Understanding the Phosphorous Issue - DEP

Environmental Protection


Programs for Communities & Businesses

Understanding the Phosphorus Issue


Phosphorus is a naturally occurring element essential for plant and animal life. It is found naturally in rocks and other mineral deposits. Due to its reactivity with air and many other oxygen-containing substances, phosphorus is not generally found free in nature but is typically in its oxidized form called phosphate.

Introduction of phosphorus into freshwater systems often leads to eutrophication, which is a condition where high nutrient levels lead to excessive production of algae and macrophytes in aquatic systems. Algae in drinking water reservoirs impair the taste and odor of drinking water and can lead to harmful disinfection by-products upon chlorination. When algae blooms die off, bacterial decomposition consumes dissolved oxygen in the deeper waters of lakes and reservoirs and can contribute to fish kills. Although eutrophication is a natural process, human activity can greatly increase the amount and rate of nutrients entering a water body, thereby exacerbating the problem.

In the Croton System, excess phosphorus loading has been identified as the major cause of reservoir eutrophication and an inability of some of the Croton System reservoirs to comply with their designated best uses. Currently, nine reservoir basins in the Croton System are classified as phosphorus -restricted, indicating that phosphorus loading from within the basins result in an exceedance of established New York State Department of Environmental Conservation (DEC) water quality guidelines. Learn more about New York City’s Water Supply System.

Anthropogenic sources of phosphorus in urban and suburban watersheds may include failing septic systems, waste water treatment plants, poorly treated stormwater, soil erosion, pet wastes, mishandling of leaves and grass clippings, misapplication of fertilizer, and/or phosphorus containing detergents. Please refer to the following materials for additional information:

Download Septic Systems as a Source of Phosphorus

Download Phosphorus Concerns with Fertilizer Use

Download Phosphorus Concerns with Grass Clippings

Download Construction Sites as a Source of Phosphorus

Download Phosphorus Concerns with Detergent Use

Download Reducing Pollutants of Concern

For more information about stormwater in the East of Hudson Watershed, visit East of Hudson Municipal Separate Storm Sewer System.