2023 CEQ Annual Report

Water Quality

The Water of Long Island Sound               Warming and Rising Waters               Swimming               Clamming and Oystering               Drinking Water

Rivers, Lakes, and Estuaries

Climate Change Indicator



The Department of Energy and Environmental Protection (DEEP) assesses* water quality for each designated use (aquatic life, recreation, and fish consumption) for some waterbodies in the state, which characterizes whether or not the water is suitable for that designated use. 

Water quality in the state has improved over the last few decades as a result of protective laws, remediation efforts, and investment in wastewater treatment infrastructure. While there has been an increase in the number of river miles assessed, there has been a slight decrease in the percentage of assessed river miles that fully support aquatic life. In addition, there has been little change in percentage of assessed lakes and estuaries that “fully support” aquatic life in recent years.33

Goal: Section 101(a)(2) of the CWA (1972) established a national goal for “water quality which provides for the protection and propagation of fish, shellfish, and wildlife, and recreation in and on the water, wherever attainable”.


Combined sewer overflows (CSOs) occur when wastewater and stormwater are conveyed together to larger bodies of water, with little or no treatment provided prior to discharge. As depicted in the chart, more than two billion gallons were released to waterbodies in 2023.

The discharge of untreated or partially treated sewage can have significant impacts on water quality. All of the volume from the CSOs come from five treatment facilities: Bridgeport East, Bridgeport West, City of Norwich, Metropolitan District Commission (MDC) and Greater New Haven Water Pollution Control Authority (GNHWPCA). Extreme weather events/precipitation in 2023 is the primary reason for the significant increase in CSO volume in 2023.** The type of precipitation event that triggers a CSO is very specific and dependent on the duration and intensity of an event, the level of saturation of ground conditions during the event, and the capacity and structural condition of a municipality’s combined sewer system.34


Technical Note: *Section 305(b) of the Federal Clean Water Act (CWA) requires each state to monitor, assess and report on the quality of its waters relative to designated uses. **In 2012, Public Act 12-11 required DEEP to post the locations and relevant information of combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs) in the state. In 2021, Public Act 21-42 added language to better clarify what types of sewage spills must be reported. This included permitted sewage bypasses and the removal of the 5,000-gallon reporting threshold, which was replaced with the requirement to report any sewage spill or permitted bypass reaching water or that may come into contact with the general public.



33 DEEP, 2022 Integrated Water Quality Report to Congress; portal.ct.gov/DEEP/Water/Water-Quality/Water-Quality-305b-Report-to-Congress.

34 DEEP, CT Sewage Right-to-Know webpage and CT DEEP Performance Dashboard, accessed 4-5-2024; portal.ct.gov/deep/municipal-wastewater/ct-sewage-right-to-know. Frequently Asked Questions about Combined Sewer Overflows (CSOs); portal.ct.gov/deep/municipal-wastewater/combined-sewer-overflows-frequently-asked-questions.