In an ideal system there would be separate systems for wastewater sewer and stormwater pipes. However, in many of the older communities 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:

In large rainstorms, CSOs routinely discharge sewage contaminated water into Alewife Brook. The volume is measured in millions of gallons of discharge. In 2019, the total at CAM401A outfall on Alewife was 7.75 million gallons.

But here is the good news: earlier this year the CSO bill H.4921 was passed--requiring water suppliers to notify the public when there’s been a sewage discharge in their area. See the timeline of the bill here. Up until now, residents had no way to know when sewage was present in their local waterways. While we wait for this notification, there are still ways to practice safe boating:

How to boat safely?

CSOs happen after a heavy rain--so MyRWA recommends not recreating on the water for up to 48 hours after a heavy rainfall.
You can check out the Boating Advisory that the Mystic River WAtershed Association created and is updated every morning--on our website or follow us on Twitter @SafeMystic.
Additionally, 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.

CSOS in the Mystic

Due to structural inequities in land use and pollution, sewage spills impact low income, minority, and linguistically isolated communities in Massachusetts at substantially higher rates than wealthier, whiter, and largely English-speaking ones. As a result, sewage spills are not only a public health risk and not only an environmental hazard, but also an Environmental Justice tragedy.

The Massachusetts Water Resource Authority (MWRA) is charged with eliminating CSOs and has updated the infrastructure of many of those going into Boston Harbor--reducing them from 84 to 50. You can learn more about MWRA CSO sites here. Additionally, each municipality with CSO sites has their own website with full details on their CSOs.

For detailed maps and information visit the municipal websites. Snapshots of the CSO maps are included below for your reference:

MWRA CSO sites