A new study published in Science has drawn a direct link between sewage pollution in the Tijuana River and hazardous air quality in nearby San Diego communities.
Researchers found hydrogen sulfide from the river transferring into the air at concentrations up to 4,500 times higher than typical urban levels. The toxic, colorless gas – which smells like rotten eggs and forms when organic matter in sewage breaks down – has long been suspected as the root of health complaints among South Bay residents, including respiratory problems, fatigue, headaches, and irritation of the eyes, nose, and throat.
The research was conducted by scientists from UC San Diego’s Scripps Institution of Oceanography, UC Riverside, San Diego State University, and the National Center for Atmospheric Research.
“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,” said lead investigator Kimberly Prather. “These findings validate the experiences of residents who have endured this crisis for decades, and underscore the urgent need for action to protect public health in San Diego and in vulnerable communities worldwide.”
The Tijuana River begins in Mexico and crosses into the U.S. before reaching the ocean south of Imperial Beach. In September 2024, air quality instruments in San Diego’s South Bay measured emissions over a three-week period. Saturn Boulevard stood out as a particularly foul-smelling stretch of the river, where turbulent conditions promote the transfer of pollutants into the air. Researchers combined these measurements with river flow records and atmospheric modeling to track how contaminants spread.
Readings showed hydrogen sulfide peaking at 4,500 parts per billion (ppb), with one-hour averages as high as 2,100 ppb. By comparison, most urban areas register less than 1 ppb, and California’s one-hour air quality standard is 30 ppb.
On September 10, 2024, river flow abruptly dropped from between 40 and 80 million gallons per day to less than five million, leading to a sharp decline in airborne toxins. Researchers believe the reduction was caused by a Mexican pump station diverting wastewater, likely in response to public outcry after levels grew so high that carbon monoxide alarms sounded in homes and student field researchers were pulled for safety reasons. Researchers say the measured decline of air pollutants further confirmed the river as the source.
“Our results show how water and air mix and exchange with one another,” Prather said. “Air, water, and soil can all influence each other, and people can be exposed through multiple pathways. We need to account for this if we are going to protect people from rising pollution.”
The researchers hope their findings push officials to improve water treatment on both sides of the border and revisit management policies of the Tijuana River. More broadly, they recommend updating air quality models around the globe to account for how polluted waterways can harm air quality.
