A study of sewage pollution off San Diego found that a wastewater-linked compound detected in La Jolla may have traveled roughly 22 miles underwater before escaping into the air near the Scripps Institution of Oceanography pier. The finding suggests the Tijuana River sewage problem can affect areas well beyond the communities closest to the border, though the study did not assess health risks from the measured concentrations.
The study, published in Scientific Reports, was led by atmospheric scientist Karolina Cysneiros de Carvalho, who conducted the work at UC Riverside and the National Center for Atmospheric Research in Colorado. The team also included UCR atmospheric scientist William Porter, UCR and NCAR atmospheric chemist Kelley Barsanti, and Scripps Institution of Oceanography scientist Kimberly Prather. The researchers’ findings and interpretations are described in a UC Riverside account of the study.
What the La Jolla Signal Can—and Cannot—Show
The team collected air samples at four locations between the Tijuana River and the Scripps pier in August and September 2024, and detected dimethyl trisulfide at the farthest site, in La Jolla. During that period, wastewater was flowing into the Pacific at an estimated 160 million to 207 million liters per day—unusually high for the dry season, when river flows typically drop to nearly nothing, Prather said, according to the UCR account.
The researchers used dimethyl trisulfide, a sulfur compound associated with decomposing organic matter and wastewater, as a tracer tied to conditions in the Tijuana River and distinguishable from other coastal sewage sources. Because the compound usually breaks down quickly in air, the team’s calculations indicated that most of it should not have survived a journey that far north. Porter said the levels detected could not be explained by wind alone.
A Possible Underwater Route
The study’s modeling offered a possible explanation: the compound could remain in the water for much of its journey north, rather than traveling through the air. At a depth of about 10 meters, the model estimated that up to 76% of dimethyl trisulfide could survive a 35-kilometer underwater trip under the conditions examined. Ocean measurements near Del Mar showed unusually fresh water along the coast during the study period, which the researchers said was consistent with a river-water plume moving north below the surface before gases escaped near La Jolla.
Researchers also examined what happened after a pump station in Mexico began diverting wastewater southward on Sept. 10, 2024: dimethyl trisulfide levels measured in La Jolla dropped 86%, according to the study. Prather said the finding shows that pumps in Mexico can affect air quality as far north as La Jolla. The team also considered whether undersea canyons near La Jolla could bring pollution to the surface, but Prather said that possibility remains uncertain and would require further measurements.
Limits of the Findings
The study did not assess whether the measured pollutant concentrations pose a health risk. Nor do researchers yet know how far the compounds travel under more typical conditions: the observations captured an unusually sustained period of dry-season discharge. Prather’s team is taking additional measurements to establish the plume’s reach. Prather said it could extend beyond La Jolla, although concentrations are likely to be much lower by the time pollutants emerge after traveling that far. She said the broader sewage crisis affects communities including Del Mar and Encinitas, not only those closest to the border.
The underwater-transport findings arrive alongside other signs of how seriously regional officials are now treating airborne and waterborne sewage exposure. The federal Agency for Toxic Substances and Disease Registry released a draft report in September 2026 finding that short-term hydrogen sulfide exposure near the river constitutes a public health hazard — the first such finding by a federal public health agency. That same month, Gov. Gavin Newsom signed Senate Bill 58, directing the California Air Resources Board to update the state's hydrogen sulfide standard, which had first been adopted in 1969. San Diego County had also contracted with UC San Diego in June 2026 for a separate Prather-led study mapping residents' exposure to airborne chemicals.
A River Already Known for Its Chemical Load
The underwater pollution route adds to a growing body of research describing the Tijuana River as chronically contaminated. A September 2026 report from San Diego Coastkeeper, which documented a year of monthly water-quality sampling at four sites in the lower watershed, found that each sampling event typically turned up 10 to 20 different industrial chemicals, according to the San Diego Union-Tribune. The same report found that copper exceeded acute federal toxicity standards for aquatic life in more than 40% of samples and chronic thresholds in nearly 70%, as reported by KPBS, with average yearly dissolved-oxygen concentrations of just 1 to 3 milligrams per liter, well below the regional minimum standard of 5 mg/L.
A separate study of the river, published in Science, found that in summer 2024, wastewater flows surged even without rain, enhancing the transfer of hydrogen sulfide and other hazardous gases from the water into the air. Near a hotspot at the Saturn Boulevard river crossing, nighttime hydrogen sulfide levels peaked at 4,500 parts per billion, that research found, and hydrogen sulfide levels correlated strongly with resident odor complaints, with a correlation coefficient of 0.92, according to Science.
Money Moving Toward a Fix
Local and federal officials have begun directing significant funding toward the problem. San Diego County supervisors voted 4-1 to spend $1.8 million designing a system to divert and treat Tijuana River water before it reaches local communities, and separately approved $4 million unanimously to address the Saturn Boulevard hot spot where river pollution becomes aerosolized, according to CalMatters. A proposed half-cent county sales tax could generate $360 million annually, with roughly $80 million per year earmarked specifically for Tijuana River improvements, the outlet reported. The federal government has separately dedicated about $650 million to repair and replace aging wastewater infrastructure serving the area.
Some relief has already materialized on the water side. San Diego Coastkeeper has noted that dry-weather wastewater flows into the United States via the Tijuana River fell from 35 to 40 million gallons per day down to roughly 15 million gallons per day after Mexico partially restarted its PBCILA pumping system. But as the new underwater-transport findings make clear, even reduced flows during unusually persistent dry-season conditions were enough to push pollutants far up the coast — a reminder that, as UCR's Porter put it, water and air pollution from the river should not be treated as separate problems.
Other Infrastructure and Monitoring Efforts
The U.S. Environmental Protection Agency announced that a completed expansion increased the South Bay International Wastewater Treatment Plant’s capacity from 25 million to 35 million gallons per day, a 40% increase. An International Boundary and Water Commission report also documents a separate February 2017 incident in which Tijuana’s water utility bypassed wastewater into the river during repairs to a line in central Tijuana; that event had a different cause and circumstance from the sustained dry-weather flows measured in 2024, according to the EPA and the IBWC.
Beach advisories and air studies track different concerns. KPBS reported that Heal the Bay’s Beach Report Card grades beaches based on bacterial pollution levels, a measure distinct from airborne dimethyl trisulfide and not a health-risk finding from the underwater-transport study. Separately, the Airborne Institute at UC San Diego describes an ongoing research campaign that includes sleep-quality and health-effects surveys and near-real-time hydrogen sulfide measurements; that work is intended to inform research, not announce a health finding from this study.
