In Philadelphia, and many urban areas across the country with sewer systems dating back to the 1800s, more frequent flooding is posing a threat not only to people and property, but also to surrounding ecosystems . In these areas stormwater and sewage drain into the same pipes and when flooding occurs they are designed to spill into nearby waterways, rather than inundating streets. Researchers at Drexel University, in collaboration with the Philadelphia Water Department, are looking at the long-term water quality impact of these events — called combined sewer overflows — to understand what steps could be taken to mitigate their negative effects.
Their latest finding, published in the Journal of Environmental Management , compared how different watershed regions in Philadelphia respond to the environmental stress of a combined sewer overflow to determine the environmental and infrastructural elements that factor most prominently into the response.
"In Philadelphia County more than 8 billion gallons of combined sewer and stormwater are discharged into rivers and streams each year during flooding events," said Amanda Carneiro Marques, PhD , an assistant professor in the Nick Howley College of Engineering and Computing's School of Engineering . "For the city to address the negative impact this has on water quality, it is important to have a full picture of what happens in an ecosystem during these events."
Marques and her team drew on more than a decade of U.S. Geological Survey water quality monitoring, combined with precipitation data from the National Oceanic and Atmospheric Administration and the Philadelphia Water Department's combined sewer overflow records to create a detailed model of the overall water quality impact of an overflow event.
Their research centered on fluctuations in dissolved oxygen, a measure of the amount of oxygen present in water, which is an important water quality metric because it indicates a body of water's potential to sustain aquatic life, like fish and macroinvertebrates.
Dissolved oxygen levels tend to fluctuate with sustained changes in temperature, flow volume and abundance of oxygen-consuming microorganisms. Because of this, it is often a lagged indicator of the residual effects of a deleterious event, such as flooding or infiltration of a pollutant.
"Dissolved oxygen is one of the most important indicators for aquatic ecosystem health as it can reflect cumulative environmental impacts over time – like recording a video, rather than just getting a snapshot of a moment in time," Marques said. "For example, an influx of organic matter or nutrients may cause rapid algal growth. When that algae dies, it may trigger a boom in aerobic microorganisms that decompose the organic matter. These microorganisms consume vast amounts of dissolved oxygen during respiration, which can lead to very low dissolved oxygen levels in the water."
The team looked at data from four Philadelphia watersheds: Darby-Cobbs Creek (in parts of Chester, Delaware, Mongomery and Philadelphia counties), Pennypack Creek (in Northeast Philadelphia), Tacony-Frankford Creek (in Montgomery and Philadelphia counties) and the portion of the Delaware River Basin that borders Philadelphia, from 2013-2024. Each watershed contains at least one designated combined sewer overflow area and Geological Survey monitoring stations both up- and downstream of it.
Looking at more than a decade of data from the same monitoring sites allowed the researchers to observe each watershed's regular dissolved oxygen fluctuations tied to seasonal temperature and flow rate changes.
For example, the Tacony-Frankford Creek watershed reported dissolved oxygen levels below the Environmental Protection Agency's minimum threshold for fish growth and reproduction in 40% of observations in July, with low levels persisting through September. By contrast, Pennypack Creek and Darby-Cobbs Creek watersheds rarely reported levels below the threshold throughout the entire year.
Overlaying this information with the Water Department's combined sewer overflow records from the same time period illuminated how each watershed responded to a rapid influx of stormwater and organic matter from sewage.
"Each watershed had a slightly different dissolved oxygen baseline profile, depending on its physical and hydrological characteristics, such as size, land coverage and flow magnitude, for example," Marques said. "These preexisting conditions also seemed to play a role in their ability to stabilize dissolved oxygen levels after a sewage overflow event."
The Delaware River watershed showed very little change in dissolved oxygen in the aftermath of overflows, likely owing to its flow rate and relatively large volume which enabled it to dilute the organic matter from the sewage, according to the researchers. Smaller watersheds, like Tacony-Cobbs Creek, saw more than a 1 mg/L decrease in dissolved oxygen that persisted for days after the discharge.
Surrounding infrastructure and land use also plays a role, according to the researchers. The flow rate and volume of the overflow plume entering the creek or river affects how well it can dilute the organic matter it carries.
"These factors suggest that combined sewer overflow impacts are far from uniform across watersheds, but they are highly dependent on variations in flow rate and volume and likely the surrounding infrastructure and geography," Marques said. "It also means that for municipalities looking to reduce the negative effects of combined sewer overflows, it is important to understand the underlying vulnerability of a watershed as they weigh mitigation strategies."
One of the strategies that urban areas with combined sewers are currently employing to reduce the frequency of overflow events is attempting to reduce or delay the stormwater entering the sewer system. This includes installing water retention areas, permeable surfaces and residential cistern and rain barrel systems – which have proven to be effective in certain areas .
The group's future research will continue to explore the interplay between the built environment and combined sewer overflow resiliency. Marques also noted that gaining a better understanding of the composition of the overflows could provide important insight into how municipalities could better manage their impact.
"This is an important first step toward providing urban areas with aging sewer systems the information they need to make data-informed decisions about infrastructure upgrades and mitigation strategies," Marques said. "More monitoring and testing of overflow water could help to identify areas that are facing the greatest ecological risk during combined sewer overflows."
