New York City: Most of Its Tap Water Is Never Filtered

About 90 percent of New York City's drinking water reaches the tap without ever passing through a filter. The system collects surface water by gravity from 2,000 square miles of upstate land, as far as 125 miles from the city. The Catskill/Delaware portion is treated only by chemical and ultraviolet disinfection, and the network as a whole runs to 19 reservoirs and three controlled lakes. 12

That runs on a waiver rather than a treatment plant. The Catskill/Delaware supply has met the criteria for filtration avoidance from January 1993 to the present, and New York State was still preparing the next determination, for 2027, in July 2026. 34

The water arrives through the Delaware Aqueduct, which carries about half of the city's supply each day. At 137 km (85 miles) from Rondout Reservoir to Hill View Reservoir, it is the longest tunnel in the world. It was built between 1937 and 1953, and in places it runs about 750 metres below the surface. 5

That same tunnel is leaking. Britannica puts the loss from two significant leaks at an estimated 20 million gallons a day, and the city's Department of Environmental Protection says its monitored leaks can release upwards of 35 million gallons per day. The repair is a 2.5-mile bypass tunnel 600 feet beneath the Hudson River, part of a $2 billion program whose final connection is not expected to be completed until after 2027. 25

The Croton system, the oldest of the city's three upstate supplies, is the part that is filtered. Its plant, beneath Van Cortlandt Park in the Bronx, was activated in 2015 and can filter up to 290 million gallons a day. 6