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Urban runoff threatens water quality. Infrastructure changes could help.

Rickie White, a bald white man in a blue shirt, looks out at the back lot of a shopping center, directly adjacent to the green wetland of Beaver Marsh.
Sophia Friesen
Rickie White, executive director of the Ellerbe Creek Watershed Association, looks out over a paved lot that borders Beaver Marsh Nature Preserve. The urban marsh helps filter polluted runoff from the surrounding area.

It’s storm season, and that means flood season.

When it rains, water sheets off the roofs, parking lots, and roads that cover an increasing portion of the landscape. To avoid flooding, city infrastructure focuses on moving all that water into pipes and streams, getting it downstream and out of town as fast as possible. But the current standard for dealing with stormwater makes pollution worse for everything and everyone depending on urban streams, including the people who get their drinking water from farther down the river.

As cities continue to develop at lightning speed, washing our problems down the river becomes an increasingly unsustainable prospect.

Rapid urban development threatens stream water quality

Urban water pollution becomes an especially acute problem when multiple cities are clustered along the same waterways, as in the Research Triangle area. That’s according to Joseph Delesantro, who studied urban stream ecology throughout the Triangle while a graduate student at UNC Chapel Hill.

Cary residents drink water coming from Chapel Hill. Raleigh drinks water from Durham. And unfortunately, Delesantro said, “There's a lot of stuff in the water of urban streams that is not water.”

Other ingredients include road salt, gasoline, medications, and sewage – the complex residue of an equally complex city landscape.

Delesantro added that the water issues that threaten Raleigh and Cary may become more widespread in the near future.

“There are problems that we're facing here in the Triangle… that maybe are kind of like an early microcosm of what we might be seeing in other places,” he said. “And that's this really rapid urban development.”

For urban streams, pollutants are dispersed over the entire city, which makes their contamination a complicated issue to understand, and a difficult one to fix. But an ongoing study at Duke University, the City and its River project, is tackling this problem in Durham’s Ellerbe Creek, which faces intense pollution from urban runoff.

One branch of the creek flows from the heart of downtown Durham, at times running orange-brown and completely opaque. Using the Ellerbe as a model, scientists and activists are starting to piece together what makes urban streams so unhealthy – and how to change them for the better.

South Branch of the Ellerbe Creek emerging from a large outflow pipe. The water is completely opaque and orange-brown. The sides of the creek are overgrown with ivy and tall grass. Ahead, the creek disappears into a dark treeline.
Sophia Friesen
Water from downtown Durham flows through a large pipe into Ellerbe Creek, which faces intense pollution from urban runoff.

Emily Bernhardt is an ecologist at Duke’s Nicholas School of the Environment and is one of the lead researchers on the study. She said that the single most important factor in urban creek health is the layout and number of roads.

Roads are designed to shed rainfall as fast as possible, channeling water down into stormwater pipes and dumping it into the creek. Bernhardt explained that when it rains, “You can just think about all those roads being part of the river network really, because they're all built to drain straight into rivers.”

Roads become tributaries, and any road salt, gasoline, or other pollution on their surfaces are washed straight into the creek. The more roads there are, and the more connected they are, the more pollution gets into the creek.

The very speed at which the water enters the creek can also harm its ecology by power-washing the creekbed, according to Bernhardt.

“Imagine that you are a little tiny bug or a fish living in that stream,” Bernhardt said. “Not only do you have these incredibly high flows coming right off pavement, but they're going to rapidly erode and move the bed, so it's like being sandblasted in a flood every single time it rains. So it's really hard to live there.”

There's a lot of different sources of pollutants to streams, especially during storms, when basically the entirety of the urban landscape is connected to these water bodies.
Joseph Delesantro, urban stream ecologist

Wastewater infrastructure also impacts the ecology of urban streams. Jonathan Behrens, an urban ecology graduate student at Duke and a team leader on the project, said that downstream of Durham’s wastewater treatment plant, the water in the Ellerbe is spiked with “this incredible complexity of contaminants.”

The water contains low levels of caffeine, diabetes medication, and illegal drugs, among other contaminants. It’s an open question how this cocktail of pharmaceuticals and poisons affects creek ecology, let alone the people who use downstream Falls Lake for drinking water. And unfortunately, not all wastewater makes it into treatment plants in the first place.

“We have sanitary infrastructure that is constantly leaking,” said Delesantro, adding that’s a common issue for urban streams in general. In Ellerbe Creek, bacteria found in the water suggest faulty sewer pipes are ubiquitous, Bernhardt said. “We do see E. coli in pretty much every single tributary of the creek that we sample. So there's leaks everywhere.”

Because the pollution in urban streams comes from across entire watersheds, it’s a difficult issue to fix. There’s no one-punch solution to a problem that spans a city. Still, that doesn’t mean that urban water quality is beyond saving, and specific infrastructure changes could make things better.

Putting the brakes on stormwater could help urban water quality

A key part of the puzzle is how to slow down stormwater before it reaches waterways. Giving water a chance to seep through a landscape more slowly not only reduces harmful creek erosion, it helps filter out contaminants in the water. Slowing water down can take many forms, from artificial wetlands to water-permeable pavements to “big containers of dirty rocks,” said Bernhardt. “Anything that slows water down, it’s going to be good for water quality.”

Rickie White, the executive director of the Ellerbe Creek Watershed Association (ECWA), is dedicated to slowing water down by making it trickle through green spaces rather than rushing through pipes. The Association works to preserve green space adjacent to the creek, and also supports small-scale solutions like “rain gardens” - depressions in the ground planted with native vegetation, designed to give runoff somewhere to run. As water absorbs into the ground, White said, “you don't get creeks that, when a thunderstorm happens, they just immediately rise and fall.” Instead, the water takes some time to move to the creek, reducing erosion and pollution.

But small-scale solutions like rain gardens can only go so far. The South Ellerbe Restoration Project aims to slow down a lot of stormwater in a big way. Currently under construction, the project will be a seven-acre artificial wetland that will collect water from downtown Durham. Stormwater will be diverted to the wetland to seep through soil and native plants before it connects with the creek, which its designers hope will reduce erosion and improve downstream water quality.

As cities continue to grow, maintaining a drinkable water supply becomes more urgent. The current strategy of funneling polluted city runoff directly into waterways is becoming less sustainable.

“It's not like we're exporting this problem way downstream,” Delesantro said. “We're exporting this problem just a couple miles away.”

Mitigating the pollution coming from an entire city is a difficult task, but a crucial one, Delesantro said. “We need this water.”

Sophia Friesen is a science writer and WUNC’s 2022 AAAS Mass Media Fellow. Before working with WUNC, they wrote for science news outlets including Massive Science, preLights, and the Berkeley Science Review, covering everything from wildfire mitigation to pterosaur flight abilities.