Toxic waste dumped into the Tijuana River can travel up to 7 times farther than previously thought before evaporating into the atmosphere, significantly degrading air quality across a 20-mile radius in San Diego, according to a new study from UC Riverside.
Published in Scientific Reports, the research details how this untreated sewage, which is a mix of household wastewater, industrial discharge, and urban runoff, enters the Pacific Ocean at Imperial Beach.
These wastewater chemicals remain submerged in ocean currents long enough to drift northward along the San Diego coastline before venting into the atmosphere.
This mechanism explains why pollution linked to the Tijuana River was found near the Scripps Institution of Oceanography pier in La Jolla, approximately 22 miles north of the river’s 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 while working at UC Riverside and as a visiting postdoctoral fellow at the National Center for Atmospheric Research (NCAR) in Colorado. She co-authored the paper alongside UCR atmospheric scientist William Porter, UCR/NCAR chemist Kelley Barsanti, and UC San Diego atmospheric chemist Kimberly Prather.
To test whether cross-border pollution reached beyond immediate border communities, the team collected air samples between August and September 2024 at four sites ranging 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,” researchers explained.
Researchers focused their tracking on dimethyl trisulfide, a volatile sulfur compound associated with decomposing organic matter and raw sewage. Air samples confirmed the Tijuana River was the primary source, with trace amounts appearing as far north as La Jolla.
The discovery uncovered a problem: dimethyl trisulfide breaks down rapidly in the atmosphere and should have dissipated long before reaching La Jolla. However, as wastewater flowed, levels rose and fell, but the sampling showed the substance persisted in the environment for around two days. This meant that the pollutants were being stored in the ocean.
“Normally, dimethyl trisulfide quickly reacts in the air and disappears. In water, however, it can survive much longer. Ocean currents can therefore carry it north before it eventually crosses the boundary between water and air. Modeling showed that at a depth of about 10 meters (33 feet), up to 76% of the compound could survive the 35-kilometer (22-mile) journey under the conditions researchers examined,” they added.
Oceanographic data corroborated the findings as the sampling period showed clear signs of freshwater along the coast, with waters off Del Mar closely matching the chemical profile of water sampled from the Tijuana River mouth.
Researchers emphasized that these findings reflect an extreme period of sustained dry-season discharge. Further research is needed to determine how contaminants far can they go under routine conditions or how seasonal shifts and shifting winds and river flows.
“The new study shows that rivers and oceans can alter that journey, preserving some chemicals and allowing them to become airborne much farther from their original source than atmospheric transport alone would predict,” they noted.
The study did not evaluate specific human health risks associated with the pollutant concentrations, however, the findings could reshape how scientists determine where people might come into contact with pollution and improve air quality forecasting. The researchers emphasized the findings demonstrate that air and water pollution can no longer be treated as isolated environmental problems.
