Stormwater Spotlight Part 3: What to Know About Combined Sewer Overflows

You’ve probably felt the particularly rainy spring and summer we’ve been having this year in Massachusetts. Lots of rain creates lots of stormwater, which is water that runs off impervious surfaces like roads or roofs into our sewer systems. And stormwater is one of the biggest sources of water pollution in Massachusetts, which means there have been lots of pollutants entering our waterways in the past couple months.

But there’s good news! Each of us can help clean up stormwater, and it starts with understanding the problem. So we’re excited to bring you the third installment in our blog series on stormwater. Throughout this four-part series, we’re exploring the fundamentals of stormwater pollution; who regulates it, how sewage overflows contribute to the issue, and tools for tackling the problem.

CSO Outfall Signage, Springfield, MA

In the last two Stormwater Spotlights we talked about how stormwater runoff harms our rivers and two different ways the Environmental Protection Agency (EPA) reduces stormwater pollution in Massachusetts. In this post we’ll explore Combined Sewer Overflows, or CSOs, and how they hurt waterways in certain areas of the state.

What is a Combined Sewer Overflow (CSO)?

CSO Diagram by Fiona Martin, for NOAA Climate.gov

Many Massachusetts cities have modern sewer systems that are designed to move stormwater and sewage in different pipes. However, in some older cities in Massachusetts, these pipes are combined - meaning stormwater and sewage flow together in one system. 

These older combined systems were built before we had wastewater treatment plants. Back then, sewage and stormwater would empty directly into nearby water bodies. When wastewater treatment plants were added, they were only designed to handle a certain amount of water. So engineers added outfalls - special pipes that act like emergency exits. When there is too much water in the system during heavy rain or snowstorms these outfalls release the excess mix of sewage and stormwater into waterbodies to avoid overloading the treatment plants or having sewage back up into peoples homes or businesses. This kind of release is called a Combined Sewer Overflow (CSO) - and it means that during storms, dirty water (including raw sewage) can end up in our rivers and water bodies.

Sewage in water sounds bad, but how much of a problem is it?

Unfortunately, CSOs still have a large negative impact on water quality in Massachusetts. In 2024 alone there were 1.9 billion gallons of untreated combined sewer water released into Massachusetts waterways from CSO events. This amount is the equivalent of 2,879 Olympic-sized swimming pools. CSO discharge is generally unsafe to come into contact with as it contains human waste and other pollutants, which can cause various illnesses. You should avoid swimming, paddling, boating, sailing, or fishing in or near sewage-polluted water for at least 48 hours after a CSO event. You also shouldn’t let your pets come into contact with the water. Additionally, CSO events are going to occur more frequently, as climate change is causing more intense storms with heavier precipitation which will increase this sewage pollution.

 Map of CSO outfalls, courtesy of Massachusetts Department of Environmental Protection (MassDEP)

How do I know if there’s been a CSO event near me?

Until 2022 there was no way for the public to know when these discharges occurred. Advocacy from an active coalition of watershed organizations and municipalities led by Mass Rivers resulted in the passage of the Sewage Notification Law in 2021. This law is designed to alert the general public to any releases of raw or partially treated sewage into a water body, not just CSOs. If you live in or adjacent to any of the municipalities with CSOs, you should go to the town or city website and opt in to their sewage notification alerts (text or email).

I don’t live in any of those municipalities, why should I care?

Many of these cities are on major waterways that discharge to coastal areas near

Beaches. So when a CSO happens, it will affect all downstream areas. For example, If there are CSO events in Lowell or Lawrence, that sewage ends up in the Merrimack River, which can close the beaches in Salisbury or Plum Island. Also, thousands of people use major rivers for boating and sailing or fishing. CSO events in Worcester or Springfield can close their nearby rivers, the Blackstone and Connecticut respectively, to paddling and fishing. CSOs in Cambridge and Somerville affect the Charles River, which are heavily used for boating and sailing. Additionally, many CSOs discharge to waterways near shellfish growing areas, which means sometimes clam, mussel, or oyster crops must be discarded because they are contaminated with sewage.

Map of Combined Sewer Outfalls and aquatic resources in MA. Courtesy of MassDEP.

What can we do about CSOs? 

Progress has already been made to reduce CSOs in Massachusetts. However, the remaining CSOs in the state are quite expensive and will require much effort to reduce or eliminate. To help mitigate CSOs, here are some steps you can take:

1. Contact your local Senator or State Representative and ask them to support CSO mitigation. There is currently a bill in the legislature, Bill H.1031/S.608, An Act relative to combined sewer overflows, which mandates that the MWRA eliminate CSOs from smaller-sized rainstorms. If you don’t know who your state legislator is, find it on Mass River’s map of State Legislators. 

2. Advocate for stormwater funding in your town. Towns can also create their own stormwater funding sources by altering water and sewer rate structures, or building a stormwater utility. 

   1. To find out how your town can build a stormwater utility, or investigate other stormwater funding sources for your town, check out Mass River’s Stormwater Financing Workshops from 2023. 

   2. The Metropolitan Area Planning Council also has a toolkit on starting a stormwater utility in your town. 

   3. To learn more about stormwater utilities and other financial or strategic options for managing stormwater, look out for Mass Rivers’ upcoming Rainwater Readiness workshop series this fall!

3. If you live in the Boston area, register for public hearings of the Combined Sewer Overflow Control Planning Program. The Combined Sewer Overflow Control Planning Program is a collaboration between Somerville, Cambridge, and the MWRA to reduce CSOs into the Charles and Mystic Rivers. Sign up here to make your voice heard in public meetings of the Program! 

All water is connected. Water pollution happening in a different area of the state can still affect you, in big and small ways. To help reduce CSOs and improve water quality in Massachusetts, lend a helping hand and get involved today!