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MWRA's Metropolitan Water Tunnel Program

Massachusetts Water Resources Authority

About the Program

The proposed Metropolitan Water Tunnel Program will consist of approximately 14 miles of deep rock tunnels and surface connections to provide redundancy to the existing Metropolitan Tunnel System in the Metropolitan Boston area.

As proposed, the new tunnel system will consist of a northern tunnel and a southern tunnel in and around the Metropolitan Boston area. Both tunnels are proposed to begin near the terminus of the Hultman Aqueduct and MetroWest Water Supply Tunnel in the town of Weston, Massachusetts. The northern tunnel is proposed to extend north with a connection to the MWRA water system in the vicinity of Waverly Oaks Road near the Belmont/Waltham line. The southern tunnel is also proposed to extend south with a connection to the MWRA water system in the Morton Street area near the American Legion Highway in Boston. The proposed tunnels will have intermediate connection points to other existing MWRA facilities along the alignment.

Two Tunnel Alternative

After the May 2010 water main break and during the mid-2010s, it became apparent that MWRA would need to execute another large water tunnel program in the near future to provide redundancy for maintenance and emergencies. On October 6, 2016, the MWRA Board of Directors held a special meeting where MWRA staff provided a briefing on the status of the existing MWRA water transmission system and the lack of redundancy for the Metropolitan Tunnel System. The preferred alternative of constructing two tunnels, one to the north and one to the south, was recommended.

At the conclusion of the special meeting, staff were directed to brief member communities and state and local officials on the Metropolitan Tunnel Redundancy initiative in order to build consensus and support for the preferred project approach. On December 8, 2016, a Long-Term Water Redundancy Forum hosted by the MWRA Advisory Board for the customer communities was held at Boston College. MWRA staff presented the history of the MWRA waterworks system, the need for Metropolitan Tunnel Redundancy and the challenges, both implementation and financial, of building redundancy.

On January 19, 2017, the MWRA Advisory Board met and voted to support moving forward with a deep rock, two-tunnel project. They also voted also to recommend: a program management division approach to manage the Program similarly to the model used for the Boston Harbor Project (BHP); concurrent construction of both tunnels rather than a phased approach; and allocation of any revenue from non-typical or one-time water users (e.g. emergency drought connections) towards the cost of the program.

On February 15, 2017, the MWRA Board of Directors approved the preferred alternative of construction of northern and southern deep rock tunnels to provide redundancy for the Metropolitan Tunnel System, and directed staff to proceed with preliminary design, geotechnical investigations and Massachusetts Environmental Policy Act (MEPA) review of the Program.

On October 17, 2022, MWRA filed its Draft Environmental Impacts Report (DEIR) for the Metropolitan Water Tunnel Program. The DEIR responds to the Secretary of EEA’s Environmental Notification Form (ENF) Certificate issued on May 7, 2021 and provides responses to all comments received on the ENF.

On December 16, 2022, the EEA issued its Certificate of the Secretary of Energy and Environmental Affairs on the DEIR for the Metropolitan Water Tunnel Program.

On September 29, 2023, the EEA issued its Certificate of the Secretary of Energy and Environmental Affairs on the Supplemental DEIR.

Proposed Plan

Given the significant community impacts and difficulties associated with large diameter surface piping, together with operational reliability concerns, MWRA staff are pursuing a preferred all-tunnel redundancy alternative. The conceptual alignment, which will be subject to more detailed review and alternatives analysis during the Massachusetts Environmental Policy Act (MEPA) public review period, is shown below.

study area

Tunnel alignment study area

This alternative consists of two deep rock tunnels, both beginning in Weston near the Massachusetts Turnpike/Route 128 interchange where they will connect to the existing water system. The Northern Tunnel generally follows the route of MWRA’s existing Weston Aqueduct Supply Main (WASM) 3 transmission main near the Waltham/Belmont line. The length of the Northern Tunnel would be approximately 4.5 miles and the tunnel would have a finished inside diameter of approximately 10 feet.

The Southern Tunnel would run east to southeast to tie into the MWRA water system in the Morton Street area near the American Legion Highway in Boston. The length of the Southern Tunnel is anticipated to be approximately 9.5 miles and would have a finished inside diameter of 10 feet. Both the normal Northern and Southern Tunnels are anticipated to include intermediate connection shafts to existing water system infrastructure.

The proposed plan limits community disruptions and construction impacts to the locations of the tunnel construction and connection shaft sites. The all-tunnel alternative meets the strategic objective of being able to make a seamless transition to a backup supply, allowing maintenance to be scheduled for the Metropolitan Tunnels, without use of a boil water order, without impacting local fire protection abilities, and without noticeable changes in customers’ water quality, flow or pressure. It has the ability to meet high water system demand conditions, which extends the potential time frame for future maintenance and rehabilitation activities.

The exact location of the tunnels and shafts will not be determined until well into the design phase. MWRA will be working closely with the host communities to ensure minimum impact to residents and businesses along the routes.

The final cost of the Tunnel Program is not yet defined because final design and construction are still years away with much work to do in the meantime. However, an estimated $1.5 billion is being carried in our Capital Improvement Program Budget. The impacts of the estimated cost are included in MWRA’s current rate projections.

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Updated October 3, 2023

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