Map: Real-time sewage overflows into Mass. rivers and bays

About 18 communities across Massachusetts — including Boston, Cambridge, Lynn, Haverhill — have some combined sewage and storm water systems, many of which were built in the mid-1800s. When it rains heavily, these systems are designed to overflow into nearby rivers and bays to avoid backing up into people’s homes.

As climate change brings more and heavier rain storms to the Northeast, it’s likely that Massachusetts will experience more combined sewage overflows, or CSOs. A 2021 state law requires cities and towns to notify residents within two hours of an overflow. However, the system only works if residents know to sign up for the text or email alerts, and for some water bodies residents would have to sign up for alerts from multiple towns to get the full picture. The Charles River, for example, has 10 different CSOs that flow into it.

WBUR has built a system to track real-time alerts for more than 180 CSOs across the state in one convenient place. Public health officials recommend waiting at least 48 hours after a CSO event to swim, boat or let pets into affected water.

Following a slate of lawsuits in the 1980s, many municipalities have worked to reengineer and shut down CSOs. Some estimates say prior to that cleanup, CSOs discharged about 3 billion gallons of untreated wastewater every year in Greater Boston. These days, that volume is closer to 400 million, and much of it is at least partially treated.

But separating sewage and stormwater pipes is very expensive — as is building a CSO water treatment plant — so tackling the remaining CSOs may be a slow process.

For more context, here are the cities that saw the most overflows in 2022.