Modeling Impacts of Various Wastewater and Stormwater Flow Scenarios on San Diego South Bay and Tijuana Beaches

The San Diego South Bay and Tijuana MX border region has significant water quality issues due to inflows of untreated sewage from two sources the Tijuana River Estuary (TJRE) and the San Antonio de los Buenos outfall at Pt. Bandera MX (SAB/PTB) located 10 km or 6 miles south of the border.  We have performed estuary and ocean modeling simulations for the San Diego South Bay and Mexico border region to evaluate the impacts of potential infrastructure solutions to improve shoreline water quality and reductions on regional beach closures.  This model leveraged previous National Science Foundation and NADB/EPA investments in model development. We examined the shoreline water quality impacts of untreated sewage under 4 different inflow scenarios denoted NADB 00, 35, 100, 163 for the 2017 year. The NADB 00 scenario represents normal baseline conditions where TJRE inflow scales with gauged river inflow and is capped at 10 millions of gallons per day (MGD) and SAB/PTB untreated sewage inflow is at 35 MGD.  The NADB 35, 100, and 163 scenarios eliminate TJRE inflows below 35, 100, or 163 MGD, respectively.   The NADB 35 scenario also reduces SAB/PTB to 10 MGD of treated sewage (5% of pathogen load of untreated sewage). Untreated sewage decays with a 7 day half life representative of noroviruses. Analysis was split into wet season (prior to 1 April and after 1 Oct), tourist (dry) season (22 May to 8 Sept), and full year. A representative untreated sewage concentration (a dilution of 1:2000) value is used to gauge beach closure limits.  Overall, the SAB/PTB source is the dominant source that leads to regional beach closures. At the Imperial Beach Pier during the tourist (dry) season, the baseline (NADB 00) scenario has the beach closed 24% of the time.  NADB 35 scenario reduces this to 0%, whereas the NADB 100 & 163 scenarios do not reduce tourist season beach closures.   At the Imperial Beach Pier during wet season, the baseline (NADB 00) scenario has the beach closed 9% of the time, ⅔ of which comes from the TJRE source and ⅓ from the SAB/PTB source.   The NADB 35, 100, & 163 scenarios reduce wet season beach closures to 4% to 6%, and are largely similar in efficacy. Thus, the NADB 35 scenario has by far the largest benefits in terms of reduced beach closures at IB Pier, because of the strong reduction of SAB/PTB inflows.  Similar conclusions are drawn at Playas Tijuana, Silver Strand State Beach, and Hotel del Coronado.  This work suggests that eliminating or dramatically reducing SAB/PTB inflows has the strongest benefit to the City of Imperial Beach, Silver Strand State Beach, and City of Coronado.

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