The unfettered flow of wastewater from the Tijuana River has not only been threatening Southern California’s beaches, but it has also unleashed alarming amounts of toxic gas in area communities, a new study has found.
The cross-border waterway, which originates in Mexico’s Baja California, has been emitting hydrogen sulfide — known as “sewer gas” due to its rotten egg smell — at peak concentrations 4,500 times greater than typical urban levels, according to the study, published on Thursday in Science.
“The community was experiencing major respiratory issues, major health issues, couldn’t sleep,” senior author Kimberly Prather, told reporters on a Wednesday press call.
“They felt they were being poisoned, and they felt they weren’t being listened to,” added Prather, an atmospheric chemist at University of California San Diego’s Scripps Institution of Oceanography.
Prather was referring to San Diego County’s southernmost residents, such as those of Imperial Beach, who have long been subject to the passage of polluted wastewater from Mexico via ocean plumes and the Tijuana River.
This fetid flow — filled with both pathogens and toxic chemicals — stems from inadequate treatment near Tijuana, shuttering beaches and sickening Navy SEALs for years.
Environmental Protection Agency Administrator Lee Zeldin and his Mexican counterpart last month took steps toward a solution by signing a memorandum of understanding. And on Thursday, Zeldin celebrated the expansion of the South Bay International Wastewater Treatment Plant — a U.S. site that treats some of Tijuana’s waste through a binational treaty.
While most research on the contamination crisis has focused on direct contact with the water, the authors of Thursday’s study warned that the pollutants have the potential to aerosolize and disperse far beyond the riverbanks.
“Once they’re in the air, they can travel for miles, and many more people can be exposed to those pollutants through inhalation,” Prather said.
This overlooked pathway, the authors explained, could mean that communities are encountering much greater exposures via inhalation than they are through direct contact with polluted wastewater.
In other words, residents “don’t necessarily have to be at the beach,” Prather continued, adding that aerosolized pollutants could potentially travel “all over San Diego County.”
“When you close the beach, you can’t really tell people, ‘Stop breathing,’” Prather said.
Although the cross-border sewage crisis has been affecting the region for nearly a century, scientists have only recently discovered the connection between river contamination and airborne emissions.
A particular turning point occurred in 2023, when Prather and her colleagues deployed a genomic technology that traced up to 76 percent of Imperial Beach’s airborne bacteria to the Tijuana River.
In a study published this past May, the researchers identified numerous aerosolized wastewater compounds — including the illicit drugs like methamphetamine, as well as chemicals from tires and personal care products — in the region’s water and air.
The latest research, however, looked to address the long voiced concerns of area residents, who have been complaining of foul smells, respiratory issues, fatigue, headaches and eye, nose and throat irritation.
Many of these same individuals reported feeling “dismissed” — up against claims that the odors were “just a nuisance,” Prather recalled. They were particularly vocal about these issues in summer 2024, a period in which she described the smells as “unbelievable.”
Having previously focused on sampling sea spray aerosols during wetter periods, Prather and her colleagues decided to track gases during the dry season — dry in the sense that there was no rain but plagued by massive nighttime river flows.
“It’s got industrial waste from all different kinds of sources, plus this raw sewage that’s emptying into the Pacific Ocean every single day,” Prather said of these so-called “turbulent” flows, labeled as such due to their chaotic and irregular nature.
Equipped with their previous data on airborne bacteria and chemicals, the researchers drew from the local complaints of foul smells and ultimately identified a specific stretch of river that might constitute a hotspot for gas emissions.
After pinpointing this section, the researchers deployed a mobile air quality lab to measure concentrations of hydrogen sulfide — the so-called “sewer gas” — as an airborne tracer of water contamination.
They discovered that during the record high dry season flows, there was a significant spike in hydrogen sulfide releases, with nighttime peaks climbing to 4,500 parts per billion — thousands of times more than typical urban levels.
Their measurements also showed that most of the hydrogen sulfide was coming from an inland location, rather than from the ocean, Prather explained.
Also remarkable to the scientists was an unexpected turnaround: after “a sudden wastewater diversion” abruptly slashed nighttime flows from up to 80 million gallons per day to less than five, odor reports also sharply decreased. These coinciding incidents, the authors noted, support the idea that the hotspot of Tijuana River turbulence was “the dominant source of malodors” and of hydrogen sulfide emissions.
Throughout the sampling experience, Prather recalled that her team members all wore respirators and biohazard suits for their personal protection.
The scientists on Wednesday also acknowledged an apparent misdiagnosis that had surfaced from their sampling in the media last fall — an announcement, which was ultimately rescinded, that hydrogen cyanide gas was also contaminating the local air.
Paula Stigler Granados, an environmental health scientist at San Diego State University’s School of Public Health, explained that the probes on the first responder protection sensors they were carrying were “being thrown off by such high hydrogen sulfide levels.”
“At that time, we felt it was super important to share that initial data that we had,” Stigler Granados said. “Even our hazardous response teams were not sure what was going on, so we had a public health and ethical responsibility to share that information.”
The researchers later retracted that initial data, revealing that these were false positive results for hydrogen cyanide, she added.
Prather reiterated these comments, while stressing that the hydrogen sulfide measurements were correct and were made with 24/7 air quality calibrated equipment, rather than the first responder protective sensors.
Evaluating their overall results, the authors emphasized the importance of considering how turbulent river segments can impact regional air quality. Because typical models don’t include emissions from polluted waterways, incorporating these sources is critical in forecasting health impacts, addressing risks and guiding mitigation policy, per the study.
The exceedingly high levels of hydrogen sulfide concentrations identified near the Tijuana River also confirmed many long-dismissed community reports, the scientists added, noting the disproportionate pollution burden borne by marginalized communities.
“Sustained monitoring, coordinated cross-border efforts and leadership from federal, state and local authorities are crucial to finally provide the protection and justice long denied to communities affected by this ongoing environmental and public health crisis,” the researchers concluded.
Prather, meanwhile, expressed some relief that in the short term, residents have the possibility of wearing appropriate masks when spending time near the river and can prioritize the purification of their indoor air.
The long-term fix, she said, will center on “cleaning the river and stopping things from getting dumped in the river.”
“It’s got to be accelerated,” Prather added. “Nobody should be breathing this mixture.”
