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What are CSOs?

•    Six urban sewage treatment systems in the Merrimack River watershed frequently discharge large quantities of raw sewage during rainstorms:  Manchester and Nashua, N.H., Lowell, the Greater Lawrence Sanitary District, and Haverhill (all on the Merrimack River), and Fitchburg (on the Nashua River, a Merrimack tributary).  These sewage treatment plants are under government order to eliminate most of their CSOs, but without additional federal and state funding, it will take as long as 25 years to get there. 
•    These sewage systems overflow because they were built years ago to bring together sewage from toilets and sinks (so-called "sanitary sewage," essentially infectious and toxic human and animal waste) and stormwater runoff.  Treatment plants cannot cope with the combined volume of raw sewage and polluted stormwater during heavy or protracted rain events, so they discharge the excess directly into the river.  This is called a "combined sewer overflow," or CSO.

•    CSOs are frequent and significant.  In 2018, these six sewage treatment systems experienced hundreds of overflow events, which MRWC estimates resulted in the discharge of an aggregate of 800 million gallons of raw sewage into the river.  In a typical year, "only" 400 million gallons are released.  

Please see below for more FAQs about CSOs. 

Combined Sewer Overflows (CSOs): FAQs

Why was 2018 such a bad year for CSOs?

•    One of the main drivers of sewage releases is substantial rainstorms that include heavy downpours—“tropical storms” as they are often called.  According to the U.S. Environmental Protection Agency, computer models show that the Northeast will experience more of these types of storms due to climate change.  In 2018 we had a few of these big storms—most notably on September 18 (3.1 inches of rain), August 11 (2.2 inches), and April 17 (2.4 inches).

•   Another factor is the unusual weather pattern we had from 2015-2017.  Our region incurred a significant drought during those years, and this cut annual sewage releases down to around 300 million gallons.  But 2018’s rain totals are more typical of our average—about 4 inches a month. 

How does this affect me?

•    Nearly all of the sewage from CSOs is discharged upstream of drinking water plants that provide Merrimack River water to residents in six municipalities: Lowell, Lawrence, Methuen, Andover, North Reading, and Tewksbury.  (A sixth, Nashua, receives about a quarter of its drinking water from the river.)

•    All told, more than 600,000 people get their daily drinking water from the Merrimack River.  Within a decade, that number is likely to increase by up to one-third, as Manchester and Haverhill tap into the river to expand local supplies and sell water to other communities.

•    The Merrimack River is the second-largest surface-based drinking water source in New England.  The largest is the Quabbin Reservoir, which, unlike the Merrimack, was built to provide pristine drinking water to Eastern Massachusetts.

•    Sewage can remain in the Merrimack for up to three days after a CSO event—a period during which, in many cases (especially in the nice weather), the skies will have cleared, leaving the river environment deceptively inviting to anglers, boaters, swimmers, and other recreational users of the river.

•    Exposure to raw sewage can cause multiple, serious health problems, particularly for the very young, the elderly, and those with compromised physiology.   According to EPA, raw sewage includes disease-causing agents falling into three general categories:

•    Bacteria:  single-celled organisms that, when pathogenic, can cause cholera, dysentery, shigellosis, and typhoid fever;
•    Viruses: parasites that grow on living tissue and, when associated with feces, include hepatitis A, Norwalk-type virus, rotavirus, and adenovirus; and
•    Protozoans : about a third of all protozoans are pathogenic, and these can cause gastrointestinal disease, dysentery, and ulceration of the liver and intestines.

•    A 2015 peer-reviewed health study that included the Lawrence area along the lower Merrimack found a statistically significant association between heavy rains (which trigger CSO events) and an increase in hospital emergency room admissions of people  complaining of gastro-intestinal disorders.

How can I find out, in real time, that a CSO is happening in my neighborhood?

Do you want to be alerted whenever a wastewater treatment plant along the Merrimack releases sewage into the river? As of now, there is no centralized system to subscribe to in order to receive alerts. But there are 2 options available if you want to stay on top of CSO events.
Option 1
: The Merrimack River Watershed Council posts alerts on our Facebook page whenever we receive an email from a sewage treatment plant reporting a sewage overflow. Four plants -- in Haverhill, Lowell, Lawrence, and Manchester -- provide these reports. The Nashua wastewater plant is required to post alerts on the city's website page, so the MRWC keeps an eye on this page and we post on Facebook as needed. 
Option 2: You can contact the treatment plants in Lawrence, Haverhill, Manchester and Lowell and ask to be put on their email alert list. It's the same email that the MRWC and many "downstream" public officials receive.
Each plant must be contacted individually. Below we've posted the contact information for each plant:
Haverhill: email Carrie Prescott <[email protected]>
Lawrence: Sign up through this link: 
Lowell: email Gregory Coyle, [email protected]
Manchester: Sign up through the city's notification system. Here's the website address --
Nashua: You can monitor the city's website immediately after a storm to see if a CSO has occurred. Here's the website address --
We should note that the emails from the plants are not always easy to understand, and each plant reports its CSO event differently. For instance, Lowell sends out 3 alerts -- one when the CSO begins, one when it ends, and a follow-up a few days later reporting how much sewage was released. Haverhill sends out one email that states which of its 15 outfall pipes activated and how much rain fell, but there is no information on how much sewage was released. Greater Lawrence sends one report that states a CSO event has begun, and a day or so later a report on how much sewage was released, and which of its 5 outfall pipes released. 

What can be done?

•    Several states and other jurisdictions have adopted programs that provide the public with real-time notice of CSO events. Massachusetts is now considering legislation to do this; New Hampshire is not.  At minimum, sewage plant operators should tell residents that a sewage overflow is happening when it is happening.

•    CSO operators also should upgrade their systems as soon as possible—and in any event no later than the schedule called for in the individual federal remediation orders already issued for each plant.

•    The federal government and the states should provide realistic funding for CSO elimination—in much the same way that the Federal Water Pollution Control Act Amendments of 1972 (the Clean Water Act) provided major financial assistance to local communities constructing modern sewage treatment plants. 

What is MRWC doing to address the issue?  

•  Building momentum by increasing public awareness of CSO events.  We have held public forums in Newburyport (with approximately 150 attendees), Andover and Lowell, and plan to hold additional forums in 2019.  Please check this website and our Facebook, Instagram and Twitter feeds for more information. 

MRWC’s campaign to bring the sewage dumping issue to the public’s attention has generated tremendous response.  Thousands of people have commented on or shared our posts on social media, and many have called their local legislators, written letters to their local newspapers, and volunteered to help spread the world.  The public’s response has fueled a variety of actions by elected officials throughout the Merrimack Valley.  Moreover, the public attention now focusing on the Merrimack’s sewage problem has caused federal lawmakers—notably Sen. Edward Markey, D-Mass.—to work toward identifying funding sources.  This is a significant development.  We’ll be closely tracking progress.

Also, local media has done an outstanding job covering the issue and calling for action.  It’s clear that a strong coalition of citizens and public officials is growing.
•  Supporting  CSO notification bills. In the summer of 2018, we came close to getting a first major piece of legislation passed in the Massachusetts Legislature—the so-called notification bill.  This bill would require sewage plants to tell the public whenever sewage is released into the river.  Currently there is no unified notification system, despite the fact that a 2015 medical study (and much anecdotal evidence) demonstrates that sewage releases can have serious public health impacts.  Though the legislation failed to pass, a similar bill has been refiled for the 2019-2020 legislative session, and it appears to have garnered far broader legislative support.  Several other bills addressing CSOs and back-up power at sewage plants were also introduced this legislative session.  You can read more about them here. We will be monitoring the situation closely. 

• Working with sewage plants:  We are working with the wastewater plants that dump sewage into the Merrimack in an effort to make progress on this issue.  And we're encouraged that three Massachusetts plants that dump sewage into the Merrimack—in Haverhill, Greater Lawrence, and Lowell—are voluntarily emailing alerts to MRWC and to certain “downstream” public officials whenever their systems experience a CSO.  Across the state, many of these plants have joined a coalition pushing for federal money to pay for the improvements needed to halt sewage dumping.  This is a significant change in direction from only a year ago.