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

Nature-based solutions (NbS) are increasingly assessed in sustainable urban projects to improve human welfare and biodiversity. Particularly, the design, implementation and assessment of NbS are in most cases spatial and context-dependent, and just NbS must also be included in the agenda. To this end, alternative paradigms such as transdisciplinary and action research can play a fundamental role in favoring local experiences where transformative changes become a reality. The aim of this work is to evaluate the results of a co-production process with a local social movement analyzing and implementing just NbS to mitigate the impact of domestic discharges on the water quality of an urban stream (Buenos Aires, Argentina). We evidenced the relative impact of domestic effluents on stream water quality, with non-negligible input flows (3-16% across seasons) and high mass loads of Chemical Oxygen Demand (COD), Total Suspended Solids, Biochemical Oxygen Demand, Oil and Greases, nutrients and pathogens, which in some cases double the content of the stream water through a one-kilometer reach. Two Biofilter prototypes were designed, implemented and assessed for the mitigation of domestic pollutants, and a participative monitoring approach was implemented. A significant percentage of the installed devices (50%) withstood heavy rainfall and remained operative for 5 consecutive months, and 25% of these remained operative for 2 years without any maintenance time. Moreover, the first prototype showed excellent efficiency in the removal of N, P and COD (>90%), with moderate efficiency in the removal of Dissolved Organic Carbon (40-67%). In contrast, the second design was easier to construct but showed low performance, with only one device displaying a moderate efficiency (20-40% according to the pollutant). We highlight and discuss the fundamental role of the participation of the local community in the whole co-production process, and key lessons to further optimize the Biofilter design.