Toxic Waters of the Tijuana River Contaminate Surrounding Air Quality

For decades, the Tijuana River has carried millions of gallons of untreated sewage and industrial waste across the U.S.-Mexico border. The river passes through San Diego’s South Bay region before emptying into the ocean, recently leading to more than 1,300 consecutive days of beach closures and water quality concerns. Residents of South Bay communities have long voiced concerns about the foul smells emanating from the river, reporting health issues including eye, nose and throat irritation, respiratory issues, fatigue and headaches. 

Now, newly published research from scientists at UC San Diego’s Scripps Institution of Oceanography, UC Riverside, San Diego State University (SDSU) and the National Science Foundation (NSF) National Center for Atmospheric Research (NCAR) reveals that the polluted Tijuana River releases large quantities of the toxic gas hydrogen sulfide — commonly known as “sewer gas” because of its association with sewage and rotten egg smell. 

Scientists measured peak concentrations of hydrogen sulfide that were some 4,500 times what is typical for an urban area. In addition, the paper identifies hundreds of other gases released into the air by the polluted Tijuana River and its ocean outflow, which can contribute to poor air quality across the region.

The study, published Aug. 28 in the peer-reviewed journal Science and supported by NSF, the National Oceanic and Atmospheric Administration (NOAA) and the Balvi Philanthropic Foundation, links the river’s poor water quality with reduced local air quality. 

“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 Benjamin Rico, an atmospheric and analytical chemistry PhD candidate at UC San Diego and lead author of the study. 

The symptoms reported by residents of South Bay communities mirror those associated with exposure to hydrogen sulfide. The health effects of long-term exposure to hydrogen sulfide are not fully understood, but the California Office of Environmental Health Hazard Assessment sets a chronic exposure limit of 7.3 parts per billion (ppb). In contrast, the California Air Resources Board’s one-hour standard is 30 ppb, a level based on odor, not health.

“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 Paula Stigler Granados, an environmental health scientist at 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.” 

To understand whether pollution from the Tijuana River was impacting air quality, the study authors set up carefully calibrated air quality instruments in the Nestor community in San Diego’s South Bay starting in September 2024. Researchers selected the specific location within Nestor in consultation with community members who identified a foamy, turbulent section of the river near Saturn Boulevard as a source of particularly strong odors.

The team measured the concentrations of various air pollutants for roughly three weeks and combined these measurements with river flow data and atmospheric modeling to track how far the air pollutants spread through nearby communities.

During the study, the air quality instruments measured concentrations of hydrogen sulfide that peaked at 4,500 ppb for at least a minute and up to an average of 2,100 ppb for one hour — the latter exceeding the California Air Resources Board’s one hour standard by nearly 70 times. The highest levels of hydrogen sulfide occurred at night, when winds usually die down. 

From September 1-10, 2024, residents near the study’s air quality measurements in Nestor — close to Berry Elementary School — were exposed to levels of hydrogen sulfide that exceeded the California Air Resources Board’s one-hour average air quality standard for five to 14 hours each day. In addition to hydrogen sulfide, the team detected hundreds of other gases, some of which had established exposure limits due to their health impacts. More 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 Kelley Barsanti, an atmospheric chemist at NSF NCAR who led the analysis of additional gases detected at the site.  

On September 10, 2024, the Tijuana River’s flow rate dropped dramatically — from 40-80 million gallons per day to less than 5 million gallons per day — reducing concentrations of hydrogen sulfide and many other gases for the remainder of the study’s duration. Though authorities have not announced any official changes in the river’s management, the study authors surmised that “on September 10, a pump station in Mexico was activated.” Activating this pump diverted the wastewater flow, keeping it on the Mexican side of the border. 

Incidentally, the rapid decline in hydrogen sulfide concentrations following this diversion helped establish the river as the air pollution’s source. Atmospheric modeling showing hydrogen sulfide’s reach into nearby communities also found that the measured pollution patterns could only be reproduced by making the river the emission source and factoring in its flow rates. Finally, the number of complaints about odor from South Bay communities spiked on days when the highest concentrations of hydrogen sulfide were measured. These multiple lines of evidence firmly establish the contaminated river as the source of the toxic gases and the foul odors residents had reported for years.

 “This study reveals a direct airborne pollutant exposure pathway — from contaminated rivers into the air we breathe,” said Kimberly Prather, the lead investigator of the study and an atmospheric chemist at  UC San Diego’s Scripps Oceanography and the Department of Chemistry. “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. These findings validate the experiences of residents who have endured this crisis for decades, and also underscore the urgent need for action to protect public health in San Diego and in vulnerable communities worldwide.”  

As a temporary but immediate solution, the researchers recommend continuing, expanding and increasing education about a program by San Diego County that provides free air purifiers to help residents breathe cleaner air at night while they sleep. Beyond air purifiers, the San Diego Air Pollution Control District (SDAPCD) has created an online air quality dashboard showing current levels of hydrogen sulfide to help residents limit their exposure when concentrations are high. The SDAPCD also sends out alerts to all residents when hydrogen sulfide levels go above 30 ppb. Additionally, SDSU researchers are continuing with their community survey which measures related health impacts. 

In the long run, the authors hope that putting numbers to an issue that residents of San Diego’s South Bay have raised for decades will help motivate government officials to address the polluted water at the root of the air quality problem. Such a solution would entail upgrading water treatment infrastructure on both sides of the border and updating policies governing the river’s management.

The study also calls for updating air quality models globally to account for emissions from polluted waterways, as more than half the world’s population lives near rivers, lakes and oceans that could face similar issues.

“Our results show how water and air mix and exchange with one another,” said Prather. “Air, water and soil can influence one another and people can be exposed through different pathways. We need to take this into account if we are going to protect people from increasing levels of pollution.”

Prather noted that federal funding was vital to connecting fundamental basic science to real-world impacts for the community. Prather’s aerosol research has been supported by NSF through the Center for Aerosol Impacts on Chemistry of the Environment. Advances in aerosol research in the lab enabled this research effort in the community. UC San Diego received federal community project funding from NOAA, secured by U.S. Representative Scott Peters (CA-50), to further investigate the conditions that lead to aerosolization of pollutants and pathogens, and how far they travel, to understand potential public health ramifications.

“For decades, our region has endured the dangerous public health effects from untreated sewage and industrial waste in the Tijuana River,” said U.S. Representative Scott Peters (CA-50). “This peer-reviewed study links water pollution to worse air quality with clear and convincing data. I secured federal funding for this study to ensure Scripps Oceanography and its partners can better understand how these toxic pollutants affect the air we breathe. I’ll keep working toward binational infrastructure solutions which strengthen public health and coastal resilience.”

In addition to Rico and Prather of UC San Diego, the study was co-authored by Barsanti of NSF NCAR, William Porter and Karolina Cysneiros de Carvalho of UC Riverside and Stigler-Granados of SDSU.