Annandale Water Hardness & Quality Report (2026)

Annandale, Virginia receives drinking water from Fairfax Water, serving over two million people in Northern Virginia including Fairfax County. Primary sources are the Potomac River, treated at the James J. Corbalis Jr. Treatment Plant (225 million gallons per day capacity), and the Occoquan Reservoir, processed at the Frederick P. Griffith Treatment Plant (120 million gallons per day). Portions of the system also incorporate water from the Washington Aqueduct's McMillan and Dalecarlia plants, which draw from the Potomac and produce up to 300 million gallons daily. Treatment involves coagulation, sedimentation, filtration, and disinfection.

The Potomac River watershed spans 14,670 square miles across four states and D.C., fed by tributaries including the Shenandoah and influenced by urban, agricultural, and forested lands. The Occoquan Reservoir lies within the smaller 573-square-mile Occoquan River basin, primarily in the Piedmont physiographic province. Geology features fractured metamorphic and igneous rocks upstream of the Potomac, limestone and dolomite outcrops contributing dissolved minerals, and Triassic sedimentary and volcanic rocks around the Occoquan — together imparting a moderately hard character through natural ion exchange and mineral dissolution.

Hard water in the moderately hard to hard range causes moderate scale buildup in pipes, water heaters, dishwashers, and washing machines, reducing efficiency and lifespan. Soap lathering is diminished, leaving residue on skin, hair, and laundry. Regular vinegar cleaning of fixtures, installing drain screens, and biannual flushing of water heaters are recommended; a water softener is advised for households noticing these effects. Fairfax Water's annual reports confirm compliance meeting or exceeding state and federal standards, with no violations noted. Treatment includes corrosion control and lead/copper rule compliance; disinfection byproducts and disinfection residuals are regulated and reported within limits.