Capitol Hill Water Hardness & Quality Report (2026)

Capitol Hill, in the District of Columbia — a DC neighborhood adjacent to the US Capitol and Eastern Market in central-east DC — receives its water from DC Water, drawing from the Potomac River at the Washington Aqueduct (McMillan and Dalecarlia Reservoirs) through the DC distribution system.

The moderately hard 133 mg/L hardness and TDS of 342.8 mg/L are slightly harder than other DC zones (Southwest Waterfront: 113.5 mg/L; Kennedy Street: 113 mg/L) — possibly reflecting Capitol Hill's older distribution infrastructure that can add mineral concentration, or a slightly different blend at this central DC distribution zone. The Potomac River at the Washington Aqueduct drains the Virginia Blue Ridge — Precambrian Catoctin Greenstone (insoluble), Triassic Culpeper Basin (calcareous — moderate hardness contributor), and Cambrian Conococheague Formation (calcareous — primary hardness contributor). The Triassic and Cambrian calcareous input produces the moderately hard DC supply.

At 133 mg/L, Capitol Hill's water is moderately hard — scale builds in kettles and appliances over months, dishwashers benefit from rinse aid, and faucet aerators need periodic cleaning. Quarterly descaling is appropriate. The PFAS level of 9.2 ppt warrants a certified reverse osmosis drinking water filter — Marine Corps Base Quantico (AFFF), Fort Belvoir (Fairfax County — AFFF), and the upper Potomac watershed military-industrial PFAS complex contribute to Capitol Hill's elevated readings.