Why CSOs (Combined Sewer Overflows) in the Merrimack River Matter

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You likely have heard about combined sewer overflows (CSOs) in the Merrimack River, but it’s difficult to find reliable, real-time information about when these happen, what exactly is the impact to us here in Newburyport, and why we should care about them. I swim in the river and off Plum Island regularly (and occasionally dunk by Salisbury campground in the depths of winter!). I love the river; I grew up here, and I live on it now. CSOs are not a new problem, but they are becoming more frequent, with greater volumes, as we get more and heavier rainfall events, and as increasing population puts more strain on wastewater treatment systems.

CSO’s originate upriver in Manchester and Nashua New Hampshire, and in Lowell, Lawrence, and Haverhill Massachusetts. These older cities do not have separate pipes for sewage and stormwater flows; their pipes are “combined” to carry both stormwater and sewage to their wastewater treatment plants. When there is not too much stormwater, and the system can handle it, the mix of sewage and stormwater is carried by the pipes to the wastewater treatment plants for processing. But with heavy rains, the amount of wastewater can overload the system. When that happens, there is a combined sewer overflow because the “outfall” pipes in the system divert the untreated sewage and stormwater into the Merrimack River. There are 13 such outfalls in Haverhill, 9 in Lowell, and 5 in Lawrence. The untreated sewage and stormwater contain, among other things, bacteria such as E. coli and Enterococcus, the “fecal indicator bacteria” which, like the name implies, indicate fecal matter in the river. Public health effects of CSOs are well-documented, including studies of adverse impacts on humans and animals of CSO events in the Merrimack River specifically.

CSOs also contain debris and other potentially hazardous substances that cause harm to people, pets and wildlife. The result, as I know from personal experience, is brownish water that does not look or smell clean, debris that clearly does not belong in the ocean, and, sometimes, floating fecal matter.

How big is the CSO problem, based on amounts reported by the upstream wastewater treatment plants? Untreated sewage releases totaled 896 million gallons in 2024, a 30% increase over annual amounts in the prior decade. The 2024 total was a reduction from 2023 — the year of the state’s wettest summer on record — when about 2 billion gallons of sewage ran into the river.

Health officials advise the public to wait 48 hours after a sewage release before engaging in “recreational contact” with the water (swimming, wading, boating or fishing). Water quality testing by the Merrimack River Watershed Council (MRWC) in 2024 showed that 48 hours or less was enough time for the plume of bacteria to flow downstream, but the plume often persisted longer than the recommended window and flowed further downstream than expected.

How much fecal bacteria (as evidenced by testing for Enterococcus) makes its way downstream to Newburyport and Salisbury after CSO events? (Enterococcus is used as indicator of fecal matter in salty or brackish water, because E. coli does not survive well there.) The recreational contact limit for salty waters such as Newburyport, Salisbury Beach, and areas with tidal influence like Amesbury, is 104 colony-forming units (CFU)/100mL in a sample of Enterococcus.

Recent testing for Enterococcus was done at Salisbury Beach State Reservation and at Newburyport’s Plum Island Beah, both sites just inside the River’s mouth on June 29, June 30 and July 1, following CSO discharges from the upstream wastewater treatment plants on June 28. Bacterial levels were elevated above EPA public health limits, indicating “extreme danger” to public health.

At the Newburyport Plum Island Beach site just inside the mouth of the River, Enterococcus levels on June 29 were 372.5 CFU/mL, on June 30 980.4 CFU/mL, and on July 1 648.8 CFU/mL, all well above the EPA public health limit of 104 CFU/mL.

The MRWC tests water quality in the Merrimack River for three days after a significant CSO. However, this non-profit organization does not have the funding or capacity to test for more than one CSO event per week; thus, not all CSOs are tested. Currently, the MRWC’s post-CSO testing is funded through a Mass DEP state budget line item; funding is contingent upon the line item surviving the state budget process.

The long-term solution to CSOs in the River is expensive upgrades to the outdated sewer systems in Manchester, Nashua, Lowell, Lawrence and Haverhill to separate stormwater and sewage. If you are concerned about this problem, advocating for change at the local, state, and federal level may help move funding forward; supporting the MRWC’s education, testing and advocacy efforts for a cleaner river is important. For more information about these efforts, see www.merrimack.org. Meanwhile, we can all help by being aware of the problem and educating others about it.

Resources:

Post-CSO Testing Program info
CSO Test Results
Merrimack River Test Sites

Jill Murphy
Newburyport resident

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