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Sewage surged into the Merrimack River in 2023 amid record rainfalls, endangering drinking water downstream

“You can see bacteria levels that exceed 10 times what’s considered safe for swimming,” said John Macone, a policy and education specialist at the Merrimack River Watershed Alliance

Three times the amount of untreated sewage that enters the Merrimack River in a typical year flowed into the water in 2023 after record rainfall.Suzanne Kreiter/Globe Staff

CONCORD, N.H. — Record rainfalls this past summer overwhelmed aging infrastructure in cities along the Merrimack River, allowing a surge of untreated water, including raw sewage, to flow into the water.

Because of the heavy rains, a record-breaking amount of untreated liquid, an estimated 1.5 billion gallons, went into the river, according to data compiled by the Merrimack River Watershed Alliance.

That’s three times the amount of untreated sewage that enters the river in a typical year, according to John Macone, a policy and education specialist at the Alliance. It can make people who swim, boat, and participate in recreation on the river sick and pose a problem to water companies that have to clean the water before it’s used to drink.


According to the Environmental Protection Agency, the Merrimack River is a source of drinking water for about 500,000 people in five Massachusetts communities: Lowell, Methuen, Andover, Tewksbury, and Lawrence. The agency said that number could grow to more than 700,000 people as more communities, including Manchester, N.H., have started using the river for drinking water. In the past five years, the river has become the top source of water for Nashua, N.H.

The amount of untreated stormwater and wastewater this year was a dramatic increase from the previous record, 823 million gallons, which was set in 2021, according to Macone. Experts said climate change could exacerbate the problem, as intense rains become more frequent.

“There are pretty significant impacts,” Macone said. “You can see bacteria levels that exceed 10 times what’s considered safe for swimming.”

Since 2013, Manchester has consistently been among the top two biggest polluters, along with Lowell, Mass. The Greater Lawrence Sanitary District, Haverhill, Mass., and Nashua also contribute to the problem. Manchester takes in its drinking water upstream from where untreated sewage is released, and sends an alert to downstream communities after it happens.


Cities like Manchester and Nashua have old sewage systems that combine stormwater and sewage. That material is usually treated before it goes into the river, but when there’s heavy rainfall, the cities don’t have enough storage space to treat the water.

If they don’t release the untreated sewage into the river, it will back up into people’s homes and businesses. To avoid that outcome, they release the water into the Merrimack.

“Basically, these older cities, the plumbing underground is bad,” said Macone. “It’s ancient. It’s plumbed completely differently than you would do if it was a modern city.”

Redoing the plumbing, which was created in the late 1800s and into the mid 1900s, is costly and time consuming. The river was historically used as a sewer, and it was only in 1972, with the passage of the Clean Water Act, that cleanup efforts were mandated by law. Water quality has improved as a result, but challenges remain.

The Environmental Protection Agency has required Manchester to upgrade its system, which the city expects to cost $300 million, according to Fred McNeill, the chief engineer for Manchester’s Environmental Protection Division. According to a 2019 city plan, it will take about 40 years to separate all of the city’s sewage from stormwater.

A new drain tunnel is supposed to be completed by 2028, which is projected to reduce the discharge by 74 percent, McNeil said, by removing drainage from Cemetery Brook from the combined system.


“That in addition allows us the opportunity to size the drainage system for new, climate-change-induced storms,” McNeill said.

In the meantime, researchers at Boston University School of Public Health are studying the impact on public health downstream.

Beth Haley and Wendy Heiger-Bernays found that within four days of sewage overflowing, more people who lived downstream along the river in Massachusetts were getting sick enough to go to the emergency room with gastrointestinal issues, like vomiting and diarrhea.

They observed a 20 percent to 60 percent increase in the risk of gastrointestinal illness within four days after an overflow event. Their findings are published in a paper that’s in the process of being peer-reviewed.

Haley said exposure happens through direct contact with the water, like swimming, kayaking, or fishing. She said people could also be exposed through drinking water, although further study is required to determine if that’s happening.

A law passed in 2021 required sewage plants in Massachusetts to alert the public whenever sewage is released into the river. Heiger-Bernays said that’s led to more public interest and awareness of the issue. No such law exists for New Hampshire, although residents can sign up for alerts about overflow events from the City of Manchester, and Nashua lists overflows on its website.

When unsafe levels of bacteria are detected at swimming beaches, Massachusetts posts advisories warning the beach is closed to swimmers and pets. In June and July, advisories were in effect for 39 days, the Daily News of Newburyport reported.


“The hypothesis would be that in times of really high concentrations of pathogens in the river, that there could be issues with treatment being as effective as it normally is,” Haley said.

Heiger-Bernays, a faculty member trained in molecular toxicology, said this research doesn’t provide answers about drinking water. “It does not give a pass. It does not say drinking water treatment is adequate. We can’t determine that from this analysis,” she said.

But, she added, larger drinking water systems are subject to rigorous federal and state requirements.

She warned that combined sewage overflows are only going to worsen, with sanitary sewer volume increasing as the region gains population — and climate change means stormwater is also going up.

“You’re going to see a continuous increase in the volume that gets discharged to these waterways,” she said.

Pennichuck Water is the company that provides drinking water to residents of Nashua. In the past five years, the Merrimack River became its top source of water, according to Larry Goodhue, the company’s chief executive officer.

The company said it monitors bacteria levels on a weekly basis, and continuously cleans the water by adding chemicals to remove organic matter before filtering the water and disinfecting it with a bleach solution. When untreated sewage enters the water, it can create challenges for treating the water downstream so it meets standards for drinking.

“It taxes our ability to actually feed enough chemical to treat the water,” said Chris Countie, Pennichuck’s director of water supply. He said the company is working to add more chemical storage to address future events.


When there was heavy rain in July and August, Countie said the system stopped pulling water from the Merrimack entirely, and instead drew water from the Pennichuck Brook system because it was less impacted.

“That’s when we saw the most difficult water to treat from the river,” he said.

Amanda Gokee can be reached at Follow her @amanda_gokee.