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Research & Monitoring


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Status and Trends: LISS Environmental Indicators

LIS Point Source Nitrogen-Trade Equalized Loads


LIS Point Source Nitrogen-Trade Equalized Loads
  CT NY Combined
Baseline 26,043 33,105 59,148
2000 20,262 23,660 43,922
2001 17,101 24,345 41,446
2002 15,855 23,326 39,181
2003 16,574 26,007 42,581
2004 14,345 25,621 39,966
2005 15,055 26,649 41,704
2006 14,738 26,298 41,036
2007 13,495 25,736 39,231
2008 13,311 27,128 40,439
2009 11,819 27,192 39,011
2010   9,911 23,792 33,703
2011 11,014 22,864 33,878
2012   8,513 20,325 28,838
2013   8,446 18,702 27,148
2014   8,899 15,962 24,861
2015   8,378 14,437 22,815
2016  7,171 13,229 20,400


Point source pollution is defined in section 502 of the Clean Water Act as “any discernible, confined and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, concentrated animal feeding operation, or vessel or other floating craft, from which pollutants are or may be discharged. This term does not include agricultural storm water discharges and return flows from irrigated agriculture.”

Point source pollution occurs when the pollutant is discharged from an identifiable source.  A major type of point source pollution in Long Island Sound is nitrogen  discharged from sewage pipes at wastewater treatment plants. This source has therefore been a major focus of nitrogen reduction efforts in the Long Island Sound region.


Trade equalization is a  calculation of the effect a pound of nitrogen leaving a point source will eventually have when it reaches  Long Island Sound.  Western Long Island Sound is the focus of interest in this calculation because the ongoing problem of hypoxia is usually confined to this part of the Sound. (In the Eastern Sound, the impact of nitrogen-enriched water is lower because of its proximity to the open ocean. There is a greater exchange of nitrogen-rich water with ocean water, which has  much lower concentrations of nitrogen.)  Furthermore, nitrogen “load” from wastewater treatment plants which is discharged directly into Long Island Sound has more impact than load discharged far up tributary rivers, where biological, physical, and chemical processes have more time to act and remove the nitrogen from the system. Using the connections among nitrogen, rivers, currents and hypoxia a calculation is made to assign a loading factor to each point source, which is meant to assess the contribution of that source relative to a source discharging directly into Western Long Island Sound (which scores a 1.00)

If a coastal wastewater treatment plant is in the eastern part of the Sound, not all of the nitrogen discharged will end up in the Western Sound. Some of the nitrogen will be carried out of the Sound by currents through the Race, The calculation of this loss due to currents is called “transfer efficiency.”   Similarly, if an inland wastewater treatment plant discharges nitrogen into a river, some of the nitrogen will be lost before the river waters reach Long Island Sound.  This “river attenuation” is also taken into account when calculating nitrogen loads.  Transfer efficiency and river attenuation are multiplied together to calculate the trade equalization.  For example, a plant located in Central Long Island Sound might have a transfer efficiency of .75 (meaning 75% of the nitrogen ends up in Western Long Island Sound, and 25% is lost) and a river attenuation factor of  0.75 (meaning 75% of the nitrogen makes it to Long Island Sound, and 25% is lost), the trade equalized load for this plant would be 0.75*0.75 or 0.56, meaning that a reduction of 100 pounds of nitrogen from this plant equates to a reduction of 56 pounds of nitrogen from a plant discharging directly into Western Long Island Sound.  This conversion factor allows managers to focus their efforts on removing the nitrogen that has the most impact on the health of Long Island Sound.

The map to the right shows the factor applied to a pound of nitrogen discharged in each of the 11 Long Island Sound management zones.


A decrease in point source nitrogen loads reflects progress by the states of CT and NY towards upgrading their wastewater treatment facilities. The ultimate nitrogen load reduction goal for wastewater treatment plants  is a reduction of 36,374 trade equalized pounds of nitrogen inputs (from 59,148 to 22,774 pounds) to Long Island Sound by 2017. The nitrogen input baseline was established in the Long Island Sound TMDL and is explained in more detail at the bottom of this page.



In 2016, New York and Connecticut reached and exceeded the goal of the Long Island Sound Total Maximum Daily Load (TMDL) to reduce nitrogen. As a result of plant modernization, wastewater treatment plants discharged 20,400 trade equalized pounds per day of nitrogen compared to the early 1990s baseline level of 59,147 pounds, a 57% decrease. The TMDL goal was to reach 22,775 trade equalized pounds. The milestone reflects progress by the states in reducing nitrogen pollution, a contributor to poor water quality.

Equalization is a mathematical calculation of the transport efficiency of a pound of nitrogen released from a source based on its geographical proximity to Long Island Sound. In real numbers (without using the equalization formula), an estimated 41.8 million fewer pounds of nitrogen were discharged in 2015 compared to levels in the early 1990s.


In general, point source monitoring data from 1988-1990 were used to calculate nutrient loads for both model development and to serve as the baseline from which reductions would be measured.  For many point sources, particularly in the upper tiers of Connecticut, nutrient monitoring was not established until 1993 or later.  For those facilities, estimated nitrogen and total organic carbon concentrations (usually 15 milligrams per liter for nitrogen and 20 milligrams per liter for total organic carbon) were applied to 1990 measured flow to develop each zone’s aggregate baseline load estimates.  When the Ocean Dumping Ban Act requirement to cease ocean disposal of sludge by 1992 created the need for the de-watering of sludge, the Long Island Sound Management Conference recognized New York City’s need to dewater its sludge at the East River facilities by increasing the nitrogen baseline in zone 8 to include the centrate of the dewatered sludge.

Source: A Total Maximum Daily Load Analysis to Achieve Water Quality Standards for Dissolved Oxygen in Long Island Sound

A Total Maximum Daily Load Analysis to Achieve Water Quality Standards for Dissolved Oxygen in Long Island Sound

Long Island Sound Nitrogen TMDL fact sheet


Show/Hide Table

This map shows the trade equalized nitrogen zones in CT and NY. The calculated impact of nitrogen inputs declines with increased distance from the Western Sound. Click photo to enlarge map.

Learn More

Hypoxia diagram

This graphic shows how nitrogen, which stimulates plant growth can contribute to hypoxia, a condition of low dissolved oxygen, in the Sound.

A Total Maximum Daily Load Analysis to Achieve Water Quality Standards for Dissolved Oxygen in Long Island Sound

Long Island Sound Nitrogen TMDL fact sheet







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