Mount Vernon Triangle Water Hardness & Quality Report (2026)

Mount Vernon Triangle, District of Columbia — a rapidly revitalized downtown DC neighborhood between Penn Quarter, NoMa (North of Massachusetts Avenue), and the Convention Center at the intersection of New York Avenue and K Street NW, named for the triangle formed by M, K, and 7th Streets — receives its municipal water from DC Water (District of Columbia Water and Sewer Authority), which draws from the Potomac River through the Washington Aqueduct operated by the US Army Corps of Engineers. The Washington Aqueduct treats Potomac River water at the McMillan and Dalecarlia plants before distribution throughout the District.

The moderately hard 113 mg/L hardness and TDS of 253.4 mg/L reflect the Potomac River watershed's mixed Appalachian geology at the Washington metropolitan reach. The Potomac River at the DC intake drains an extraordinarily diverse watershed — the Blue Ridge Province (Precambrian Catoctin metabasalt and Harpers Formation phyllite — soft crystalline terrain), the Valley and Ridge (Ordovician Martinsburg Shale, Helderberg Group limestone — moderate carbonate input), and the Piedmont crystalline basement. This mixed Blue Ridge-Valley and Ridge-Piedmont watershed produces moderately hard water, slightly harder than the purely crystalline Blue Ridge supplies of western Virginia, reflecting the carbonate Valley and Ridge contribution to the Potomac's dissolved mineral budget.

At 113 mg/L, Mount Vernon Triangle's water is moderately hard — comfortable for all household applications. Scale builds slowly in kettles and appliances over months, dishwashers benefit from rinse aid, and faucet aerators need periodic cleaning. Quarterly descaling of heating appliances is the standard schedule. The PFAS level of 7.5 ppt warrants a certified drinking water filter — the Potomac River watershed's upstream PFAS sources (military installations including Fort Detrick in Frederick MD, Marine Corps Base Quantico in Virginia, and numerous Shenandoah Valley agricultural and industrial PFAS sources) contribute to DC Water's consistent PFAS challenge and active monitoring program.