NYCDEP Bronx River/MS4 Outfall Study

The Bronx River, which flows through urban areas of Westchester County and the Bronx until it reaches the East River, has been significantly impacted by anthropologic influences. The entirety of this river is classified as “impaired” according to New York State water quality classifications. Stormwater runs down through urban areas in this watershed and may collect pollutants, including pathogens, that are discharged directly into the Bronx River through Municipal Separate Storm Sewer System (MS4) outfalls, potentially contributing to these water quality impairments. In response to this, New York City Department of Environmental Protection (NYCDEP) conducted an MS4 flow monitoring study as part of the Bronx River Combined Sewer Outfall (CSO) Long-Term Control Plan. This study was carried out by NYCDEP until March 2025 and included sampling at five sites in the Bronx River along with three MS4 outfall points in wet weather and dry weather conditions. Sites were selected to capture water quality at MS4 outfalls and the receiving water. However, initial data gaps have led to the need for additional sampling under this study. In order to fill these gaps, Interstate Environmental Commission is partnering with NYCDEP and HDR Engineering and is projected to perform microbiological analyses on the additional samples necessary to complete this study.

Additional sampling will include at least two wet weather events with local rainfall greater than 0.5 inches. Both of these events will include sampling during rainfall and for three days following in order to understand the path of bacteria after a storm. It may also include additional dry weather sampling. Samples will be collected by HDR Engineering and brought to the IEC laboratory for analysis, where IEC staff will perform fecal indicator bacteria testing, namely fecal coliform and Enterococcus, on all provided samples.

The results of this study will be used by NYCDEP to guide outfall long-term control planning. As part of the Bronx River CSO Long-Term Control Plan, NYCDEP must identify appropriate controls to achieve water quality standards in the Bronx River. Identifying bacteria levels in both the Bronx River and MS4 outfalls into the river will provide a better understanding of the contribution of stormwater discharge on elevated bacteria levels in the Bronx River and help to identify a possible source of contamination. Sampling in both dry weather and wet weather may provide a greater understanding of spatial bacteriological patterns in the Bronx River and characterization of differences between wet weather and dry weather bacteria concentrations. This study will help inform management decisions aimed at meeting water quality standards and reducing pollution into this waterway.