Sea Spray at California Beach Comes with Aerosolized Raw Sewage

There's something sublime about sea spray—the salty mist that drifts from breaking waves and gives beachgoers a kiss of the ocean. But unfortunately, that kiss could be tainted with bacteria, viruses, and chemical compounds from untreated sewage.

A new study led by researchers from the Scripps Institution of Oceanography at UC San Diego has confirmed that pollution from the coastal waters isn't just a problem for swimmers and surfers, but can transfer to the atmosphere in sea spray aerosol.

To investigate the potential airborne transport of coastal waste pollution, the team took samples from Imperial Beach and the Tijuana River, close to the California-Mexico border. Imperial Beach is notorious for its pollution problem. As Robert Monroe from Scripps explains in a press statement:

"Rainfall in the US-Mexico border region causes complications for wastewater treatment and results in untreated sewage being diverted into the Tijuana River and flowing into the ocean in south Imperial Beach. This input of contaminated water has caused chronic coastal water pollution in Imperial Beach for decades."

The team sampled the coastal aerosols between January and May 2019 and were able to link bacteria and chemical compounds in the sea spray back to the sewage-polluted Tijuana River flowing into coastal waters.

“We’ve shown that up to three-quarters of the bacteria that you breathe in at Imperial Beach are coming from aerosolization of raw sewage in the surf zone,” said lead researcher Kim Prather, a Distinguished Chair in Atmospheric Chemistry, and Distinguished Professor at Scripps Oceanography and UC San Diego’s Department of Chemistry and Biochemistry. “Coastal water pollution has been traditionally considered just a waterborne problem. People worry about swimming and surfing in it but not about breathing it in, even though the aerosols can travel long distances and expose many more people than those just at the beach or in the water."

This winter, an estimated 13 billion gallons of sewage-polluted waters have entered the ocean via the Tijuana River since Dec. 28, 2022, according to Prather.

The researchers stress that this does not mean people are getting sick from sewage in sea spray aerosol. "Most bacteria and viruses are harmless, and the presence of bacteria in sea spray aerosol does not automatically mean that microbes become airborne. Infectivity, exposure levels, and other factors that determine risk need further investigation," writes Monroe.

And thankfully, most beaches aren't seeing the kind of pollution that Imperial Beach does. But the implications are notable: Dumping stuff into coastal waters doesn't mean said stuff gets carried away to sea (which is also awful)—and coastal water pollution affects more than just beachgoers. As the study notes, more than 100,000 cases of illness and tens of thousands of deaths occur annually worldwide due to people entering contaminated waters or eating tainted seafood. It seems prudent to better understand the full impact of coastal pollution.

“This research demonstrates that coastal communities are exposed to coastal water pollution even without entering polluted waters,” said lead author Matthew Pendergraft, a recent graduate from Scripps Oceanography. “More research is necessary to determine the level of risk posed to the public by aerosolized coastal water pollution. These findings provide further justification for prioritizing cleaning up coastal waters.”

The study, "Bacterial and Chemical Evidence of Coastal Water Pollution from the Tijuana River in Sea Spray Aerosol," was published in the journal Environmental Science & Technology.