Stormwater Retrofitting to Protect Drinking Water Reservoirs from the Impacts of Urban Runoff - Part 1

May 29, 2002
To meet federal Surface Water Treatment Rule requirements & U.S. Environmental Protection Filtration Avoidance mandates, the NYC Environmental Protection Dept developed a proactive program

About the author: James D. Benson, AICP, CPESC, is a supervisor for the NYC Department of Environmental Protection, 465 Columbus Avenue, Valhalla, N.Y. 10595, phone: 914-742-2034, e-mail: [email protected]. Melissa Beristain, CPESC, works with Benson in the New York City Department of Environmental Protection’s Bureau of Water Supplies.


The prime components of the program are stormwater and waterfowl management, sewer system inspection and repair, an in-reservoir turbidity curtain, reservoir dredging and hazardous spill containment. Protecting water quality in the Kensico Reservoir is particularly important because it is the final impoundment for more than one billion gallons (90 percent) of New York City’s unfiltered water supply before it enters the distribution system.

This paper summarizes the stormwater management element of the program and its control of the two key pollutants regulated by the SWTR: fecal coliform bacteria and turbidity that are conveyed to the reservoir by stormwater.

The first phase of the project, watershed assessment, site selection and stormwater management facility screening and design, is complete. Facility construction began in the spring of 1999 and was completed in 2001.

DEP also developed a protocol to assess the effectiveness of the program, collected baseline and storm event water quality data prior to facility construction, and will collect data from representative stormwater facilities once they are operational to assess the effectiveness of the nearly $15 million program.

New York City (the City) has placed great emphasis on protecting and improving the quality of its drinking water supply through watershed protection and management programs. One such program was developed and implemented by DEP’s Bureau of Water Supply.

The City’s drinking water supply system is one of the largest in the world, supplying approximately 1.33 billion gallons of potable water each day to some 9 million City and upstate residents. The entire watershed covers 1,969 square miles on both sides of the Hudson River and is composed of 19 reservoirs, three controlled lakes and numerous wetlands, watercourses and intermittent streams. Land use, topography, hydrology and political climates in the system’s three watersheds (i.e., the Delaware, the Catskill and the Croton [Figure 1]) vary dramatically. One reservoir, the Kensico, is integral in managing unfiltered systems because it serves as the final impoundment for water from the Catskill and Delaware watersheds before it enters the City’s distribution system through Kensico’s two effluent chambers. On average, approximately 1.3 billion gallons flow through the Kensico Reservoir each day, accounting for 90 percent of the system’s daily demand. For this reason, it is critical to protect water quality in the Kensico. Controlling stormwater entering the reservoir is an important part of the City’s Kensico Water Quality Protection Program.

The United States Environmental Protection Agency (EPA) recognizes the importance of the Kensico Reservoir and has required that the DEP implement an aggressive watershed management and protection plan that reduces fecal coliform bacteria and turbidity inputs into the Kensico Reservoir. Elements of DEP’s plan include aggressive stormwater and waterfowl management programs, sewer and septic system inspection and repair, an in-reservoir turbidity curtain, hazardous spill containment and a variety of other regulatory and nonregulatory nonpoint source pollution control programs. Other components of DEP’s Kensico Program include the following.

Sewer Inspection and Repair

The sewer system within the watershed, including type and size of pipe and manhole locations, was mapped by the City. Of the 95,000 feet of sewer line in the watershed, 55,000 feet were installed before 1970 and are more prone to defects. The older sections of sewer line were inspected using a video camera to locate potential sources of exfiltration. The inspection program found 39 segments and three manholes in need of repair. The Town of Mount Pleasant and Westchester County completed the repairs under intermunicipal agreements with City funding.

Stormwater Infrastructure Inspection/ Sewer System Disconnection

To comply with the FAD requirement to track down discharges of wastewater into storm sewers, DEP is digitally mapping the storm sewer system in the Kensico watershed. When the mapping is completed, DEP will video inspect the storm sewer system in areas of the Kensico watershed served by sanitary sewers and locate any illicit wastewater connections that discharge into the storm sewer system. The results of the inspection will be referred to DEP and County enforcement officials for appropriate enforcement and remediation actions.

Waterfowl Management

The gull and waterfowl management program is designed to reduce the number of geese and gulls roosting and defecating in or near the surface water. Through hazing, shoreline meadow management, egg addling, physical barriers and Canada geese egg depredation, the City has dramatically reduced the amount of bird waste that enters the reservoir. The program, implemented August 1 through March 31 each year, also includes research into new methods of bird control and ongoing assessments of program effectiveness. Although labor intensive, the waterfowl management program has been an overwhelming success in eliminating the greatest source of fecal coliform bacteria to the Kensico Reservoir.

Turbidity Curtain

A turbidity curtain installed at the mouth of Malcolm Brook in the southwest section of the Kensico Reservoir successfully directs turbidity and fecal coliform bacteria conveyed to the reservoir by two watercourses away from the Catskill Upper Effluent Chamber. Maintaining high quality water in the effluent chamber is critical as water is conveyed directly to the distribution system from the chamber. Water entering the chamber is constantly monitored to determine compliance with the SWTR. Due to the curtain’s effectiveness, the City will maintain it indefinitely.

Reservoir Dredging

The channels leading to the reservoir’s two effluent chambers were dredged in 1999 to eliminate the potential for accumulated sediments to be resuspended during storms and impact the quality of water entering the effluent chambers. Some 1,777 cubic yards of sediment were excavated, dewatered and removed from the Kensico Reservoir, significantly reducing the potential for elevated turbidity in the reservoir.

Failing Septic System Detection and Remediation

DEP is conducting a house-to-house survey to identify any watershed residences with septic systems to determine if any systems are failing. DEP conducted a similar survey in 1991 and is applying a May 1998 “Methodology for Prioritizing Routine Inspections to Detect Septic System Failures” to stream and reservoir water quality monitoring data to identify potential septic failures. DEP also routinely patrols the watershed for potential water quality threats, including failing septic systems. Potential septic failures are thoroughly investigated and promptly remediated.

Part 2 will detail the three main phases of stormwater management at the Kensico Reservoir and also will focus on the inspection and maintenance of facilities.

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