This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In Philadelphia and many urban areas across the U.S. 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, which are designed to spill into nearby waterways when flooding occurs 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 study, 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 (30 billion liters) of combined sewer and stormwater are discharged into rivers and streams each year during flooding events," said Amanda Carneiro Marques, Ph.D., 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 those algae die, 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, Montgomery 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 to 2024.
Each watershed contains at least one designated combined sewer overflow area and Geological Survey monitoring stations both upstream and downstream of it. Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights. d research that matter—daily or weekly. 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.
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