Arlington Water Hardness & Quality Report (2026)

Arlington, Virginia receives its drinking water from the Arlington County Department of Environmental Services, supplied by the Washington Aqueduct — a federal water supply facility operated by the US Army Corps of Engineers. The Washington Aqueduct draws raw water from two Potomac River sources: the Great Falls Intake on the upper Potomac and the Little Falls Intake downstream, treating and delivering water to Arlington and Washington DC through the Dalecarlia and McMillan treatment plants. Arlington County in Virginia is one of the most densely populated counties in the US and relies entirely on this surface water system. Water hardness measures 85.5 mg/L — classified as moderately soft.

The moderate softness of Arlington's Potomac River supply reflects the geology of the upper Potomac watershed. The river drains the Blue Ridge Mountains and Valley and Ridge physiographic province — predominantly Precambrian gneiss, Cambrian quartzite, and Silurian limestone — before entering the Piedmont. While some limestone outcrops in the Valley and Ridge contribute calcium, the dominant granitic Blue Ridge terrain is calcium-poor, producing a moderately soft, well-buffered supply. Washington Aqueduct's treatment process further refines water quality to consistently deliver a clean, palatable product.

At 85.5 mg/L, Arlington residents enjoy fairly soft water with minimal scale challenges. Faucet aerators and showerheads may develop occasional light deposits over several months, but routine cleaning keeps them clear without frequent descaling. Soap and shampoo lather well, and dishwashers produce clean, largely spot-free glassware. Hot water systems experience minimal long-term scale build-up. The Washington Aqueduct is one of the oldest continuous water utilities in the US, and its supply to Arlington consistently ranks among the highest quality in the Washington metro region.