San Diego County faces multifront health threat amid transboundary sewage flow from Mexico

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A transboundary sewage stream that regularly flows from Tijuana, Mexico, into San Diego County may be creating a multifrontal public health crisis — as a noxious mix of chemicals and pathogens makes their way into households not just via water, but also through air and soil.  

The cross-border contamination — a result of inadequate infrastructure and urbanization — poses a persistent public health threat with significant socioeconomic and legal implications, according to a new white paper, shared with The Hill prior to its public release on Tuesday.  

Of particular concern is the possibility of the reemergence of diseases that had previously been eradicated in California, microbes carrying antibiotic-resistant genes and industrial chemicals that have long been banned in the U.S., per the authors.  

“I don’t go to Imperial Beach anymore — I used to go all the time, quite frequently,” lead author Paula Stigler Granados, an associate professor at San Diego State University’s School of Public Health, told The Hill.  

“As a scientist, when you know, it’s really hard to turn that brain off,” she continued. “Especially now that we’re talking about the contaminants becoming aerosolized, it really has me paused.” 

Aerosolization refers to the suspension of waterborne pathogens and compounds in the air — a mounting concern in Southern California’s Imperial Beach, a border-adjacent city that has borne the brunt of an unrelenting transboundary sewage crisis. 

The fetid flow, which results from insufficient sewage treatment on the Mexican side of the border, ends up in San Diego County both via ocean plumes and the Tijuana River Watershed — which passes through Baja California before reentering its U.S. counterpart.   

Area residents have had some room for hope in recent weeks, after Mexico began overhauling an obsolete facility that releases millions of gallons of sewage daily into the Pacific Ocean.   

But on the U.S. side of the border, the South Bay Wastewater Treatment Plant — which treats some of Tijuana’s waste through an international treaty — is also failing to pull its weight.  

Congress in 2020 allocated $300 million toward renovating the site, but officials warned that the plant requires $150 million more to function properly. President Biden then asked lawmakers this past fall to authorize an additional $310 million, but that approval has yet to occur. 

Rep. Scott Peters (D-Calif.), who serves San Diego County, last month called upon Congress to approve Biden’s request, describing a situation in which sewage is spewing “dangerous levels of hydrogen sulfide into the air around people’s homes.” 

The congressman also requested that the San Diego-based Prebys Foundation commission the new white paper, which synthesizes about 60 reports related to the region’s wastewater woes and sheds lights on the extent of the public health crisis.  

“What we realized was over the years, a lot of people had been doing research on this, but the body of it was fairly opaque,” Peters told The Hill.  

The congressman said his office first became involved in this issue following the 2017 breakdown of Tijuana’s sewage infrastructure, which prompted a swell of comments on the matter from his constituents.  

“We started out just getting beach closures every once a while, but it was hundreds and hundreds of days a year,” Peters said.  

While border communities like Imperial Beach face the most chronic effects of the crisis, other parts of San Diego County — including the peninsular resort city of Coronado — also endure regular episodes of contamination. 

Peters acknowledged that as opposed to Imperial Beach, most of his district, which includes Coronado, cannot be defined as an environmental justice community — a largely low-income or marginalized population with a disproportionate pollution burden = 

But he stressed that Coronado’s beaches do serve the Latino residents of the South Bay, while also hosting Navy SEAL, U.S. Border Patrol and U.S. Coast Guard operations.   

To that end, Peters and fellow San Diego County Democrats — Reps. Sara Jacobs, Mike Levin and Juan Vargas, the latter of whom represents Imperial Beach — recently sent a letter to Navy leadership about how the pollution is affecting SEAL training.  

The writers expressed their concern that if the contamination is not curtailed, further training cancellations could occur and “harm the Navy and our military readiness.”  

Since 2018, the International Boundary and Water Commission has documented more than 100 billion gallons of wastewater entering the U.S. through the Tijuana River, according to the authors of the white paper.  

“It’s just a toxic soup,” Stigler Granados said.  

The continuous stream of sewage, the authors stressed, has led to more than 700 consecutive days of beach closures and taken a toll on the local economy and tourism.  

The contamination not only poses public health risks but also creates environmental justice issues, as border-adjacent communities are often equipped with fewer resources and face a heightened risk of chronic diseases, the authors noted. 

Most under threat are vulnerable populations, including pregnant women and children, as well as lifeguards Navy personnel, first responders and border patrol agents, according to the paper.  

Within the region’s soil sediments, scientists have identified more than 170 compounds — such as toxic polycyclic aromatic hydrocarbons (PAHs), prohibited pesticides like chlordane and DDT, polychlorinated biphenyls (PCBs), heavy metals and phthalates. 

“The pesticides that we’re finding in the environment and soil — they are banned in the U.S.,” Sigler Granados said.  

Many of these chemicals “are known to be persistent, bio-accumulative, carcinogenic, toxic and can be resuspended in water and air during weather events in both the wet and dry seasons, exposing nearby communities,” according to the white paper.  

Levels of arsenic and cadmium in area soil samples exceeded Environmental Protection Agency (EPA) thresholds, while concentrations of bisphenol A (BPA) and triclosan were comparable to those in sewage sludge, per the paper.   

As far as the region’s water is concerned, the white paper catalogs a smorgasbord of viruses, bacteria and parasites found in samples, including HIV, Hepatitis B and C, salmonella, vibrio, streptococcus, tuberculosis, listeria, trichomoniasis. 

Also of concern to the researchers was the heightened presence of microbes carrying antibiotic-resistant genes, as well as antibiotic-resistant strains of E. coli and Legionella bacteria.  

In addition, they flagged an emergence of zoonotic pathogens — those that can jump from animals to humans — in bottle nosed dolphins that have died of bacteria-induced sepsis.  

Water tests have also confirmed the presence of pesticides, herbicides, volatile organic compounds, acetone, methanol, xylene, plasticizers, hormones and flame retardants. Out of 392 total organic chemical contaminants identified, 224 appeared on regulatory lists, while 175 were indexed under the EPA’s Toxic Substance Control Act. 

“A substantial number of contaminants of emerging concern were detected in the water for the first time,” the authors stated.  

The researchers stressed that pollutants and pathogens can become airborne through aerosolization and travel long distances — entering homes, schools and businesses and reaching those who have had no direct contact with the water.  

Reiterating the risks posed by this exposure pathway, the writers cited a March 2023 study, in which a University of California San Diego team showed that polluted coastal waters are ending up in the atmosphere as aerosol. While the public health threat is difficult to quantify, the researchers found that the “sea spray” mix contains bacteria, viruses and chemical compounds. 

“You can respire them and breathe them in and become ill as a result of that,” Stigler Granados said, noting that these pathogens can also settle on objects like playground equipment.  

Although linking environmental exposures to specific illnesses remains a challenge, Stigler Granados said that an urgent care clinic in San Diego County has been reporting upticks in gastrointestinal illnesses following storm events.  

But because many of these diseases are self-limiting, county-level epidemiological surveillance would be needed to explore any potential correlations, she added.  

Going forward, Stigler Granados and her colleagues called for increased air quality monitoring, community exposure investigations and U.S.-Mexico strategic plans that prioritize infrastructure.  

“Investments by Congress and federal and state agencies are desperately needed,” the authors concluded.  

The white paper’s conclusions may have a local, San Diego region focus, but they are indicative of a broader national problem, according to Eli Dueker, a microbe aerosolization expert who was not involved in the research. 

“This is actually happening across the United States and has been a very long time,” Dueker, an associate professor of environmental and urban studies at Bard College, told The Hill. 

His research focuses on the connections between water and air quality in New York’s Hudson River, as well as hazard-designated sites like the state’s Newtown Creek and the Gowanus Canal.  

“It is a new way to think about water quality,” said Dueker. “Whether or not you engage with the water, if you’re also breathing the water, that’s a whole different ballgame.”  

Dueker credited the white paper’s authors for “laying out the kinds of things that people can be exposed to,” while noting that antibiotic-resistant pathogens are present in sewage nationwide. 

“I also really appreciated the fact that they center in on the communities that are most vulnerable to this,” he added. “That’s how policy should be generated.”  

With regards to federal policy and the congressional funding question, Peters said he’s “pretty hopeful” that his fellow lawmakers will approve Biden’s $310 million request. 

Paloma Aguirre, mayor of Imperial Beach, told The Hill in an email that while her city is grateful for the president’s appeal, they need more federal and state support “to tackle this public health ticking time bomb.” 

From an engineering perspective, Peters explained, fixing the South Bay plant is simple, but the international border and federal funding aspect creates a complicated renovation process.  

“We’ve taken as many federal officials out to the plan as we can — get them out there on a nice, stinky day,” he continued. “It’s pretty remarkable.”  

If the money comes through, Peters said it will double the size of the facility and provide for ongoing maintenance. Yet in the interim, he described a status quo in which Mexico is now ahead of the U.S. in terms of treatment plant reconstruction efforts.  

“It’s very fashionable to blame Mexico from Washington,” Peters said, noting that Mexico is meeting its “side of the bargain” in terms of treaty obligations.   

“I don’t want to be lagging behind Mexico,” the congressman added. “They’re setting the pace, and we need to catch up.” 

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