 million gallons of CSO and stormwater from discharging into Massachusetts Bay during more than 60 rainfall events. With the exception of Hurricane Irene in August 2011, when separate stormwater flows were allowed to discharge to the Bay and to Savin Hill Cove via the Morrissey Boulevard storm drain, MWRA directed all flows to the tunnel and prevented any discharge of CSO, stormwater or dry weather flow from the five remaining outfalls to South Boston beaches.
Water quality tests back up the vast improvements on South Boston beaches. Daily sample results show a reduction in the number of violations of the bacteria standard from 17 days in 2008, 2009, and 2010 to just four days in 2011 — despite a comparatively high rainfall that year. Except on those four days, water samples met the swimming standard 95 percent of the time at Pleasure Bay Beach and 99 percent of the time at McCormack Bathhouse, M Street Beach and City Point Beach.
Earlier this year, the North Dorchester Bay CSO tunnel received the Operations and Environmental Performance Award at the 2012 conference of the National Association of Clean Water Agencies in Los Angeles. The design and construction of this complex project had to accommodate variable geology, buried obstructions, sensitive buildings and utility structures, contaminated soils and groundwater nearby, and be mindful of high sensitivity to noise and vibration. Despite all that, the project was completed six months ahead of schedule with a minimal 1.1 percent rate of change orders. It was also completed without any claims or any major injuries.
The North Dorchester Bay CSO storage tunnel has achieved the intended results of eliminating CSO, stormwater and dry weather infiltration discharges to the beaches of South Boston in all but extreme storm events. The construction has also had a measurable impact on water quality. But most obviously (and perhaps most importantly for local residents), it has created a marine environment that is once again safe and enjoyable, making South Boston beaches, according to EPA, among the cleanest urban beaches in the nation.
About the Author: Christopher J. Barnett is a Senior Engineering Manager in the Boston, Massachusetts, office of Parsons Brinckerhoff, which, in joint venture, designed the North Dorchester CSO tunnel and related combined sewer system improvements for the Massachusetts Water Resources Authority.