Water Quality

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The Water of Long Island Sound

 

 

The area of Long Island Sound with hypoxic conditions decreased in 2022.

 

The maximum area of Long Island Sound with hypoxia, which is water with a dissolved oxygen (DO) concentration less than 3.0 milligrams per liter (mg/l), decreased from 142 square miles in 2021 to approximately 87 square miles in 2022. The area of hypoxia for 2022 was also approximately 22 percent lower than the ten-year average. However, the duration of the hypoxic conditions increased from 47 days in 2021 to 57 days in 2022. Most, if not all, of the hypoxic conditions are found in the western basin of the Sound, which is also affected by contributions from New York State. The primary cause of hypoxia is nutrient pollution, primarily nitrogen and phosphorus, from runoff and wastewater treatment effluent that fuels the growth of phytoplankton in the Sound. The average dissolved nitrogen concentration at the bottom of the Sound was approximately 0.2 mg/l in 2022.³¹   

_{Goal: The goal line on the top chart is an approximation of the maximum area of the hypoxia target adopted in the 2015 edition of the Long Island Sound Study's Comprehensive Conservation and Management Plan to "measurably reduce the area of hypoxia in Long Island Sound … by 2035, as measured by the five-year running average size of the zone."}  

Nitrogen discharged in 2022 was almost 11 percent lower than in 2021 and 28 percent lower than the previous ten-year average.

Connecticut has reduced nitrogen discharges over the last decade by investing in nitrogen-removal technology at sewage treatment plants and implementing a Nitrogen Control Program; however, reducing nitrogen discharges from non-point sources remains a challenge. The decline in nitrogen discharged in 2022 is attributed to warmer weather, which is more conducive to nitrogen removal, and because 64 municipalities out of 78 participating in nitrogen credit exchange program upgraded projects that enhance nitrogen removal.³²

 

_{Goal: Substantial reduction of nitrogen discharges is a goal that is shared by Connecticut and New York. Connecticut established a reduction goal of about 6,670 tons annually by 2014, which is the result of a 63.5 percent reduction from the point source baseline of 10,500 tons per year. Therefore, Connecticut’s goal was established as a maximum of 3,830 tons per year.³³  Nitrogen discharges “upstream” of Connecticut also contribute to the nitrogen loading.}

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³¹ _{DEEP; Long Island Sound Water Quality and Hypoxia Monitoring Program, personal communication from K. O’Brien-Clayton, December 5, 2022 and M. Lyman, May 1, 2023; portal.ct.gov/DEEP/Water/LIS-Monitoring/LIS-Water-Quality-and-Hypoxia-Monitoring-Program-Overview}
³² _{DEEP, Nitrogen Control Program for Long Island Sound; personal communications from I. Raffa, March 17 and 24, 2023}
³³ _{DEEP, Nitrogen Control Program for Long Island Sound; The Long Island Sound TMDL Frequently Asked Questions; portal.ct.gov/-/media/DEEP/water/lis_water_quality/nitrogen_control_program/tmdlfaqpdf.pdf.}