Abstract
A model study has been conducted to understand the influence of hydrodynamic features, environmental conditions as well as bather shedding and animal fecal sources on the fate and transport of indicator microbe enterococci at a subtropical marine beach in South Florida. The model being used is based on an existing finite element hydrodynamic and transport model CAFE3D to which a first order microbe deactivation function due to solar radiation is added. The decay coefficient is assumed to be linearly proportional to the solar insolation value, while the constant coefficient linking the two is determined to be 0.368[m2/MJ] using local experimental data. This value corresponds to decay coefficients in the range of 0 to 1 hr-1 and is of similar magnitude to values found in the literature. The hydrodynamic model produces water current velocities, which are used in the transport model to simulate water enterococci concentration in space and time. Previous measurements have suggested that enterococci originate from the beach shoreline area. In this case the simulated velocity fields and Lagrangian particle trajectories indicate that the small velocity (generally less than 0.05m/s) and weak dilution at the water?s edge may cause enterococci accumulation and elevated concentration. Among human and animal sources, the impacts on maximum enterococci concentration in descending order is a dog fecal event, human bather shedding and seagull fecal events. A single dog fecal event at the middle part of the beach is found to cause enterococci far field concentrations to reach hundreds of CFU/100ml, which exceeds the EPA water quality standard (104 CFU/100ml). These high concentrations, however, only impact a small area. Concentration due to tens of bathers shedding enterococci during a course of 4 hours reaches 0.01 CFU/100ml, while the concentration due to thousands of bather shedding during a Memorial Day weekend reaches about 1 CFU/100ml. Concentration due to tens of seagull fecal events reaches only 0.0001 CFU/100ml. Thus, only the presence of dogs could explain the high enterococci observations infrequently found in monitoring data. The limited spatial extent compared to beach survey data of the enterococci plume from a dog fecal event suggests that other microbial sources are present, most likely microbes released from the beach sand.