Real-Time Empirical Risk Assessment From Recurrent Coastal Sewage Plumes

Untreated wastewater enters the ocean at an outfall in Mexico and spreads to the San Diego-Tijuana (USA-Mexico) border region, posing significant risks to human health. Here, we developed a risk assessment tool for coastal communities, leveraging hindcast oceanographic simulations (2017-2019), to link changes in temperature and salinity at the coastline to high wastewater concentrations. We first calculated the modeled timescales (i.e., duration and return time) of wastewater exposure for popular beaches in the region. Most high wastewater exposure events occurred about once a month and lasted less than a week at the southern locations (e.g., Imperial Beach), and occurred less frequently and for shorter periods of time further north (e.g., Coronado). Using the same hindcast simulations, we then identified relationships between anomalous environmental conditions and wastewater concentration along the coastline. High wastewater concentrations were typically associated with lower salinity and temperature, reflecting the low salinity of wastewater and the colder temperatures of water originating south of the USA-Mexico border. Statistical models with only parameters of salinity and temperature anomalies captured a large proportion of the variation in wastewater-associated risk of illness (R ² = 0.63-0.78). We tested the risk assessment approach with several months of recent observations (January-December 2024) to show how this tool may be practically applied. This study provides an efficient method for developing risk models that utilize commonly measured environmental data, with applications to other pollution-impacted coastal locations.