McMillan Park and Sand Filtration Site is a twenty-five acre green space and decommissioned water treatment plant in northwest Washington, D.C. connected to the McMillan Reservoir. It is bound on the north by Michigan Avenue, on the east by North Capitol Street, on the south by Channing Street and on the west by First Street. Two paved courts lined by regulator houses, tower-like sand bins, sand washers and the gated entrances to the underground filter cells provided a promenade for citizens taking the air in the park. Below grade there are twenty catacomb-like cells, each an acre in extent, where sand was used to filter water from the Potomac River by way of the Washington Aqueduct. The purification system was a slow sand filter design that became obsolete by the late 20th century. In 1985 a new rapid sand filter plant replaced it across First Street beside the reservoir. The treatment system is operated by the Army Corps of Engineers. Public access to the site has been restricted since World War II when the Army erected a fence to guard against sabotage of the city's water supply. Specially arranged biannual tours are supported by scores of visitors curious about the odd-looking structures. In 1991, the D.C. Historic Preservation Review Board designated McMillan Park a Historic Landmark and nominated the site for the National Register of Historic Places. It included the site on their "List of Most Endangered Properties in 2000" and again in 2005.

The Booterstown nature reserve is centred on a marsh that derives from the building of the Dublin and Kingstown railway, one of the first in the world, in 1834-35. Railways prefer to go in straight lines, so that the engineers decided to build an embankment rather than follow the complex curve of the shoreline, and this cut off two arcs of the tidal shore. By 1876 the land at Booterstown was in use for agriculture, made possible by the simple but ingenious water control system. A large gate valve was installed at the Williamstown outlet to the sea, which was lowered when the tide came in and raised when it fell. The Williamstown lagoon acted as a sump to collect the water draining from the agricultural land when the valve was closed, and this emptied over the strand when the gate was raised. Some soil must also have been imported, as the level of the marsh is higher than the strand outside the railway, and the land was irrigated with fresh water from the Trimlestown Stream that now flows through a culvert, and from a stream flowing under The Punchbowl that was sealed off when the car park for the station was created. The Nutley Stream also flows along the railway line from behind Merrion House.