- A new Science study links sewage pollution to toxic air in San Diego.
- Hydrogen sulfide levels exceeded state standards by up to 70 times.
- EPA and Mexico signed an agreement to accelerate wastewater projects.
- The South Bay plant expansion increases treatment capacity by 40 percent.
Friday, August 29, 2025 — For decades, the Tijuana River has carried untreated sewage, industrial chemicals, and stormwater runoff from Mexico into Southern California. The contamination has forced more than 1,300 consecutive days of beach closures in recent years. While much of the concern has focused on water quality, a new study in Science
shows that the crisis extends into the air.
Researchers from the University of California, San Diego, and other institutions measured hydrogen sulfide (H₂S)—a toxic gas with a distinctive “rotten egg” odor—at levels far beyond public health standards. In the Nestor neighborhood of San Diego, nighttime peaks reached 4,500 parts per billion (ppb). For comparison, typical urban levels are less than 1 ppb, and California’s one-hour air quality standard is 30 ppb.
The study documented that the foul odors reported by South Bay residents closely matched spikes in hydrogen sulfide, with a correlation coefficient of 0.92. Even after a wastewater diversion in September 2024 reduced flows in the river, communities continued to face hours of nightly exposure to toxic levels of the gas.
The researchers also found other hazardous compounds released into the air alongside hydrogen sulfide, raising concerns about long-term health risks. Residents reported respiratory illnesses, skin infections, headaches, and stress directly tied to the sewage crisis. A federal survey last year showed nearly 80 percent of households in the affected area believe their quality of life has been harmed.
Environmental Justice Concerns.
The neighborhoods most affected by the pollution—including Nestor, San Ysidro, and Imperial Beach—are home to predominantly Hispanic and Latino residents, many of whom live below the poverty line. The study’s authors highlighted that communities facing these conditions have endured environmental burdens for decades with limited government response.
While immediate steps such as air filtration and local monitoring could provide some relief, the report stressed that only large-scale infrastructure improvements and cross-border cooperation will address the crisis at its source.
EPA and Mexico Announce Agreement.
On August 28, 2025, the U.S. Environmental Protection Agency (EPA) and the U.S. International Boundary and Water Commission (IBWC) announced a major expansion of the South Bay International Wastewater Treatment Plant in San Diego. The facility’s capacity has grown from 25 million gallons per day to 35 million gallons per day, a 40 percent increase. Federal officials described the rapid expansion—completed in 100 days instead of the originally scheduled two years—as a milestone in addressing the decades-long problem.
The expansion is part of a broader Memorandum of Understanding signed on July 24, 2025, in Mexico City by U.S. EPA Administrator Lee Zeldin and Mexican Secretary of Environment and Natural Resources Alicia Bárcena Ibarra. The agreement commits both nations to accelerate sewage infrastructure projects, share monitoring data, and establish new safeguards for the Tijuana-San Diego region.
Key Provisions of the MOU.
Under the agreement, Mexico has pledged to divert 10 million gallons per day of treated effluent away from the Tijuana River and rehabilitate the aging Parallel Gravity Line by the end of 2025. Mexico has also committed $93 million across its 2026 and 2027 budgets for additional infrastructure improvements.
The United States has pledged to release Border Water Infrastructure Program funds to support construction projects once Mexico begins work on its commitments. Future plans include expanding the South Bay treatment plant further to 50 million gallons per day by December 2027.
Both governments agreed to negotiate a new binational Minute by the end of 2025 that will establish at least a dozen additional actions. These include evaluating an ocean outfall pipe at the San Antonio de los Buenos treatment plant in Tijuana, creating a special maintenance account at the North American Development Bank, and establishing a real-time binational monitoring system.
Looking Ahead.
The EPA has described the expansion and the MOU as the beginning of a permanent resolution to the Tijuana River sewage crisis. Scientists and community members, however, stress that the crisis involves both water and air quality, with direct impacts on public health.
As the binational projects move forward, residents along the border will be watching closely to see whether the promises translate into long-term relief from the sewage and air pollution that have defined life in the region for generations.
