Tijuana River sewage may be contaminating air along Southern California coast: study

Chronic coastal contamination from the Tijuana River can end up in the atmosphere as “sea spray aerosol” — spreading far beyond the San Diego County beaches where it has long polluted the water, a new study has found.

For decades, storms occurring along the U.S.-Mexico border have been diverting sewage through the Tijuana River and into the ocean in south Imperial Beach, according to a study published on Thursday in Environmental Science & Technology.

But researchers have now determined that sewage-polluted coastal waters can transfer to the atmosphere as aerosol — generated “by breaking waves and bursting bubbles.”

And while the level of threat to human health remains uncertain, this so-called “sea spray aerosol” contains bacteria, viruses and chemical compounds.

“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,” Kim Prather, an atmospheric chemist at the University of California San Diego’s Scripps Institution of Oceanography, said in a statement.

While residents typically consider coastal pollution to be a waterborne issue, Prather warned that there may be additional cause for concern beyond the swimming and surfing communities.

Contaminated coastal waters have been found to cause more than 100 million annual illnesses worldwide, but such pollution could also be reaching an uncounted number of individuals through the air, according to the study.

“Aerosols can travel long distances and expose many more people than those just at the beach or in the water,” Prather said.

The findings are particularly timely after wet winter weather last week exacerbated a sewage spill that had recently occurred across the border.

In addition to coping with storm-related wastewater issues, the region has long been enduring infrastructural breakdowns that shuttle Tijuana’s sewage toward San Diego.

The Tijuana River Watershed originates in the U.S. before crossing the border into Mexico and then returning to California.

A pipeline rupture near Tijuana last month led Mexican authorities to shutter pumping stations in its water conveyance systems for two weeks — creating a “transboundary flow” of wastewater, the International Boundary and Water Commission explained at the time.

While that acute problem was temporarily resolved late last week, a storm had already begun by the time the initial repair concluded.

Since Dec. 28, an estimated 13 billion gallons of sewage-polluted waters have flowed into the ocean from the Tijuana River, according to Prather.

On Thursday, the International Boundary and Water Commission updated that flow to 20 billion gallons.

The atmospheric chemist and her colleagues sampled coastal aerosols at Imperial Beach and water from the Tijuana River between January and May 2019.

The scientists then used DNA sequencing and mass spectrometry to connect bacteria and chemical compounds in sea spray aerosol back to the polluted river water. They are now conducting follow-up research to identify viruses and other airborne pathogens.

Although the authors identified the presence of certain bacteria and chemicals in the sea spray, they stressed that this does not necessarily mean people are getting sick from their exposure, as most bacteria and viruses are harmless.

Meanwhile, the presence of bacteria in these aerosols does not automatically indicate that pathogenic microbes become airborne, according to the scientists.

Characteristics like infectivity, exposure levels and other risk factors require further investigation, they concluded.

“More research is necessary to determine the level of risk posed to the public by aerosolized coastal water pollution,” lead author Matthew Pendergraft, a recent Ph.D. recipient from Prather’s group, said in a statement.

“These findings provide further justification for prioritizing cleaning up coastal waters,” Pendergraft added.