About 90 percent of the total New York City water supply is derived from the City's six West-of-Hudson reservoirs in the Catskill Mountains and the Upper Delaware River Basin (the Catskill/Delaware system). Water from those reservoirs is delivered via the Catskill and Delaware aqueducts, which cross beneath the Hudson, to a common destination at Kensico Reservoir, 15 miles north of New York City. There, water from both aqueducts mixes and, after 15–25 days of storage time, it is chlorinated and delivered to the City and other user communities. The state of water quality in Kensico Reservoir is thus critical: if it becomes degraded, it would contaminate the high-quality water derived from the other Catskill/Delaware reservoirs. Although both aqueducts are designed to permit bypassing of Kensico, bypassing is undesirable because the settling time, mixing, and flow control that Kensico provides would be lost.

Kensico Reservoir, with a capacity of 30 billion gallons, was placed in service in 1915 with the damming of the Bronx River. Although most water in Kensico Reservoir is delivered by the two trans-Hudson aqueducts, it also receives runoff from its immediate drainage area of 13.1 square miles. Largely forested, this area contains about 1,500 dwelling units, a number of office parks and other commercial facilities, as well as transportation facilities. The latter include Westchester County Airport, Interstate 684, and state highway 22, which borders the reservoir and crosses it at one point. Stormwater from each of these facilities and structures drains into Kensico.

Recent violations in stormwater runoff quality from the airport, the proposed widening of Routes 22 and 120, and new office construction have made the Kensico watershed the subject of much discussion (Marx and Goldstein, 1999). These projects pose serious threats to water quality in Kensico Reservoir if stormwater management is not undertaken to counteract associated increases in pollutant loading. In particular, wetlands bordering the reservoir will likely be removed to accommodate roadwidening, thereby reducing the ability of the surrounding buffer land to diminish pollutant loading from stormwater. If such projects go forth, stormwater management must (1) control all new pollutant loadings, (2) compensate for losses in nonpoint source pollutant control via wetland functioning, and (3) mitigate current problems with runoff from the Westchester County Airport.

To confront these challenges, NYC DEP has developed the Kensico Stormwater Management Plan, which currently includes 44 stormwater facilities in 16 subbasins to reduce loadings of turbidity and fecal coliform bacteria (and associated pollutants) to the reservoir. The stormwater