Quantifying the benefits of nature-based solutions in urban drainage on headwater stream water quality

In urban areas lacking adequate sanitation infrastructure, greywater flows into streams via stormwater drainage networks. This study evaluates the impact of these discharges on water quality in a headwater stream in the Metropolitan Area of Buenos Aires, Argentina, comparing vegetated drainage ditches (VDDs)-a type of green infrastructure (GI)-with traditional concrete drainage (grey infrastructure) across seasons. Pollutant loads-including organic matter, nutrients, and faecal bacteria-were analysed and modelled to assess their relative contribution to stream pollution. Effluent flows represented 3-16 % of total stream flow, introducing substantial pollutant loads. Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), Biochemical Oxygen Demand (BOD), oil and grease, nutrients, and faecal bacteria increased markedly downstream, in some cases doubling stream concentrations within 1 km. Linear models indicated mostly independent and distinct effects of seasonality and infrastructure type on effluent characteristics. Grey infrastructure exhibited significantly higher discharge flows and lower nitrate and phosphorus concentrations. Seasonally, BOD and COD peaked in winter, while Dissolved Organic Carbon (DOC) and nitrates increased in summer. Bayesian modelling highlighted that during the critical winter season, pollutant loads from grey infrastructure exceeded those from VDDs by 1-97 %, emphasising grey infrastructure's higher environmental impact. Results demonstrate that VDDs and higher temperatures mitigate pollution impacts by enhancing infiltration and microbial activity, facilitating organic matter breakdown. This study underscores the ecological benefits and potential of Nature-based Solutions (NbS) like VDDs for improving urban water quality, supporting biodiversity, and strengthening ecosystem resilience.