Human Norovirus GII is Widespread in the Coastal Marine Waters of the San Diego-Tijuana (US-Mexico) Border Region

Human norovirus GII (HuNoV GII) causes millions of gastrointestinal infections annually and persists for days to weeks in marine waters after wastewater discharge, raising transmission concerns in polluted coastal regions such as the San Diego-Tijuana border between the US and Mexico. We conducted a six-month ddPCR-based monitoring campaign in the Tijuana River Estuary and adjacent coastal ocean to quantify HuNoV GII's spatial and temporal distribution. This is one of the first such spatially and temporally resolved studies in a coastal marine environment. Mean concentrations were ∼102 copies L-1, reaching > 105 copies L-1 near the mouth of the Tijuana River. HuNoV GII declined with increasing distance from shore and the pollution source, varying across gradients in shoreline proximity, river-mouth distance, alongshore wave-driven surfzone velocity, and river discharge. Concentrations correlated strongly with the widely used fecal indicator Enterococcus (ρ = 0.768), pointing to a shared wastewater source for both. Mean concentration-based swimmer illness risk ranged from 200-350 per 1000 swimmers, far exceeding the EPA benchmark of 32. These findings establish HuNoV GII as a persistent, spatially structured contaminant in this transboundary system and provide a baseline for tracking wastewater infrastructure improvements and informing pathogen risk forecasting models under development.