Fresh, disinfected, from the Catskills: How your tap water gets to Westchester

The water that fills most Westchester residents’ drinking glasses arrives there after a journey south from Catskills reservoirs.

As a multi-year project launched this month to add a new tunnel to the water supply system, here’s a (very) simplified look at how things work in what is the nation’s largest such system.

Where does Hudson Valley get their drinking water from?

Gravity-fed, the system serves 8.8 million New York City people and a million people in Westchester, Putnam, Orange and Ulster counties. Ninety percent of Westchester tap water is from it. (People not on the system get water from wells or other sources).

The water's journey starts well north of the highly populated areas that the supply system serves. The system supplies roughly half the state's population.

The water gets to the Kensico Reservoir in Westchester via two routes: One is the Catskill Aqueduct, which draws from Ulster County’s Ashokan Reservoir, and the other is the Delaware Aqueduct, which draws from Ulster County’s Rondout Reservoir. Various reservoirs involved feed directly into those two reservoirs, according to a spokesperson for the New York City Department of Environmental Protection, which oversees and operates the water system.

The 85-mile Delaware Aqueduct, deep underground, is the world’s longest tunnel. The Catskill Aqueduct is not a tunnel, but, rather is at surface and spans 92 miles.

They bring water to Kensico. A number of municipalities receive water that is treated at Kensico, while others purchase water wholesale from the system and can do their own treatment of it.

How your drinking water is disinfected

The city DEP's treatment includes chlorine to meet disinfection standards, fluoride to help prevent tooth decay, and orthophosphate that creates a coating to reduce release of metals, such as copper and lead, from household plumbing, according to a DEP fact sheet.

All water is treated for enhanced disinfection at a location close to Kensico: The world's largest ultraviolet water treatment facility — the Catskill-Delaware Ultraviolet Light Disinfection Facility — which started operating in 2012 in Eastview, in the town of Mount Pleasant.

Project to link water system serving Westchester will bolster resiliency

Now, work has launched on a new, $1.9 billion, two-mile connection of Kensico Reservoir and the Mount Pleasant treatment facility.

A machine will help to create the 27-foot diameter tunnel, to be dug hundreds of feet underground, that will link with the system of reservoirs, tunnels, aqueducts, and water mains that brings roughly a billion gallons of drinking water a day to Westchester and New York City.

The project, which consists of four contracts for various aspects, is expected to span 10 years or so. The reasons given for the project are:

• A section of Catskill Aqueduct from the Kensico Reservoir has insufficient pressure to supply the treatment facility in Eastview/Mount Pleasant.

• Delaware Aqueduct is the lone aqueduct to supply the Mount Pleasant facility but there’s limited ability for the aqueduct to be shut down for an extended period.

The new pipeline is meant to bolster resiliency.

In the project’s first phase, two large shafts will be connected by a tunnel. The Kensico-to-Eastview tunnel will be 27 feet in diameter. The idea is it to allow flexibility to take other parts of the water supply system out of service for inspection and maintenance.

And a 100-year-old intake chamber at Kensico Reservoir will be enlarged to bring water into the new tunnel. Shoreline around the chamber will be improved to keep sediment from entering the tunnel.

The Catskill and Delaware aqueducts end at Yonkers' Hillview Reservoir, a kind of holding area that is the final point before drinking water enters three main New York City tunnels for residents there.

USA Network New York state team reporter Tom Zambito contributed to this article.

This article originally appeared on Rockland/Westchester Journal News: Fresh, disinfected, from the Catskills: How your tap water gets to Westchester