Cambridge Water Hardness & Quality Report (2026)

The Cambridge Water Department (CWD) operates the city's independent water supply, serving approximately 118,000 residents across 7.1 square miles in Middlesex County, Massachusetts. Water is sourced from the Stony Brook Reservoir and Fresh Pond Reservoir, with treatment at the J.W. Wilbur Treatment Plant adjacent to Fresh Pond. Unlike neighboring communities supplied by the Massachusetts Water Resources Authority (MWRA), Cambridge maintains its own reservoirs, pumping, and distribution system, providing cost-effective water without regional blending. Weekly per-neighborhood testing is conducted, with free home lead testing offered to residents.

The Stony Brook/Irish Brook watershed spans about 4.5 square miles northwest of Cambridge, bordered by urban areas including Route 128. It overlies Pleistocene glacial till and outwash deposits, with underlying Cambrian-Ordovician metasedimentary bedrock including the Cambridge Argillite and Roxbury Conglomerate. These formations release minerals into the surface water, yielding a soft supply. Winter road salt runoff from nearby highways elevates chloride and sodium levels, with recent data showing chloride at 235 ppm — near the 250 ppm guideline — and total dissolved solids at 469 ppm.

At 60 mg/L (soft), Cambridge water produces minimal scale buildup in dishwashers, kettles, and showerheads; glassware spotting and laundry stiffness are minor concerns. Maintenance includes regular vinegar descaling, rinse agents in dishwashers, and high-phosphate detergents. A water softener is optional, not a necessity, at this hardness level. Elevated TDS (469 ppm) and chloride (235 ppm) from de-icing salts are notable, though all contaminants comply with EPA standards. Conventional treatment at the J.W. Wilbur Treatment Plant involves coagulation, sedimentation, filtration, and chloramination; pH is neutral-alkaline with no specific PFAS violations noted.