Bright yellow beach closure sign that says “Keep out. Sewage contaminated water. Exposure may cause illness.” The sign is leaning against a gate outside of Border Field State Park, which is located in Imperial Beach, Calif.
Bacteria and pollutants found in sewage-contaminated waters can become airborne, potentially exposing people living downwind. Credit: Tony Webster/Flickr, CC BY 2.0
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Public health officials have long known that sewage-ladened coastal waters threaten the health of swimmers and surfers. However, new research shows that this common form of water pollution might also pose health risks to those who stay ashore.

Bacteria and pollutants found in sewage-tainted water can be ejected into the atmosphere via sea spray aerosols—tiny droplets that form when waves break—according to a study published in Environmental Science and Technology.

“Once they get into the air, those aerosols can travel miles,” said study coauthor Kimberly Prather, an atmospheric chemist at Scripps Institution of Oceanography at the University of California, San Diego. As a result, “many, many more people are exposed.”

These findings come from air and water samples collected at and around Imperial Beach, Calif., a residential community in San Diego County that is no stranger to living near a sewage-filled ocean. Because of inadequate wastewater infrastructure on the Mexican side of the border, the nearby Tijuana River regularly discharges millions (sometimes billions) of gallons of polluted water into the ocean near the southern part of the city.

“A high fraction of the time, [pollution] gets trapped in the surf zone,” the sliver of turbulent ocean that hugs the coast, Prather explained.

When that happens, local officials often trigger advisories or closures to prevent beachgoers from swimming in contaminated waters, which could result in diarrhea, skin rashes, and respiratory infections. However, these measures don’t consider whether the pollution in the water is, in fact, staying in the water. “Most people do not think about what that would do to air quality,” Prather said.

“We don’t want this to create fear. We just want to make people aware of what’s in the air.”

Using DNA sequencing, Prather and her colleagues found that up to 76% of the bacteria in aerosols collected at Imperial Beach could be traced back to sewage in the polluted river. The researchers also identified chemicals commonly found in flame retardants, cleaning products, and pharmaceuticals, but they could not determine the degree to which those compounds originated from the sewage in the water as opposed to other pollution sources present on land.

Regardless, this study “is demonstrating exposure to coastal water pollution without even going into the water,” said lead author Matthew Pendergraft, a recent Ph.D. graduate of Scripps Institution of Oceanography. The researchers caution that further research is needed before they can say whether aerosolized sewage might affect human health.

“We don’t want this to create fear,” Prather said. “We just want to make people aware of what’s in the air.”

Assessing Health Effects

The researchers focused their attention on Imperial Beach because the area’s main source of water pollution—the Tijuana River—is well characterized. However, “it’s important for any coastal region to consider the transfer [of pollution] from sea to air,” said Crystal Weagle, an atmospheric scientist and former postdoctoral fellow at Dalhousie University who was not affiliated with the study.

Existing research clearly shows that bacteria, viruses, and chemicals floating in the water can transfer into the air via sea spray aerosols, which scoop up substances sitting on the surface of the ocean as they’re flung into the air, according to Weagle. The current study is novel, she said, because the researchers took the extra step of tracing these substances back to their source.

Weagle said she hopes that other researchers will be inspired by the new study and start investigating this phenomenon in other regions of the world. Meanwhile, Prather is working with medical and public health experts to better understand how aerosolized sewage might affect the people breathing it in.

“Viruses and bacteria are in the sewage,” she said. “But whether they remain infectious after they’ve flowed through the salt water and been aerosolized remain the first big questions that we’re going after.” Future work will also take a closer look at some of the chemical pollutants the researchers identified in the study to better understand how much is coming from the sewage.

“The first step in eliminating water and air pollution is to control it—to never let it get there in the first place.”

In the interim, Prather suggested that those living near coastal areas can still take steps to limit their exposure. “We know enough to know when the water is polluted; you should also be thoughtful of the air,” she explained. She recommended making use of indoor air filters, especially on days when the surf is large and beaches are closed because of water pollution.

Weagle agreed, recommending that people living near the coast should treat these sea spray aerosols like wildfire smoke. “What would you expect somebody to do in those situations? Stay inside, maybe keep your windows closed.” She also suggested that those with outdoor gardens thoroughly wash any produce they plan to eat because these aerosols can easily deposit on their surfaces.

Of course, the real solution is to reduce the amount of sewage getting into our waterways, Prather said. “The first step in eliminating water and air pollution is to control it—to never let it get there in the first place.”

—Krystal Vasquez (@caffeinatedkrys), Science Writer

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Citation: Vasquez, K. (2023), Spring and sewage are in the air near San Diego, Eos, 104, Published on 5 April 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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