Polluted Tijuana River Causing Significant Air Quality Concerns on Both Sides of Border: New Study Demonstrates That Toxic Gases in the Water are Transferred to the Air in Turbulent Sections of River

      Air pollution from the heavily polluted Tijuana River is affecting both sides of the border area. The river passes through San Diego's South Bay Region before reaching the ocean. South Bay area residents have long complained about foul smells, and the water quality measured in the river has been very poor. The main causes of water and air pollution are untreated raw sewage and industrial waste. Highly elevated levels of hydrogen sulfide (H2S) have been found, likely due to the sewage. Peak concentrations of H2S that were some 4,500 times higher than what is considered typical for an urban area, along with the presence of hundreds of other gases, were found.

     The study, published in August in the journal Science, analyzed the results of air quality monitoring in the area. 

     Alex Fox of UC San Diego Today notes:

“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.”

“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.”

     The abstract below explains some of the results and implications, including the implication that water pollution can be transferred to the air and become air pollution in turbulent sections of rivers.

     The authors note that:

“…many processes can transfer water pollutants to the air, including bursting bubbles in waterfalls, turbulence in rivers, aeration in wastewater treatment plants, toilet flushing, and breaking waves in coastal surf zones, which form aerosols in a process known as aerosolization.”

     The researchers identified a turbulent section of the river as an air pollution hotspot. This is where there are the most malodor complaints.

“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.”

     After the water was diverted, the H2S concentrations dropped, definitely tagging the river as the source of the airborne H2S and other gases. They also found that the number of complaints about maladies such as headaches, as well as complaints about odors, spiked on days when the H2S levels were highest. The graph below shows hourly and daily changes in H2S concentrations.

     The graph below shows changes (lower levels) in gaseous oxygenated hydrocarbons after the water was diverted.

     San Diego County has a program that provides free air purifiers for residents to use at night when H2S levels tend to be highest and when the wind speed is lowest. They also developed an online dashboard and an alert system that alerts residents when H2S levels exceed 30 parts per billion (ppb).

     This issue is also considered to be an environmental justice issue where disenfranchised and marginal communities tend to be most affected. 

    

References:

 

Researchers issue warning about toxic gases crossing US border: 'Hazardous'. Tina Deines. The Cool Down. September 16, 2025. Researchers issue warning about toxic gases crossing US border: 'Hazardous'

Tijuana River’s Toxic Water Pollutes the Air: New peer-reviewed study shows hydrogen sulfide levels exceeded California air quality standards. UC San Diego Today. Alex Fox. August 28, 2025. Tijuana River’s Toxic Water Pollutes the Air

Heavily polluted Tijuana River drives regional air quality crisis. Benjamin Rico, Kelley C. Barsanti, William C. Porter, Karolina Cysneiros de Carvalho, Paula Stigler-Granados, and Kimberly A. Prather. Science. Vol 389, Issue 6763. August 28, 2025. Heavily polluted Tijuana River drives regional air quality crisis | Science