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What is stormwater and why does it matter?

When it rains (or snows) in urban areas, precipitation that cannot be absorbed into the ground “runs off” and flows into the sewer system. This unabsorbed water is referred to as stormwater or stormwater runoff. If this stormwater is not properly managed it can lead to pollution of local waterways as well as flooding in low-lying parts of the city.  

As a densely-populated and growing city, Somerville is working to build and maintain highly-effective stormwater management systems through a combination of traditional “grey” infrastructure and green stormwater infrastructure (features that mimic natural drainage processes). Somerville is also working closely with the neighboring municipalities, as well as state and federal agencies to develop regional approaches to solving flooding and water quality challenges.  

Scoop the Poop

Child walking a small dog down a city sidewalk

Did you know that stormwater can carry bacteria and nutrients from your dog’s poop from anywhere in the watershed to the Mystic or Charles River? Pick up your dog’s poop and properly dispose of it in the trash.

Learn More

 Our System At-a-Glance

Like many cities in the northeast, Somerville’s stormwater and sewer infrastructure was built primarily in the late 19th and early-to-mid-20th century. Somerville’s combination of ongoing development and aging infrastructure, exacerbated by the impacts of climate change, requires significant investment toward modernizing and upgrading our stormwater drainage and sewage systems. 

Combined vs. Separated Systems

The majority of Somerville is served by a combined sewer system, as is common in older cities. Combined sewers collect both sanitary sewage (or wastewater) and stormwater in the same pipe. In most cases, Somerville’s combined sewers transport all wastewater and stormwater collected from our streets, driveways, and parking lots to the Massachusetts Water Resource Authority’s (MWRA) Deer Island wastewater treatment plant. However, during heavy storms the capacity of the system is sometimes exceeded resulting in a discharge into a nearby waterbody prevent flooding. This is known as a Combined Sewer Overview. (Learn more about CSOs and how Somerville with working to reduce them.

On the other hand, a separated sewer generally has two sets of pipes under the streets. One pipe collects stormwater, which is discharged at a stormwater outfall, and the other collects sanitary sewage, which is ultimately discharged to a wastewater treatment plant. 

Municipal separate storm sewer systems (MS4) use separate pipes for sanitary sewage and stormwater flows. Combined sewer systems (CSS) use the same pipes.



The City’s sewer system is divided into seven sewer flow collection areas. These areas are generally known as sewersheds. While there are some connections between these flow collection areas, their sewer and stormwater systems operate mostly independent of one another.  


Factors influencing flood-risk and water quality  

Understandings the way the limitations of our current systems and other factors contribute to flood-risk and water quality challenges throughout the city is key to planning improvements.  

Aging Infrastructure and System Limitations - The prevalence of combined sewers and aging infrastructure contribute to flood risk and water quality issues. 

Development and Impermeable Surfaces - Much of Somerville’s land (both public and private) is made up of impermeable surfaces (concrete, buildings, etc.) that do not absorb stormwater naturally.  

Climate Change - Current climate change projections show an increase in frequency of heavily rainfalls. 

Topography - Somerville contains multiple hills and valleys. During heavy rainfall events stormwater can concentrate quickly in the valleys. Additionally, multiple areas within the city were once tidal rivers or marshes. These areas remain vulnerable to flooding. 

What the City is Doing

Somerville is committed to modernizing and maintain our city’s stormwater management. The city is working alongside our network of partners to invest in present and future community needs. 

The steps Somerville is taking to reduce the impact of stormwater pollution are being undertaken through the Environmental Protection Agency (EPA) National Pollutant Discharge Elimination System (NPDES) permitting program. 

Somerville’s Stormwater Management Plan (SWMP) describes and details the activities and measures that will be implemented to reduce pollutants that enter the drainage system to the maximum extent practicable (MEP). These control measures are based on the requirements of the National Pollutant Detection Elimination System (NPDES) Municipal Separate Storm Sewer System (MS4) permit.  

More on Somerville's Stormwater Management Plan (SWMP)

The table below outlines some of the requirements of the MS4 permit as well as highlights the City's ongoing efforts in this area. Any questions or comments can be sent to Lucica Hiller, Stormwater Program Manager, at                                                                                                                                                                                                                         

Minimum Control Measures


What Is the City Doing?

Public Education and Outreach Implement an education program that includes educational goals based on stormwater issues of significance within the MS4 area Check out the What Can You Do tab, Twitter account, and mailers for more information on stormwater messages for residents, businesses, industrial users, and developers.
Public Involvement & Participation Provide opportunities to engage the public to participate in the review and implementation of the City’s Stormwater Management Plan (SWMP) Every year the public is provided opportunities to review and submit comments on the SWMP through public meetings and events. Check out the most recent SWMP under the Annual Reports and Resources tab.
Illicit Discharge Detection Elimination (IDDE) Implement a program to systematically find and eliminate sources of non-stormwater discharges to the City’s separate storm sewer system and implement procedures to prevent such discharges

The City has:

  • mapped over 99% of the City’s MS4 system                
  • completed outfall screening
  •  continued annual staff training  
  • continued ongoing catchment investigations to identify and correct illicit connections.                       

Check out the Final IDDE Plan and results from past IDDE work under the Annual Reports and Resources tab.

Construction Site Stormwater Runoff Control Minimize or eliminate erosion and maintain sediment on-site so that it is not transported in stormwater and allowed to discharge to the Mystic River or Charles River through the City’s MS4 system and outfalls The City has implemented Site Construction Permit Rules and Regulations to review site plans, inspect active sites, and enforce site plan review requirements.
Post Construction Stormwater Management for New Development & Redevelopment Reduce the discharge of pollutants found in stormwater through the retention or treatment of stormwater after construction on new or redeveloped sites

            The City has:   

  • approved Stormwater Ordinances to enforce post-construction stormwater management                
  • developed a street design report               
  • developed a green infrastructure report with retrofit opportunities           


Good Housekeeping & Pollution Prevention for Permittee Owned Operations Implement an operations and maintenance program for City-owned operations that has a goal of preventing or reducing pollutant runoff and protecting water quality from all City-owned operations

Every year, the City:                    

  • removes debris from catch basins             
  • sweeps the streets between April and November         
Additional Control Measures Objective What is the City doing?
Discharges to Water Quality Limited Waterbodies

If there is a discharge from the MS4 area to a water quality limited waterbody, additional requirements are required to reduce pollutants in discharges from the impaired catchment(s). Somerville currently only discharges to the Mystic River and Alewife Brook (a tributary of the Mystic River). The Mystic River and Alewife Brook have the following impairments:

  • Phosphorus  
  • Bacteria/Pathogens                
  • Solids/Oil/Grease/Metals            


The City has:                    

  • increased public education and outreach for specific messages, such as pet waste and leaf/yard waste          
  • increased street sweeping frequency of all municipal-owned streets and parking lots              
  • identified a prioritized list of catch basins for inspection and maintenance to ensure catch basins are no more than 50% full         



Additional City Planning and Infrastructure Improvement Efforts 

Explore below to learn more about projects and improvements we’re working on. 

City Planning Projects Related to Stormwater  

CSO Control Plan Update - The City of Somerville, the City of Cambridge, and the Massachusetts Water Resources Authority are collaborating to improve water quality in the Alewife Brook and Mystic River through the development of an updated combined sewer overflow (CSO) control plan. 

Flood Mitigation and Water Quality Master Plan - The City developed a Citywide Drainage and Water Quality Master Plan, a collection of infrastructure projects that will reduce flooding, improve water quality, and mitigate combined sewer overflows. After years of modeling and evaluation, the Engineering Division and its consultants, with input from the pubic, have identified 22 projects that would address the root causes of flooding and prepare Somerville for climate change. 

City Infrastructure Projects Related to Stormwater  

Somerville Ave. Utility and Streetscape Improvements – Drainage upgrades will increase capacity and reduce flooding with the installation of a 14-foot wide by 6-foot tall box culvert (a rectangular concrete structure) under Somerville Ave. from Union Square to Medford St. The 800,000 gallon box culvert will remove storm flows from the 100-year old, 72-inch brick combined sewer underneath Somerville Ave.  

Spring Hill Sewer Separation  - The City is working to upgrade Spring Hill’s sewer and drainage systems by installing new storm drains that separate stormwater from the existing combined sewer. In addition to the underground utility improvements, this project will also install street safety improvements and green infrastructure.  

Poplar St. Pump Station - The Poplar Street pump station and accompanying underground storage tank will fundamentally change the way stormwater drainage is managed for approximately 60% of the city. It represents a major investment in modernizing the City’ stormwater management infrastructure and increasing preparedness for extreme weather events. 



What You Can Do

Bag of lawn fertilizer

Use Fertilizer Responsibly

Curious about what you can do to keep our water clean? Watch this video! And then visit to learn more! Keep our waterways clean, use fertilizer responsibly! (Video: English | Español)

City-issued trash tote

Keep a (Trash) Lid on It

Trash can be picked up by runoff when it rains and ends up in the Mystic River or Charles River. Properly dispose of trash in your trash carts or dumpsters and keep the lid securely on. (Video: English | Español)  

Leaf bags lined up for collection

Leave the Leaves & Yard Waste

Be a Leaf Hero and leave leaf and yard waste in your yard as a natural habitat for native pollinators. Any excess leaf and yard waste can be collected and placed at the curb for City pick up between April and December! Check out the yard waste collection calendar!




Adopt-a-Drain Program

Help keep our waterways clean by adopting a drain in your neighborhood! Through Somerville’s Adopt-a-Drain program, residents and business owners can volunteer to care for a local storm drain and check on it before and after heavy rain, wind, or snow.

What is an Adopt-a-Drain program? What does it mean to adopt a drain?
An Adopt-a-Drain program is a volunteer program that helps keep our local environment clean and healthy. Participants sign up to adopt a drain (or drains!) and then check on that drain before and after heavy rain, wind, or snow. Any extra care is even better! Anything that goes down the drain can end up in the Mystic River, the Alewife Brook, or the Charles River. Keeping the drains clear can also help prevent streets from flooding as it ensures that water can freely flow down the drain.

What am I adopting? What is a storm drain?
Storm drains (or catch basins, as engineers like to call them) are the grates you see in our streets and roads. They’ll be to the side of the road, along the curb, and in Somerville are typically square. Since asphalt and pavement can’t absorb water, any rain and melting snow flow along the street and into these drains. 

A basin below the grates, called a catch basin sump, is meant to collect this water and any debris but it can fill up quickly and doesn’t capture everything. 

Trash and leaves clogging a drain.

Trash and leaves can clog drains, cause flooding, and pollute our waterways.

A clear drain with no obstructions.

A clear drain means water can flow into the catch basin sump.


How can I get involved?

Just head to the Somerville Adopt-a-Drain page to sign up! Find a drain by typing in a nearby address or by dragging the map. If the drain you want to adopt is available, you can claim it by clicking on it. Sign up for the program, give your drain a name, and keep an eye out for a welcome email with more info! You can adopt as many drains as you would like to care for.

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