The secret scourge of climate change? More raw sewage in Philadelphia’s waterways.

A sudden, intense storm whipped up over Philadelphia on Labor Day. It dumped less than a quarter-inch of water. But it made a big impact.

Frankford Creek, which normally flows at a lazy 10 cubic feet per second on its way to the Delaware River, roared to 130, according to a gauge in Juniata Park.

Water carrying trash from city streets mixed with diluted raw sewage from nearby homes as stormwater blew past the treatment capacity of the city’s aging sewer systems. Clear water turned muddy brown as it carried human waste, plastic bottles, and other debris along.

Water quality plummeted as microbes began to feed on fecal bacteria and other contaminants, sending life-giving oxygen levels plummeting. Short as the storm was, it took days for the water to recover.

The event was far from unusual — the same scene repeated about every six days this summer, which has seen far bigger rainfalls than that Labor Day cloudburst.

Some of the oldest parts of Philadelphia have a long-buried past that is posing a present danger expected to get even worse in the future: an aging sewage system designed to overflow during storms into local waterways, including the Delaware River and the Schuylkill. Decades ago, that system made sense.

These so-called combined sewer overflows, or CSOs, might be ground zero for gauging the effects of climate change in Philadelphia. The antiquated system is dealing with more intense rainfalls as more heat and moisture in the air produce stronger, wetter storms. City officials say they are lessening the impact through “green” programs that reduce runoff. Sewage that does get released is highly diluted, they stress.

The problem is not unique to Philadelphia but is exacerbated in urban areas because so much of the surface is hard. Stormwater flows quickly over sidewalks, parking lots, streets, and roofs, and directly into storm drains. Almost three-quarters of Philadelphia is impervious surface. In more suburban or rural settings, much more water gets absorbed by the ground instead of flowing straight into a waterway, carrying in pollutants.

Serving more than three-quarters of the city’s residents, the combined sewer system runs under many of the oldest and most developed parts of the city, including Center City, South Philadelphia, West Philadelphia, North Philadelphia, Bridesburg-Kensington-Richmond, East Mount Airy, and East Germantown, as well as parts of Northeast Philadelphia. The rest of the city uses a more modern system that sends all sewage to treatment plants and only surface runoff goes directly to the waterways.

In a combined system, stormwater runoff flows into the same pipes as everything that gets flushed down toilets. In dry weather, they both flow together to one of the city’s three plants that treat the water before releasing it back into the Delaware River. However, during heavy rains, the plants can’t handle the volume. So, as a safety valve, the combined stormwater and sewage automatically diverts to outfalls that spill into creeks and streams during storms.

The overflows are permitted and intentional, as they prevent what could be catastrophic issues at sewage treatment plants.

At a whopping 24 feet at its widest, one pipe at Frankford Creek, known as CSO T14, normally diverts waste water to the city’s Northeast Water Pollution Control Plant during a small rain. But during moderate to heavy rains, it spews about 42 million gallons into the creek on average.

It is just one of the 164 CSOs in the city, though most are much smaller.

Philadelphia water officials have never hidden what’s going on — the federal Clean Water Act requires local governments to make stormwater plans public. Most residents, however, likely are not aware of what’s happening.

Just two days after the Labor Day storm, Dana D’Angiolini was playing with her daughter, Violet, and their beagle puppy, Cocoa, at the Juniata Park playground, which rises above Frankford Creek near the T14 outflow.

Asked if she knew what was in that water, she wrinkled her nose.

“I live right there,” D’Angiolini said, pointing toward J Street, “and I didn’t know that sewage flows in the creek. That’s disgusting.”

The city has 3,200 miles of underground pipes that handle runoff and sewage, making it impractical to even consider ripping them all up. In a typical year, about 13 billion gallons of combined sewer overflow dumps into waterways. But the last two years — especially 2018 — have been extraordinarily rainy.

Combined systems like Philadelphia’s exist all over the country, as part of a national problem that hundreds of utilities are working to solve. About 820 municipalities spread across 32 states use combined sewer outflows.

Delaware and Montgomery Counties have them. And there are 22 alone in Camden City emptying into the Delaware and Cooper Rivers, as well as seven in Gloucester City emptying into the Delaware.

In New Jersey, Gov. Phil Murphy this year signed a law that allows municipalities and counties to create stormwater utilities to deal with the problem, financed with fees levied on property owners with large paved surfaces. Opposition began referring to it as a “rain tax.”

But Philadelphia has by far the largest problem regionally because of its size and age. In 2011, the Water Department launched its “Green City, Clean Waters” program, a 25-year plan featuring “green” stormwater infrastructure projects like rain gardens, stream cleanouts and restoration, and rain-barrel giveaways. Philadelphia also uses porous paving when possible. The goal is to reduce stormwater flow while beautifying parts of the city. Environmental groups praise the city’s efforts.

» READ MORE: Those surprising gardens along I-95 in Fishtown? They'll manage massive amounts of stormwater

For example, on Sept. 6, the city opened a $1.6 million project using elements such as a large rain garden and trenches around trees lined with gravel rather than solid pavement in a North Philadelphia neighborhood. The tools were designed to manage the approximately 158,000 gallons of runoff that a 1-inch rainstorm would generate, helping to reduce sewer overflows by millions of gallons a year.

Marc Cammarata, a deputy commissioner for planning at the Water Department, said the combined sewer overflows send about 13 billion gallons a year into five creeks. Normally, he stressed, water quality in city creeks and streams is “much, much better” than it is during storms.

Intense storms also can create runoff issues in the more modern parts of the system, where stormwater and wastewater travel in separate pipes to treatment plants.

On top of visible trash, stormwater can include gas, oil, and antifreeze, plus whatever other chemicals residents dump on the streets. Traps stop plastics from flowing through stormwater pipes, but can’t prevent bottles and bags from washing in directly from the streets.

Cammarata said the city’s Green City, Clean Waters program has resulted in 700 green projects in the combined sewer section of the city, along with improvements to treatment plants and pipes, and has helped reduce the city’s combined sewer overflow by 1.7 billion gallons during a typical year of rainfall.

However, the last few years have not been typical. On average, Philadelphia gets about 42 inches of rain in a year, according to the National Weather Service. In 2018, it got 62 inches. This year is also proving to be wet, with about 36 inches so far — 8 inches more than normal for the same period.

Cammarata added that the intense storms are dangerous in other ways, too. In 2018, a woman shooting photos with her boyfriend from inside a storm drain near Roosevelt Boulevard was swept away by Pennypack Creek during a flash flood.

Erik Silldorff, a scientist with the Delaware Riverkeeper Network, happened to be collecting data at the Frankford Creek on Labor Day morning when the water there began raging. Near Aramingo Avenue, he captured video showing the water rising and turning brown.

“In the morning, the water was running clear,” Silldorff said. “Then came the intense storm. It only lasted probably less than half an hour.”

Julie Slavet, executive director of the nonprofit Tookany/Tacony-Frankford Watershed Partnership, has been working on projects to address stormwater.

“And to think kids swim in this,” Slavet said, referring to kids who jump from a bridge near T14 into Frankford Creek, though swimming is illegal there. “It shouldn’t be like this anymore. There shouldn’t be sewage going into the water.”

» READ MORE: Is it safe to swim in the Delaware or Schuylkill? It’s complicated.

Slavet’s group is starting to restore 42 acres off Jenkintown Creek at the Charles D. Conklin Jr. Pool and Recreation Center. There, 225 feet of concrete will be ripped out and replaced with 7,500 square feet of turf grass that will become a meadow. Overall, the project will manage 42 acres of stormwater runoff. It’s one of multiple stream restoration projects going on throughout the region.

On a recent day, Slavet walked with the group’s Robin Irizarry along another outflow known as Rock Run. A concrete culvert designed to funnel the overflow there often fills, leaving a strong sewage odor and plastic bags strewn about the trees in its wake.

“The more rain, the more runoff,” Slavet said. “The CSOs just can’t handle it. The water rises so fast. It’s dangerous and scary.”

Editor’s note: “From the Source: Stories of the Delaware River” is produced with support from the National Geographic Society, the Lenfest Institute for Journalism, and the William Penn Foundation. Editorial content is created independently of the project’s donors. This article also is part of Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.