Study of three domestic sewage submarine outfall plumes through the use of numerical modeling in the São Sebastião channel, São Paulo state, Brazil

Abstract The dispersion of the thermotolerant coliform plumes emitted from the Araca, Itaquanduba and Cigarras domestic sewage submarine outfalls in the Sao Sebastiao Channel was modeled. The plumes were simulated as the only pollution sources considering bacterial decay for the summer and winter of 2016. The main goal was to determine if and which areas were impacted by the disposal systems. The simulated concentrations were quantitatively compared with Enterococos and thermotolerant coliform data provided by sanitation agencies in Sao Paulo state (CETESB and SABESP). The UM3 model from Visual Plumes software and the ECOM were used as the near-field and far-field models, respectively. The ECOM was configurated with 37 sigma levels and its horizontal resolution varies from 0.2 km to 5 km. The modeled concentrations did not exceed the limit stated by CONAMA Resolution 274 (2000), with the exception of that in June 2016. The main conclusion of this study is that channel hydrodynamic patterns are sufficient to disperse the plumes from sewage disposal outfalls. The areas adjacent to Araca outfall presented the highest persistence of coliform concentrations; however, the modeled concentrations did not exceed 1000 MPN/100 mL for more than 0.1% of the time. The simulations satisfactorily reproduced the coliform variability and dispersion in the region.