DISCLAIMER: The present Project Deliverable has been submitted to the European Commission for review. The information and views set out in this report are those of the author(s) and do not necessarily reflect the official opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained therein. CoAct (Co-designing Citizen Social Science for Collective Action) aims at co-designing concepts, methods and tools for citizen social science (CSS) around ���wicked��� global social issues. Four different research and innovation (R&I) actions organise the empirical work of CoAct. This deliverable D5.3 ���Open Source Digital Platform��� describes ��Qu�� Pasa, Riachuelo? (QPR) alpha version, which is the cornerstone of the research co-design activities in R&I Action #3 in the frame of CoAct WP5, coordinated by the National University of San Martin (UNSAM) and the Fundaci��n Ambiente y Recursos Naturales (FARN). QPR is a geo-referenced website that presents several layers of information, which can be chosen to be visualised on cartographic maps. The information available in the platform is both public information from different sources and citizen-generated data shared by platform users about different socio-environmental themes. R&I Action #3 aims at co-designing CSS tools to facilitate actions contributing towards Environmental Justice in the Matanza-Riachuelo river basin, where 6 million people live, many of them in vulnerable socio-environmental situations. This report describes this platform's main characteristics, reflects on its process of co-creation and presents the expected uses and purposes. Stakeholders who may benefit from QPR are different actors interested in transforming the basin towards environmental justice, particularly in relation to the platform current themes: basin water quality, conservation of natural areas and resettlement and redevelopment plans. The platform was co-designed in 15 workshops organised by theme. 51 co-researchers participated in those workshops. The main purposes of the workshops were to collect impressions, potential uses and expectations regarding the digital platform and to discuss and improve of the data-collection tool for each theme. In these workshops different stakeholders identify different potential uses. For this reason, we developed a landing page where four different users��� profiles (community, research, education and policy) can identify the platform potential benefits and the type of data they could produce and access. This can be accessed at https://quepasariachuelo.farn.org.ar. The platform organises existing information and enables opportunities for sharing experiences and can be accessed at: https://mapaqpr.farn.org.ar/. The platform is open source, published at GNU General Public License version 3, and its code can be found at https://github.com/farn21/platform-qpr. This document is organised as follows: Section 2 presents an introduction to CoAct and the R&I Action #3 on Environmental Justice; Section 3 analyses the platform co-design process; Section 4 describes the platform characteristics and technology, Section 5 comments on how the Covid-19 pandemic affected the development of this delivery, Section 6 presents the future activities using the platform and Section 7 presents the conclusions.