Philadelphia is home to many beautiful waterways, from the Delaware and Schuylkill rivers to the Wissahickon and Cobbs creeks. A visit to their banks affords city dwellers a chance to escape the concrete jungle to fish, hike, picnic, or let their dogs romp and roam.

Yet after a heavy rainfall, these may not be the safest places for urbanites to get their nature fix. The reason? Sewage.

“Here in Philadelphia, the majority of residents are served by combined sewers,” says David Hewitt, a lecturer in PennDesign and the Department of Biology in the School of Arts & Sciences. “That means stormwater and sanitary sewers join in one pipe and go to the wastewater treatment plant. But when it rains a lot and the sewers overflow—which they’re designed to do—sewage goes straight into the waterways.”

In a new project, Hewitt is collecting data from all around the city to find out how long bacteria from the combined sewers persist on land after overflow events. In the longer term, he hopes to help land managers keep park visitors safe from potentially disease-causing pathogens carried in sewage.

From today’s standpoint, combining stormwater runoff and sanitary sewage may seem rather crude. But when combined sewers were designed and installed in Philadelphia in the late 19th and early 20th centuries, the infrastructure was considered “a good solution” for managing wastewater, Hewitt says. Channeling the two into one pipe can keep sewage pipes washed out and freely flowing, and can prevent stormwaters from washing pollutants into waterways during low-intensity rains.

Combined sewer systems tend to be found in older parts of Philadelphia—including the lower Northeast, Germantown, West and North Philly—that also often tend to be poorer. For this reason, Hewitt sees the impacts of combined sewer overflows as partly an environmental justice issue.

“If you have a flood and it exceeds the stream banks, there are pathogens in that water,” he says. “And even if you wouldn’t go swimming in the water, there are probably pathogens remaining at certain levels on the ground where the water has receded. Those might be places where kids want to play, people want to picnic, and dogs want to run around in the mud and on the grass.”

As a first step, Hewitt is surveying floodplains of Philadelphia rivers and creeks following combined sewer overflow events to see if he can find evidence of dangerous bacterial contamination. That’s precisely what he did one chilly November afternoon, taking the trolley from Penn to Bartram’s Garden in Southwest Philadelphia to collect soil samples. Tromping along the Schuylkill with a view of Center City in the distance, he scooped up muddy clumps from the shores and secured them in sealed tubes.

In this early stage of his research, Hewitt, in collaboration with Jeffrey Townsend of Yale University, will use simple genetic tests to determine whether pathogens are present in the soil, how long they persist after combined sewer overflows, and whether they are resistant to antibiotics. The presence of antibiotic resistance genes poses a threat to public health, as they provide openings in the wall of modern medicine that protects us from infectious diseases. After initial results are in, he hopes to expand his studies, in partnership with local and federal agencies, to establish whether pathogens are present in great enough quantities to make someone ill.

“Just finding bacteria there doesn’t mean someone will get sick from it,” Hewitt says.

His results may lay the groundwork for studies that look for a link between combined sewer overflows and increased incidence of gastrointestinal diseases.

“One plan is to track the sales of anti-diarrheals at area drug stores following overflow events,” Hewitt says. “If there are spikes in those sales after overflow events and they are concentrated in areas with combined sewers, that is a real smoking gun.”

Fortunately, reducing the risk of illnesses can be relatively straightforward.

“Interventions can be simple—for example, you post signs for additional caution after flood events,” Hewitt says. “You can also talk to park designers; maybe employing more open planting designs in flood-prone areas will help the pathogens degrade faster by allowing exposure to sunlight and hastening desiccation.”

The bottom line, Hewitt says, is finding a way to balance public health and recreation.

“We want these natural areas to be enjoyed and we don’t want people scared away or avoiding them,” he says. “But we have to overcome the idea that ‘nature’ is always pure and clean. Sometimes what looks beautiful can make people sick. While we don’t want people frightened of nature, we do most certainly want them to respect it.”