Submarine outfall relocation and the reduction of microbiological risk in bathing waters.

Urbanisation of coastal zones increases sewage loads and microbiological risks in bathing waters. This study evaluates whether relocating a planned submarine outfall can reduce these risks while preserving suitable bathing conditions along the Itaipuaçu coast, Rio de Janeiro, Brazil. A coupled framework integrates three-dimensional hydrodynamics, near-field plume simulations, and a Lagrangian far-field transport model with an E. coli decay formulation driven by solar radiation, temperature, and salinity. Hydrodynamic fields forced by tides, large-scale circulation, and local and remote winds were validated against in situ current measurements during winter conditions. Deterministic simulations and a probabilistic analysis quantified E. coli concentrations and the percentage of time that bathing-water thresholds (800 MPN/100 ml) are exceeded in the vicinity of the outfall diffuser and along the shoreline. Results show that an alternative outfall location eliminates plume contact with Maricas Islands, maintains compliant shoreline bathing waters, and reduces outfall length, providing a transferable framework for optimising regional submarine outfall design.