The 120-mile Tijuana River flows from Baja California into the United States and discharges millions of gallons of wastewater—including sewage, industrial waste and runoff—into the Pacific Ocean every day, making it the dominant source of coastal pollution in the region.

Wastewater pollution has been an ongoing problem for decades and is so severe that the nonprofit environmental group American Rivers recently named the Tijuana River America’s second most endangered river.

A new study from the University of California San Diego examines how pollutants in wastewater travel and are transmitted in the atmosphere through coastal aerosols.

In the study, researchers found that a mixture of illicit drugs, drug metabolites, and chemicals from tires and personal care products aerosolize from wastewater and are detectable in both air and water. The results appear in Science Advances.

The paper’s lead author, Adam Cooper, collected samples from the air and water at various points along the coast of San Diego County, including the U.S.-Mexico border, Imperial Beach and the Scripps Institution of Oceanography in La Jolla.

Cooper, who graduated last spring with a doctorate in chemistry, was a member of Assistant Professor of Chemistry and Biochemistry Jonathan Slade’s group, and collected the samples as part of a field study with Distinguished Professor of Atmospheric Chemistry Kimberly Prather’s lab.

Prather, who holds a joint appointment at Scripps Oceanography and the Department of Chemistry and Biochemistry at UC San Diego, had been studying the air pollution impacts of the Tijuana River for several years. Cooper was able to join her team’s comprehensive sampling campaign across multiple sites from January to March of 2020.

Even though the samples were taken in 2020, the findings are still relevant because little has changed in how sewage released from the river is processed. In fact, pollution from the river has been an ongoing problem for more than 50 years.

“The Tijuana River region is a very dynamic environment with implications for public health, environmental policy and international relations between the United States and Mexico,” stated Cooper.

“Ours is one of the most comprehensive studies to date investigating water-to-air transfer of these pollutants.” The study looked at two aspects of wastewater pollution: the source and the concentration of pollutants along the San Diego County coastline.

To determine the origin of the pollutants, Slade’s lab used a compound they knew came from sewage: benzoylecgonine (BZG), a stable metabolite of cocaine, primarily produced when people use cocaine and then excrete it in urine.

They found that after rainfall, BZG levels in Imperial Beach ocean water spiked in correlation with increased Tijuana River flows, while BZG levels in aerosols spiked in correlation with enhanced sea spray aerosol emissions.

Correlating 11 other pollutants to BZG in aerosols allowed the team to determine which ones behaved similarly in the environment and likely originated from the same wastewater source. The results showed a high correlation between BZG; methamphetamine; octinoxate, a UV filter used in sunscreen; and dibenzylamine, a compound used in tire manufacturing.

The second part of the study measured pollutant concentrations along the coastline in the water and air. Overwhelmingly, they found that these pollutants were higher in the Tijuana River water than in the ocean, and higher in the water and aerosols in the Imperial Beach region than in La Jolla.

Although the amounts of some pollutants, like cocaine, were minuscule, others were more prominent, like octinoxate, which can break down into more toxic components.

In some cases, the octinoxate levels were comparable to measurements made directly above wastewater treatment plant vats, meaning that in some ambient conditions at the coast, the concentrations of pollutants that people are inhaling can be comparable to a worker at a wastewater treatment plant.

“It’s been shown that octinoxate can degrade DNA when exposed to light,” stated Slade. “And if it’s in these tiny aerosols we’re breathing in, it can get deep into our lungs and pass into our bloodstream. That’s very concerning, especially considering the high levels at which we found it in the air.”

The study shows that the closer you are to the Tijuana River, the more likely you are to be exposed to the pollutants it carries, even though the amounts are still relatively small—on the scale of tens of nanograms per hour. This may not seem like much if your exposure is limited to a few hours, but residents living close to the border are inhaling these chemicals over years, even decades.

Many residents have complained of respiratory illness, insomnia and headaches, and several San Diego beaches have been closed almost continuously for the last three years because of high levels of bacteria from wastewater runoff.

Although the paper doesn’t draw any conclusions about the detrimental effects on the environment or human health, Slade and Cooper emphasize the need for more research, better infrastructure and cross-border collaboration.

“Often the sewage crisis is considered a water issue—and it is—but we show that it’s in the air too. Truthfully, we don’t yet know the acute health effects,” stated Slade. “But the numbers we report can be incorporated into models to help us better understand what we’re breathing in and how much we’re exposed to.”

Cooper was so influenced by his work at UC San Diego that he is now a Science and Policy Technology Fellow with the California Council on Science and Technology, working with State Senator Ben Allen.

“The solutions to the cross-border sewage crisis aren’t constrained by technical challenges,” stated Cooper. “They’re constrained by political challenges and policy issues. We have to motivate decision-makers to make the right investments.”

In addition to better infrastructure, more public awareness is crucial to improving the region’s water and air quality, including understanding the downstream effects of the products we use, such as sunscreens and tires.

“Although our study focuses on the Tijuana River, there are other notable sources of wastewater and pollution run-off in Southern California, including wastewater treatment outfalls, the San Diego River and the Los Angeles River,” said Slade, who also noted that “turbulence in rivers and streams may aerosolize wastewater, requiring further study.”

Coastal port environments are extremely dynamic and complex, but these pollution issues are not relegated just to the San Diego-Tijuana region. They pose a global hazard.

An estimated 80% of all global wastewater is untreated. Of the portion that is treated, many plants remove bacteria, but not chemical pollutants. These chemicals remain in the water, which is released into rivers, lakes and oceans, traveling around the world through waterways and in the atmosphere.

“The global surge of untreated wastewater entering lakes, rivers and oceans poses a growing health threat. Aerosolization of this polluted water exposes billions of people through airborne transmission, reaching far beyond those in direct contact and impacting countless others who inhale contaminated air that can travel for many miles,” stated Prather.

“We are continuing our studies in this region to better understand the short and long-term health impacts of inhaling this newly identified source of airborne pollution.”