Urban stormwater presents pollution challenge

On the wildest, stormiest nights in the San Francisco Bay Area, scientists from the San Francisco Estuary Institute (SFEI) go out on the prowl. Nighttime is when storm intensity in the Bay Area is generally highest, and the team gets going only when a storm is predicted to dump more than 2 cm over 6 h at a particular site.

The researchers fan out to different sites where they know stormwater flow is especially strong. At each site, they take samples to assay the levels of five classes of chemicals: tire- and vehicle-derived chemicals such as 6PPD-quinone; bisphenols, a starting material in manufacturing plastics;organophosphate esters, a key component of flame retardants; per- and polyfluoroalkyl substances (PFAS); and ethoxylated surfactants from paints, coatings, and floor polish. Over several hours, they nab some samples in 50 mL tubes and others in 2 L glass jugs.

This year will be the fourth and final wet season that SFEI scientists conduct these night runs before synthesizing their findings. “We’re laying the groundwork for understanding what’s out there in stormwater,” chemist Rebecca Sutton says. SFEI launched the monitoring project after a chemical analysis of samples collected in 2016 revealed untreated stormwater runoff from populated areas as an overlooked source of chemicals that water-monitoring agencies have only recently begun to track (Environ. Sci.: Processes Impacts 2021, DOI: 10.1039/d0em00463d). “It really opened our eyes to stormwater as an underexplored pathway in terms of emerging contaminants,” Sutton says.

In the US, the passage of the Clean Water Act in 1972 mandated federal, state, and local agencies to prevent chemicals from polluting the nation’s streams, rivers, lakes, and coastline. Sewers and factories posed the biggest problem, but as that pollution got cleaned up, attention turned to stormwater runoff, which in many ways is more challenging to address.

Wastewater and industrial effluent generally come from specific locations. But runoff, which is primarily carried by stormwater, is what environmental engineers call nonpoint source pollution—in other words, it flows in from all over the place. “It’s pollution coming from a whole bunch of small sources that individually create maybe a larger-than-expected issue because none of those individual sources looks important by itself,” says Edward Kolodziej, a civil and environmental engineer at the University of Washington Seattle and Tacoma.

Initially, environmental scientists studying the chemical pollution spawned by urban runoff focused their concern mostly on nutrients, such as nitrogen and phosphorus, and metals. But studies over the past 5 years or so have begun picking up a wide variety of organic chemicals. The sheer diversity of compounds that this flow carries is dizzying, and researchers are only beginning to appreciate the scope of the problem, says Allen P. Davis, a civil and environmental engineer at the University of Maryland, College Park. Davis has been studying the issue in the Chesapeake Bay region for close to 3 decades. “At this point, if you can name a chemical, it can probably be found in stormwater,” he says. “As we continue to identify more and more different pollutants that we want to remove from our waters, we are going to have to look at novel ideas for making that happen.”

In the natural environment, water from heavy rain gets absorbed by soil, trickling down to replenish groundwater. But most pavement and other features of the built environment are impervious to liquid. Consequently, says Jessica Ray, an environmental engineer at the University of Washington, “we get high volumes of runoff that are going over these engineered surfaces. They go into storm drains, and eventually the storm drains convey the stormwater to a nearby body of water.” Climate change compounds the problem, adding more intense rainfall and weather events, she says.

Efforts to stem pollution from urban runoff have largely focused on creating what civil and environmental engineers call green infrastructure—things like rain gardens, stormwater ponds, and other vegetation-based systems that capture stormwater and act as a filter for pollutants. Another option is permeable pavements that allow stormwater to drain into the ground so it doesn’t hit waterways in concentrated form.

For many pollutants in urban runoff, rain gardens work great, says Andy Erickson, a civil engineer at the University of Minnesota Twin Cities. The organic matter in such gardens removes metals and provides a substrate for microbial communities, which can break down polycyclic aromatic hydrocarbons that accumulate from gasoline and oil. A layer of sand also extracts particulate matter.

Other pollutants need a bit more than a simple garden can provide. Minnesota set out to tackle phosphorus pollution, which comes from yard waste like hedge trimmings and grass cuttings as well as from many soaps and detergents. As a graduate student, Erickson found that iron shavings mixed into the sand at a mass fraction of 5% can pull about 90% of the phosphorus out of stormwater. Since Erickson first published the work in 2007, about 150 full-scale, iron-enhanced sand filters treating runoff have been incorporated into rain gardens across the state, and other regions are adopting them too.

But these systems are not designed to handle many of the newer substances appearing in stormwater, such as pharmaceuticals, flame retardants, PFAS, and microplastics. “The problem with these newer, long-chain complex chemicals is that in the environment they can do some pretty strange things, like transform into other chemicals, volatilize, or stick to particles,” Erickson says. “Having materials that remove those chemicals is going to be a huge thing for stormwater.”

One solution researchers are experimenting with is adding biochar, typically made by pyrolyzing agricultural waste products such as wood and corn husks, to rain gardens and other green infrastructure. Biochar’s charcoal-like, high-carbon composition creates a material that is loaded with tiny pores, giving it a high surface area with which to draw pollutants out of water. The trick is to figure out which type works for what chemicals, how to configure it in green infrastructure and how to switch it out when it is “used up,” Ray says. Her lab at the University of Washington is studying a biochar made from coffee grounds to determine the types of contaminants it can remove and how long it functions under a continuous flow of stormwater. The researchers are also testing the properties of an iron oxide-coated sand that degrades compounds on contact.

Another approach is to augment the ecosystems that emerge in green infrastructure. Gregory H. LeFevre, a civil and environmental engineer at the University of Iowa, is studying microbial communities in these structures. He says that boosting specific bacteria to promote biofilm formation or adding particular plants and fungi may help degrade especially recalcitrant chemicals. His lab is also examining what exactly happens to contaminants as they mix in systems enhanced in such ways. Chemical mixtures could be “creating a whole plethora of different metabolites, some of which nobody has ever seen before,” he says, and understanding how they interact and evolve is an emerging area of environmental science.

And then there’s the ongoing problem of identifying which chemicals to target. Often, chemicals’ effects on wildlife are subtle—causing changes in fertility or immune response, for example—and it’s not obvious which ones are most damaging. “As a society we are making several hundred thousand of individual chemicals, so the potential portfolio of chemical pollutants is very, very large,” Kolodziej says. “How do we prioritize the most important ones in this huge chemical soup?” It doesn’t help, he notes, that companies classify their chemical recipes as trade secrets. “To protect the environment, we need to know what's there," he says.

Occasionally, clear culprits emerge. In 2020, Kolodziej and his colleagues identified 6PPD-quinone, a chemical derived from tire plastic, as the primary cause of mass die-offs of coho salmon in the Pacific Northwest. The chemical, a component of roadway runoff, had been ending up in waterways for decades, yet it still took 5–6 years of concerted effort and some intense chemistry to identify it. This effort involved narrowing down from some 2,000 compounds in tire plastic leachate. It’s still unclear how the chemical is transported through the environment. Some is leaching from bits of rubber, and those bits could be filtered out by roadside rain gardens or captured by buffer strips or retention basins that highway crews clean periodically. Some is also dissolved in water and will likely require a different solution.

Because runoff from roadways and other sources is so chemically complex and largely uncharacterized, Kolodziej and others advocate testing the water not just for specific known chemicals. Some researchers are exploring using a suite of biological assays, such as ones that measure changes to estrogen receptors, to monitor aquatic environments where runoff is discharged. Thomas Young, an environmental engineer at the University of California, Davis, calls this approach “an insurance policy” that can “detect problems in an environment where you don’t know, a priori, what chemicals might or might not be there.” He notes, though, that so far the field hasn’t settled on which assays to use.

Regardless of how many pollutants researchers can identify and ways they find to remove them, what’s clear is that treatment and remediation alone won’t solve the problem, SFEI’s Sutton says. The level of infrastructure required to treat stormwater everywhere in the US would require “an incredible investment,” she says. “That would be unrealistic.”

Indeed, even maintaining existing infrastructure will take more than is currently being done, the University of Maryland’s Davis says. With proper maintenance, rain gardens and similar constructions were designed to last for about 20–30 years, and some of the earliest installations are reaching the end of their natural life span. How exactly to refresh, rebuild, or update them is a question the field has yet to tackle. “Nobody has really thought that far ahead,” Davis says. And what, chemically speaking, happens to many of the compounds being captured is unknown. “That’s also an area where we need to continue to do research to make this a sustainable process.”

But just as important, Sutton says, are longer-term efforts such as working with manufacturers to reduce the variety of chemicals they use and develop less toxic replacements for some, she says. Efforts might also involve reformulating products in such a way that they shed fewer contaminants or finding ways of capturing them before they enter the environment. “It would be much smarter,” she says, “to address this pollution at the source.”

Alla Katsnelson is a freelance writer based in Northampton, Massachusetts, who covers the life sciences.