LIS Point Source Nitrogen-Trade Equalized Loads

LIS Point Source Nitrogen-Trade Equalized Loads
CT NY Combined
Baseline 26042 33105 59147
2000 20262 23660 43922
2001 17101 24345 41446
2002 15855 23326 39181
2003 16574 26007 42581
2004 14345 25621 39966
2005 15055 26649 41704
2006 14738 26298 41036
2007 13495 25736 39231
2008 13311 27128 40439
2009 11819 27192 39011
2010 9911 23792 33703
2011 11014 22864 33878
2012 8513 20325 28838

What are Point source nitrogen loads?

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.

What does trade equalized mean?

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.

what does this indicate

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 is a 58.5% reduction in point source nitrogen inputs to Long Island Sound by 2014. The nitrogen input baseline was established in the Long Island Sound TMDL and is explained in more detail at the bottom of this page.



Point source nitrogen loads have been reduced over the last 25 years in great part due to wastewater treatment plant upgrades that have been performed in Connecticut and New York.  In total, of the 103 municipal wastewater treatment facilities which discharge into Long Island Sound  or its tributaries, 73 have plans to upgrade to advanced wastewater treatment techniques to remove nitrogen from their effluent.  A total of 65 of these plants have already completed some form of upgrade as of 2011, with 5 more planning upgrades by 2014.  Two very large plants in New York with high trade equalized loads, Wards Island, Hunts Point, recently completed upgrades, with a third, Bowery Bay, slated to come online in 2012.   In total, as of 2011, New York and Connecticut are 79% of the way to their 2014 goal of  reduction of 58.4% in trade equalized load, which is equivalent to a reduction of 20,000 trade equalized pounds of nitrogen per day discharged into the Sound.

Data Note

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

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