Almost 900 million gallons of untreated sewage flowed into the Merrimack River last year.
That was 30% more than the usual overflows over the past 10 years, according to the Merrimack River Watershed Council. But it 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.
The sewage pouring into the river came from five cities along the Merrimack that use combined sewer systems, an older design that can overflow during rainstorms.
In 2024, about half of the sewage came from the city of Manchester, with other overflows from Nashua, and Lowell, Lawrence and Haverhill in Massachusetts.
Curt Rogers, the executive director of the Merrimack River Watershed Council, says the overflows have big impacts for those downstream.
“That sewage in the river creates toxic levels of bacteria that are harmful to humans and wildlife,” he said.
About 500,000 people in Massachusetts get drinking water from the Merrimack River, according to the Environmental Protection Agency, along with some New Hampshire communities. The number of people with drinking water coming from the river is set to increase as Manchester and Haverhill, Massachusetts add wells.
Researchers from the Boston University School of Public Health found that emergency room visits for gastrointestinal illnesses for people living along the Merrimack River rose 22% after large sewage discharges, and 62% after very large releases.
Most officials recommend not swimming or recreating in the river for 48 hours after a sewage discharge, but Rogers says for downstream communities the waiting period may need to be longer.
The issue of overflowing sewers may be getting worse with climate change, Rogers said. The warmer atmosphere is bringing heavier rainstorms to New England.
“If you get the same amount of rainfall in a drizzle over 10 days, that may not cause a CSO [combined sewer overflow]. But if all of that rain falls within three hours, you could be triggering a CSO,” he said.
Manchester city officials said rainfall has been significantly above average for the last two years, increasing discharges.
The city has been working on a project to separate its sewer system and stop the overflows based on a long-term plan created in 1999 and enforced through a consent decree with the Environmental Protection Agency.
Right now, the city is on year five of a 20-year project to reduce overflows on the east side of Manchester, after reducing discharges on the west side by 99%, according to chief engineer Robert Robinson.
