Updated November 1, 2026

The capital's water is treated by the Army

Washington's drinking water starts in the Potomac River. Two layers of government handle it. The U.S. Army Corps of Engineers' Washington Aqueduct draws and treats the water, and DC Water distributes it through 1,300 miles of pipe, nearly 44,000 valves and more than 9,500 hydrants.

Great Falls to your tap

Raw water enters at intakes at Great Falls and Little Falls, then moves through the Dalecarlia and Georgetown reservoirs to the Dalecarlia and McMillan treatment plants. Treatment includes:

• coagulation, flocculation and sedimentation to settle out river silt
• filtration, which also removes Cryptosporidium
• pH adjustment and fluoridation
• disinfection with free chlorine, then chloramine (chlorine plus ammonia) for the trip through the pipes
• orthophosphate for corrosion control

DC Water's water quality team ran more than 40,000 tests across the District in 2025. Upstream, the Interstate Commission on the Potomac River Basin coordinates work on farm runoff, spill response and watershed education. The report calls today's watershed cleaner and healthier than ever.

Disinfection byproducts

Potomac water carries organic matter, especially after storms, and chlorine reacts with it. In 2025 the highest locational running annual average was 50 ppb for total trihalomethanes (single results 17–73, limit 80) and 34 ppb for HAA5 (14–48, limit 60). Those numbers are higher than in many groundwater cities but within the law. Chlorine residual peaked at a 3.1 ppm running annual average. Of 3,011 coliform samples, 11 were positive. The worst month reached 1.2%, well below the 5% trigger.

PFAS: low and voluntary

The Aqueduct sampled for PFAS in 2025 at both plants, before federal compliance deadlines:

PFHxS, PFNA and GenX were not detected. To push PFAS close to zero at home, see our PFAS and lead filter selector.

Lead

Lead is the defining water issue in DC because of the pipes. Tap monitoring in 2025:

• January–June: 90th percentile 1 ppb, 2 of 105 samples above 15 ppb, highest 243 ppb.
• July–December: 90th percentile 2 ppb, 2 of 106 above, highest 123 ppb.

The system as a whole passes easily, but a few homes have very high results. That's typical where lead service lines are still in the ground. Lead also builds up in faucet aerator screens, so cleaning them is worth doing. Hardness averaged 139 ppm (8 gpg) and sodium 32 ppm, which explains the white crust on kettles.