Photo by The Great Photographicon on Flickr.

Grassy fields disguise century-old waterworks at one of DC’s most interesting local historic sites, the McMillan Sand Filtration Plant.

The plant is located just north of DC’s Bloomingdale neighborhood, surrounded by North Capitol Street, Channing Street NW, 1st Street NW, and Michigan Avenue NW. From 1905 to 1985 it was used to purify water for many of Washington’s taps.

The city recently released a preliminary development plan for the site. With changes coming, now is a good time to note the history of the plant, and what exactly lies under those grassy fields you see from the street.

The plant sits behind locked fences, but this past weekend two ANC commissioners were allowed in to give a rare tour of the dormant site.

Beneath the grassy fields visible from the surface there are 25 acres of underground concrete chambers, where the process of water purification was carried out.

Before water could be purified, it had to be delivered to the reservoir. It is still delivered in much the same way.

Water flows through aqueducts all the way from Great Falls to the Dalecarlia Reservoir, and then to the Georgetown Reservoir. From there water flows from the “castle” on McArthur Boulevard NW, at the reservoir’s edge, through an arrow-straight tunnel to the pumping house on 4th Street at the McMillan Reservoir.

Diagram of the Washington City Tunnel by the US Army Corps of Engineers.

The reservoir, which is still active, opened in 1902 and is actually a dammed stream valley. The streams that used to flow here eventually formed Tiber Creek, which ran along what is now Constitution Avenue toward the Potomac.

Since the reservoir stores untreated river water, the water must be cleaned before it can be distributed to residents’ taps.

At the turn of the 20th century a debate ensued regarding the best way to purify water, between proponents of chemical purification and slow sand filtration. Slow sand filtration won out, and Congress provided money to build a sand filtration plant for DC.

McMillan sand filtration site under construction. Photo by the US Army Corps of Engineers.

The process of slow sand filtration is pretty simple. Water fills a cell that contains 2 feet of sand sitting at the bottom. The water percolates through the sand, which traps contaminants. When the water reaches the floor under the sand, it is clean. The water then exits the cell and is distributed into the city’s pipes.

Sand Pit

Sand Pit.

The sand itself required routine cleaning to remove the contaminants. Clean sand was stored in the concrete silos that still stand in rows on the site, visible above ground.


Clean sand silos.

Workers replenished the cells by dumping clean sand through access holes on the roof of each cell. You can still see the circular access covers from the street, and even from satellite photos.


Sand pit.

This early photo shows fresh sand recently dumped into a cell.

Photo from the US Army Corps of Engineers.

Regulator houses such as this one contained valves for controlling the flow of water through each cell.

Regulator House

Senator James McMillan (R - Michigan), famous for his ambitious McMillan Plan to beautify Washington, proposed turning the ground level of the filtration site into a park. The idea found support, and a park was later designed by Frederick Law Olmsted, Jr.

Park lightThe park unfortunately closed to public access during World War II, but many of the original park lamps, walkways, and staircases remain.

In the 1980s the US Army Corps of Engineers built a more modern rapid sand filter adjacent to the reservoir, west of 1st Street NW. With the new rapid filter in place, the old slow filters east of 1st Street became obsolete. The western section of the site still holds the active open-air reservoir and rapid sand filters that today supply clean water to much of Washington.

The western section containing the active reservoir and water treatment plant is closed to the public. What’s most unfortunate is that the western section also contains the most notable feature of the old park.

Shortly after Senator McMillan’s death in 1902, Congress and donors from his home state of Michigan honored the senator with an ornate fountain adorning the park that bears his name. The 1912 fountain, designed by Herbert Adams, contains a bronze sculpture of 3 nymphs on a pink granite base.

Photo by the US Army Corps of Engineers

In 1941 the fountain was dismantled, left in storage, and mostly neglected. In 1983 the top portion of the fountain was moved to Bloomingdale’s Crispus Attucks Park, and in 1992 that section was moved again to its current location at the active reservoir site, where it is locked away from public access.

One can still see the top portion of the fountain by glancing through the fence on 1st Street NW.

McMillan Fountain

The base of the fountain is today somewhere in Fort Washington National Park, in Prince George’s County. Perhaps someday the District, the federal government, and neighbors can raise the funds to reunite and restore the fountain for public enjoyment.

Cross-posted at Left for LeDroit.

Eric Fidler has lived in DC and suburban Maryland his entire life. He likes long walks along the Potomac and considers the L’Enfant Plan an elegant work of art. He also blogs at Left for LeDroit, LeDroit Park’s (only) blog of record.