200 million gallons of combined sewage and stormwater released into the Mystic River and Alewife Brook in 2023
The summer of 2023 was Boston’s second rainiest on record, with over 20 inches of rain pounding the city. Across Massachusetts, towns experienced unprecedented levels of flooding, perhaps most dramatically in Leominster, where flash floods washed out bridges, damaged homes and businesses, and opened up sinkholes beneath the roads. In December, early winter rains caused road closures across the state. Alongside flooding, heavy rain can cause another issue that is less visible, except to those who experience it in their neighborhood and backyards - Combined Sewer Overflows (CSOs) - which send untreated and partially treated sewage into rivers and streams.
All told, over 200 million gallons of combined sewage were discharged into the Mystic River and Alewife Brook in 2023 during at least 99 separate CSO discharges. That’s around the volume of 300 Olympic-size swimming pools. The Alewife Brook, a small tributary of the Mystic that receives the highest volume of untreated CSOs in the watershed, received over 26 million gallons of CSOs.
“We track Combined Sewer Overflows to bring attention to one of our most significant drivers of poor habitat quality, public health and aesthetics.” says Patrick Herron, Executive Director of MyRWA. “Also, we track this information to build public support for the policies and investments needed to eliminate these discharges and create a healthy environment for all of us.”
WHAT ARE CSOs?
Fig. 1 - During heavy rain, combined sewage and stormwater discharges via an outfall pipe into the river
CSOs are intentional outflows of sewage and rainwater discharged into rivers and streams during very wet weather. They are the result of mid-19th century water infrastructure that is built with a connected sewer and storm drain system. During dry weather, this system works just fine, sending wastewater down the pipe and off to a treatment facility where waste and pollutants are removed. During very wet weather, though, especially when a lot of water falls in a very short period of time, the amount of water coming through the system is more than the pipes can handle, so they are designed to overflow rather than back up into people’s homes, sending both the stormwater and the sewage into rivers and streams. These events are called Combined Sewer Overflows, and they pose a risk to the health of both the people and the ecosystem downstream.
Fig. 2 - Total CSO volumes in the Mystic River watershed by month. This data is downloaded from the MA EEA Data Portal for CSOs. Data in the portal extends backwards to mid-2022.
Fig. 3 - Number of separate CSO events in the Alewife Brook and the Mystic River by month in 2023. The CSO discharges on Alewife Brook are untreated: raw sewage makes its way directly to the stream. The CSO discharges on the Mystic River, while larger, are treated or partially treated CSOs.
Unfortunately, the types of storms that cause CSOs are getting more frequent as our climate changes, and 2023 was a particularly wet year. Compared to 2022, during which much of Massachusetts experienced drought conditions, 2023 saw the volume of CSOs increase dramatically. While the number and volume of CSOs in a given year can fluctuate based on the weather, we can expect to see a similar pattern in years to come with wet years generating an increased frequency of CSO discharges and higher total volumes of CSOs.
CSOs are a barrier to fishable and swimmable rivers as pledged in the Clean Water Act. CSOs affect the ecological health of our water bodies, lower property values, and pose a public health concern to residents who reside downstream of outfalls. For example, sewage contamination in swimming and drinking water is associated with increased rates of gastrointestinal illnesses and CSOs have an outsized impact on environmental justice communities. Due to CSOs and other pollutants, public health officials recommend avoiding contact with water bodies during rainstorms and for 48 hours afterward.
“The Clean Water Act has been a boon to the people and natural resources of greater Boston, but the remaining CSOs on Alewife and the Mystic represent a major unfinished chapter of that story,” says Andy Hrycyna, MyRWA’s Watershed Scientist. “If there were ever a time to invest in infrastructure that protects public health and ecosystems, it is now. With will and resources, we can stop putting sewage into rivers and streams that we all share in common.”
TACKLING CSOS IN GREATER BOSTON
MyRWA Executive Director Patrick Heron testifies on CSOs in the Mystic River watershed, November 15, 2023. Image via the Charles River Watershed Association YouTube.
CSOs must be fully eliminated in order to ensure the health of our communities and ecosystems. The ultimate solution is to fully separate stormwater and sanitary sewer pipes and close the CSOs that remain throughout Greater Boston. This will be an expensive process that involves a collaborative effort between cities and public pressure to address the problem.
Thankfully, the Massachusetts Water Resources Authority (MWRA) and the cities of Somerville and Cambridge are already engaged in a process of controlling CSOs in the Mystic and Charles River watersheds. Over the past four decades, many CSOs have been closed and more CSOs are being partially treated before they are released into our waterways.
Photo courtesy of Save the Alewife Brook
Still, CSOs remain a pressing issue for all of us who live, work, and recreate near our rivers and streams. This summer, the Alewife Brook overflowed its banks during a CSO discharge sending sewage-laden water into backyards and the popular Alewife Brook commuter path. Resident David Stoff, a member of Save the Alewife Brook, captured this photo of an unknowing resident pushing a baby stroller through a puddle containing raw sewage from the brook.
For those residents who continue to be directly exposed to the hazards of CSOs and for the health of our ecosystems and waterways, we must continue to advocate for the full elimination of CSOs across Greater Boston.
TRACKING CSOs
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. You can sign up for alerts to be informed of CSOs when they occur near you.
Thanks to public reporting on CSOs, MyRWA is able to track CSOs in the Mystic River watershed and better understand the threat they pose.
HOW CAN YOU HELP?
Add Your Voice to the CSO Conversation
Subscribe to the MyRWA e-news to receive updates about CSOs in the Mystic and to stay in the loop about opportunities for public involvement in the CSO Long-Term Control Plan.
Contact your legislators to let them know that you care about CSOs and want to see investment in the improvement of stormwater infrastructure. Find your legislator here.
When the opportunity arises, vote yes on increased stormwater funding in your municipality. Better stormwater infrastructure can reduce the flow of rainwater through combined systems and help mitigate CSOs.
Stay Informed: Sign up for CSO Notifications
Use the MWRA 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 can sign up for those alerts separately through their respective websites.
