COMBINED SEWER OVERFLOWS

WHAT IS A COMBINED SEWER OVERFLOW (CSO)? 

Combined Sewer Overflows (CSOs) are overflows of untreated sewage and stormwater into local waterbodies.

WHY DOES THIS HAPPEN?

Combined sewer systems collect rainwater runoff, domestic sewage, and industrial wastewater in a single pipe system. Typically, combined sewer systems transport all of their wastewater to a sewage treatment plant, such as Deer Island. However, during periods of heavy rainfall or snowmelt, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, or other waterbodies.

Courtesy US Environmental Protection Agency

Courtesy US Environmental Protection Agency

CSO's AND THE MYSTIC RIVER WATERSHED

CSO's are a major pollution concern for the Mystic River Watershed. CSO's contain not only stormwater but also untreated human and industrial waste, toxic materials, and debris. Pollutants associated with such overflows pose a risk to public health, impact water uses (swimming, boating, and fishing), and stress the aquatic environment.

Based upon water quality data collected on the Mystic and its tributaries, the Mystic River Watershed Association recommends that whenever it rains people should revise their water-based recreation plans such as boating, fishing, or swimming on the Mystic.  In general “wet weather” conditions are a high level predictor for poor water quality in the Mystic River Watershed from various pollutant sources. These poor water quality conditions can occur whenever it rains regardless of a CSO event due to stormwater runoff and local plumbing problems (illicit connections).  Bacterial contaminants typically last for 48 hours.

Where does this happen?

Currently, the City of Cambridge reports CSO's to Alewife Brook most frequently within the Mystic River Watershed. CSO's are also reported in Chelsea Creek.

WHAT IS BEING DONE?

The sewer system in the watershed is managed by the Massachusetts Water Resources Authority (MWRA), who is actively addressing CSO's and has closed several CSO's along Alewife Brook. Read more about the MWRA's plans. MyRWA's Policy Committee advocates for improvements to the sewer and stormwater system to minimize CSO occurances.

CSO ALERTS

Get informed! Please email MyRWA if you would like to be notified about future CSO alerts. Contact your local municipality for any concerns you may have.

It should also be noted that the MA Department of Public Health has issued a fish consumption advisory for several rivers and ponds in the Mystic River Watershed against consuming fish caught in these waters. For more information, contact MDPH Bureau of Environmental Health at 617-624-5757.

RESOURCES


WHAT IS A SANITARY SEWER OVERFLOW?

Sanitary Sewer Overflows - or SSOs - are illegal discharges or raw sewage that are often set off during heavy rains because of stormwater infiltrating and overloading the sewage system. 

Sanitary sewer overflows (SSOs) are discharges of raw sewage into surrounding water sources and communities and can be caused by severe weather, improper maintenance, and vandalism. The EPA estimates that there are at least 40,000 SSOs each year across the US. Recurring SSO sites exist throughout the Mystic River Watershed, and usually occur during heavy rains because of stormwater infiltrating and overloading the sewage system. Although these overflows are illegal, they continue to occur.

HEALTH ISSUES

Sanitary sewer overflows pose a serious health risk. SSOs contaminate the surrounding environment and threaten anyone who comes in contact with the runoff. They degrade the quality of water sources and can cause flooding into homes, basements, and low-lying areas. Raw sewage can carry bacteria, viruses, protozoa, intestinal worms, and fungi. Contact with these contaminants can lead to diseases ranging from mild gastroenteritis (the stomach flu) to severe water-borne illnesses such as cholera and dysentery. Community members may come in contact with contaminated water through direct contact in residential areas, water-related activities such as canoeing or swimming, or through drinking water. For your safety MyRWA suggests avoiding contact with river water after large storm events.