Barbara Marion Gomez , Nelson Samuel Diaz, Martin Saraceno , Rodrigo Sinistro, Martin Graziano

In urban areas lacking adequate sanitation infrastructure, greywater flows into nearby urban streams through the stormwater drainage network. This study evaluates the impact of this drainage on water quality in a headwater stream in the Metropolitan Area of Buenos Aires (AMBA), Argentina. We analysed and model pollutant loads — organic matter, nutrients, and allochthonous faecal bacteria — entering the stream via the stormwater drainage network, comparing vegetated systems (green infrastructure) and concrete systems (grey infrastructure) across seasons. We observed the relative impact of domestic effluents on stream water quality, with significant input flows (3-16% across seasons) and high mass loads of Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), Biochemical Oxygen Demand (BOD), Oil and Grease, nutrients, and allochthonous faecal bacteria, which, in some cases, double the concentrations of these substances in stream water over a one-kilometre reach. Linear models showed that both features, season and type of infrastructure, have mostly independent and differential effects on effluent quality. Grey infrastructure showed a significant increase in flow across seasons, together with a reduction in nitrate and total phosphorus levels. On the other hand, an increase in effluent BOD and COD levels was observed in winter, while DOC and nitrate concentrations increased in summer. Finally, bayesian models of effluent impact on stream water quality showed that during the most critical season (winter), the impact on the organic load of the stream is predominantly higher in a scenario with only grey infrastructure, with estimated increases between 1-96% for the different pollutants compared to green infrastructure. Overall, our results suggest that higher temperatures and the presence of green infrastructure, such as vegetated ditches, result in a lower impact of domestic effluents on stream water quality through mechanisms such as improved infiltration and enhanced biotic activity that contribute to metabolizing the organic load present in the effluent. This study highlights the ecological benefits of Nature-based Solutions (NbS) for urban water management, underscoring the potential of green infrastructure to improve water quality, support biodiversity, and strengthen the resilience of urban ecosystems