Washington Water Hardness & Quality Report (2026)

Washington, D.C.'s drinking water is supplied by DC Water (District of Columbia Water and Sewer Authority), drawing almost entirely from the Potomac River at a filtration intake near Great Falls, Virginia. The McMillan Sand Filtration Site (historic, now decommissioned) and the current Washington Aqueduct — operated by the US Army Corps of Engineers — direct Potomac River water to the Dalecarlia Water Treatment Plant in northwest D.C. and the McMillan Reservoir distribution system. The Potomac, one of the cleanest large rivers in the Eastern United States following decades of restoration effort, drains a 14,670-square-mile watershed extending into West Virginia, Maryland, Virginia, and Pennsylvania.

Washington's soft-to-moderate hardness of 75.5 mg/L reflects the mixed geological character of the upper Potomac watershed. Above Great Falls, the river drains the Blue Ridge Province and Appalachian Valley and Ridge — terrains of Cambrian and Ordovician carbonate rocks interbedded with Precambrian metabasalt and schist — before cutting through the Piedmont metamorphic zone dominated by crystalline gneiss and phyllite. The partial carbonate contribution from the Valley and Ridge province mildly elevates hardness above what pure metamorphic-rock watersheds would produce, while the substantial Blue Ridge crystalline component keeps it relatively soft overall.

At this soft level, D.C. residents benefit from good soap and detergent performance, minimal limescale buildup on fixtures and appliances, and water that tastes clean after treatment. Glassware comes out of dishwashers with little to no mineral spotting. The most common water quality concerns in Washington are related to chlorine and chloramine treatment byproducts rather than hardness. A carbon-block filter pitcher or under-sink filter addresses taste and odor issues effectively, and no descaling routine is necessary for appliances under normal conditions.