Modeling Untreated Wastewater Evolution and Swimmer Illness for Four Wastewater Infrastructure Scenarios in the San Diego‐Tijuana (US/MX) Border Region

Abstract

The popular beaches of the San Diego‐Tijuana (US/MX) border region are often impacted by untreated wastewater sourced from Mexico—via the Tijuana River Estuary (TJRE) and San Antonio de los Buenos outfall at the Pt. Bandera (SAB/PTB) shoreline, leading to impacted beaches and human illness. The US‐Mexico‐Canada trade agreement will fund border infrastructure projects reducing untreated wastewater discharges. However, estimating project benefits such as reduced human illness and beach impacts is challenging. We develop a coupled hydrodynamic, norovirus (NoV) pathogen, and swimmer illness risk model with the wastewater sources for the year 2017. The model is used to evaluate the reduction in human illness and beach impacts under baseline conditions and three infrastructure diversion scenarios which (Scenario A) reduce SAB/PTB discharges and moderately reduce TJRE inflows or (Scenarios B, C) strongly reduce TJRE in inflows only. The model estimates shoreline untreated wastewater and NoV concentrations, and the number of NoV ill swimmers at Imperial Beach CA. In the Baseline, the percentage of swimmers becoming ill is 3.8% over 2017, increasing to 4.5% during the tourist season (Memorial to Labor Day) due to south‐swell driven SAB/PTB plumes. Overall, Scenario A provides the largest reduction in ill swimmers and beach impacts for the tourist season and full year. The 2017 tourist season TJRE inflows were not representative of those in 2020, yet, Scenario A likely still provides the greatest benefit in other years. This methodology can be applied to other coastal regions with wastewater inputs.

Bacterial and Chemical Evidence of Coastal Water Pollution from the Tijuana River in Sea Spray Aerosol

“…CWP at the Mexico–USA border between Tijuana (TJ), Mexico, and Imperial Beach (IB), USA, has persisted for decades and has been officially declared a state of emergency. ,, Whereas fecal and chemical pollution from stormwater runoff has been detected at various beaches in San Diego (SD), there is persistent and severe CWP at IB and TJ. ,,, Rains and inadequate infrastructure result in untreated sewage flowing into TJ–IB coastal waters. Hepatitis A virus and bacteria from TJ sewage have been detected in IB coastal waters. , The Tijuana River (TJR) is a major pollution conduit that sends 100-million-gallon sewage spills into South IB coastal waters. ,, SARS-CoV-2 has been detected in TJR waters at concentrations matching those at wastewater treatment plants . These problems caused IB beaches to be closed to water contact for 295 days 81 in 2020 .…”

“…These problems caused IB beaches to be closed to water contact for 295 days 81 in 2020 . This problem will likely persist after implementation of planned infrastructure due to multiple sources and continued diversion of high flow stormwater and sewage directly to the ocean . Climate change is expected to cause more extreme precipitation events, which may further exacerbate the problem …”

Quantifying the pollutant load into the Southern California Bight from Mexican sewage discharges from 2011 to 2020

“…A model by Feddersen et al (2021) combined for the first time hydrodynamic, pathogen, and human illness models to predict human health and beach impacts in the California Bight, impacted by untreated or poorly treated wastewater in Tijuana. In the Baseline, the percentage of swimmers becoming ill was 3.8% over the year, increasing to 4.5% for the tourist season (Memorial to Labor Day) due to south-swell driven SAB discharge.…”

Performance of a One-Dimensional Model of Wave-Driven Nearshore Alongshore Tracer Transport and Decay with Applications for Dry Weather Coastal Pollution

“…The condition C BAC = 5 × 10 −4 was chosen as a cut off value, referred to as the beach advisory condition. This C BAC was chosen by converting the dye concentration in the PB outfall (set to 0.7, where 0.01 is 1 part dye to 100 parts water, in the SD Bight model to represent the untreated sewage fraction of the effluent) first to the norovirus abundance in fresh untreated sewage, then to the likelihood of swimmer illness given exposure to the norovirus abundance following Feddersen et al 16 Here, C BAC corresponds to a 10% likelihood of swimmer illness. 10,16 Specificity and Sensitivity are calculated,…”

“…Among other issues, HFR does not sample within 1 km of shore, where pollution plumes are often located. , HFR therefore cannot resolve the relevant nearshore processes to accurately estimate plume transport . A hydrodynamic model of the coastal ocean near San Diego, USA, and Tijuana, MX, that resolves both the shelf and the nearshore and tracks plumes from both TJRE and PB (hereafter “SD Bight model”) was built by coupling an ocean model to a wave model using the COAWST framework. , However, the SD Bight model is computationally expensive (a year of model output requires weeks of runtime on a supercomputer cluster) and currently exists as a hindcast. Conversion to an operational forecast would require significant funding and effort.…”