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DC H2O/150

DC Water Facts

Clean, drinkable water has been pouring through DC pipes and taps for 150 years, since the Washington Aqueduct began operation in 1863. How does this behemoth of an operation work and what role does nature play in supporting it? To answer this, we did our homework. 

  • The Washington Aqueduct project began in 1853 and began distributing water in 1863. All of DC’s drinking water comes from Washington Aqueduct's production facilities at Dalecarlia and McMillan.
  • The first pipes in DC were hollowed-out logs.
  • The Potomac watershed upstream of Washington is about 11,500 square miles and is home to 6.11 million people.
  • Rock Creek was originally proposed as the source for DC’s water. We’re fortunate that planners instead looked to the much more voluminous Potomac. The man in charge of building the aqueduct, Montgomery Meigs, planned for DC’s water supply and envisioned a city as bedecked with fountains as Rome.
  • Montgomery Meig’s name is engraved in the side of the Washington Aqueduct. So is the name Jefferson Davis. (He was the secretary of war when the project began.) Davis’s name was erased when he assumed the presidency of the Confederacy. It was re-engraved by order of President Theodore Roosevelt in 1908.
  • The original aqueduct that brought water from the Potomac River to DC runs through the superstructure of what is now called Cabin John Bridge. When it was built, it was the world’s longest masonry bridge (at 220 feet) and remained so for 40 years.
  • The aqueduct is nine feet wide, almost 10 miles long, and is meticulously inspected every two years.
  • The Potomac River is pretty muddy. Originally, the engineers of the aqueduct allowed river water to just settle in the two reservoirs—the Dalecarlia and the Georgetown—because there was no capability for filtration to remove the sediment particles.  Water quality was better and, importantly, the water system now provided fire protection from a series of hydrants, but until the end of the 18th Century, DC’s water was frequently an unappealing muddy yellow.  In 1905 the McMillan Water Treatment Plant was built in the vicinity of Howard University and then in 1927 the Dalecarlia Water Treatment Plant on MacArthur Boulevard at the Maryland/DC boundary was constructed.
  • Recently, DC conducted blind taste tests, giving people both bottled and DC tap water. In a 2010 study, more than half of tasters preferred the taste of DC tap water. Many couldn’t tell the difference between the two.
  • A pipe, or water service line, connects your house or apartment building to the water main in the street. DC Water is responsible for the quality of the water in the water main, but their responsibility ends at the curb. Most lead that may get into water comes from old plumbing connections here, which are the responsibility of property owners.
  • Many of DC's pipes are between 30 and 50 years old. Some parts of the water and sewer system are more than 100 years old.
  • More than 115 million gallons of water flow through DC’s pipes every day; and an additional 40 million gallons go to Washington Aqueduct's Virginia customers each day.
  • Many older city water systems like DC’s have a combined sewage overflow problem. Usually combined sewer systems transport all their wastewater to a sewage treatment plant, where it is treated and then released into a body of water. But in times when too much rain falls too fast, stormwater and sewage combine and flow together, untreated, into rivers. DC Water is working to address this problem.
  • The DC Water Supply system is downstream of a dozen or so combined sewer overflow systems in West Virginia and western Maryland.
  • Jennings Randolph Lake, an artificial reservoir 200 miles upstream of DC holds 13 billion gallons of water, some of which is used in times of drought. Releases from this reservoir can take up to nine days to reach DC.
  • It’s not something most people enjoy thinking about, but much of our water comes from recycled wastewater from sewage treatment plants. With less than one percent of Earth’s water accessible, we can’t afford to be picky. Plus, it’s safe. Both natural filtration and sophisticated modern techniques keep the water clean and safe.
  • The largest wastewater-based drinking water supply plant is on the Occoquan in Centerville. It pumps treated effluent into Bull Run, which helps dilute it further, then into the Occoquan Reservoir. It’s treated again at Fairfax Water’s treatment plant before coming through your pipes. If it’s good enough for astronauts, it’s good enough for us
  • The changing climate may affect our water system: producing more frequent or extended periods of flood and drought.
  • By 2040, water demand and climate change projections suggest that the DC metro region will either need mandatory water-use restrictions or additional supply in order to make sure everyone has access to clean drinking water.
  • Today, the EPA regulates about 90 potential water contaminants—three times what it regulated just 30 years ago. This is only a tiny fraction though of the total number of man-made chemicals in production and use in the United States.
  • Water utilities have to assess the weak spots of their systems for potential bioterrorist threats. Except we never really know how they do it—to keep the information out of the hands of potential terrorists, the reports are classified. It’s difficult to know exactly how the water is protected or even, sometimes, where the water treatment plants are.
  • Luckily, we have specialized professionals standing sentinel to protect our water. Except some of those sentinels actually swim. The US Army at Fort Detrick in Frederick uses bluegill to test the water. Bluegill are naturally very sensitive to toxicity. Several bluegill tanks guard our water. Their number and location is a classified secret.
  • Oysters help, too! When they filter water for snippets of food, they also strain out sediments and pollutants. One oyster can filter more than 50 gallons of water every day. Mussels, clams, and other marine invertebrates purify freshwater in the same way.
  • Amphibians, like frogs, are the most famous indicator species for water quality, but studies indicate that fish and invertebrates may be better “canaries".
  • Wetland plants can remove toxic chemicals from water. Special pros are water hyacinths, duckweed, water ferns, cattails, and reeds. Some can even absorb toxic heavy metals!
  • Wetlands help purify water. Wetland plants trap pollutants and reduce nutrients including phosphorus and nitrogen.

Join the conversation on Twitter: #DCH2O

The Nature Conservancy protects clean drinking water across the Potomac watershed in Virginia, Maryland, Pennsylvania, West Virginia, and Washington, DC. Using novel - often innovative - techniques, we’re working to ensure clean water for our region for at least another 150 years, and generations beyond. Learn more at

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