Sewage-polluted Southern California coastal waters become airborne in sea spray aerosol, and can spread to people on land, even beyond the beach, according to a new study.
For this research, scientists sampled coastal aerosols at Imperial Beach in San Diego County and water from the Tijuana River between January and May 2019.
Rainfall in the U.S.-Mexico border region complicates wastewater treatment, leading to untreated sewage being diverted into the Tijuana River and flowing into the ocean in south Imperial Beach.
This has been a problem at Imperial Beach for decades, researchers noted.
Sea spray is formed when waves break and bubbles burst. The aerosol that results contains bacteria, viruses and chemical compounds from the seawater.
Using DNA sequencing and mass spectrometry, the researchers found ocean aerosols contained bacteria and chemicals originating from the polluted Tijuana River.
This winter, about 13 billion gallons of sewage-polluted waters have entered the ocean through the Tijuana River, according to lead researcher Kim Prather, an atmospheric chemist and professor at the University of California, San Diego's Department of Chemistry and Biochemistry and the university's Scripps Institution of Oceanography.
“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,” Prather said in a university news release.
“Coastal water pollution has been traditionally considered just a waterborne problem," she noted. "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 does not mean people are getting sick from sewage in sea spray aerosol, the researchers said.
Most bacteria and viruses are harmless. The presence of bacteria in sea spray aerosol does not automatically mean that disease-causing microbes or others will become airborne. Infectivity, exposure levels and other factors that determine risk need more investigation, the authors said.
Researchers are now following up in an attempt to detect viruses and other airborne pathogens.
"This research demonstrates that coastal communities are exposed to coastal water pollution even without entering polluted waters,” said lead author Matthew Pendergraft, who obtained his PhD from Scripps Oceanography under Prather's guidance.
“More research is necessary to determine the level of risk posed to the public by aerosolized coastal water pollution," he said in the release. "These findings provide further justification for prioritizing cleaning up coastal waters.”
Study findings were published March 2 in the journal Environmental Science & Technology.
The U.S. Environmental Protection Agency has more on what affects human health at the beach.
SOURCE: University of California, San Diego, news release, March 2, 2023