Pollutant export under a new regime of continuous transboundary sewage flow in the Tijuana River

Transboundary rivers worldwide face escalating wastewater pollution, highlighting the need for multinational monitoring and mitigation. The Tijuana River, shared by Mexico and the United States, is contaminated by untreated sewage, industrial waste, trash, and polluted runoff. In December of 2022, damaged wastewater infrastructure and wet weather shifted transboundary flow from intermittent to perennial. Average dry-weather transboundary flows increased from 3.18 × 10 4 m 3 /day during water years (WYs) 2021–2022 to 19.1 × 10 4 m 3 /day for WYs 2023–2024, a 6.3-fold increase. To evaluate the effects of the altered flow regime on pollutant export, over 280 water samples were analyzed for chemical constituents and fecal indicator bacteria. Chronic cross-border flows increased pollutant loading to the Pacific Ocean by an order of magnitude, with annual exports of ~ 1.06 million kg of total dissolved nitrogen (TDN) and 1.15 million kg of dissolved organic carbon (DOC). Chronic dry weather flows averaged 7.8 × 10 6 MPN/100mL for Escherichia coli , equivalent to untreated wastewater, and resulted in continuous beach closures since 2023. Under this continuous transboundary flow regime, dry-weather baseflow had significantly (p < 0.01) higher DOC, TDN, and tryptophan-like fluorescence than wet-weather, storm-driven runoff. Attenuation of chemical constituents, microbial source tracking markers, and enteric virus pathogens across the ~ 12 km from border to ocean was essentially absent, likely due to limited photodegradation in highly colored water. These findings establish a baseline for evaluating infrastructure investments, reducing public health and ecosystem risks, and supporting sustained binational monitoring and data sharing.