Combined Sewer Overflow

The 1.8-mile-long Gowanus Canal was constructed in the 1860s on the site of a former salt marsh and creek. It has a long history of environmental issues, including industrial pollution and combined sewer overflow (CSO). Combined sewer systems carry sewage from homes and businesses, as well as stormwater that flows over streets and other paved surfaces. A typical rainstorm in NYC generates more water than our sewer system can handle, which causes raw sewage and untreated stormwater to overflow into our waterways. CSOs are the greatest source of ongoing pollution to the Gowanus Canal. The overloaded sewer system currently discharges about 363 million gallons of raw sewage and polluted run-off, or combined sewer overflow (CSO), into the Gowanus Canal each year, spread over about 40 rainfall events.

Reducing CSO

The NYC Department of Environmental Protection (DEP) is designing and constructing grey and green infrastructure across the Watershed to reduce CSO and decrease the amount of raw sewage flowing into the Canal. DEP installations currently underway include a growing number of curbside rain gardens, or bioswales; a high level storm sewer system; and two large underground sewage detention tanks, as part of the Superfund remedy. Learn more about DEP’s work in Gowanus here. Visit the DEP’s Green Infrastructure page to learn more about green infrastructure across the city.

While planned grey and green infrastructure investments are part of the solution for improving water quality in the Gowanus Canal, they do not fully address CSO. Once these projects are complete, there will still be about 115 million gallons of annual CSO discharge left unmanaged. While we recognize that 100% CSO elimination in the Gowanus Canal may not be feasible without the complete separation of more than 7,000 miles of sewer pipes that make up NYC’s combined sewer system, GCC’s long-term vision for a clean and resilient Gowanus Canal and Watershed is to achieve as close to zero CSO as possible. Through advocacy and action towards comprehensive watershed planning, we can strategically address most existing and future sources of pollution to reach this goal.

Net Zero CSO Rezoning

We are advocating for a net zero CSO rezoning in Gowanus where new development does not increase pollution by contributing additional sewage and stormwater to the Gowanus Canal. This will be a challenge given the number of new residents expected to inhabit the neighborhood (along with their estimated waste generation), so additional CSO reduction measures will likely be necessary. 

The City is in the process of developing a Unified Stormwater Rule that will create and enforce stricter requirements for stormwater management on new development sites. The Unified Stormwater Rule is a step in the right direction, as we believe it will help to reduce the amount of combined sewer overflow (CSO) that enters into the Gowanus Canal. Our community needs to further advocate that the new rule work for Gowanus by acknowledging prevailing environmental conditions, such as the high groundwater table and history of contamination on many sites, and the need to develop locally adaptive green infrastructure solutions for mitigation.  

In Spring 2021, the City will release the Draft Environmental Impact Statement (DEIS), a statement that will report all estimates of environmental harm as a result of the proposed Gowanus rezoning. In advance of this release and before the proposed rezoning is approved and certified by the Department of City Planning (DCP), it is critical that the City address specific community concerns and questions that have been posed since May 2019, including: 

  • How will new development in Gowanus impact the amount of CSO that enters into the Gowanus Canal? Will it increase, and if so, by how much, and will the Unified Stormwater Rule address this?
  • How will new development impact the existing capacity of the already overburdened sewer system, which contributes to both CSO and neighborhood flooding during rain events?
  • How will the City measure the development impacts on CSO and sewer capacity against the existing conditions to assess CSO net increases and future neighborhood flooding?
  • How will the City manage the concentration of hazardous substances and solids from additional sewage resulting from new development? Will the additional sewage compromise the effectiveness of the EPA Superfund clean-up? 
  • How will we know if the Unified Stormwater Rule is working? What sort of monitoring will the city be doing to ensure that new development isn’t increasing pollution? What will enforcement consist of?
  • Will there be additional gray and green infrastructure investments as part of the rezoning?

On December 2, 2020, DCP discussed the predicted infrastructure impacts resulting from the proposed Gowanus rezoning, along with potential management strategies. Watch the full presentation recording here.

After this presentation, we sent a letter to DEP with outstanding questions that need to be answered in order to evaluate whether the Unified Stormwater Rule will ensure that new development does not further pollute the Gowanus Canal. 

Read more from GCC

GCC Draft Scope of Work Testimony

Memo RE Unified SW Rule

Follow-up to Dec. 2 Infrastructure Meeting


NYC Soil & Water Conservation District
S.W.I.M. Coalition