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Water Quality

Two people canoeing on a calm river near an industrial building during golden hour.
Canoeing on Newtown Creek

Water Quality In NYC

How Clean Is the Water?

The City has come a long way to improve the quality of our rivers, creeks, the harbor and the ocean in the last several decades since the passing of the federal Clean Water Act. Many of our waterways are now swimmable. Except when it rains.

Much of the City is served by what is called a combined sewer system in which one set of pipes carry both household waste water and runoff from the streets. On a dry day the system works fine but on rainy days the increased volume of water (a mixture of waste water and stormwater) overwhelms the treatment plants and this mixture is often discharged into our waterways without any treatment. This is called Combined Sewer Overflows, or CSOs. About two thirds of the City is served by the combined system, making this a serious problem that prevents us from meeting the federal Clean Water Act goal of fishable and swimmable waters.  

NYC Department of Environmental Protection (DEP) has been testing water quality since 1909 and has made its entire dataset available online. Since 1986, DEP has tested for Fecal Coliform, a sewage-indicating bacteria. Fecal Coliform is found in human and animal feces, and is not necessarily harmful itself but indicates the presence of sewage and potentially disease-causing bacteria (pathogens) in the water. The interactive map below shows Fecal Coliform results since 1986. According to New York State standards, the number of Fecal Coliform colonies in a 100 milliliter sample should not exceed 1,000 colonies (shown in red) and the 30-day average* should not exceed 200 colonies (shown in yellow). Samples with less than 200 colonies indicate good water quality (shown in green). The temporal map shows that while water quality has dramatically improved over the decades, there are many tributaries that continue to show consistent failure to meet water quality standards.

​Another measure of water quality is dissolved oxygen (DO); fish require a certain level of DO in the water column to survive. CSO events bring excessive levels of nutrients into the waterway, causing algal blooms. When the nutrient load stops, the algae die and decompose, taking up DO in the process. DEP has DO test results available since 1909, shown in the interactive map below. According to New York State standards, DO should never dip below 3.0 mg/L (shown in red) and the 30-day average should not be below 4.8 mg/L (shown in yellow). DO above 4.8 mg/L indicates good water quality (shown in green).

​*The 30-day average is calculated using the geometric mean.

Green infrastructure helps reduce combined sewer overflow by detaining, absorbing, and infiltrating stormwater on site. Green roofs, rain gardens and cisterns are all effective practices for managing stormwater on your property. 

Resources for Green Infrastructure 
Green Infrastructure Guide: For Private Property Owners in New York City

produced by Riverkeeper and the NYC Soil & Water Conservation District

NYC Soil & Water Conservation District featured in videos! 

Stormwater Pollution and Green Infrastructure Solutions
produced by Nassau County Soil & Water Conservation District

How Important is Green Infrastructure?
produced by Transformation Design Team at Pratt Institute

Why Does Sewage Flow Into Rivers Anyway?
produced by WNYC Radio

Useful Links 

For water quality data in the Hudson River and the Harbor
Hudson River Water Quality Data by the Riverkeeper

NYC Water Trail Association's Citizens' Water Quality Testing Program

NYC Department of Environmental Protection: Harbor Water

Urban Ocean Observatory at Davidson Laboratory

To learn what the City is doing to improve water quality
NYC Department of Environmental Protection: Stormwater

​To visually understand the CIty's sewer infrastructure
Open Sewer Atlas NYC

To learn about the State's water quality programs
NYS Department of Environmental Conservation: Water Quality Information 

To learn the role of the federal government and the Clean Water Act
Environmental Protection Agency Region 2

To learn how natural systems can make our city more resilient
Naturally Resilient Communities

To learn about the National Estuary Program in our region
NY-NJ Harbor & Estuary Program 

For up-to-date information on what is happening with water quality improvement programs in NYC
Stormwater Infrastructure Matters (S.W.I.M.) Coalition