Water Supply-Water Quality Model Framework for Watershed and Reservoir Planning and Management

The Catskill District of New York City's West-of-Hudson water supply system is one of three major reservoir systems that supply water to New York City and includes Schoharie Reservoir, Shandaken Tunnel, and Ashokan Reservoir. Approximately 40% of the City's average demand is provided by the Catskill System, with an average of 50% and 10% provided by the Delaware and Croton Systems, respectively. The New York City Water Supply System (Figure 1) is operated by the New York City Department of Environmental Protection (NYCDEP) to provide more than 1.1 billion gallons per day of water to more than 9 million customers in the City and the surrounding communities. In the Catskill System, Schoharie Reservoir is fed by a 314-square mile watershed, and diverts up to 615 mgd to Esopus Creek via the Shandaken Tunnel. Esopus Creek drains a watershed of 200 square miles, and flows into Ashokan Reservoir. Water from Ashokan Reservoir is conveyed via the roughly 600-mgd Catskill Aqueduct to Kensico Reservoir, where it mixes with water from the Delaware System before disinfection and delivery to NYC. NYC has maintained a continuous waiver from the federal filtration requirements for its Catskill and Delaware surface water supplies since 1993. In 2002 and 2007, the US EPA issued Filtration Avoidance Determinations (FADs) that classify the Catskill and Delaware Systems as meeting the requirements for an unfiltered water supply given in the Surface Water Treatment Rule (SWTR) and the Interim Enhanced Surface Water Treatment Rule (IESWTR), contingent upon construction of a UV disinfection facility. Due to development in the Croton watershed, the Croton System was not eligible for filtration avoidance; construction is underway on a filtration plant to treat Croton water. Despite the typically pristine nature of the West-of-Hudson supplies, the Catskill System is subject to occasional periods of elevated turbidity levels following major storm events. NYCDEP developed the water supply — water quality modeling framework presented here in order to evaluate alternatives for controlling turbidity in the Catskill reservoirs. This paper describes the development of this modeling tool and its application to Catskill turbidity control issues.