Where New York City's water comes from

New York City gets its drinking water from 19 reservoirs and three controlled lakes spread across a nearly 2,000-square-mile watershed. The watershed is located upstate in portions of the Hudson Valley and Catskill Mountains that are as far as 125 miles north of the city. New York City's water supply system is composed of three surface water supplies called the Catskill, Delaware and Croton.

The Croton system, in Putnam and Westchester, is the nineteenth-century original. The Catskill and Delaware systems, built across the early twentieth, now carry roughly nine gallons of every ten.

In all, this system serves half the population of New York State. The Catskill and Delaware systems provided 91.3 percent of the water and the Croton system provided 8.7 percent.

Half the state, one system. The city is the anchor customer, but the pipes do not stop at the city line.

The system also provides about 110 million gallons a day to one million people living in Westchester, Putnam, Orange, and Ulster counties.

The upstate counties that host the reservoirs drink from them too, by law and by pipe.

Due to the very high quality of our Catskill and Delaware supply, New York City is one of only five large cities in the United States that does not filter its surface drinking water supply. The Catskill/Delaware supply operates under a filtration waiver, known as the "Filtration Avoidance Determination" (FAD).

The waiver is re-earned, not permanent. State health officials revisit its terms on a schedule; the current revisions run the arrangement through at least 2027.

To comply with the FAD, DEP spends over $100 million annually on watershed protection programs that conserve watershed lands, upgrade wastewater infrastructure for local municipalities, implement clean water strategies on watershed farms, and manage streams, forests, and other natural resources that affect water quality.

This is the deal underneath your tap: buy land, pay for upstate sewer upgrades, and work with watershed farms so the reservoirs stay clean enough that no filter is needed.

It is then treated with ultraviolet (UV) light at the Catskill/Delaware Ultraviolet Disinfection Facility in Westchester County. The facility, the largest of its kind in the world, can disinfect more than two billion gallons of water per day.

The world's largest UV plant, and most New Yorkers have never heard of it. It kills what chlorine alone might miss, without changing the water's taste.

Water from the Croton supply is treated at the Croton Water Filtration Plant, an underground facility in the Bronx. The plant can treat up to 290 million gallons of drinking water each day

Croton water gets coagulation, dissolved air flotation, carbon-and-sand filtration, and then the same chlorine, UV, and additives as the mountain supply.