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…