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 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 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 further. 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 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. Dimethyl trisulfide doesn't last long once it is airborne. Calculations showed that if it had traveled entirely through the air from the Tijuana River, most of it should have disappeared long before reaching La Jolla.
"We found pollutants far from the source that couldn't be explained by just the wind," Porter said. "We wondered, how did this happen?"
A major change in river flow provided another clue. On Sept. 10, a pump station began diverting wastewater southward, dramatically reducing the amount entering the Tijuana River. Afterward, levels of dimethyl trisulfide measured in La Jolla dropped 86%. Some of the chemical lingered for about two days, suggesting the pollution had been stored somewhere before reaching the air.
The researchers found that the ocean could provide that storage and transportation system.
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, up to 76% of the compound could survive the 35-kilometer journey under the conditions researchers examined.
Some chemicals can also temporarily attach themselves to organic particles in the water. These particles can give the chemicals another way to hitchhike along the coast, protecting them from immediately escaping into the atmosphere.
"It's a dynamic process," Porter said. "You have suspended particles, compounds dissolved in the water, and compounds in the air, and they keep exchanging."
Ocean measurements provided additional evidence. During August and September 2024, researchers detected unusually fresh water extending along the coast. Water near Del Mar increasingly resembled water measured farther south, near the Tijuana River discharge region. Ocean circulation models also showed currents moving northward during the period.
The phenomenon was not limited to a single chemical. The researchers measured hundreds of volatile organic compounds and identified a group of them whose behavior tracked both the wastewater tracer and changes in river flow. Their chemical properties supported that these compounds were also able to remain in the water long enough to travel, but volatile enough to eventually escape into the air.
The researchers caution that their observations came during an extraordinary period of sustained dry-season wastewater discharge. They do not yet know how far these pollutants travel under more typical conditions, or how seasons, ocean currents, winds, and changing river flows affect the process.
But the finding could have implications beyond the Tijuana River. Traditional air quality models largely focus on what happens to pollutants after they enter the atmosphere. 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.
The study did not examine whether the concentrations of pollutants detected pose a risk to human health. But the study could change how scientists determine where people might come into contact with pollution. It could also improve air-quality models, which generally do not account for pollutants traveling long distances through water before entering the atmosphere.
For Porter, the findings reinforce the idea that water and air pollution should not be treated as separate environmental problems.
"Water quality has been treated, traditionally, in its own silo, and air quality separately as well. But the universe doesn't work that way. The realms cross," Porter said. "You can't just shut down the beach and call it good. We need to clean up the water not just for swimming but for breathing."
