Heavily polluted Tijuana River drives regional air quality crisis

INTRODUCTION: The Tijuana River watershed is the epicenter of an environmental and public health crisis. For decades, millions of gallons per day of untreated sewage, industrial waste, and contaminated runoff have been released into the river, subsequently flowing through the Tijuana River Estuary and into the Pacific Ocean. These heavily contaminated waters recently caused nearly 1300 consecutive days of Southern California beach closures, negatively affecting both public health and the economy. Whereas health concerns have typically focused on direct exposure to contaminated waters, this study addresses the often-ignored impact of polluted waters on air quality. Pollutants are transferred from water to air—a process that is enhanced by turbulent flows—where they can be dispersed over tens of kilometers. As water pollution increases globally, understanding the atmospheric consequences of contaminated water bodies is critical to safeguarding public health. RATIONALE: In summer 2024, even in the absence of rain, wastewater flows surged to millions of gallons per day, enhancing water-to-air transfer of hydrogen sulfide (H₂S)—a toxic gas and key sewage tracer—and other hazardous gases. Despite community concerns, the pollutants causing malodors and adverse health effects had never been identified or quantified. In this study, we combined advanced measurements and trajectory modeling to demonstrate that poor water quality can substantially affect air quality, highlighting inhalation as a major exposure pathway while validating long-dismissed community voices. RESULTS: Neighborhood-based nighttime H₂S levels near a riverine hotspot located at the Saturn Boulevard river crossing peaked at 4500 parts per billion (ppb)—with hourly averages orders of magnitude above typical urban levels (<1 ppb) and California’s 1-hour ambient air quality standard (30 ppb). A sudden wastewater diversion abruptly reduced peak nighttime flows from between 40 and 80 million gallons per day (MGD) to <5 MGD. H₂S levels and odor reports were strongly correlated (correlation coefficient r = 0.92), both sharply decreasing (~95%) after the wastewater diversion. The strong correlation supports that the turbulent riverine hotspot was the dominant source of malodors and H₂S. Air samples collected before and after the diversion also identified numerous compounds with known toxicity that decreased in abundance after the wastewater diversion, underscoring that exposure is not limited to a single toxic gas. An air trajectory model was able to reproduce the observed H₂S trends only when meteorology and river flows were represented, further supporting the riverine hotspot as the primary source. The model demonstrated the potential for widespread dispersion of airborne pollutants well beyond the edge of the contaminated Tijuana River. CONCLUSION: Communities along the Tijuana River Valley have endured decades of environmental pollution, with a public health crisis now clearly linked to airborne exposure of wastewater-derived pollutants. Such prolonged exposure without adequate protection or timely intervention highlights an environmental injustice that demands action. Immediate actions—such as diverting wastewater away from the Tijuana River and implementing localized air monitoring—can sharply reduce toxic exposures, but long-term solutions require upstream treatment infrastructure, strengthened regulatory oversight, and binational collaboration to reduce pollutant discharges at their source. As the number of heavily polluted water bodies grows with ever-increasing inputs, ignoring water-to-air contaminant emissions in air quality management allows hidden risks to persist. Recognizing and addressing this airborne exposure pathway is critical to protecting vulnerable populations and advancing global health equity.