The United States Environmental Protection Agency (EPA) is working to establish an appropriate water quality guidance value, and resultant Total Maximum Daily Load (TMDL), to control phosphorus pollution within New York City drinking water reservoirs. The TMDL "Phase II" regulatory process currently underway is a major component of the program to improve drinking water quality specified by both the Clean Water Act and the 1997 New York City Watershed Memorandum of Agreement. The TMDL process is implemented when other pollution controls mandated by the Clean Water Act are not sufficiently effective in removing pollution that is degrading water quality.



The Effects of Excessive Phosphorus

Phosphorus Pollution in the New Croton Reservoir

Water Quality Criteria Applicable to the TMDL Process

Phosphorus Poem

The New Croton Reservoir in Westchester county provides drinking water to approximately 10% to 12% of the individuals served by the New York City water supply system, or about 900,000 persons daily. This reservoir may serve as the source of up to 25% of the City’s water during periods of drought. The New Croton reservoir currently suffers from many serious  phosphorus-triggered water quality problems (frequently referred to as "use impairments") even though its phosphorus levels have been averaging approximately 17.2 ug/L each year. Given the troublesome condition of the New Croton, it is clear that the higher 20 ug/L phosphorus guidance value established by EPA in the earlier Phase I TMDL program will not suffice to successfully achieve the Clean Water Act’s goal -- removal of pollution and protection from further degradation in New York City's water system.  

The Effects of Excessive Phosphorus

The unfavorable impact of excessive phosphorus on the water quality of the New Croton Reservoir serves as an example for the entire Watershed. Each year during the summer and fall seasons, phosphorus in the New Croton sets off a biological chain reaction. It promotes algae blooms that result in poor water taste, odor and color.  Phosphorus induced algae blooms also reduce dissolved oxygen in the bottom waters (due to increased bacteria ingesting dead algae), cause increased levels of the heavy metal pollutants iron and manganese, and increase levels of organic carbon. The chlorine-based disinfection of waters that are high in organic carbons results in the formation of chemicals that are suspected of having a number of serious impacts that are threatening to our health.  

These water quality problems at the New Croton have created an "operational nightmare" for DEP. As water quality degrades each summer, DEP has to shut down the flow from the New Croton or blend New Croton waters with higher quality waters from the Catskills to dilute the pollutants. These reservoir shut downs often occur for months at a time. Such actions by DEP support the discovery that the New Croton water quality does not meet its New York State standards and does not serve as an appropriate source of drinking water.  This problem could be significantly exacerbated under drought conditions and flooding scenarios, if it remains unaddressed.  

It is recommended that EPA adopt a water quality value for phosphorus that is no higher than 15 micrograms per liter (ug/L) for all source water reservoirs within the New York City Watershed (Watershed). All seven source water reservoirs, from both the Catskill/Delaware and Croton portion of the Watershed, currently serve as a source of unfiltered drinking water. It is recommended that no regulatory distinction be drawn between any of these water bodies simply because Croton waters are set to be chemically filtered by 2007. A 15 ug/L phosphorus TMDL for all source water reservoirs is also being recommended and that level would be consistent with the "multiple barrier" approach to the protection of drinking water, whether or not filtration ultimately occurs. As explained in the National Research Council’s report on the New York City Watershed, public health literature, and EPA assessments, the multiple barrier approach to water supply protection -- involving full water supply protection at the source of the water -- is more effective than relying on filtration alone. The nine million people who expect safe drinking water from the Watershed deserve nothing less.
If adopted by EPA, this water quality criterion of 15ug/L would represent a full 25% reduction from the earlier Phase I TMDL guidance value of 20 ug/L for phosphorus in these reservoirs.

Phosphorus pollution is a serious threat to the purity of the New York City drinking water supply. Within a reservoir a complex ecosystem exists, including plants, microorganisms, and fish, which rely on nutrients from upstream sources. The growth of organisms within a reservoir is directly related to the amount of nutrients flowing into the water body. The "limiting nutrient" in the New York City Watershed reservoirs is phosphorus. If allowed to increase, phosphorus would generally allow an increase in biological life in these water bodies during the warm weather growing season. Simply stated, phosphorus levels control the extent to which plant life can grow in the New York City reservoirs.



The New Croton Reservoir portion of the New York City Watershed has a relatively high level of phosphorus. These excessive phosphorus levels result in "eutrophic" conditions, characterized by algae blooms and limited water transparency in the warmer weather.  A visual indicator to the rapid growth of algae is that the water becomes green in color, and mats of colored plant and bacterial life will appear on the surface.  A large portion of the phosphorus loading to the reservoirs is controllable (because it is either man-made or man-altered) and, with the proper management techniques, the levels of phosphorus in a reservoir can be significantly reduced.

Over time, the individual algae die off, and while the bloom itself continues in the surface waters, the dead algae will fall to the bottom of the reservoir’s water column. The dead plant material is consumed by an increasing population of bacteria and other animal life. A rapid decline in the levels of dissolved oxygen in the water results because the bacteria that consume the dead algae also consume oxygen as they breathe.  Therefore, the water becomes severely deprived of dissolved oxygen, which results in an anaerobic condition. Other forms of life can live in this anaerobic environment, such as bacteria, but almost all fish cannot. The fish living there will either die or move elsewhere.



This anaerobic environment causes serious problems when the water is to be used as a drinking water supply. Generally, drinking water is drawn from the bottom of a reservoir, since this water will less likely contain algae.  This practice can avoid the algal mats, but it is more likely to draw the anaerobic water that results from an algal bloom. Anaerobic water contains bacteria that generate serious odor, taste problems, and poor water color. In addition, anaerobic conditions cause contaminants such as iron, manganese, hydrogen sulfide and even additional phosphorus to be released from reservoir bottom sediments into the water, further decreasing the quality of the water.

Eutrophic water conditions triggered by excess levels of phosphorus also result in increased levels of organic carbon in the water. As in most other drinking water supply systems, chlorine is used to disinfect water from New York City reservoirs prior to distribution to consumers. The chlorine-based disinfection of waters that are high in organic carbon results in the formation of chemicals classified as "disinfection byproducts" -- chemicals that are suspected of being carcinogenic and of increasing the risk of early term miscarriages.

High levels of phosphorus also encourage the rapid growth of "blue-green" algae (cyanobacteria). This group of organisms, which contributes significantly to distaste and odor problems, presents an even more serious problem than other forms of alga.  Blue-green algae also produce toxins which, at high levels, pose a concern for human and animal health. Before significant levels of blue-green algae are present in a reservoir, the use of that water body as a water supply is suspended to minimize risks. Thus, if an algae bloom persists, DEP is forced to switch to alternative water sources due to the multitude of negative impacts of eutrophic conditions. This is a frequent occurrence with the New Croton reservoir.

Phosphorus Pollution in the New Croton Reservoir

Typically, the concentration of phosphorus within the New Croton Reservoir ranges between 16 and 18 ug/L during the growing season, with the average phosphorus levels for 1992 through 1996 being 17.2 ug/L for the entire reservoir. This phosphorus concentration is already significantly below the 20 ug/L phosphorus criterion that was enforced by EPA in the previous Phase I TMDL process. Despite having a lower phosphorus concentration than the previously identified guidance value, the New Croton still suffers from algae blooms, anoxia (low dissolved oxygen), poor taste, increased color and other problems associated with serious eutrophication -- requiring the reservoir’s use to be limited or temporarily suspended during certain periods of the growing season.


Even when algae blooms induced by excessive phosphorus are not severe enough to warrant a complete shutdown of the water supply, higher than normal algae levels can nevertheless impair drinking water disinfection.

 Water Quality Criteria Applicable to the TMDL Process

Expert research, analysis and scientific discretion are crucial when EPA adopts a specific numeric water quality criterion for phosphorus for the Watershed’s source water reservoirs based upon the State’s water quality standards. The numeric water quality guidance value is then used by EPA to calculate a specific limit on the total reservoir loading of phosphorus (the TMDL) that will improve or maintain the reservoirs as a drinking water source, with an appropriate "margin of safety."

On January 31, 1997, State DEC submitted the Phase I TMDLs for the Watershed to EPA for review. On April 2, 1997, EPA approved TMDLs for eight City reservoirs, finding that they were excessively polluted with phosphorus and would require reductions in their levels.

In making this Phase I determination, EPA calculated the TMDLs for phosphorus levels in the reservoirs containing a phosphorus guidance value intended to hold an appropriate water quality for recreational uses. This guidance value was not developed to ensure safe and appropriate quality for drinking water. Nevertheless, EPA relied upon a State DEC guidance value of 20 ug/L phosphorus as an interim numeric criterion.

 DEP has recommended that EPA establish a phosphorus criterion of no higher than 15 ug/L in the Watershed’s source water reservoirs when determining the Phase II TMDLs. The need for phosphorus reductions is well demonstrated by the detailed studies provided by DEP scientific research staff, as well as by the assessment conducted by the National Research Council. The need for a more stringent TMDL is further evidenced by the serious phosphorus-induced water quality degradation of the New Croton reservoir, a reservoir with phosphorus values already below the current 20 ug/L Phase I guidance value.

The official Phase II TMDLs as well as DEC's response to comments are on the web at:
 Proposed Phase II Phosphorous TMDL Reports

  After a detailed assessment, the National Research Council concluded that: "The new 15-ug/L phosphorus guidance value is appropriate. The Phase I [TMDL] goal of 20 ug/L was not adequately conservative for a drinking water supply, as it is based on ecological and aesthetic considerations."

For the Drinking Water Supply and Quality Report refer to: New York City Water Supply Statement

Lowering the phosphorus concentration criterion to a maximum of 15 ug/L would be fully consistent with the Clean Water Act’s TMDL provisions. Section 303(d) of the Clean Water Act requires that TMDLs be set "at a level necessary to implement the applicable water quality standards."  Therefore, to better conform with the Clean Water Act’s mandate, EPA should implement a phosphorus concentration of 15 ug/L when establishing the pending Phase II TMDLs for the Watershed’s seven source water reservoirs.