The Mystic River is a great place to get out on the water this summer! However, while the river is usually clean and safe for paddling, Combined Sewer Overflows and other forms of wastewater pollution remain a barrier to fully safe recreation on the Mystic. It’s best to avoid recreating on the water for 48 hours after heavy rain, and you can check the Mystic River Boating Advisory before heading out on the river. Here’s what else you should know before you boat…
Combined sewer overflows on the mystic river
Combined Sewer Overflows are the result of outdated systems for managing sewage and stormwater. In an ideal system there would be separate pipe networks for sewers and stormwater. However, in several older neighborhoods in the Mystic River watershed these systems are not separated. On a normal day this is not a problem. But — as you may know, during heavy rainfall the amount of rain water in a combined sewer system can overwhelm the system. In these cases, the combined sewer systems are designed to overflow, instead of backing up into houses and streets, and to discharge directly to nearby rivers — like the Mystic River and Alewife Brook.
Total CSO volumes in the Mystic River watershed by month in 2023. This data is downloaded from the MA EEA Data Portal for CSOs. Data in the portal extends backwards to mid-2022.
In 2023, over 200 million gallons of CSOs discharged into the Mystic River and Alewife Brook — a volume roughly the size of 300 Olympic swimming pools. While some CSOs receive partial treatment, many contain completely untreated raw sewage spilling directly into the river, particularly in Alewife Brook.
CSOs are a major pollution concern for the Mystic River watershed and are a barrier to fishable and swimmable rivers as pledged in the Clean Water Act. CSOs contain not only stormwater but also untreated human and industrial waste, toxic materials, and debris.
CSOs are a public health threat. WBUR recently reported on a study from the Boston University School of Public Health that provided evidence for an association between CSO discharges and acute gastrointestinal illnesses in communities on the Merrimack River.
CSOs also have an outsized impact on environmental justice communities, occurring disproportionately in communities with lower incomes, more linguistic isolation, and more people of color.
HOW TO BOAT SAFELY
In general MyRWA recommends not recreating on the water for 48 hours after heavy rainfall. To see if it’s safe to boat, you can check out the Boating Advisory on our website, which is updated every morning.
You can also keep track of CSO discharges in your area. Thanks to the CSO bill H.4921, waste treatment operators are required to notify the public when there’s been a sewage discharge in their area. MWRA has created a notification sign up. You need to create an account — but then you will be notified of any CSOs in the area. The cities of Boston, Cambridge, Chelsea, and Somerville manage their own combined sewer outfalls, including CSO reporting, so you would want to sign up for those alerts as well.
At the map below you can find all active CSO sites in the Mystic River, Alewife Brook, and Chelsea Creek. WBUR also maintains a real-time sewage overflow map for the entire state of Massachusetts.
CSOs are not the only source of bacterial pollution in the Mystic. Bacterial pollution can also be driven by pet waste, sanitary sewer overflows, illicit connections, and leaky pipes (learn more here). For that reason, even if you did not receive a CSO alert, it may still be best to avoid the water after heavy rain.
Don’t let CSOs ruin the fun this summer! Check the boating advisory and then get out and explore!
