Because of the construction of the Walnut Hill Water Treatment Plant in Marlborough, the Cosgrove Tunnel was shut down in 2003 in order to make the large piping connection between the new treatment plant and the MWRA's new MetroWest Water Supply Tunnel.

The MetroWest Tunnel extends east 18.6 miles (29.9 km) from Walnut Hill to Weston.

During that shutdown, the Wachusett Aqueduct was the primary method of transmission[2][3] of about 240 million US gallons (910,000 m3) of water per day.

[citation needed] The section of the Wachusett Aqueduct between the Wachusett Reservoir and the treatment plant remains a standby backup transmission path in the event the Cosgrove Tunnel is unavailable, while the open channel between the treatment plant and the Sudbury Reservoir remains an emergency backup transmission path in the event of a major problem with the treatment plant or the primary transmission pathways between the treatment plant and the water system.

The aqueduct's route and supporting structures were added to the National Register of Historic Places in 1990.