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MS4 Survey of Croton-Kensico Watershed

Stormwater and MS4s

Stormwater runoff flowing to a storm drain.
Stormwater runoff often collects
debris and pollutants on its way to
storm drains and local waterways.

When it rains, or when snow melts, where does the water go? Called stormwater, some of it soaks into the ground, while the rest flows across streets, sidewalks, parking lots, driveways, and lawns before entering local waterways or storm drains. As it flows over these surfaces stormwater collects trash, oils, sediment, nutrients and other pollutants. In urban areas, stormwater that flows into a storm drain usually moves through a network of pipes and ditches called a Municipal Separate Storm Sewer System (MS4) before entering a nearby waterbody.

In New York, urban areas with MS4s need a State Pollutant Discharge Elimination System (SPDES) permit for their stormwater discharges. The SPDES permit requires the city or town to develop a Stormwater Management Program to help keep harmful pollutants from entering the sewer system and flowing to a local waterbody. One important piece of the Stormwater Management Program is identification and mapping of storm sewer features within the MS4. Identifying and mapping storm sewer features helps MS4s find illegal connections and discharges and maintain their system.

Project Overview

In 2009, the Interstate Environmental Commission (IEC) received a $28,500 American Recovery and Reinvestment Act (ARRA) grant from the DEC to to develop a regional Geographic Information System (GIS)-based map of the stormwater conveyance features of an MS4 in a subwatershed of the Croton/Kensico watersheds. The map will help local officials detect the source of flows other than stormwater, known as illicit discharges, in the system. Once identified, the illicit discharges can ultimately be eliminated, improving the quality of the waterways that the stormwater discharges into.

Project Status

GPS unit on storm drain
A GPS device was used to verify
locations of storm sewer features.
Photo courtesy of IEC.

IEC finished work on this project in July, 2011. The project consisted of two phases:

  1. IEC selected the Purdy Lake MS4 community in Somers as the project location and used existing data to identify all storm sewer features, including outfalls, catch basins and manholes, within the subwatershed.

  2. IEC field verified and gathered data about each storm sewer feature. This included: locations of outfalls; type of conveyance system; pipe material, shape and size for closed pipe systems; channel/ditch lining material, shape and dimensions for open drainage systems; location and dimensions of culvert crossings; drop inlet, catch basin and manhole locations; number and size of connections (inlets/outlets) to catch basins and manholes and direction of stormwater flow.

    Map showing locations of stormwater features.
    Locations of stormwater conveyance features in the
    Croton River North Basin displayed on the GIS-
    based map created by IEC.

    IEC used the storm sewer feature data to create a map that will help the Purdy Lake MS4 community find illegal connections and discharges and maintain the system.

Final Progress Report

When each ARRA 604(b) project is complete, DEC requires a final report summarizing the entire project to be submitted. The report includes a description of the project's goals, work accomplished, and final project outcomes.

To view the final progress report for this project, click the following link:

MS4 Survey of the Croton-Kensico Watershed, Westchester County, New York - Final Report (PDF) (126 KB)

About the Interstate Environmental Commission and the Croton-Kensico Watershed

IEC is an interstate water and air pollution control agency serving New York, New Jersey and Connecticut. Originally formed in 1936 by New York and New Jersey, Connecticut joined in 1941. In coordination with its member states and the U.S. EPA, IEC enforces water and air quality regulations in the three states, with the authority to enforce laws across state lines. Through promotion of public participation, outreach activities, and research initiatives, IEC has expanded its initial scope of activities to advance water quality education efforts.

The Town of Somers is located in northern Westchester County, in the Croton River North Basin, which is a subwatershed of the Croton-Kensico Watershed. The Croton-Kensico Watershed is part of the New York City water supply system that supplies drinking water to over nine million people. The Purdy Lakes community is located between the Croton Falls and Muscoot reservoirs, both important reservoirs in the New York City East-of-Hudson water supply system.

Contact Information

Interstate Environmental Commission

311 West 43rd Street, Suite 201, New York, NY 10036