 The aqueduct serves as a standby backup to the Cosgrove Tunnel.The Wachusett Aqueduct extends from the Wachusett Reservoir in Clinton, through Berlin and Northborough, to the John J. Carroll Water Treatment Plant at Walnut Hill in Marlborough, Massachusetts.This long water system consists of 2 miles (3 km) of hard rock tunnel and 7 miles (11 km) of high horseshoe-shaped underground aqueduct constructed of non-reinforced concrete with a brick-lined invert.