This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Waste dumped into the Tijuana River can travel seven times farther than expected before becoming airborne and affecting San Diego's air, at least 20 miles (32 kilometers) from its source, according to new UC Riverside research. The river carries a complex mixture of untreated sewage from toilets, industrial waste and urban runoff into the Pacific Ocean at Imperial Beach.
The study, published in Scientific Reports, found that wastewater chemicals can remain in the ocean long enough to then travel north along the San Diego coast before escaping into the atmosphere. The finding helps explain why researchers detected Tijuana River–linked pollution in the air near the Scripps Institution of Oceanography pier in La Jolla, roughly 22 miles (35 kilometers) from the river mouth. "It was already known that compounds from this waste could enter the air, but we didn't realize until now how far it could travel underwater before emerging," said atmospheric scientist and study lead author Karolina Cysneiros de Carvalho.
Cysneiros de Carvalho conducted the research at UCR and the National Center for Atmospheric Research (NCAR) in Colorado, where she was a visiting postdoctoral researcher. She worked on the study with UCR atmospheric scientist William Porter, UCR and NCAR atmospheric chemist Kelley Barsanti, and Kimberly Prather of UC San Diego. Previous research demonstrated that human waste flowing through the Tijuana River affects air quality in communities near the border.
For this study, the researchers wanted to know whether the pollution might extend farther. In August and September 2024, they collected air samples at four locations stretching from the Tijuana River to La Jolla. The timing offered a natural experiment.
Summer is normally the dry season, when Tijuana River flows can fall nearly to zero. But in 2024, unusually large volumes of wastewater were flowing through the river despite the lack of rain. Roughly 160 million to 207 million liters (42 million to 55 million gallons) per day were entering the Pacific, creating an extraordinary dry-season pollution event.
The researchers began by tracking dimethyl trisulfide, a volatile sulfur-containing compound associated with decomposing organic matter and wastewater. They took multiple measurements per day at the four sites and identified the Tijuana River as a strong regional source of dimethyl trisulfide, detecting the compound even at the most distant site in La Jolla. That presented a problem.
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