New data on the conditions of the sewage-laced Tijuana River confirms that polluted water substantially affects air quality. Researchers said the findings strongly validate long-dismissed concerns residents have raised.
“This study reveals a direct airborne pollutant exposure pathway — from contaminated rivers into the air we breathe,” said Kim Prather, one of the study’s authors and a professor of Atmospheric Chemistry at UCSD’s Scripps Institution of Oceanography. “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."
Scientists from UC Riverside, San Diego State University and the National Science Foundation National Center for Atmospheric Research also contributed to the study published Thursday in the peer-reviewed journal, Science. They focused on measuring hydrogen sulfide, a toxic gas that has a distinct rotten egg-like odor. It is a byproduct of the millions of gallons of untreated wastewater from Tijuana that reaches Coronado shorelines via the river.
The river routinely carries flows with untreated sewage and industrial waste during the dry season. Pollution has worsened in recent years due to growth in Tijuana’s population, heavy rain events, sediment, and years of underinvestment in wastewater infrastructure on both sides of the U.S.-Mexico border.
Data revealed that when wastewater flows increased in the river and winds were low, gas levels spiked, and reports of odors from people living in and around the Tijuana River Valley also jumped. And when flows dropped, so did the concentrations of the gas and the number of odor reports.
The paper highlights data from September 2024, when researchers recorded a spike in hydrogen sulfide levels at a river crossing on Saturn Boulevard near Sunset Avenue.
On the night of Sept. 8, when about 55 million gallons flowed through the river, gas levels far exceeded the state’s 1-hour odor control threshold of 30 ppb for several hours. Researchers, advocates, and local doctors have raised concerns about the potential long-term health effects of exposure to low levels of hydrogen sulfide on nearby residents.
On Sept. 10, wastewater was diverted from the river, which significantly dropped flows to 8 million gallons. What followed on Sept. 11 was a steep drop in hydrogen sulfide levels.
That correlation between poor water and air quality, Prather said, validates what residents have said for years: exposure to the pollution is not just a nuisance, it’s causing their headaches, nausea, coughs, fatigue, and other health issues.
“The community was acting as the (air quality) sensors, but unfortunately, they were not being heard or … they felt (they were) being dismissed,” she added. “What they were saying was directly linked to the measurements of at least this one gas.”
Hydrogen sulfide is not the only gas released into the air by the river. The study identifies hundreds of others linked to industrial waste. In May, Scripps published a separate study that found that chemicals from tires, illicit drugs and personal care products in the Tijuana River become airborne.
Researchers have found that these pollutants are higher in the Tijuana River than in the ocean. Researchers said the river crossing on Saturn Boulevard, located near homes and schools, is the largest source of these toxic gases and chemicals, which is why the area is considered a “hotspot.” A culvert at the site creates turbulence and foam that is even visible in satellite imagery.
Last week, the county and city of San Diego placed “warning” signs at the hotspot to inform the public about emissions found there, and that some people exposed to them have experienced headaches, nausea or respiratory issues.
One short-term solution the paper calls for is the elimination of the turbulent drop at the hotspot.
On Wednesday, County Supervisor Paloma Aguirre, whose District 1 includes the communities in and around the river valley, urged the San Diego Regional Water Quality Control Board to collaborate with the county on expediting a maintenance project to address the turbulence.
“This project will reduce turbulence, restore natural flow, and directly lessen airborne hazards that are making our children, seniors, and visitors sick,” she said in a statement. “The Water Board should work hand-in-hand with the County to cut red tape and deliver these protections as quickly as possible.”
Meanwhile, the U.S. and Mexico are working on upgrading treatment infrastructure on both sides of the border, including expediting a partial expansion of a treatment plant in the river valley.
Prather said she hopes the findings will further show that there’s a “broader problem” that “cannot be ignored. So, now it needs to be fixed.”
People living and working near the pollution are encouraged to inform their healthcare providers about their location and inquire about long-term care if they experience symptoms, said Paula Stigler Granados, an Environmental Health professor at San Diego State University and co-author of the study.
Researchers added that they will continue to monitor water and air quality, as well as study the potential long-term impacts of pollution exposure.
