town lines of Marlborough, Northborough, and Southborough, Massachusetts. The plant replaced one used previously only for pH control. It comprises four ozone generators with diffusers and five concrete contact chambers with a volume of . The plant has a capacity of per day, on an average day or per day, at peak level. It cost US$340 million. The MetroWest Water Supply Tunnel (MWWST) carries water further east, passing the Norumbega Reservoir, Schneck's Pond, and Norumbega Covered Storage in Weston. Near Route 128 and the Charles River, it splits in two, feeding regional distribution lines at the Loring Road Tanks and an interconnection with the City Tunnel passing into Newton. In the Chestnut Hill area, the City Tunnel splits into the City Tunnel Extension (northeast) and Dorchester Tunnel (southeast), which act as backbones for smaller distribution mains and feed various regional storage tanks. Westbound water flow The Chicopee Valley Aqueduct carries water from the Quabbin Reservoir to the Western Massachusetts communities of Chicopee, Wilbraham and South Hadley (Fire District No. 1). It passes through the Ware Water Treatment Facility and the Nash Hill Covered Reservoir in Ludlow. Pressure zones The water system is divided into seven pressure zones, needed because different consumers are at different altitudes. The seven zones, measured from "Boston City Base" level are approximately: 185 ft – Low Service – Cambridge and lower portions of Boston, Somerville, Medford, Malden, Chelsea, Everett, and Winchester 280 ft – Northern and Southern High Service – Downtown Boston towers, southern and western Boston, part of Milton, Quincy, Needham, Weston, Wellesley, Watertown, southern Waltham, Marblehead, Swampscott, Revere, Melrose, Peabody; parts of Newton, Brookline, Arlington, Somerville, Medford, Malden, Everett, Wakefield, and Stoneham 320 ft – Intermediate High Service – parts of Belmont, Watertown, and Newton 330 ft – Northern Intermediate High Service – Reading, Woburn; parts of Stoneham, Winchester 400 ft – Southern Extra High Service – Stoughton, Canton, Norwood, Westwood, Dedham; parts of Milton, Boston, Brookline, and Newton 440 ft – Northern Extra High Service – Bedford, Lexington; parts of Winchester, Arlington, Belmont, and Waltham Water storage facilities The major MWRA water storage facilities outside of the source reservoirs are listed below. Covered storage facilities ( total capacity) are in primary use, and surface reservoirs are used as backup only. (Uncovered reservoirs cannot store potable water without the need for later treatment). Redundancy The Wachusett Aqueduct is an older parallel conduit to the