Potomac Water Hardness & Quality Report (2026)

Potomac, Maryland is served by the Washington Suburban Sanitary Commission (WSSC Water), supplying drinking water to Montgomery County and Prince George's County. Water sources include the Potomac River, treated at the Potomac Water Filtration Plant near Rockville, and the Patuxent River via the Triadelphia Reservoir (also known as T. Howard Duckett Reservoir or Rocky Gorge), processed at the Patuxent Water Filtration Plant. Treated water is distributed through pressurized mains and storage tanks to residents in Potomac, Bethesda, Rockville, and Gaithersburg on the west side of Montgomery County.

The Potomac River watershed spans from the Appalachian Mountains to the Chesapeake Bay, with upstream influences from limestone and shale-dominated Appalachian Plateau and Valley & Ridge provinces, transitioning to Coastal Plain sands and clays downstream. The Patuxent River draws from Piedmont metamorphic rocks and Coastal Plain sediments. Geology featuring carbonate rocks — including limestone and dolomite from the Devonian Marcellus Formation and Cretaceous Potomac Group — leaches alkaline earth metals, yielding a moderately mineralised, hard supply prone to natural mineral variations with seasonal flow and precipitation.

Hard water causes limescale buildup in pipes, heaters, and fixtures, most affecting water heaters, dishwashers, washing machines, and coffee makers by reducing efficiency and lifespan. Soap scum forms more readily, requiring extra detergent. Regular descaling of appliances, installing drain screens, and flushing hot water tanks annually are advised; a water softener is recommended for households with hard water to extend appliance life. WSSC Water conducts over 500,000 tests annually; treatment includes filtration, disinfection, lime for pH balance, and orthophosphate for corrosion control. The supply complies with lead and copper rules, though older homes may need pipe checks.