Thanks to advocacy from MyRWA, the Charles River Watershed Association, Save the Alewife Brook, and others, regional planning work on combined sewer overflows can soon be informed by the latest climate science.
Here’s why that’s important.
The framework around CSOs
As many know, “combined sewer overflows” still happen on Alewife Brook, the Mystic River, and Charles River, among other places in greater Boston. These events occur in neighborhoods in Cambridge, Somerville, Chelsea, Boston, and other cities, where rainwater from roads and rooftops (also called “stormwater”) goes into the same pipe network as sewage from bathrooms. In the normal case, all that “combined” water goes to the regional wastewater treatment plant at Deer Island, run by the Massachusetts Water Resources Authority (MWRA).
But when it rains heavily, that pipe to Deer Island can sometimes fill to capacity. In that case, in order to avoid sewage spilling into streets or into homes, the system is designed to overflow directly into waterways like Alewife Brook. The reason these overflows still exist — many have been eliminated or reduced over the years thanks to the Clean Water Act — is that the remaining changes necessary to separate the two pipe networks would be extremely expensive to accomplish.
What exists today is a legal framework, called a Long-Term Control Plan, emerging from the federal lawsuit that mandated the cleanup of Boston Harbor, for making improvements over time, setting minimum thresholds for performance, and making sure that the public is informed about the threat to clean water that these overflows present.
How CSO modeling happens
Incremental improvements to the system are being made all the time. How do we know if we are making progress?
Imagine that engineers make an improvement to the system that is designed to reduce CSO volumes. How do you quantify what kind of impact that physical change to the system will have on the amount of untreated sewage that makes its way to rivers and streams?
You can’t just look at how much is released in the year after the improvement is installed. Because that year might just happen to be a year with very little rain. Or it might happen to be a year with anomalously large amounts of rain (like 2021).
In order to chart progress over time, you need some benchmark set of conditions that you can use to compare performance between one year and the next. So engineers pick a so-called “typical year,” a year where the frequency, duration, and intensity of storms best captures average conditions.
A lot of work goes into modeling how the current system—with everything we know about its capacity and limitations and its performance under past conditions—would respond to a typical year in the future.
But notice, engineers pick a year with “typical” rainfall patterns, to best serve as an average year in terms of not just total rainfall, but intensity and number of storms (because large storms are vastly more likely to cause overflows than small ones).
Many advocates stepped up and said, “but climate change science is telling us that typical conditions are changing. The new normal will not look like the past. Don’t CSO plans need to take that into account?”
The Mystic Steering Committee, co-hosted by MyRWA and EPA, hosted a meeting to talk through how this might happen. Comment letters from a variety of stakeholders were submitted. And to the great credit of many partners — including and especially the cities of Cambridge and Somerville — consultants, including a leading climate expert as reviewer, went through the painstaking process of defining what a new “typical year” should be in light of the latest science.
A presentation on the (complex) technical details can be found here. More about the public meeting devoted to the unveiling of this analysis in December, hosted by the City of Cambridge, is here.
Why climate change matters
We know that climate change is changing precipitation patterns in New England. In particular, it’s well understood that climate change will make big storms — larger extreme precipitation events — more common in New England. Figure 1 shows that the new normal will almost certainly mean bigger storms. Storms that only happen once every two years now will likely happen every year in the future.
Figure 1.
The final stage of the technical work will be to specify exactly the conditions that should characterize a typical year of rainfall under future conditions.
This is important, because what we really want to know is not how our CSO systems behaved in the past, but how they will behave in the future. If big storms are becoming more frequent, then it becomes even more important that the system be able to address big storms. It is important when running models to know that the scenarios you’re throwing at the model are realistic. If models do not reflect future reality, you might think you’ve solved the problem when you haven’t, and you won’t have an accurate picture of the choices facing the public in terms of water quality and public health.
Thanks to the process begun this year, greater Boston will be in a much more confident place in predicting how our old cities will react to climate change, so we can make effective, informed decisions about our futures.
