Boston Water Hardness & Quality Report (2026)

Boston's water is supplied by the Massachusetts Water Resources Authority (MWRA), drawing from two major upland reservoirs in central Massachusetts. Quabbin Reservoir — created in the 1930s by flooding the Swift River valley and four towns — is one of the largest unfiltered water supplies in the United States, holding approximately 412 billion gallons and providing roughly 70% of MWRA supply. Wachusett Reservoir in central Worcester County supplements the supply and serves as a balancing reservoir. Water flows east by gravity through the Cosgrove Tunnel and MetroWest Tunnel — major aqueduct tunnels beneath the Boston metro area — before reaching the Carroll Water Treatment Plant in Marlborough for final treatment and UV disinfection.

Boston's very soft water at 43.5 mg/L reflects the geology of the Quabbin and Wachusett watersheds. These central Massachusetts highlands are underlain by Precambrian and Devonian granite, gneiss, and schist — crystalline metamorphic and igneous rocks of the Merrimack and Nashoba terranes that are highly resistant to chemical weathering. Rainfall and snowmelt passing over these silicate-rich formations absorb only trace calcium and magnesium, producing consistently soft water. MWRA adds lime and carbon dioxide for pH adjustment after treatment, slightly elevating alkalinity while maintaining the naturally low hardness character of the upland sources.

Boston's soft water produces excellent soap and detergent performance — lather is abundant and rinses cleanly in showers and laundry. Appliances accumulate essentially no limescale, and glassware comes out of dishwashers spot-free without rinse-aid. The main water quality concern for Boston residents is not hardness but the historic legacy of lead service lines in older neighborhoods, for which MWRA operates an active replacement program. Flushing the cold tap for 30 seconds before drawing drinking water is recommended in pre-1980 buildings with lead solder or lead service connections.