Research

Sewage Overflows Linked to Increase in Gastrointestinal Illnesses

A new study has found that the risk of people developing acute gastrointestinal illness is significantly higher in Massachusetts communities that border the Merrimack River, following extreme combined sewer overflows. This risk increased regardless of drinking water source, suggesting that exposure to sewage-associated pathogens may occur through other pathways such as recreation in the river.

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 School of Public Health 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 Beth Haley (SPH’23), postdoctoral researcher and a doctoral student at SPH 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, Haley and colleagues from BU, 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,” 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 Wendy Heiger-Bernays, clinical professor of environmental health. 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.

The EPA expects CSO data to be publicly available in a national electronic database by the end of 2025, which would allow for additional research to occur on a national scale.

In the meantime, residents in the Merrimack Valley communities can track real-time updates about sewage overflows into Massachusetts rivers and bays on a map developed by WBUR. The researchers say people can also reduce their potential exposures to CSO-related pathogens by limiting their use of the Merrimack River for recreational purposes in the days following rainstorms.

At SPH, the study was coauthored by Yuantong Sun, research data analyst in the Department of Environmental Health; Jessica Leibler, associate professor of environmental health; and Gregory Wellenius, professor of environmental health and director of the Center for Climate and Health.