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CSO Wet Weather Advisory

Advisory for CSO outfalls during and following rainfall and snowmelt events

Facilities with permitted CSO discharges are required by law to post signs at all CSO outfalls to alert the public that the water may be contaminated with untreated sewage after a rainfall event.

Combined sewer overflow (CSO) sign advising people to avoid contact with the waterbody during and following wet weather events.
Example of a sign posted at a combined sewer overflow
(CSO) outfall. The sign advises people to avoid contact
with the water during and following wet weather events.
Photo: NYC Department of Environmental Protection.

CSO outfalls may discharge rainwater mixed with untreated sewage during or following rainfall or snowmelt events and may contain bacteria that can cause illness.

Avoid contact or recreation (swimming, boating, and fishing) within the waterbody during or following a rainfall or snowmelt event.

Find out if you live, or recreate in a CSO community. For more information about CSOs visit DEC's Combined Sewer Overflow (CSO) web page.

Google Earth icon

Combined Sewer Overflow (CSO) Outfalls Google Earth Map (requires Google Earth to be installed on your computer)-This map provides location and general facility information, receiving waterbody, and information about overflow events. Learn more about the map.

Several municipal facilities have web pages dedicated to their CSO programs with more detailed information: New York City Department of Environmental Protection, Buffalo Sewer Authority, and Onondaga County Department of Water Environment Protection. Links to their CSO program web pages are in the right-hand column under "Links Leaving DEC's website".

Potential Impacts of CSOs discharges

Discharges from CSO outfalls may contain mixtures of domestic sewage, storm water runoff, and sometimes, industrial wastewater, including: high levels of suspended solids, toxic chemicals, floatable material and other pollutants.

A brown and yellow sign warning of discharge of untreated wastewater
Another example of a sign posted
at a combined sewer overflow (CSO).

The polluted CSO outfall discharge may:

  • cause a waterbody to exceed NYS water quality standards,
  • impact human and aquatic health,
  • lead to bathing beach closures due to bacterial contamination
  • aesthetic impacts due to floating debris or slicks,
  • close shellfishing beds,
  • promote the growth of algae, and
  • reduce oxygen levels in the water.

What does the CSO Outfall map tell you?

The map shows the locations of permitted CSO outfalls in NYS. CSOs are part of a combined sewer system, which are required to have a State Pollutant Discharge Elimination System (SPDES) permit issued by DEC.

The information displayed for each CSO outfall location contains the following:

  • Name of the municipal wastewater treatment facility and/or collection system owner.
  • Name of the municipality that owns the wastewater treatment facility and/or collection system.
  • Total number of outfalls listed in the facility's permit.
  • The county and DEC region the wastewater treatment facility or collection system is located.
  • SPDES permit identification number.
  • The outfall number. This is the number listed in the SPDES permit.
  • The name of the waterbody that the CSO outfall may discharge into.
  • Method the system owner uses to detect overflows at the outfall (see descriptions below).
  • Time frame of the overflow information.
  • Number of overflows that occurred within the time frame.

How can this information be used?

The CSO outfall information can help the public to make an informed decision before recreating in a waterbody by knowing:

  1. where the outfalls are located
  2. the number of times the outfall has typically discharged
  3. that recreation in waterbodies with CSOs is not advised during and following wet weather

CSO overflow detection methods

There are three types of detection methods: model, monitoring, and observation. The models used by wastewater treatment facilities are predictive mathematical calculations that are based on watershed and rainfall information. Wastewater treatment facilities that use monitoring have equipment installed at the outfall that collects real-time water flow information that the facility can download and analyze. Facilities with observation listed as the detection method are usually smaller communities; at these facilities staff visually checks the outfall to determine if the CSO has discharged.

How is the number of overflows determined for the time frame?

Permitted wastewater treatment facilities are required to submit an annual report to DEC (Combined Sewer Overflows Annual Report (PDF, 592 KB). The annual report includes the frequency of CSO overflows within the calendar year. Number of overflows reported by facilities with monitoring and observation detection methods is typically derived from the CSO annual report information. Overflow information for facilities with models are usually based on an average year. A few facilities provide real-time information about CSO overflows; links to the facility's website are provided where available.