Philadelphia sends more than 8 billion gallons of sewage and stormwater into its waterways a year

More than 8 billion gallons of combined sewer and stormwater pour into Philadelphia’s rivers and streams every year, according to Amanda Carneiro Marques, an assistant professor in Drexel University’s School of Engineering. In one creek, that burden shows up as a basic failure of the water: dissolved oxygen sank below the level fish need in 40% of July observations.

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The creek is Tacony-Frankford, which runs through Montgomery and Philadelphia counties, and the numbers come from a study that tracked a decade of monitoring data around the city’s overflow points. Its authors argue that the damage from overflows differs sharply from one waterway to the next.

Combined sewer overflows in an old city

Much of Philadelphia is served by combined sewers, pipes that carry household wastewater and rainwater together to a treatment plant. When heavy rain overwhelms the system, the excess is released directly into waterways to prevent backups. The Drexel news release on EurekAlert describes the paper, “Seasonal drivers and infrastructure impacts on dissolved oxygen dynamics in complex environmental systems,” published September 22, 2026 in the Journal of Environmental Management.

As the original Drexel announcement explains, Marques led the team, which also included Jake Black and Gbenga Daniels of Drexel and Atiq Rahman of the Philadelphia Water Department, with support from the U.S. Geological Survey. Four study areas were examined: Darby-Cobbs Creek, which spans parts of Chester, Delaware, Montgomery and Philadelphia counties; Pennypack Creek in Northeast Philadelphia; Tacony-Frankford Creek; and the portion of the Delaware River Basin that borders the city. The researchers worked from more than a decade of Geological Survey water quality monitoring, precipitation data from the National Oceanic and Atmospheric Administration, and the Water Department’s overflow records. The full paper is listed on ScienceDirect.

Dissolved oxygen as a measure of stream health

Dissolved oxygen is the oxygen available to aquatic life, and the Environmental Protection Agency’s parameter factsheet notes that fish need it for spawning, growth and activity. Low levels stress fish and can impede reproduction, while very low or near-zero levels cannot support fish or the small invertebrates they feed on. Marques called it “one of the most important indicators for aquatic ecosystem health.”

She said the long record works like “recording a video, rather than just getting a snapshot of a moment in time.” Each studied watershed had at least one designated overflow area with Geological Survey stations both upstream and downstream, and the data covered 2013 through 2024. The team modeled the overall water quality effect of overflow events, concentrating on swings in dissolved oxygen after storms. That focus lets the study compare how quickly oxygen falls and how long it stays low, which is closer to what fish experience than a single reading taken on one summer afternoon.

Tacony-Frankford Creek against Pennypack, Darby-Cobbs and the Delaware

The contrasts are stark. Tacony-Frankford Creek fell below the EPA minimum for fish growth and reproduction in 40% of July observations, with low levels persisting through September. Pennypack Creek in Northeast Philadelphia and Darby-Cobbs Creek rarely dropped below that threshold during the year. Marques stressed in the Drexel release that these differences show impacts depend on the flow rate and volume of the overflow plume, plus the surrounding infrastructure and geography, and that each watershed has its own baseline oxygen profile shaped by size, land cover and flow. Smaller watersheds saw oxygen decline by more than 1 milligram per liter after discharges, and the drop lasted for days.

The Delaware River, the portion of the basin bordering Philadelphia, showed very little change after overflows. The researchers attribute that to its high flow and large volume, which dilute the organic matter in sewage before it can strip the water of oxygen. Marques said these factors suggest that combined sewer overflow impacts are “far from uniform across watersheds,” and that understanding a watershed’s underlying vulnerability is important for planning.

What Green City, Clean Waters has delivered

Philadelphia is not standing still on the problem. The Water Department’s Green City, Clean Waters program, which began in 2011, pairs green stormwater infrastructure with traditional sewer upgrades. As of the last required reporting period in 2021, the annual average overflow volume had fallen by 3.08 billion gallons, above the 2.044 billion gallon target cited at the time, and the program lists more than 3,850 greened acres from over 1,100 completed projects.

Those gains sit alongside a larger remaining figure. The Water Department’s stated overflow reduction target is 3,619 million gallons per year, and it expects to finish the remaining year-15 projects by April 2027. The work is meant as groundwork for cities with aging sewers: knowing which streams are vulnerable lets planners put limited money where captured stormwater protects the most aquatic life, and repeated measurements over many seasons reveal cumulative effects that single sampling days miss. For waterways like Tacony-Frankford, the Drexel study offers a way to rank where the next dollar matters most: places whose baseline oxygen is already low, whose flows are small, and whose surroundings send the most sewage when it rains.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.