Sewage discharges increase risk of hospital visits for residents near Merrimack River, study finds

People living in communities along the Merrimack River are at increased risk of developing acute gastrointestinal illnesses in the days following large combined sewer overflows, according to a study by Boston University researchers School of Public Health.

Overflows, which often occur during heavy rains, discharge untreated sewage into waterways. The findings add to evidence suggesting they pose a public health risk for millions of residents in Massachusetts and other states, primarily in the Northeast and Midwest.

Researchers examined data from combined sewer overflows — or CSOs — along the Merrimack River, as well as medical records from nearby hospitals. They found a 22% increased risk of going to the emergency room for gastrointestinal illness after large wastewater discharges, and a 62% increased risk after very large discharges.

“Even though one might assume that sewage from a river or lake could make people sick, there are actually relatively few studies that have looked at this connection,” said Beth Haley, postdoctoral researcher and lead author of the study.

The researchers also found that the highest risk of gastrointestinal illnesses, such as severe vomiting and diarrhea, occurred four days after discharge. Haley said this could indicate that people are getting sick from viruses, such as norovirus, that have a short incubation period.

The Merrimack River, like other rivers in Massachusetts, is also a source of drinking water. Haley said she expected to see a higher risk of gastrointestinal illnesses in communities that get their drinking water from the Merrimack, but that's not the case. She said this suggests that exposure to sewage-related pathogens may occur through other routes, such as recreational activities near the river.

About 18 Massachusetts communities, including Boston, Cambridge, Lynn and Haverhill, have combined sewer and stormwater systems, many of which were built in the mid-1800s. The systems were designed to overflow into rivers and neighboring bays in the event of heavy rain, in order to avoid backflow into houses.

Storms causing overflows are becoming more common with climate change.

“If we look at data from the past year, we see more frequent rainstorms and more frequent releases,” said Wendy Heiger-Bernays, a professor at the Boston University School of Public Health and co-author of the article published this month. in the journal Environmental Health Perspectives. With greater volumes of rain, she said, “greater volumes are dumped into rivers.”

A 2021 state law requires cities and towns to notify residents within two hours of an overflow. WBUR built a map to track overflows, with updates based on real-time alerts for more than 180 CSOs across the state.

Public health officials recommend waiting at least 48 hours after a CSO discharge to swim, boat, or allow pets into affected waters.