Combined Sewage Overflow (CSO) events are a common and concerning occurrences in the Merrimack River. CSOs occur during instances of heavy rainfall when the volume of sewage and rain exceed system capacity, causing untreated stormwater and wastewater to discharge directly into the river.  Through our involvement in the Merrimack River Collaborative (MRC), MVPC partners with organizations actively tracking CSO discharge in the Merrimack River. The new report by the Merrimack River Watershed Council details 2023 as a record-breaking year for CSO discharge. 

Due to the intense rainstorms experienced across the Valley throughout 2023, the total CSO volume surpassed 2 billion gallons. This amount far exceeds the previous record of 823 million gallons set in 2021.  The frequent occurrence of CSOs in 2023 greatly limited access to the River due to the recommended 48-hours of “no contact” following a CSO event. Between June and July (61 days), the river was under advisory for a total of 39 days (64% of the time).   

Communities along the river are actively working to reduce their CSO discharge volume, though solutions are massively expensive and technically challenging. In many cases, substantial infrastructure projects to separate combined sewage pipes, which run below key community infrastructure, is needed to fully address the problem. Communities are conducting infrastructure work, as well as exploring other solutions including building underground storage tanks to temporarily hold stormwater and expanding the capacity of wastewater treatment plants. 

With climate change expected to cause more intense and frequent precipitation events, CSO discharge will only become a more pressing challenge for our River. MVPC is working with partners to bring more attention and resources to the region to address this issue. To read MRWC’s full 2023 CSO report, click here. For more information on what MVPC is doing to support this issue, learn more about the Merrimack River Collaborative.