Sewage Overflows Linked to Increase in Gastrointestinal Illnesses

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Jillian McKoy,

Michael Saunders,


During extreme rain or rapid snowmelt, the high volume of water that enters sewer systems can cause untreated sewage to flow into waterways in US cities that have combined wastewater and stormwater sewer systems, including the Merrimack River in Massachusetts.

A new study led by Boston University School of Public Health (BUSPH) researchers has found that residents living in the downstream communities that border the Merrimack River are at a substantially increased risk of developing acute gastrointestinal illnesses (AGI) in the four days following large-volume combined sewer overflows (CSOs).

Published in the journal Environmental Health Perspectives, the study found that risk of AGI was elevated among all of the Massachusetts cities observed in the analysis, regardless of whether communities sourced their drinking water from the Merrimack River, suggesting that residents may be exposed to sewage-associated waterborne contaminants through pathways beyond drinking water.

Health experts worry that more extreme rainfall events due to climate change will spur additional large-scale CSOs, putting residents’ health at risk even further.

“The largest sewage releases occur when precipitation is most intense, as this produces a large volume of rainfall over a short amount of time,” says study lead and corresponding author Dr. Beth Haley, postdoctoral researcher and a doctoral student at BUSPH at the time of the study. “Our findings are concerning because climate change predictions suggest that heavy rainfall events will become more frequent and intense in the Northeast, which means that residents of communities exposed to sewage-contaminated waterways may experience increased risk of severe gastrointestinal illnesses.”

The study lends critical insight into the health implications of CSO events, as there is very little research on the health outcomes of populations who are exposed to the waterways affected by CSOs. The analysis is also the first study to investigate the role of drinking water sources in these possible connections between CSOs and AGI.

For the study, Dr. Haley and colleagues from Boston University, the University of Chicago, and New Leaf Energy utilized federal and state data to examine the volume and intensity of CSOs, along with hospital records of emergency department visits for AGI, based on diagnoses of infectious intestinal diseases, noninfectious gastroenteritis, vomiting, and diarrhea. The analysis included communities in all of the 17 northern Massachusetts cities that border the Merrimack River. 

After adjusting for precipitation, the results showed that residents living in CSO-impacted communities were 62 percent more likely to develop AGI within four days after the largest sewage overflow events in the study period occurred compared to days with no CSO events. But there was no meaningful difference in AGI risk based on whether residents relied on drinking water from the Merrimack River, or other sources.

Exposure to sewage-associated pathogens in drinking water is particularly concerning because of the large number of people who do rely on public drinking water, the researchers say—so the lack of evidence connecting CSO-impacted drinking water sources to AGI risk in the study was a relief. But they are careful to note that the results do not necessarily indicate that drinking water sourced from CSO-impacted waterways is harmless.

“We cannot conclude that drinking water is not an exposure pathway following CSO events, but this study does suggest that other pathways—for example, exposure during water recreation—likely contribute more to the observed association,” Dr. Haley says.

More research is needed to better understand the role of drinking water on CSO-associated AGI risk, especially for populations who are more susceptible to microbial pathogens, such as immunocompromised people, older people, and young children, the team says.

Although cities such as San Francisco, Seattle, and Portland, Ore. have combined sewer systems, this type of infrastructure is more prevalent in the Northeast and Midwest, where systems were built during the mid-1800s to early 1900s in the country’s oldest cities.

“At that time, there was little concern that combining sanitary and storm sewers would be so problematic,” says study senior author Dr. Wendy Heiger-Bernays, clinical professor of environmental health at BUSPH. One of the main reasons CSO events still occur today, she says, is that combined sewer systems are expensive to separate, as are measures to minimize CSO occurrences. “Many cities that have combined sewer systems do not have the resources to make these large infrastructure investments without state or federal assistance.”

There is some movement to address these issues at the federal level now, Heiger-Bernays says. The Environmental Protection Agency (EPA) is working to implement long-term control plans and water quality improvement—and to make CSO data more readily available to the public—in accordance with the Clean Water Act.

“Massachusetts has done this at the state level following the passage of a 2021 law requiring notification of sewage overflow events, but a national system for notifications and data access does not currently exist,” she says.


About Boston University School of Public Health

Founded in 1976, Boston University School of Public Health is one of the top ten ranked schools of public health in the world. It offers master's- and doctoral-level education in public health. The faculty in six departments conduct policy-changing public health research around the world, with the mission of improving the health of populations—especially the disadvantaged, underserved, and vulnerable—locally and globally.


Environmental Health Perspectives

Method of Research

Observational study

Subject of Research


Article Title

Association between combined sewer overflow events and gastrointestinal illness in Massachusetts municipalities with and without river-sourced drinking water, 2014-2019

Article Publication Date


COI Statement

Dr. Wellenius serves as a consultant to the Health Effects Institute (Boston, MA) and Google, LLC (Mountain View, CA).

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.

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