In a new study that tracks air pollution, researchers confirmed a definitive link between the alarming levels of toxic gas in South County and the untreated sewage and wastewater that enters the Tijuana River from Mexico.
Scientists found high concentrations of hydrogen sulfide and hundreds of toxic gases in the air, originating from untreated waters in the Tijuana River, posing a county-wide environmental and public health crisis. The study, which analyzes measurements from last August and September, identified a riverine hotspot on Saturn Boulevard as the primary source of malodors residents have raised concerns about.
The study’s lead investigator, Kimberly Prather, is an atmospheric chemist at UC San Diego’s Scripps Oceanography and the Department of Chemistry. Prather said this is a pretty big discovery, considering the project intended to understand the connection between water and air quality, something that has never been done before.
“This study reveals a direct airborne pollutant exposure pathway — from contaminated rivers into the air we breathe,” said Prather. “For the first time, we’ve shown that poor water quality can profoundly degrade air quality, exposing entire communities to toxic gases and other pollutants.”
The study, published on Aug. 28, is co-authored by Prather and Benjamin Rico of UC San Diego, Kelley C. Barsanti of the National Science Foundation’s National Center for Atmospheric Research, William C. Porter and Karolina Cysneiros de Carvalho of UC Riverside, and Paula Stigler-Granados of San Diego State University.
“Our results validate the community voices that have been saying that air quality near the Tijuana River has been a problem for many years,” said Rico, the study’s lead author who is an atmospheric and analytical chemistry doctoral candidate.
Air concerns
A Centers for Disease Control and Prevention (CDC) health survey conducted last fall found that more than half of 189 households report the sewage crisis has negatively affected their quality of life, worsened health conditions, and increased stress levels about what they agree is a worsening crisis.
Apart from untreated sewage, millions of gallons of untreated industrial waste containing heavy metals and harmful chemicals have for decades entered the Tijuana River from maquiladoras, or manufacturing facilities in Mexico, and eventually enter the Tijuana River Estuary and Pacific Ocean.
A winter season study in 2023 found that 76% of airborne bacteria can be traced to the Tijuana River, said Prather.
Another UCSD study published earlier this year found illicit drugs, drug metabolites, and chemicals from tires and personal care products in water and aerosol samples taken during the winter in 2020 across San Diego shorelines, from Imperial Beach to Coronado and La Jolla.
Prather, who also co-authored that study in May, said the former study focused on the aerosolization of ocean spray and chemicals that people are breathing as a result during the winter season, also known as the rainy season from November through March.
On the contrary, the new study focuses on the toxic gases and aerosol measurements in the same region during the summer, or the river’s dry-season from June through September, when minimal flows are expected.
This year American Rivers, an environmental advocacy organization, named the Tijuana River the No. 2 most endangered river in the nation following its listing as No. 9 in 2024.
“It’s not a river at this point,” said Prather. “Most of this is actually industrial waste and raw sewage that’s making its way through the community. … When it’s concentrated, even though the volumes are less, that sounds better, but to the air, it’s actually worse. We see far more pollution in the air under these concentrated conditions.”
According to the study, humans inhale about 11,000 liters of air daily, which is significantly more than the 2 liters of (usually filtered) water people drink.
This points to inhalation as the primary exposure pathway for many water-derived pollutants.
“This study offers a real-world example of how pollutant exposure can extend well beyond the banks of a contaminated river or waterway,” says the study. “The health impacts of pollutants and pathogens transferred from heavily contaminated waters into the atmosphere remain a substantial and largely unaddressed gap in our understanding.”
A ‘hotspot’ identified on Saturn Boulevard
Data from the San Diego County Air Pollution Control District shows more than 200 complaints were submitted daily by the community last summer.
“A sharp increase in community odor complaints indicates that the riverine hotspot — characterized by high dry-season flows (low dilution, untreated sewage) and elevated turbulence — was active by July 2024,” the study says.
This is why Prather and her team decided to go down and take a look at what was in the air, she added.
“Odors are linked to gases. And the gas that we focused on initially was hydrogen sulfide gas, which is that rotten egg smell that you get when you’re near sewage,” said Prather.
The study says researchers focused on hydrogen sulfide (H2S) — a toxic gas and key sewage tracer — and other hazardous gases like volatile organic compounds (VOCs).
The U.S. Environmental Protection Agency says VOCs are human-made chemicals that are emitted as gases from certain solids or liquids and may have short- and long-term adverse health effects.
The researchers collected samples using monitoring stations across five locations including: Border Field State Park, the community of Nestor in the City of San Diego, a Tijuana River site at Saturn Boulevard, the Tijuana River Estuary, and a coastal site at Imperial Beach.
The first two air sampling sites were hydrogen sulfide sampling locations, while the remaining three sites sampled volatile organic compounds.
The scientists set out to measure hydrogen sulfide beginning at Border Field State Park, near the U.S.-Mexico border, but input from South Bay residents informed researchers about a “riverine hotspot” on Saturn Boulevard.
The study describes the hotspot as the only visibly turbulent portion of the Tijuana River on the United States side of the border. This information led to the Nestor sampling site.
Researchers found that hydrogen sulfide levels at Border Field remained below the state’s exposure level limit of 30 parts per billion — California’s one-hour ambient air quality standard of 30 parts per billion — throughout a 24-hour period beginning at 6 p.m. on Aug. 24. Meanwhile, hydrogen sulfide levels exceeded the state threshold from 6 p.m. to 12 p.m. on Sept. 7-8 in Nestor.
The study says high wastewater flows and low winds led to nighttime hydrogen sulfide peaks, reaching 4,500 parts per billion in summer 2024.
“This level is too high for chronic exposures as 30 parts per billion is already associated with headaches, nausea, respiratory symptoms and other adverse health effects, particularly among vulnerable populations,” said Stigler-Granados, associate professor and chair of Environmental Health in SDSU’s School of Public Health. “Framing this as merely an odor issue dangerously understates the real public health risks of repeated exposure to toxic gases at such concentrations.”
The study detected hundreds of other gases in addition to hydrogen sulfide, but scientists say work is needed to measure the concentrations of these other gases to determine if they exceeded these exposure limits.
“We show here that while hydrogen sulfide is an excellent marker of the sewage impacting area residents, there are multiple sources of waste entering the Tijuana River and a multitude of other hazardous gases that area residents are potentially inhaling,” said Barsanti, who led the analysis of additional gases detected at the site.
Proximity to the hotspot led to much higher hydrogen sulfide levels, particularly due to low-wind, stagnant-air nighttime conditions, says the study, “further confirming the Saturn Boulevard riverine hotspot as the primary source region driving local emissions of (hydrogen sulfide) and other airborne pollutants.”
Sept. 10
The Tijuana River saw unprecedented high river flows throughout the dry-season in 2023 and 2024.
The study says the river had 40 to 80 million gallons per day in 2024 due to an infrastructure failure at PBCILA.
On Sept. 10, 2024, scientists say the river was diverted, which means the flows were kept on the Mexico side with wastewater being diverted to the San Antonio de los Buenos Wastewater Treatment Plant and released into the Pacific Ocean at Punta Bandera.
After this, the river’s flow was less than 5 million gallons per day, reducing concentrations of hydrogen sulfide by 95%, but levels still peaked at night.
“People hike here, … they horseback ride here. They know it smells bad, but they weren’t aware that if they start feeling certain symptoms, they should get help,” said Prather, noting new signs from the county that inform the public about hydrogen sulfide in the area. “People are being made more aware of this issue and it’s finally being acknowledged and addressed. So this is a really positive step forward.”
Binational environmental leaders recently signed a memorandum of understanding on behalf of the U.S. and Mexico.
Among over one dozen actions to be included in a new minute by the end of this year is to “develop a Tijuana water infrastructure master plan to ensure that sufficient water infrastructure is planned and constructed commensurate with anticipated population growth.”
The study says communities on both sides of the border will continue to bear the health burden of this preventable crisis without urgent interventions.
Prather said air quality measurements are ongoing, but funding limitations could prevent future work.
Researchers are now conducting a research study in San Diego County to better understand how the air quality is impacting people’s sleep in the South Bay area. Interested participants can learn about the survey at: https://bit.ly/yoursleepucsd.
