338 million gallons released–a steep increase from years past
If we wanted proof that the greater Boston water infrastructure needs investment, 2021 provided it. Frequent and intense rainstorms put our stormwater systems to the test. The results? A dramatic increase in sewage pollution in the Mystic River and Alewife Book from combined sewer overflows (CSOs). The total annual precipitation in 2021 was not higher than we have experienced before (Figure 1) and the problem of CSOs is not new, but 2021 showed us that new patterns of precipitation may make the CSO problem worse. Our stormwater infrastructure is arguably not prepared for future climate scenarios where extreme weather and intense periods of rain are only going to get more common.
Figure 1. Total annual precipitation 2000-2021 (as recorded by NOAA sensors at Logan Airport)
Heavy Rainfall Triggers Combined Sewer Overflows
Figure 2. A depiction of what happens in a combined sewage and stormwater system during dry and wet weather.
Here’s the source of the pollution problem: significant areas in the Mystic watershed have a combined sewer pipe system, where stormwater from roads and sewage flow through the same pipes. During dry weather or moderate rain events, the combined stormwater and sewer pipes deliver the flow to the Massachusetts Water Resources Authority (MWRA) Deer Island Treatment Plant. When the combined system gets overwhelmed – usually because of heavy rainfall – Combined Sewer Overflows occur to divert the flow directly into nearby rivers, like the Mystic River and Alewife Brook, to insure that it doesn’t back up into people’s homes (Figure 2). This releases raw or partially treated sewage into local water bodies, which is a major source of bacterial pollution, threatening human health.
Combined Sewer Overflows (CSOs) are a threat to clean water and safe recreation. The negative public health and environmental effects of CSOs disproportionately impact low income, minority, and linguistically isolated communities in Massachusetts, making CSOs an environmental justice tragedy and a pressing equity and climate change issue.
Tracking Public CSO Data
The MWRA and the cities of Somerville, Cambridge, Chelsea, and Boston all have combined sewer overflow pipes that go into the Alewife and Mystic system and they also track and report when these CSOs occur. With the recent passage of the CSO bill H.4921, which was sponsored by Senator Patricia Jehlen, and Former Representative Denise Provost, waste treatment operators are now required to notify the public when there’s been a sewage discharge in their area.
As part of MyRWA’s efforts to improve water quality in the Mystic, we keep a close eye on CSOs throughout the year. By knowing when and where CSOs occur, we can track their occurrence over time and advocate for more infrastructure improvements. The information below was collated from CSO events reported on the MWRA, Cambridge, Chelsea, and Somerville CSO websites.
A total of 108 CSO events happened on the Mystic in 2021 compared to 67 events in 2020 (Figure 3). The majority of CSO events in 2021 occurred during July through October – when we saw the heaviest rainfall. This past July was the second wettest July on record, with the Boston area seeing three times more rain than average during this month.
Figure 3. The number of CSO events per month during 2020 and 2021.
What’s more, a total volume of approximately 338 million gallons of combined stormwater and sewage was discharged during CSO events in 2021 compared to approximately 41.5 million gallons discharged in 2020 (Figure 4). A 40% increase in precipitation from 2020 to 2021, led to a 700% increase in volume of CSO released.
Figure 4. Volume released (in millions of gallons) per month during CSO events in 2020 and 2021.
Climate change scenarios for New England predict increased extreme precipitation events, the trigger for CSOs. Even if total precipitation didn’t change, if the frequency of extreme precipitation goes up, CSOs will go up – as we saw this past July.
(More information on the exact CSO events can be found on the reporting websites of Somerville, Cambridge, Chelsea, and the MWRA.)
We Need to Invest in Improving Stormwater Infrastructure
The overall solution to CSO pollution is clear: separate (rather than combined) stormwater and sanitary sewer pipe networks. In fact, much of the cleanup of Boston Harbor over the past 40 years is due to the closing of CSOs on the Mystic and Charles River systems, among other changes, such as investment in green infrastructure.
The remaining CSOs will, however, be very expensive to replace. The missing pieces are funding and political will.
“Toward the reduction of discharges of combined sewer systems, the MWRA has led a $1b effort across their system and they should be commended for their efforts and strong management”, said Patrick Herron, Executive Director of the Mystic River Watershed Association. “But as 2021 showed, the system remains vulnerable to the extreme precipitation events that we will see more frequently in the future. This is a wake-up call for the need for another major round of investment.”
The data presented here speak to the importance of modeling what frequency and volumes of CSO overflows are expected in a future precipitation regime.
The next two years present an opportunity to use data and community voices to advocate for increased funding for the improvement of stormwater infrastructure. “2022 is the 50th anniversary of the Clean Water Act,” notes Andy Hrycyna, watershed scientist at MyRWA. “As the federal Infrastructure Bill and other federal programs make once-in-a-generation infrastructure funding available over the next few years, wouldn’t it be fantastic if we could collectively choose to make a huge advance on solving the CSO problem once and for all?”
How Can You Help?
Add Your Voice to the CSO Conversation
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.
Stay Informed: Sign up for CSO Notifications
MWRA has created a 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.
During the boating season, you can also track boating safety on the Mystic by checking out the Boating Advisory that the Mystic River Watershed Association created and is updated every morning--on our website or follow us on Twitter @SafeMystic.
Check out Friends of Alewife Reservation for opportunities to protect and restore the Alewife Brook Stormwater Wetland.
