New York City has the largest unfiltered surface water supply in the world. Every day, some 1.3 billions gallons of water from this vast system is delivered to eight million New York City residents, one million more consumers in four upstate counties and hundreds of thousands of commuters and tourists. The New York City Water Supply System includes a watershed of 1,969 square miles across eight counties north and west of the City: Westchester, Putnam and Dutchess on the east side of the Hudson River and Delaware, Greene, Schoharie, Sullivan and Ulster in the Catskill Mountains, west of the Hudson. The system's 19 reservoirs and three controlled lakes contain a total storage capacity of 580 billion gallons.
With the pending adoption of the Surface Water Treatment Rule by the USEPA in 1989, New York City was required to either take steps to provide filtration for their 1.3 Billion gallons/day supply or meet a series of water quality, operational and watershed control criteria referred to as the filtration avoidance criteria.