What DC Water's reported hardness means for shower doors, faucets, kettles and aerators in Washington DC, and which utility supplies the suburbs around it.
Potomac River water is not the hardest in the country, but it is hard enough to leave a mark. DC Water's 2025 report lists total hardness averaging 126 parts per million, about 7 grains per gallon, with a range from 78 to 175 ppm. The higher end of that range shows up as white rings and cloudy glass within weeks.
This guide explains where Washington's water comes from, which utility supplies each part of the region, and how to keep mineral film off glass and fixtures without damaging older finishes.
DC Water's drinking water comes from the Potomac River. The Washington Aqueduct withdraws about 140 million gallons a day from intakes at Great Falls and Little Falls and treats it at the Dalecarlia and McMillan plants. The U.S. Army Corps of Engineers owns and operates the Aqueduct, and DC Water buys the treated water and delivers it to homes.
Hardness is mostly dissolved calcium and magnesium, and that mineral load is what you see when a droplet dries on glass. DC Water reports a range rather than a single figure, so some weeks leave more film than others.
The Washington Aqueduct also supplies Potomac water to Arlington County, Falls Church and part of Fairfax County, where local utilities bill customers. Arlington homes therefore see water from the same treatment plants as the District.
In Maryland, the City of Rockville provides water to about 70 percent of the city, around 13,000 accounts, and WSSC Water serves the rest. WSSC Water, which also covers Silver Spring, draws from both the Potomac River and reservoirs on the Patuxent River. Check your own utility's annual water quality report for its hardness figure, since the numbers on this page are DC Water's.
Mineral film forms wherever water evaporates instead of draining: the lower third of a glass door, the base of a faucet, the rim of a shower head and the grout lines at the bottom of a tile wall. In small bathrooms with weak exhaust fans, the water sits longer and the film builds faster.
A squeegee after each shower prevents most of it. For buildup, a mild acidic cleaner such as diluted white vinegar dissolves the minerals; rinse well and dry. Never use acid on marble, travertine or other natural stone, which etches, and test any product on old porcelain fixtures before using it widely.
DC Water advises customers to remove and clean faucet aerators every three months as one step to reduce possible lead exposure. Aerators also catch mineral flakes, so cleaning them on that schedule helps water pressure and keeps the faucet tip from crusting over.
DC Water notes that lead service lines were mostly installed in the District before the mid-1950s, with records of some installed as late as 1977. With 69 percent of District homes built before 1980, it is worth looking up your own service line with DC Water. Cleaning does not change what is in the pipes, but the aerator habit is easy to keep.
At around 7 grains per gallon, kettles and coffee makers scale noticeably over a few months. Descale them with a vinegar or citric acid cycle, then run plain water through twice. Glassware that comes out of the dishwasher cloudy usually means mineral film, which a rinse aid reduces.
Each spring the Aqueduct temporarily switches the disinfectant from chloramine to chlorine, and DC Water notes the water can taste and smell more of chlorine. It is not a hardness change, but many people notice it in coffee and tea while it lasts.
A One-Time Deep Clean treats shower glass for hard water spots and details faucets around the handles and base, which takes care of weeks or months of film. A recurring plan afterward keeps it from building back. Glass that has been etched by years of minerals can look cloudy even when it is clean, and that damage cannot be reversed by cleaning. See the deep cleaning guide for the full scope.
The questions Washington DC homeowners ask most often about this topic, answered plainly.
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