Thinking about recreating on the Mystic River? Read on to learn about Combined Sewer Overflows (CSOs) and how you can sign up to get an email notification when a CSO occurs near you:

Figure 1. A depiction of what happens in a combined sewage and stormwater system during dry and wet weather.

WHAT CAUSES CSOS AND WHY ARE THEY A HEALTH CONCERN

Here’s the source of the pollution problem: significant areas in the Mystic River 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 1). This releases raw or partially treated sewage into local water bodies, which is a major source of bacterial pollution, threatening human health.

for safe recreation, sign up for cso notifications

To know when and where a CSO has occurred, click these links to sign up for email notifications. *Note: the cities of Boston, Cambridge, Chelsea and Somerville manage their own combined sewer outfalls, including CSO reporting; to receive information about outfalls in those cities, you must sign up for those alerts separately through their respective websites.