Heavy storms can overwhelm older sewer systems, causing a spike in sewage-borne bacterial and viral infections
In older U.S. cities, heavy rainfall doesn’t just flood basements. It can also send a wave of dangerous pathogens into municipal water sources. Lakes, rivers, streams and even reservoirs may see an uptick in sewage-borne bacteria and viruses after heavy rains, infecting boaters and swimmers and overwhelming the ability of treatment plants to purify drinking water, according to Jyotsna Jagai, MPH05, N09.
Jagai, who earned her doctorate at the Friedman School and is a research assistant professor of environmental and occupational health sciences at the University of Illinois at Chicago, said much of the problem is caused by aging infrastructure. Instead of having two separate pipelines—one for human waste and one for storm runoff—older sewer systems route both waste streams into a single underground pipe, aptly called a “combined sewer.”
When unusually heavy rains fill these sewers past their capacity, water and human waste can sometimes back up out of storm drains, spilling into rivers and streams. In some cases, Jagai said, sewer operators may be forced to cause an intentional overflow in order to release excess pressure in the sewage lines after a storm.
Jagai is studying these events, which are known as “combined sewer overflows” (CSOs). They’re a distinct problem in more than 70 major American cities with aging sewage systems and have a significant effect on public health in those regions, she said.
In 2014, she examined daily historical weather data across Massachusetts during a five-year period (2003 to 2007), noting regions where sudden, heavy rainfall occurred. She then compared those dates to hospital records from the same area, looking for any change in the number of patients diagnosed with illnesses caused by bacteria and viruses carried in human waste. Sure enough, she said, after heavy rains, she saw a distinct spike in patients diagnosed with gastrointestinal disorders—nearly 13 percent more than usual in areas where the CSOs affect drinking water sources.
“This study looks at extremely heavy rainfall events as a proxy for combined sewer overflow events—we don’t make the connection 100 percent, but it definitely shows there’s a significant impact,” she said. “These sorts of gastrointestinal or diarrheal diseases have a huge impact on health, especially for patients who are malnourished, or whose immune systems are compromised in some way.”
Replacing the Pipes
Environmental Protection Agency guidelines call for the removal of combined sewers over the next 20 years, Jagai said, but many major cities have yet to make the change because such projects are extremely costly. Until that infrastructure is replaced, Jagai said that new data linking CSOs to illness could help warn residents to boil drinking water or to avoid swimming and boating in certain areas.
“If climate change predictions are true, we’ll be seeing more heavy rainfall and more CSO events in the next 10 to 20 years,” she said. “This study suggests the need for increased messaging to communities to warn people not to play in the water, or to boil and filter drinking water after major rainfall as a precaution.”
Preventive measures like these may work in the short term, but in some cases, the effects of a CSO event can linger for weeks or months after a heavy rain. Certain pathogens can thrive inside aging pipes and water transport systems within some buildings. Microbes like Legionella pneumophila, Mycobacterium avium and Pseudomonas aeruginosa, all of which cause serious respiratory and systemic infections, can form sticky biofilms on interior surfaces of pipes, infecting water that passes over them and prolonging residents’ exposure to those pathogens.
A study published in 2016 by Elena Naumova, director of the Initiative for the Forecasting and Modeling of Infectious Disease at Tufts, estimated that those three pathogens alone may result in the hospitalization of as many as 80,000 elderly each year, at a cost of $2 billion.
While this is a serious localized concern, Naumova and Jagai are quick to note that water quality in the United States is still exceedingly good overall. Pathogens released during CSO events are often removed from the community water supply during normal treatment processes, and microbes growing in a building’s water mains usually infect only people with compromised immune systems.
“While public drinking water is safe, it is clearly more safe if you are healthy than if you have medical conditions that enhance your vulnerability to infections,” said Jeffrey Griffiths, a professor of public health and community medicine at Tufts School of Medicine and a coauthor on Naumova’s study. “The risk of becoming ill from drinking water is much less than the risk of becoming ill from food, but it is not zero.”
Jagai wants to extend the scope of her research into a broader public health arena. She’s developing a cumulative index for exposure to environmental pollutants—not just pathogens in water, but industrial chemicals, pesticides and more.
“I want to build a much broader notion of exposure,” she said. “Everyone should be able to live in a place with access to clean water and clean air. There shouldn’t be an uneven distribution of pollutants. The big theme to me is one of environmental justice.”
David Levin is a Boston-based freelance science writer.