ARPA Puglia carried out a numerical study to simulate the evolution of the turbidity plume geometry and to describe the bacterial dispersion caused by the discharge of a mixture of raw sewage and surface runoff from the so called condotta Matteotti into the marine-coastal waterbody close to the urban beach (bathing area) called “Pane e Pomodoro”. The condotta Matteotti is an underground concrete pipe connected with a spillway to the main sewer system: it acts as an emergency weir to prevent flooding during or soon after intense rainfall events. During these rainfalls the seaward sluice gates open, and the untreated sewage is discharged into the sea thus decreasing the water quality. The simulation, according to a heavy rainfall and overflow event occurred in July 2018, was implemented using the DHI's Mike 3 suite enhanced with the ECOlab module, both organized as step-by-step processes. The validation of the final model was performed by comparing the microbiological concentration resulted from the model application with the on-field measurements and samples collected by ARPA Puglia during its ordinary water monitoring activities.