Alewife Brook, Upper Mystic, and Charles River Draft Updated CSO Control Plan
August 10, 2026
On April 30th, 2026, the Cities of Somerville and Cambridge, along with the MWRA, submitted their Draft Updated Combined Sewer Overflow Control Plan (Draft Plan), designed to address the remaining CSOs in the Charles, Mystic, and Alewife waterways. This blog post provides some background on CSOs, what is in the Draft Plan, reactions to it, and potential next steps.
CSO Outfalls in the Draft Plan Area, their Impact, and Why they are a Public Health Risk
Most communities have separate sewer and stormwater systems: wastewater is taken to a waste treatment plant; stormwater is carried to a local waterbody. In combined sewer systems, stormwater and wastewater both flow through one pipe. During dry weather, wastewater flows to a treatment plant. However, during wet weather, particularly heavy storms, stormwater also enters the pipe. If runoff exceeds the capacity of the system, untreated stormwater and wastewater flow into nearby waterbodies. Combined Sewer Overflows (CSOs) prevent backflow into homes and other critical infrastructure, usually existing along waterbodies, serving as relief points for capacity overflows.
In the area covered by the Draft Plan, there are approximately 16 open CSO outfalls (see Table 1) on the Charles, Mystic, and Alewife waterways, zoned to either Cambridge, Somerville, or the Massachusetts Water Resources Authority (MWRA). The outfalls within Boston are zoned to the MWRA. See Map 1 for locations. Although the zoning of these CSOs is the responsibility of Cambridge, Somerville, and the MWRA, the impacts from polluted water (see Map 1) affect the entire watershed area for each waterbody.
Table 1: Open CSO Outfalls Subject to the Draft Plan
Source: Draft Plan, April 30, 2026: https://voice.somervillema.gov/joint-cso-planning p. 1-6
Map 1: Permitted CSOs Discharging in the Alewife Brook, Mystic River, and Charles River
Source: Draft Plan April 30, 2026: https://voice.somervillema.gov/joint-cso-planning p. 1-6
CSOs are a public health risk because they expose people and ecosystems to pathogens associated with sewage, including E. coli and Enterococcus. E. coli is an indicator species identifying wastewater contamination. The Draft Plan details the levels of E. coli by source in the Charles River, Alewife Brook, and Upper Mystic River. Although the Draft Plan doesn’t attribute any E. coli to CSOs in the latter, a small amount is attributed to untreated CSOs in the Charles River, and a significant amount is attributed to untreated CSOs in the Alewife Brook. Other sources of E. coli include stormwater, upstream inputs, and dry weather.
The Draft Plan Recommendations
The Draft Plan incorporates increased precipitation associated with climate change in a 2050 Typical Year (TY) with projections aligning with those in Boston, Cambridge, and Somerville. Five planning scenarios were analyzed for a 2050 TY: limited CSOs; no CSOs; no CSOs with a large (5-year) storm event; no CSOs with a very large (25-year) storm event; and full sewer separation.
The Draft Plan recommends the 2050 TY no CSOs scenario that eliminates CSOs for a 2050 typical year, with limited CSOs for the 5 and 25-year storm scenarios. It has some sewer separation and storage but has incomplete CSO control. Intense storms are still expected to result in limited CSO leakage after the project is complete.
The recommended plans for the waterways are as follows:
- Charles River: 2 storage tanks (12.6 MG), 446 acres of sewer separation, 2 storage conduits. $690 million plan that will take 28-33 years to complete.
- Mystic River: 1 storage tank (7.4 MG) and 95 acres of sewer separation. $260 million plan that will take 5-7 years to complete.
- Alewife Brook: 2 storage tanks (3 MG), 8 acres of sewer separation, 0.75-mile-long conveyance pipe, and 1 mile-long microtunnel. $340 million plan that will take 13-18 years to complete.
The Draft Plan’s Financial Impacts
The Draft Plan is estimated to cost $1.3 billion, split between the MWRA ($763.7 million), Cambridge ($257 million), and Somerville ($257.3 million). MWRA’s cost will be paid by debt servicing through the 2070s, split between MWRA’s 43 Sewer Service Area communities, identified in pink in the Figure below (Figure 1-2).
Map 2: MWRA Service Area
Source: Draft Plan, April 30, 2026: https://voice.somervillema.gov/joint-cso-planning p. 1-4
Residents of Cambridge and Somerville will pay twice: for their portion of the Draft Plan costs: 1) as MWRA ratepayers; and 2) through paying for their cities’ share of the cost. The Draft Plan estimates Cambridge residents’ annual bill will increase 171% by 2056[1], while Somerville’s will increase 205% by 2055[2]. With the 25-year plan, these increases surge to a projected 523%[3] and 265%[4], respectively.
Watershed Organization Recommendations
Grassroots organizations such as the Charles River Watershed Association (CRWA), Mystic River Watershed Association (MyRWA), and Save the Alewife Brook disagree with some of the recommendations made in the Draft Plan. CRWA has promoted a Cut the Crap Campaign, MyRWA and CRWA published a joint press release condemning the Draft Plan after its release, and Save the Alewife Brook released its own alternative plan, the Alewife Brook Community Combined Sewer Overflow Elimination Plan, which includes sewer separation and green stormwater infrastructure recommendations.
Their main concerns are that the scenario chosen in the Draft Plan does not do enough to keep the rivers and surrounding areas safe and clean for local communities. The organizations ask that:
- The project team incorporate more long-term, climate-resilient solutions, including sewer separation and green stormwater infrastructure, into the Draft Plan (Save the Alewife Brook, P. 14-16)
- MWRA utilize its own funding to pay for the Draft Plan, rather than gather funding through debt service (Save the Alewife Brook, P. 17-22)
- MWRA accurately and clearly describe the issue to the public, as they perceive that important information such as the sources of E. coli has been incorrectly presented. (CRWA/MyRWA). To back up their E. coli concerns, MyRWA has produced its own graphics (see Figure 1), indicating particularly high levels of E. Coli outside CSO outfalls after heavy storms.
Figure 1: Wet Weather Median E.Coli Results from Alewife & Mystic by Location and Year
Source :Ending Combined Sewer Overflows: The Time Is Now! Presentation. 06.09.2026, Charles River Watershed Association and Mystic River Watershed Association
(Note: E. coli concentrations during dry weather (gray) vs. after a CSO event (red)).
The organizations are advocating for the TY 2050 no CSOs with a very large (25-year) storm event scenario, which would cost an additional $99 per household per year (for all ratepayers in the MWRA Sewer Service Area. CRWA and MyRWA argue that $99/year is not a large price to pay when considering the associated contributions to the elimination of CSO.
Next Steps
Public comments are being accepted until September 30, 2026. A Better City will draft comments by working with members, municipalities, and advocates to better understand the concerns and how it will affect the Greater Boston urban environment. From November 2026 through early 2027, the Draft Plan project team will incorporate public comments. The final submission to MEPA is due by January 31, 2027. The MEPA Office will review the Draft Plan from January 2027 through August 2029.
[1] City of Cambridge, City of Somerville, and Massachusetts Water Resources Authority, Draft Updated Combined Sewer Overflow Control Plan (April 30, 2026), Table 11-14, https://voice.somervillema.gov/19901/widgets/115506/documents/80799.
[2] City of Cambridge, City of Somerville, and Massachusetts Water Resources Authority, Draft Updated Combined Sewer Overflow Control Plan (April 30, 2026), Table 11-21, https://voice.somervillema.gov/19901/widgets/115506/documents/80799.
[3] City of Cambridge, City of Somerville, and Massachusetts Water Resources Authority, Draft Updated Combined Sewer Overflow Control Plan (April 30, 2026), Table 11-14, https://voice.somervillema.gov/19901/widgets/115506/documents/80799.
[4] City of Cambridge, City of Somerville, and Massachusetts Water Resources Authority, Draft Updated Combined Sewer Overflow Control Plan (April 30, 2026), Table 11-21, https://voice.somervillema.gov/19901/widgets/115506/documents/80799.
