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Hours after a November storm, the Tijuana River flooded a grove of trees in Imperial Beach, gushed through a row of culverts and exploded into mounds of fetid foam.
This is ground zero for the contaminated river, which sickens thousands of people in southern San Diego County.
“The Tijuana River is one of, if not the most polluted, river in the entire United States,” said San Diego County Supervisor Paloma Aguirre, who viewed the overflowing river wearing black rain boots and a hot pink respirator mask. “The river is carrying dangerous chemicals, pollutants, pathogens and toxic gases that are impacting South San Diego communities.”
The site, known as the Saturn Boulevard hot spot, is part of a system of polluted waterways and failed sewage treatment plants in the cross-border region. In the ocean, the contamination leaves swimmers and surfers with breathing problems, digestive illness and rashes. Unsafe conditions have closed parts of the Imperial Beach shoreline for three years.
Last year, researchers discovered that the pollution is airborne as well. Foul-smelling hydrogen sulfide emissions near the river sometimes rise hundreds of times higher than the state’s odor threshold. At those levels the gas triggers headaches, nausea, eye irritation and respiratory distress. And there are other chemicals, viruses and bacteria in the mix.
For children, the effects are worse, said Tom Csanadi, an Imperial Beach physician who has been active in the issue. Their lung surface area to body size is higher, which means they absorb more toxins. Children breathe faster than adults and they’re still growing, so it can affect their body tissues more severely. There are 11 schools within three kilometers of the hot spot.
“It could lower IQ, stunt cognitive development,” Csanadi said.
As a surfer, activist and elected leader, Aguirre has spent two decades tackling this problem, which she considers one of the worst environmental crises in the country.
“She’s been at the forefront of the advocacy side of this for a long, long time, before her political career even started,” said Falk Feddersen, an oceanographer with Scripps Institution of Oceanography who has mapped sewage flows up the coast from Mexico.
A cocktail of chemicals
While storm water seeped across the road at the hot spot, a swiftwater rescue truck drove through puddles, scanning for stranded motorists. The culverts under the crossing were installed to keep flooding under control, but they also churn the water, spewing noxious gas and other pollutants.
“The unintended consequence is that it's exacerbating the release of all the molecules and aerosols into the air,” Aguirre said. “It's literally rocketing them into the environment.”
Hydrogen sulfide, with its distinctive rotten egg odor, is an indicator of that toxic brew, said Kim Prather, an atmospheric chemist at Scripps Institution of Oceanography. She raised the alarm about airborne pollution from the Tijuana River last year.
Flooding caused by the Tijuana River covers a section of Saturn Boulevard after a rainy day in San Diego on Nov. 21, 2025. Photo by Adriana Heldiz, CalMatters
Layers of foam caused by sewage and chemicals bubble up along a section of the Tijuana River after a rainy day in San Diego on Nov. 21, 2025. Photos by Adriana Heldiz, CalMatters
“That’s one in a cocktail of thousands of compounds,” she said. “It’s a blessing that it smells. I know it sounds strange, but it tells you to get away.”
Aguirre described her own struggles with Tijuana River pollution, including migraines, chest pain, shortness of breath, and waking in the middle of the night to an odor she likened to a “porta potty.”
Recent improvements to wastewater treatment plants in the U.S. and Mexico have reduced water pollution by keeping tens of millions of gallons of sewage out of the ocean each day. Aguirre and others celebrate that news, but note the river still contaminates surrounding areas.
More big upgrades are in the works on both sides of the border, but fixing the Saturn Boulevard hot spot quickly could offer immediate relief, Aguirre said.
“This is a very specific and low hanging fruit that will at least begin to mitigate the amount of gases being released into the air and benefit tens of thousands of people that live here,” she said.
Waves of pollution
Tijuana River pollution dates back to at least the 1930s, when the U.S. and Mexican governments built the first cross-border sewage plants. As Tijuana’s population soared with its booming industry, the city’s waste outstripped its treatment systems. Plant failures and sewage spills became common in the early 2000s, along with frequent beach closures along the south San Diego coast.
That’s when Aguirre encountered cross-border pollution in the surf at Imperial Beach. Growing up in Puerto Vallarta Mexico, she was used to surfing in muddy water after rains, so the discolored waves didn’t seem worrisome.
“I remember going out here in Imperial Beach while the water was chocolate brown, not knowing that it's nothing like what I was used to, because that was sewage,” she said.
She was the only one at the beach that day, except for a man posting signs stating “Clean water now.” He was Serge Dedina, executive director of the environmental group WildCoast, and he enlisted her in the fight against sewage pollution.
Aguirre first volunteered for the organization and soon joined its staff. She worked there for more than a decade, while earning a master's degree in marine biodiversity and conservation at Scripps Institution of Oceanography. At WildCoast she organized a citizens’ group, advocated for improved water testing using DNA analysis, and served on working groups for a binational agreement on cross-border pollution, called Minute 320.
When Dedina was elected mayor of Imperial Beach in 2014, Aguirre saw a path to solving the sewage problem.
“I thought, well, if he can do it I can do it,” she said. “And I built on the momentum that he was able to create on this issue.”
First: San Diego County Supervisor Paloma Aguirre wears a respiratory filter mask while standing near a section of the Tijuana River in San Diego on Nov. 21, 2025. Last: A warning sign about sewage and chemical contamination is posted along the shore of Imperial Beach on Nov. 21, 2025. Photos by Adriana Heldiz, CalMatters Aguirre won a seat on the Imperial Beach City Council in 2018 and was elected mayor in 2022, when Dedina left office. With a bigger platform, she called on California and the federal government to declare a state of emergency over the border pollution problem and lobbied to classify the area as a Superfund site.
Those efforts haven’t gained traction, but other angles yielded results. Imperial Beach sued the International Boundary and Water Commission with the city of Chula Vista and Port of San Diego in 2018, alleging that it violated the Clean Water Act and other federal laws by failing to control coastal sewage pollution. They settled the lawsuit in 2023 with a promise of more resources and binational cooperation.
“My tenure as mayor of IB really focused on advocating and working in a bipartisan fashion to secure the additional funding that was needed,” to fix cross-border pollution, she said.
A person walks their dog near the Imperial Beach Pier in Imperial Beach on Nov. 21, 2025. Photo by Adriana Heldiz, CalMatters Aguirre led delegations of local officials to Washington, D.C. to drum up money for costly infrastructure upgrades needed to get the sewage problem under control. She met with White House officials in both the Biden and Trump administrations, and with lawmakers who had served as Navy SEALS and had experienced the pollution problem at BUD/S, the Navy SEAL training program in Coronado.
In July, Aguirre won a special election for an open San Diego County Board of Supervisors seat. She immediately led county plans to study the health effects of cross-border pollution and asked the state for $50 million to fix the Saturn Boulevard hot spot.
“She’s moved a problem that has been stuck, when other people could not,” Prather, the Scripps atmospheric chemist, said.
Sewage spills prompt quick fixes
The long-standing pollution problem came under new scrutiny in 2017, when a spill from a damaged line in Mexico dumped an estimated 143 million gallons of wastewater into the Tijuana River, sending foul odors wafting through the region. That accident revealed just how dilapidated the aging infrastructure had become.
“That’s one of the reasons why things are so horrific, because they're playing catch up on fixing these things when they have catastrophic failures,” said Feddersen, the Scripps oceanographer.
In early 2022, another major spill released hundreds of millions of gallons of sewage-tainted water across the border for two and a half weeks.
That summer, San Diego congress members freed up more than $300 million that had been authorized for wastewater treatment upgrades through the United States-Mexico-Canada Agreement. Mexico committed $144 million to replace failing sewage treatment facilities in Tijuana, with an updated treaty between the two countries known as Minute 328.
In 2024, the lawmakers persuaded the Biden administration to add another $370 million to repair the aging South Bay International Wastewater Treatment Plant near the border, Rep. Scott Peters said.
After decades of deterioration, major improvements came online this year. The South Bay International Wastewater Treatment Plant, which was barely operable, is now fully functioning and expanded its capacity from 25 million to 35 million gallons of wastewater per day. The project was expected to take two years, but was completed in 100 days, according to the U.S. International Boundary and Water Commission.
By the end of next year that will climb to 50 million gallons per day, with higher capacity for peak wastewater surges. The commission, which manages the wastewater systems, has spent $122 million on the first series fixes, and the full project will cost $650 million.
Although the Trump administration has clawed back federal funding for many projects, it has doubled down on the cross-border sewage problem, Aguirre said. In July U.S. Environmental Protection Agency Administrator Lee Zeldin met with his Mexican counterpart to seal the environmental deal.
In April Mexico repaired its Punta Bandera plant, located on the coast about six miles south of the border. The plant had failed completely in 2020 and was dumping raw sewage into the ocean. It now handles 18 million gallons of wastewater per day. That’s a big boost for beach safety, said Feddersen, whose research tracked the flow of sewage in ocean currents and modeled scenarios for reducing it.
“The best bang for the buck, the greatest reduction in beach closure and reduction in human illness, was fixing Punta Bandera,” he said.
Yet, the Tijuana River still threatens residents in its watershed with untreated sewage and industrial chemicals from maquiladoras in Tijuana. That includes solvents, heavy metals and toxins known as PFAS, or “forever chemicals,” Prather said.
“The river right now is a wastewater treatment plant without any processing,” she said.
Removing the culverts would eliminate the turbulence that sprays out hydrogen sulfide and other toxins. The county plans to finish a feasibility study on the project by January. That project would keep contaminants out of the air, but not out of the water.
Aguirre also wants new infrastructure to clean up the Tijuana River on the U.S. side. The recent binational Treaty, Minute 328, includes that option, and the International Boundary and Water Commission is exploring what it would take to divert and treat the river flows. There’s no funding for the project yet, but Aguirre says it’s on her agenda.
“Rivers are diverted up and down,” she said. “It's doable. Is it expensive? Yes. Are our lives in South San Diego worth it? Yes.”
This article was originally published on CalMatters and was republished under the Creative Commons Attribution-NonCommercial-NoDerivatives license.
