1937 Plan
A redundant tunnel system was proposed as early as 1937. The plan included a proposed pressure aqueduct and tunnel system with a tunnel loop beginning in Weston near the Charles River and running east into Boston, turning north to Everett, looping west to Belmont and connecting back to Weston.
1937 Northern Tunnel Loop Plan
While much of the 1937 plan for pressure aqueducts and tunnels was implemented from 1937 to present day, the proposed tunnel loop was never completed.
Redundancy Planning 1990 through 2016
In 1990, a plan was proposed to construct a tunnel from Marlborough to Weston (the MetroWest Water Supply Tunnel) to provide redundancy for the Hultman Aqueduct and a future northern tunnel loop from Weston to Stoneham and Malden The MetroWest Water Supply Tunnel was approved for construction and was completed in 2003. However, the proposed northern tunnel loop was not constructed.
1990 Northern Loop Tunnel Plan
In 2011, the MWRA completed a new evaluation of alternatives for redundancy within the Metropolitan Boston area. This evaluation included surface pipe alternatives in addition to tunnel alternatives with an objective of incorporating redundancy planning into the existing pipeline asset management program (ie. allocating funds already budgeted for rehabilitation of existing pipelines toward replacing the existing pipelines with larger pipelines). The result of that evaluation was a plan of constructing primarily large diameter surface pipes to provide redundancy. However, as the planning for this program progressed, it became apparent that the construction of large diameter pipelines through dense urban areas would cause unacceptable community disruption and had serious implementation challenges.
2011 Surface Pipe Plan
Finally, in 2016 MWRA revisited the all-tunnel approach (PDF) to providing redundancy to the Metropolitan area. Over 30 alternatives were screened based on level of redundancy, constructibility, cost and operation and maintenance. Based on this evaluation, an all-tunnel alternative was recommended for redundancy.
