Combined sewer systems increase risk of illness after heavy rains, study finds

April 20, 2015

A new study finds that combined sewer systems can contaminate drinking water supplies with bacteria and viruses, causing severe community health impacts.

Combined sewer systems collect both sewage and stormwater runoff before reaching treatment facilities. When heavy rains overload the capacity of such systems, untreated contents may be discharged into local bodies of water in events known as combined sewer overflows (CSOs).

Kyle Messier

Kyle Messier

Kyle Messier, a doctoral student in the Department of Environmental Sciences and Engineering at the UNC-Chapel Hill Gillings School of Global Public Health, is a co-author on the study. He conducted research in the United States Environmental Protection Agency facility on UNC’s campus, working with a team that also includes Timothy Wade, PhD, adjunct assistant professor of epidemiology at the Gillings School.

The resulting paper, titled “Extreme Precipitation and Emergency Room Visits for Gastrointestinal Illness in Areas with and without Combined Sewer Systems: An Analysis of Massachusetts Data, 2003–2007,” was published online April 9 by Environmental Health Perspectives.

Messier, who recently won a second UNC-Chapel Hill Graduate Student Impact Award for research on nitrate contamination of groundwater, helped examine the daily rate of emergency room visits for gastrointestinal illness, over a five-year period, during the eight days following each incidence of heavy rain in forty-four Mass. towns.

Overall, emergency room visits related to gastrointestinal illness increased by 13 percent following extreme precipitation in areas with combined sewer systems that discharged untreated flow into drinking water sources.

In control areas, where overflow was discharged into recreational waters, or where combined sewer systems were not in use, there was no significant increase in emergency room visits.

These findings indicate a link between releasing untreated overflow into drinking water supplies and community members suffering gastrointestinal illness. Since not all individuals with gastrointestinal distress visit emergency rooms, the true number of people affected by this practice is likely much higher.

“Mitigating exposure from CSOs is not easy, due to the cost of significant infrastructure changes,” Messier explained. “As evidenced by these findings, however, there are already very real public health impacts from CSOs. These will likely only worsen with future, more extreme precipitation events due to climate change.”

Combined sewer systems are especially common in the United States’ northeast, the Great Lakes region and the Pacific Northwest.

Gillings School of Global Public Health contact: David Pesci, director of communications, (919) 962-2600 or

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