Washington Water Hardness & Quality Report (2026)

DC Water (D.C. Water and Sewer Authority) supplies drinking water to approximately 681,000 residents in Washington, District of Columbia, through a distribution system spanning 1,300 miles of pipes, 44,000 valves, and 9,510 fire hydrants. Water is sourced exclusively from the Potomac River at intakes at Great Falls and Little Falls, operated by the U.S. Army Corps of Engineers' Washington Aqueduct. The Aqueduct treats about 140 million gallons daily at its facilities in Arlington, Virginia; DC Water itself operates no local treatment plants.

The Potomac River watershed covers 14,670 square miles across Maryland, Virginia, West Virginia, and Pennsylvania, draining into the Chesapeake Bay. Water chemistry is shaped by the river's path through limestone and sedimentary rock formations in the Piedmont region, dissolving minerals from Paleozoic dolomites and calcium carbonates of the Frederick Valley. This geology results in moderately mineralized surface water averaging 126 mg/L, with seasonal hardness peaks in warmer months when lower flow and higher evaporation concentrate dissolved ions from these rock types.

At 126 mg/L hardness, scale buildup affects dishwashers — spotting glassware and leaving residue on fixtures — while water heaters, washing machines, and coffee makers experience reduced efficiency from mineral deposits. Showers develop soap scum and pipes can accumulate deposits over time. Regular vinegar descaling for appliances, drain screens, and detergent boosters help mitigate issues; a water softener or conditioner is recommended for households with persistent spotting, dry skin, or appliance wear. The 2025 Consumer Confidence Report confirms compliance with all EPA primary standards, with over 40,000 tests conducted in 2024 and total hardness averaging 126 ppm within legal limits.