 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 of about of water per day.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.