Combined Sewage 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). 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.

Green and Grey Infrastructure

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.

Net Zero CSO Development

To ensure that new development does not further contribute to pollution, we are calling for a Net Zero CSO rezoning. To realistically achieve this, we need a multi-pronged approach that includes new developer requirements for storm and wastewater management as well as City capital investment in critical infrastructure. 

We are encouraged by the City’s work to date on the 2021 Unified Stormwater Rule, which will improve citywide stormwater controls on new development sites in combined sewer areas. In August 2020, step one for enacting the new rule took place when City Council and the Mayor held a public hearing and passed legislative Intro Bill 1851, which expands existing stormwater rules to the Combined Sewer System area of the city and authorizes DEP  to begin the rulemaking process for a new Unified Stormwater Rule. NYC DEP will begin developing the new rule this fall and once fully enacted, it will require developers to manage on-site stormwater using green infrastructure (GI) infiltration practices to ensure that runoff is either detained during wet weather events or completely retained or re-used on new development sites. 

While these proposed regulations could result in Net Zero CSO in Gowanus, further steps will be required to ensure this strategy will achieve the goal including: 

  • The proposed new rule only mandates more stringent stormwater controls on development sites with a lot size of 20,000 square feet or greater. In Gowanus, this lot size threshold limits the potential to address a large number of smaller, upland sites slated for high-density development that will contribute additional CSO. As NYC DEP moves forward with the rulemaking process, a smaller lot size threshold must be evaluated to realistically achieve a Net Zero CSO rezoning.
  • Furthermore, lot size should not be the only criteria that determines compliance under the new rule. For successful implementation, known barriers to green infrastructure implementation must be addressed, including a high groundwater table and proximity to contaminated sites. In Gowanus, these conditions are prevalent on large, waterfront sites which will severely limit infiltration opportunities and the overall impact of new regulations. As the rulemaking process moves forward, these factors must be evaluated to determine where additional stormwater controls are needed and alternative practices, such as wet swales, stormwater direct discharge, and black water reuse, should be permitted and counted towards compliance. 
  • NYC DEP must assess the impacts of the Unified Stormwater Rule on future new development sites in Gowanus as part of a comprehensive hydrology study that is accessible to the public before ULURP begins or provide proof of Net Zero CSO impact as part of the forthcoming Gowanus Draft Environmental Impact Statement (DEIS). Results must indicate Net Zero CSO or CSO reduction at each outfall on the Gowanus Canal to ensure sufficient requirements and infrastructure upgrades are in place. 
  • While the Unified Stormwater Rule addresses stormwater management on new development sites, there is still a need to manage stormwater in the public right-of-way. The City should facilitate allowances for new development practices to incorporate stormwater management in adjacent streets and street ends. 

Gowanus Infrastructure Updates

April 9, 2018:  CityLimits: CityViews: Council Can Foster Clean Water, Healthy Residents Vote on Gowanus Tank


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