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	Washington Aqueduct
	U.S. National Register of Historic Places
	U.S. National Historic Landmark
	The Union Arch Bridge carries the Washington Aqueduct across Cabin John Creek in 2008
	Dalecarlia Reservoir & Treatment Plant
Location	5900 MacArthur Blvd., NW; Washington, D.C.
Coordinates	38°56′15″N 77°6′51″W / 38.93750°N 77.11417°W
Built	1853-1864
Architect	Montgomery C. Meigs
NRHP reference No.	73002123
Significant dates
Added to NRHP	September 8, 1973
Designated NHL	November 7, 1973

The Washington Aqueduct is an aqueduct that provides the public water supply system serving Washington, D.C., and parts of its suburbs, using water from the Potomac River. One of the first major aqueduct projects in the United States, it was commissioned by the U.S. Congress in 1852, and construction began in 1853 under the supervision of Montgomery C. Meigs and the U.S. Army Corps of Engineers. Portions of the aqueduct began operation on January 3, 1859, and the full pipeline began operating in 1864.^[1]^: 68

The system is owned and operated by the U.S. Army Corps of Engineers and has been in continuous use ever since. It is listed as a National Historic Landmark. The Union Arch Bridge, which carries a portion of the aqueduct, is also listed as a Historic Civil Engineering Landmark.

Design and facilities

The centerpiece of the Aqueduct is a 12-mile (19 km) pipeline that connects the system's dam at Great Falls with the Dalecarlia Reservoir on the border with Montgomery County, Maryland. Portions of the Aqueduct went online on January 3, 1859, and the full pipeline began operating in 1864.^[1]^: 68 The pipeline runs along what is now MacArthur Boulevard, traversing some of the higher cliffs along the Potomac River.

The Union Arch Bridge carries the pipeline and MacArthur Boulevard over Cabin John Creek and the Cabin John Parkway near the community of Cabin John, Maryland. This bridge was the longest masonry arch bridge in the world for 40 years.^[2] ^[3]

The Dalecarlia Reservoir serves as a primary sedimentation basin. A portion of the water from the reservoir is treated at the nearby Dalecarlia Water Treatment Plant and distributed to municipal water mains. The remainder of the water from the reservoir flows to the Georgetown Reservoir in the Palisades neighborhood of Washington. This facility serves as an additional sedimentation basin, and then the water flows through the Washington City Tunnel to the treatment facility at the McMillan Reservoir, after which it is pumped through city mains. The Dalecarlia Reservoir was modified in 1895 and 1935 to improve water quality and increase water supply.^[1]^: 99

System expansion

The system originally used a single pipe for water delivery, and did not have any water purification plants, relying instead on the reservoirs to act as settling basins. By the turn of the 20th century, however, Washington's growth and the high amount of sediment in the Potomac's water kept the reservoirs from doing their jobs well, and so the first treatment plant, a massive slow sand filter bed system, was installed at McMillan Reservoir, and was completed in 1905.^[4]^: Ch.4 Improvements in the early 1900s were planned and supervised by Army engineer Henry C. Newcomer.^[5] The regular use of chlorine as a disinfectant began in 1923 at the McMillan plant. The McMillan plant was not replaced until 1985, when a rapid sand filter plant was opened next to it.^[1]^{: 124–125}

In the 1920s, the aqueduct was upgraded with the addition of a second pipe from Great Falls to Dalecarlia, along with several new reservoirs and a pumping station, A rapid sand filter plant was built at Dalecarlia Reservoir, which went online in 1927.^[4]^{: 204–205} The Dalecarlia plant is the larger of the two plants in the system, having been upgraded in the 1950s, and is the plant that serves the Virginia communities that use the Aqueduct.

In 1926, Congress approved selling water from the aqueduct to Arlington County, Virginia.^[1]^: 105 A new water supply pipe was constructed at Chain Bridge and service to Arlington began in 1927. Additional pipes were built as Arlington's population grew, including one under the Potomac River. In 1947, Congress approved adding the city of Falls Church, Virginia, to the aqueduct system, and nearby portions of Fairfax County, Virginia, were added in the 1960s.^[1]^{: 129–130}

The U.S. Army Corps of Engineers built an additional intake and pumping station at Little Falls in 1959.^[4]^: 256

Operations and service area

The Aqueduct is a wholesale water supplier, and the communities it serves are responsible for billing customers and managing water mains. The service area is: