Cambridge, Somerville, and MWRA Submit Draft Updated CSO Control Plan
A Five-Month Public Engagement and Comment Period to Follow
Massachusetts - On Thursday, April 30, 2026, the Cities of Cambridge and Somerville, along with the Massachusetts Water Resources Authority (MWRA), collectively referred to as the Partners, submitted the Draft Updated Combined Sewer Overflow (CSO) Control Plan to the United States Environmental Protection Agency (EPA) and the Massachusetts Department of Environmental Protection (MassDEP). Importantly, the recommended alternatives in the Draft Plan are predicted to result in zero CSO discharges in the Charles River, the Mystic River, and the Alewife Brook in a 2050 Typical Year, meaning that projected increases in precipitation due to climate change are accounted for, offset, and largely mitigated through the implementation of the recommendations. As the first CSO plan in the nation to explicitly incorporate climate change, it sets a new standard for integrated, resilient water management.
Waterbody | Alternative Description | Cost | Estimated Duration |
| Alewife Brook | 2 storage tanks (3 MG) + 8 acres of sewer separation + 0.75-mile-long conveyance pipe + 1 mile-long microtunnel | $340M | 13-18 years |
| Mystic River | 1 storage tank (7.4 MG) + 95 acres of sewer separation | $260M | 5-7 years |
| Charles River | 2 storage tanks (12.6 MG) + 446 acres of sewer separation + 2 storage conduits | $690M | 28-33 years |
The Draft Plan’s estimated design and construction is expected to cost approximately $1.3 billion (in current dollars). While some of the previously examined alternatives could achieve higher levels of control (i.e. in storm events that occur infrequently), the recommended alternatives provide additional benefits, such as:
- Earlier reductions in CSO discharges into the waterbodies due to faster design, construction, and completion of several of the projects;
- Less disruptive construction impacts to the region, including less truck traffic;
- A reduction in temporary and permanent loss of invaluable open space, including public parkland; and
- Significant reductions in CSOs commensurate with the significant financial investment.
Significantly, the Draft Plan builds upon more than four years of robust technical analysis, collaboration, and a meaningful public engagement process. During this time period, six public meetings were held with an average attendance of 247 people. Additionally, 29 community events were hosted, seven online surveys and questionnaires were conducted, and 19 meetings between the Partners and the EPA/MassDEP were held.
Over the next five months, from May 1, 2026, to September 30, 2026, the Partners will seek further feedback from the public, advocacy groups, elected local and state officials, and other organizations on the Draft Plan. Visit the Partners’ Joint CSO Planning website to access the Draft Plan and learn how to provide comments.