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New York City Waterbody Advisories

Combined Sewer Overflow Monitoring Page

Combined Sewer Overflows (CSOs) happen when a wastewater treatment plant or parts of the sewer get too full, usually when there’s a big rain event or snowstorm. When this occurs, wastewater is sent straight into our waterways, without being treated. DEP supplies an online Waterbody advisory application to see how rainfall has impacted NYC Waterways.

Launch NYC Waterbody Advisory Program

combined sewer system

What is a combined sewer system?
A combined sewer system is a sewer system where wastewater and rainwater are collected in the same sewers and then conveyed together to the City's treatment plants.


What is a combined sewer overflow (CSO)?
During heavy rains or snow, combined sewers can fill to capacity and are unable to carry the combined sanitary and storm sewage to the plants. When this occurs, the mix of excess storm water and untreated sewage flows directly into the City's waterways.

map of waterways

Why is there such concern with CSOs?
These overflows concern us because they can increase the number of harmful bacteria and pollutants being released into our open waters. They can also carry trash, litter and toxic chemicals from the streets

CSO Abatement Program

How is DEP addressing the issue of CSOs?
DEP is in the midst of a $1.8 billion CSO Abatement Program, building retention tanks to hold the overflows near heavily impacted bays and tributaries. In addition to construction of these retention tanks, CSO pollution has already been noticeably controlled through improvements in DEP's operation of its treatment plants and sewer system during wet weather.
Managing Combined Sewer Overflows
CSO Projects at DEP