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Boston Harbor Outfall Tunnel


The Boston Harbor Effluent Outfall Tunnel was a key part of the Boston Harbor Project, a $4 billion effort to clean up Boston Harbor by providing new wastewater infrastructure including a new sewage treatment plant at Deer Island in Boston Harbor and an associated 9.5-mile (15.2-kilometer) outfall tunnel. 

WSP USA, as a subconsultant, designed the 25-foot-diameter (7.6-meter) tunnel on behalf of the Massachusetts Water Resources Authority (MWRA). One of the longest and largest outfalls in the world, the tunnel was a key part of the MWRA’s Boston Harbor Project, a court-ordered program to improve the water quality of Boston Harbor, making it swimmable and fishable. As part of our services, we provided extensive geotechnical exploration and geophysical exploration prior to construction of the tunnel. 

The outfall includes a 400-foot-deep (121-meter-deep) onshore shaft at Deer Island and 55 riser shafts that connect the final section of the tunnel to diffuser caps in the deep waters of Massachusetts Bay. 

In September 2000, the outfall began discharging twice-treated wastewater nine miles (15 kilometers) into Massachusetts Bay. “This is a major step in our effort to clean up Boston Harbor and to provide first-class recreational opportunities for our citizens and unpolluted habitat for our wildlife,” said the Massachusetts Secretary of Environmental Affairs.