It’s not just in the water.
A UC San Diego study discovered a new connection between the decades-long Tijuana River sewage crisis and air quality in South Bay.
The study, titled “Heavily polluted Tijuana River drives regional air quality crisis,” was conducted over a three-week period from August to September 2024 and published Thursday in the journal Science.
A team of researchers from UCSD, UC Riverside, San Diego State University and the National Science Foundation Center for Aerosol Impacts on Chemistry of Environment found that hydrogen sulfide levels were dangerously high in inland regions, despite having no direct contact with the polluted ocean itself.
Hydrogen sulfide is the gas responsible for the foul odor associated with sewage, often referred to as a “rotten egg smell.” According to study co-author and assistant professor of environmental health at SDSU, Paula Stigler Granados, the toxic gas is associated with headaches, nausea, and respiratory issues at levels even smaller than the one-hour standard threshold of 30 parts per billion.
However, lead author Dr. Kimberly Prather, an atmospheric chemist at UCSD’s Scripps Institution of Oceanography and the Department of Chemistry, explained that the pollution goes beyond the smell.
“Hydrogen sulfide is a great tracer for sewage, but … there’s all these other industrial sources,” Prather said in a media briefing. “So what other gases are there? Well, we detected over 1000 different other gases present in that air.”
Prather and her team focused on aerosolization of the Tijuana wastewater, or how chemicals and substances in water are dispersed into the air, typically through water movement that creates bubbles or foam.
“I think one of the big punchlines is connecting water quality with air quality, which has not been done in the past,” Prather said. “So, it’s a pretty big discovery.”
When Prather and her team originally began their research, they followed their noses to set up along Imperial Beach, believing that was where the odor originated. However, after analyzing air flow against hydrogen sulfide levels, the team identified a “hotspot” near Nestor and relocated.
One foamy section of the river on Saturn Boulevard is responsible for much of the air contamination and related health issues in Nestor and the surrounding areas.
In the region, the research team measured a peak hydrogen sulfide concentration of 4500 times that of average urban levels. During an hour with the highest concentration levels reached, the average concentration was 2,100 ppb; nearly 70 times the 30 ppb safety threshold.
The study reported that Nestor residents were exposed to these high levels for five to 14 hours a day in early September 2024.
However, in the middle of the study, on Sept. 10, 2024, hydrogen sulfide and other volatile gas levels dropped dramatically. This, the authors concluded, was the result of the activation of a pump station in Mexico, which diverted the water to stay on that side of the border.
“Air pollution [and] water pollution, they don’t stop at the border,” Prather said in the briefing.
“So, when I say it’s diverted, it helps people on the U.S. side of the border, but it didn’t help the people on the Mexico side of the border. So, we need to work together to really deal with this, as it is a binational problem.”
On July 24, the U.S. and Mexico signed a memorandum of understanding to expedite various improvement projects.
The good news, Prather said, is that the researchers were able to pinpoint the wastewater as the cause of the air pollution. The bad news is that, despite the drop in hydrogen sulfide levels, current levels are still toxic.
“[The diversion’s] not really a long-term solution, because it’s kind of kicking the can down the road. It’s just pushing the problem over to someone else,” Prather said. “And, by the way, the air travels right back across the border, so it’s less than it was, but it’s not gone … It was the best hint we have that it was the water that was leading to the air pollution, but now we have to clean up the water.”
The South Bay community is still feeling the effects, too. According to an ongoing SDSU survey, 71% of residents report experiencing upper respiratory health issues, and 99% report being concerned about local air and water pollution.
“Knowing is half the battle with this information, and so understanding that their voices have been heard and that they’re not crazy, that this is actually making them sick, is important for them to communicate with their healthcare providers,” Granados said in the briefing.
As for next steps, the team is still looking to study the 1,000 other gases emitted by the wastewater, as well as the long-term effects of exposure. They are also continuing their sleep study survey, as exposure levels are highest at night when there is less wind.
The Air Improvement Relief Effort Program is also still available, providing free air filters to South Bay residents.
On Aug. 20, city and county officials put up warning signs at various toxic hotspots to warn visitors about the dangerously high hydrogen sulfide levels.
“They say you have to acknowledge you have a problem in order to fix it,” Prather said. “And I think now there’s a broader problem, and it’s a problem that the community knew was there, and it was not being acknowledged.
“Now it cannot be ignored. So now it needs to be fixed.”
