Modeling and Design of the Buenos Aires Outfalls

Field studies, modeling, and the design of two major outfalls to discharge domestic wastewater from Buenos Aires, Argentina, into the Río de la Plata are described. Extensive data were gathered from current profilers, Lagrangian drifters, meteorological stations, and water column profiling. The river is essentially freshwater and is the main water source for the city; it is tidally dominated, shallow, and well-mixed vertically. The data were used to develop a two-dimensional hydrodynamic model of the river that closely simulated its major features. The wastewater disposal strategy and special design and modeling considerations of the outfalls and diffusers are discussed. The tidal currents will recirculate the wastewater back and forth over the diffuser several times before it is flushed away by the mean river flow. Because of the shallow water and low flushing currents, it is not possible to achieve the high near-field dilutions typical of deep-water marine outfalls, and other design constraints include the proximity to water intakes. Lagrangian particle-tracking modeling approaches were used to predict the fate and transport of the effluent and its dilution. The models reproduce near-field mixing at moderate to high current speeds and also reentrainment due to the reversing tide without the need for special model coupling. However, dilutions at low current speeds were corrected to account for the jet momentum and reentrainment. The near-field jet modeling to determine the riser and port designs is discussed. Bacterial impacts decreased rapidly with distance from the diffuser and no impacts on the shoreline or water intakes were predicted. The effects of sudestadas were also modeled and it was concluded that they will not significantly affect water quality for the proposed outfalls.