Washington Metropolitan Area Water Supply

The Washington Metropolitan Area draws drinking water from a single dominant surface source — the Potomac River — serving a combined population exceeding 6 million people across the District of Columbia, suburban Maryland, and Northern Virginia. That concentration of dependency on one river system creates a structural vulnerability that has driven nearly 150 years of coordinated infrastructure investment, interstate compact agreements, and federal oversight.

Primary Water Sources

The Potomac River supplies approximately 80 percent of the metropolitan area's treated drinking water, according to the Interstate Commission on the Potomac River Basin (ICPRB). Three major intake points — Great Falls (Maryland side), Little Falls (D.C.), and Occoquan Reservoir (Virginia) — divide the regional draw among the major utilities. The Occoquan Reservoir, a managed impoundment on a Potomac tributary, provides a secondary buffer for Fairfax Water and portions of Prince William County.

Groundwater plays a limited but measurable role. Loudoun County and portions of the Virginia Piedmont draw from fractured-rock aquifers, though yield rates from those formations are substantially lower than surface-water systems and insufficient to serve dense suburban municipalities independently.

The Three Major Utilities

Washington Aqueduct

The Washington Aqueduct, operated by the U.S. Army Corps of Engineers, is the oldest component of the regional water system. It intakes raw Potomac water at Great Falls and Little Falls, treating it at the Dalecarlia and McMillan treatment plants before distributing finished water wholesale to DC Water, Arlington County, and the City of Falls Church. Capacity at the two plants totals approximately 270 million gallons per day (MGD), according to the Corps of Engineers.

DC Water

DC Water (District of Columbia Water and Sewer Authority) serves approximately 700,000 residents within the District and purchases treated water wholesale from the Washington Aqueduct. DC Water also operates a Blue Plains Advanced Wastewater Treatment Plant on the Anacostia River — the largest advanced wastewater treatment facility in the world by capacity at 370 MGD — which discharges treated effluent to the Potomac. This dual role as both a drinking water distributor and a major wastewater discharger places DC Water at the center of the regional water quality cycle.

WSSC Water

Washington Suburban Sanitary Commission (WSSC Water) serves Montgomery and Prince George's counties in Maryland, collectively among the most densely populated suburban jurisdictions in the mid-Atlantic. WSSC Water operates 2 water filtration plants — the Potomac and Patuxent facilities — with a combined permitted treatment capacity exceeding 280 MGD. The Patuxent plant draws from the Patuxent River watershed, giving WSSC a partial source diversification that DC Water and Arlington County do not have.

Interstate Coordination and the ICPRB

No single state authority governs the Potomac. The river forms the boundary between Maryland and Virginia and flows through the District of Columbia before reaching the Chesapeake Bay. The Interstate Commission on the Potomac River Basin (ICPRB) coordinates flow allocation, drought response, and low-flow supplementation across all jurisdictions. The Section for Cooperative Water Supply Operations on the Potomac (CO-OP program), managed by ICPRB, enables the three major utilities — Washington Aqueduct, WSSC Water, and Fairfax Water — to shift intake operations in real time based on river quality and flow conditions. During drought or high-turbidity events, utilities can draw from reservoir storage at Jennings Randolph Reservoir on the North Branch Potomac or Little Seneca Lake in Montgomery County.

Regulatory Framework

Drinking water quality standards derive from the federal Safe Drinking Water Act, administered by the EPA with delegated enforcement authority in Maryland assigned to the Maryland Department of the Environment and in Virginia to the Virginia Department of Environmental Quality. The District of Columbia operates under direct EPA oversight because it lacks statehood.

Treatment requirements under the Surface Water Treatment Rule and the Lead and Copper Rule set the baseline standards all three major utilities must meet. The 2004 lead contamination episode in DC — in which elevated lead levels exceeding 300 parts per billion were detected in some homes — prompted a decade of pipe replacement and monitoring reforms that have since been cited in EPA enforcement guidance as a national reference case (according to the EPA).

Water Quality Monitoring

The U.S. Geological Survey Virginia and West Virginia Water Science Center maintains continuous monitoring stations on the Potomac that track turbidity, dissolved oxygen, conductivity, and nutrient loading. These real-time data feeds directly inform intake decisions by the CO-OP utilities. Nutrient loading — particularly nitrogen and phosphorus from agricultural runoff in the upstream watershed — remains an active management challenge. The Chesapeake Bay Program has set Total Maximum Daily Load (TMDL) limits for both nutrients under EPA authority, requiring point-source dischargers including DC Water's Blue Plains plant to meet specific nitrogen effluent thresholds.

Regional Planning and Infrastructure Gap

The Northern Virginia Regional Commission identifies water supply capacity as a tier-one infrastructure concern for its member jurisdictions, which include Arlington, Fairfax, Alexandria, and Prince William counties. Population growth in the outer jurisdictions — Loudoun County added over 40,000 residents between 2010 and 2020 (according to U.S. Census Bureau data) — is placing incremental pressure on existing treatment and distribution capacity. Fairfax Water has responded with a capital improvement program that includes expanded raw-water storage at the Griffith facility. Prince William County's service areas overlap between Fairfax Water and the Western Prince William Service Authority, creating a distribution patchwork that regional planners have flagged as a coordination challenge.

References

The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)

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