Pathogen Forecast Model Version 1.0: A Process-based Five-Day Beach Advisory Forecast System for the San...

The San Diego/Tijuana (US/Mexico) coastal region has immense water quality issues due to untreated wastewater sources. Regular ddPCR Enterococcus sampling provides same-day water quality information for beachgoers but does not provide forecasts for decision making. We describe the Pathogen Forecast Model (PFM) V1.0, providing daily 5 day forecasts of swimmer illness risk along 30 km of shoreline on a web dashboard. PFM is based upon a hydrodynamic model using forecasted waves, winds, offshore currents, and Tijuana River flow as inputs. PFM accurately forecasts tides and waves and forecasts depth-averaged shelf currents moderately well. Modeled untreated wastewater concentration tracer C evolves with a constant decay rate representing Norovirus. Over 13 km of shoreline, forecasted C and observed EnterococcusE are moderately log-correlated (≈0.5) for day-1 and -5 forecasts. The C and E power-law relationship has an exponent of ≈0.5, possibly suggesting the tracer decays 2× slower than Enterococcus DNA, among other potential explanations. Using the cutoff C = 10-5, PFM classified advisory exceedances with nearly 80% raw accuracy, high sensitivity, and lower specificity. This is the first forecast model that resolves estuarine, surfzone, and shelf regions. PFM is a useful public health tool and is constantly being improved.