Cities of Cambridge, Somerville, and MWRA Submit Draft Updated Combined Sewer Overflow Control Plan

The Cities of Cambridge and Somerville, together with the Massachusetts Water Resources Authority (MWRA), have submitted a joint Draft Updated Combined Sewer Overflow (CSO) Control Plan to the U.S. Environmental Protection Agency (EPA) and the Massachusetts Department of Environmental Protection (MassDEP) to protect local waterways from pollution. The Draft Updated Plan is the first in the nation to go beyond EPA guidance and prepare for climate change.

The Draft Updated Plan is the first to be taking bold, progressive next steps to eliminate CSOs in a typical 2050 year, and the first to plan for heavier rainfall due to climate change. It is the result of more than four years of detailed analysis, engineering evaluation, and broad public engagement. The draft recommends a set of projects-including new storage tanks, sewer separation, and upgraded conveyance systems-that together are projected to eliminate CSO discharges during a typical 2050 year in the Alewife Brook, Charles River, and the Mystic River. A Combined Sewer Overflow is an overflow of stormwater and sewage into nearby rivers and streams that happen during heavy rain.

This plan prepares for increased and heavier rainfall expected in a typical 2050 year due to climate change, while also balancing water quality, construction impacts, public open space, project timelines, and costs.

Climate change is expected to overwhelm sewer and drainage systems in area communities. This would result in local flooding, health risks, and water pollution equal to or worse than CSOs. To reduce these risks, it is critical that the MWRA, Cambridge, Somerville and other Massachusetts communities continue resiliency planning and investing in major infrastructure projects that manage stormwater and help protect homes, rivers and public health.

After reviewing 39 potential approaches, the following recommendations were made for each body of water.

Alewife Brook:

Two storage tanks (three million gallons total), eight acres of sewer separation, a 0.75-mile conveyance pipe, and a one-mile micro tunnel. The estimated design and construction cost is approximately $340 million, and the estimated completion timeframe is 13-18 years.

Mystic River:

One storage tank (7.4 million gallons) and 95 acres of sewer separation. The estimated design and construction cost is approximately $260 million, and the estimated completion timeframe is 5-7 years.

Charles River:

Two storage tanks (12.6 million gallons), 446 acres of sewer separation, and two new storage conduits. The estimated design and construction cost is approximately $690 million, and the estimated completion timeframe is 28-33 years.

The recommended projects offer major advantages to other alternatives, including:

• Reduce pollution faster due to shorter construction timelines
• Less disruption to neighborhoods, including reduced truck traffic
• Protect parks and open spaces by limiting how much land is needed

The Draft Updated Plan incorporated extensive engagement by residents, advocates, and technical experts. Over the past four years, Cambridge, Somerville and the MWRA hosted six large public meetings (with an average of 247 participants), held nearly 30 community events, conducted seven online surveys, and held nearly 20 meetings with the EPA and MassDEP.
This input helped shape a plan that balances environmental responsibility with affordability, community goals, and long-term resilience. It also builds on decades of investments by Cambridge, Somerville, and MWRA to modernize sewer and stormwater systems and protect local waterways.

"This Draft Plan is the most technically sophisticated and boldest CSO control plan in the country," said Cambridge City Manager Yi-An Huang. "It proposes a level of control that goes beyond regulatory requirements and EPA guidance. It would eliminate CSOs in a typical 2050 year and factors in future rainfall projections that account for the impacts of climate change. Overall, this represents an important next step toward a cleaner, more climate resilient future and builds upon years of investment and collaboration to improve our sewage and stormwater infrastructure. We are grateful for the ongoing support and partnership of Somerville and the MWRA during this critical process."

"This new draft recommends bold action on Combined Sewer Overflows and highlights the scale of the challenge for communities across the state who are looking to responsibly manage stormwater in the era of climate change," said Somerville Mayor Jake Wilson. "I am grateful to our partners at the City of Cambridge and the MWRA, and the many resident voices that helped shape the recommendations. My hope is that this plan will be a valuable contribution to our regional discussions of how we can best prepare our communities for a resilient climate future and rally together as a region to equitably share the financial costs of progress."

"The Draft Updated CSO Control Plan is a significant and responsible investment of rate payer dollars that balances real environmental benefits and actual water quality improvements," said MWRA Interim Executive Director Stephen Estes-Smargiassi. "The Draft Plan is just the first step in a thorough public process. Today, we begin a five-month public comment period to solicit feedback from stakeholders and members of the public. The best public policy decisions are made when all perspectives are heard, and we hope to hear from all interested parties during this review."

Next Steps: A Five-Month Public Feedback Period

Between May 1 and September 30, 2026, community members, advocacy groups, and organizations are invited to review the Draft Updated CSO Control Plan and share feedback with Cities of Cambridge and Somerville, along with the MWRA.

• A virtual public meeting on June 2, 2026, will provide an overview of the Draft Plan.
• Two public hearings will be held in September, where residents can provide formal verbal comments.
• Local office hours will be hosted by project staff and technical teams.
• A dedicated website will be available throughout the project and members of the public can submit comments on the plan through September 30.

In addition, Cambridge will launch a new CSO and Stormwater Management Advisory Committee this summer to support continued community involvement. The volunteer committee will consist of members appointed by the City Manager. It represents residents, technical experts, and representatives from several stakeholder groups, who will advise on the planning and implementation of programs, infrastructure projects, and other initiatives related to stormwater quantity, stormwater quality, sewer separation, wastewater, and more.