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- Chemicals from wastewater, such as illicit drugs and byproducts, have been found in the air along the coast near the polluted Tijuana River.
- Researchers said the pollutants are carried in wastewater and stormwater runoff, and become airborne in spray where the Tijuana River meets the Pacific Ocean.
- Pollutant concentrations were significantly higher at Imperial Beach than farther north in La Jolla.
Researchers have found that pollutants in the Tijuana River, which carries raw sewage and industrial waste from Tijuana, are also turning up in the air along the coast near the U.S.-Mexico border.
After collecting samples from air and water along the coast, scientists from UC San Diego determined that fine particles of various pollutants from wastewater are in the air in parts of San Diego County. They found that sea spray aerosols contain illicit drugs and drug byproducts that occur in human urine, as well as chemicals from tires and personal care products.
The researchers said the pollutants are carried in wastewater and stormwater runoff, and become airborne in spray where the river meets the crashing waves near the border. Pollutants also likely enter the air from churning waters in the river itself, they said.
The findings show that the river, in addition to carrying water pollution that has forced beach closures and odors that have plagued nearby communities, is releasing fine particles of air pollution beyond what people can see or smell. The potential health effects of breathing these pollutants aren’t yet known, the researchers said.
“Often the sewage crisis is considered a water issue — and it is — but we show that it’s in the air too,” said Jonathan Slade, a co-author and associate professor of chemistry at UC San Diego. “This study points to it being an air quality issue, which requires more focus and more study in the future to understand the effects of exposure to these chemicals.”
The study, published Wednesday in the journal Science Advances, involved testing air and water samples collected in 2020 at locations including the U.S.-Mexico border, Imperial Beach and the Scripps Institution of Oceanography in La Jolla.
“We’re only looking at just a dozen or so wastewater chemicals, but there’s hundreds, potentially thousands, of these different wastewater chemicals in the air,” Slade said. “We don’t know how they interact with each other and what they can do together in terms of how they get processed in our body and affect our health.”
The researchers used a compound called benzoylecgonine, a byproduct of cocaine use found in urine, to trace the pollutants to wastewater. They tested for about a dozen chemicals found in wastewater and urban runoff, including cocaine and methamphetamine, octinoxate from sunscreen and dibenzylamine from tires.
They found concentrations were significantly higher in the air near the river mouth at the border and Imperial Beach than farther north along the coast at the Scripps Pier in La Jolla.
“Some of these compounds, we measured concentrations that are as high as what a worker would be exposed to at a wastewater treatment plant if they’re standing directly over an aeration tank,” Slade said, “where it’s often recommended that you wear personal protective equipment like masks.”
American Rivers ranks the Tijuana River No. 2 on its list of most endangered rivers. Advocates are urging the U.S. and Mexico to address the flow of raw sewage.
The research team, which included scientists from UC San Diego and Scripps Institution of Oceanography, wore respirators and protective gear while collecting samples near the Tijuana River.
“We see some of this stuff coming out at levels that can be concerning,” Slade said. “But we don’t know the effects of chronic exposure, long-term exposures of people living in the South Bay region, breathing in this air on a daily basis for years or decades.”
Adam Cooper, the lead author, said the research is “one of the most comprehensive studies to date investigating water-to-air transfer of these pollutants.”
Additional research is underway examining other types of air pollution from the Tijuana River, including bacteria and other pathogens from sewage.
Kimberly Prather, a UC San Diego professor of atmospheric chemistry and founding co-director of the Airborne Institute, has been researching air pollution from the river for years.
“This is the No. 1 way that this pollution from the water is actually making its way into your body, even if you’re not at the beach,” Prather said. “The beaches can be closed, but people are still breathing that air.”
The Tijuana River flows 120 miles, starting in Baja California and crossing into Southern California, then reaching the Pacific Ocean in Imperial Beach. The river, which has been plagued with raw sewage from Tijuana for decades, was recently named the country’s second most endangered river by the group American Rivers.
Some beaches in southern San Diego County have been closed for nearly all of the last three years because of high levels of bacteria flowing from the river.
Residents who live near the river in Imperial Beach have complained of respiratory illnesses and headaches, and have said they believe the air is making them sick.
When aerosols such as sea spray or smoke are inhaled, the fine particles can reach deep into the body, entering the bloodstream and even the brain when the particles are small enough.
Prather said she experienced health effects personally while working near the river for a month for research.
“It took me a few months before my lungs felt normal again,” she said.
“People have been complaining about issues for decades now,” Prather said. “And it’s just not right. And all we’re doing is coming in and providing the data that back up what the people have been saying all along, and they need help.”
EPA Administrator Lee Zeldin visited the polluted Tijuana River on the U.S.-Mexico border, calling for a ‘100% solution’ to clean up raw sewage that has fouled the waterway for years.
Trump administration officials say they have been discussing ways of cleaning up the river with the Mexican government.
During a visit to the river in April, U.S. EPA Administrator Lee Zeldin pledged to work with Mexico on a solution to halt the flow of untreated sewage in the river. Zeldin recently told Congress the Trump administration in early May submitted to Mexico a plan that “the United States believes to be the 100% solution for the future.”
“If they are only willing to agree to an 80% solution, then we are not there yet and we will need to use other tactics to get a 100% solution done,” Zeldin said. “Americans on our side of the border are fed up.”
The scientists said they hope their findings will help encourage more collaboration and investments in improving infrastructure to clean up the river, as well as greater awareness of the environmental effects of chemicals from tires and sunscreens.
The researchers collected samples before and after rains, when they hypothesized runoff would sweep more pollution into the river. They confirmed that levels of the cocaine metabolite benzoylecgonine spiked in ocean water and in sea spray in Imperial Beach after the river swelled with rainwater.
The levels of some pollutants, such as cocaine, were minuscule. But the tests also found methamphetamine in water at Imperial Beach at levels that other studies have shown are high enough for fish to develop an addiction and suffer withdrawal symptoms, Slade said.
Such pollutants accumulate on the ocean’s surface near the Tijuana River, and then are released as wave action pounds the water and forms bubbles that burst.
The tiny particles can travel for miles in the air. The results suggest that the closer people are to the Tijuana River, the more likely they are exposed to air pollution from wastewater, though the amounts are relatively low, in the tens of nanograms per hour.
California officials sent water flowing to create new wetlands along the shores of the shrinking Salton Sea. The project is intended to control dust and provide habitat for birds.
The research was supported by state and federal funding, including from the National Science Foundation and the Environmental Protection Agency.
Slade said the data can be used in additional studies to better understand the air pollution and its potential effects.
The authors said their findings also have broader implications beyond the Tijuana River, in other areas where wastewater effluent is discharged into the ocean. They said in the study that “this airborne pathway highlights a largely overlooked source of atmospheric pollution, emphasizing the need to reassess health risks in coastal regions.”
Elsewhere in Southern California, they said, wastewater and polluted runoff make their way into the ocean from sources such as the San Diego River and the Los Angeles River.
They noted that an estimated 80% of the world’s wastewater goes untreated. And where wastewater does undergo treatment at a plant, the process often removes bacteria but not chemicals.
“The global surge of untreated wastewater entering lakes, rivers and oceans poses a growing health threat,” Prather said. “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.”
