Friends of the Croton Watershed

The Croton Aqueduct Degrades Water Quality

by Paul Moskowitz


Millions of gallons of ground water flow unchecked into the New Croton Aqueduct each day because thousands of openings were built into the 100-year-old conduit at its time of construction. Croton water meets primary health standards. The motivation for the construction of a filtration plant for the the Croton system is to improve the taste, color, and odor of the water for the end user. Evidence indicates that degradation of the water results from the transport of ground water carrying dissolved minerals and organics, including contamination from leaking underground storage tanks, into the aqueduct. The conclusion is that sealing the aqueduct over its entire length is a better solution than the construction of a filtration plant.

Original drawing of the Croton Aqueduct from the 1896 publication. The weep holes are on the lower left of the design cross section.

Infiltration of the Aqueduct:

The New Croton Aqueduct was designed and constructed over 100 years ago. Today it carries ten to twenty-five percent of the New York City water supply. The structure is a brick-lined "D" or horseshoe-shaped tunnel with the flat of the "D" serving as the floor. The tunnel is not impervious. After many hours of research, Robert Kornfeld has located a description of the design and construction of the New Croton Aqueduct. Published in 1896, "The Water Supply of the City of New York", by Edward Wegmann, states: "The ground water draining into the tunnel was not excluded as it was of good quality. Small "weepers" (openings) placed about twenty feet apart, when required, allowed this water to flow into the aqueduct. About 4,000,000 U. S. gallons per day enter the conduit between Croton Lake and the Harlem River in this manner" (page 126). The openings are clearly seen in the design cross section, Plate 60, of Wegmann's work.

One hundred years after the 1890 opening of the New Croton Aqueduct the New York City Department of Environmental Protection commissioned Harza Associates to inspect the aqueduct. Their first observations published in a 1994 DEP report (Phase I - Preliminary Report, Contract CRO-196, August 1994) indicate that even though the water flow from Croton Lake was stopped, inspection of the floor "was not always possible due to water flowing" (page 18). Water was entering through the original weep holes. Some were found to yield as much as 20 gallons per minute or 30,000 gallons per day (page 19). Harza Associates estimate 3.2 million gallons of water infiltrate the aqueduct each day (page 32).

Although Harza did not evaluate the quality of the water entering the aqueduct, they did make some relevant observations (page 18): leaching through the brick lining; stalactites of up to 15 inches in length; leaching residues colored orange and brown.

Ground and Surface Water Contamination:

The engineers who built the aqueduct in the 1880's had constructed not just a tunnel to move water, but the world's largest curtain drain. They could be satisfied that they had captured millions of gallons of surface water to add to the water supply.

Today, we have reason to mistrust the quality of ground water. Although more than 100,000 people in northern Westchester and Putnam County drink well water, no municipality allows its residents to drink unregulated surface water because of the dangers posed by pathogens carried by runoff from road surfaces and septic systems. Because of variations in local topography, the Croton Aqueduct is found only a few feet below the surface for some of its length. Water that infiltrates the aqueduct in those regions is not deep well water. It is surface water. Ground water also carries dissolved minerals, such as those observed by Harza, that can discolor the water.

More important is the contamination of ground water by underground storage tanks. In the 1880s, since the horseless carriage was not yet invented, there were no automobiles on the roads and homes were heated by coal. There were no underground gasoline or oil storage tanks. Today the areas under which the Croton Aqueduct passes are thickly populated. From north to south the path of the aqueduct includes Yorktown, Briarcliff Manor, Mt. Pleasant, Greenburgh, Ardsley, Yonkers, and the Bronx.

The contamination of ground water by leaking gasoline storage tanks in our area has been described in a recent article, "Wells contaminated by gas tanks may trigger bigger problem", by Tom Anderson, Gannett Suburban Newspapers, April 12, 1998. Anderson points to incidents within the past ten years at Baldwin Place, Mahopac, Pocantico Hills, Banksville, Scotts Corners, Croton Falls, Ramapo, and recently at Lake Secor which provides drinking water to Carmel. Anderson notes that the New York State DEC reports there are 6,500 underground tanks in Westchester and Putnam and an average of 70 leaks each year.

The vulnerability of the Croton Aqueduct was highlighted by a recent oil spill reported in the Gannett Suburban Newspapers on December 18, 1996 ("3 Agencies probe oil spill into aqueduct" by Mitch Lipka). Over five hundred gallons of oil leaked from a newly-installed storage tank at the Ardsley Library. This oil was discovered and flushed from the system, but ONLY because the aqueduct had been shut down for phase two of the Harza inspection.


The continuing, designed, infiltration of the aqueduct has been documented by the observations of Harza Associates and confirmed by the oil spill at Ardsley. De facto recognition of the problem is given by the Joint Venture (Metcalf and Eddy - Hazen and Sawyer) in their "Conceptual Designs for the Croton Water Treatment Plant", February 1998. For each of the seven sites the New Croton Aqueduct is to be rehabilitated from the location of the filtration plant to Jerome Park. See Table 1.6-1. Tainted water including leaked petroleum products are to be allowed to continue to enter the aqueduct above the filtration plant.

There is a better solution for improving the quality of water delivered to New York City than the construction of a $1 billion filtration plant. The water must be protected at its source and throughout the delivery system. The New Croton Aqueduct must be lined and sealed along its entire length. This will improve the color, odor, and taste of the water by eliminating the infiltration of millions of gallons per day of ground water.


The author acknowledges the contributions of Robert Kornfeld who researched the orignal plans for the New Croton Aqueduct, and Tina Argenti who provided a copy of the Harza Report.

A version of this report was presented at DEP Scoping Hearings: Yorktown on April 29, 1998; and Lehman College on May 7, 1998.