| Summary: | Mexico City faces multiple environmental challenges. Studying these challenges separately is a common practice, ignoring the interdependencies between the sectors of the city and limiting the environmental impact assessments to the local level. This perspective has led to biased assessments and an underestimation of urban environmental impacts because interactions between the city and other territories have been omitted. Therefore, in this work OLCA is proposed to be used as a systematic and holistic approach to support decision-making in cities. The goal of this study was to evaluate Mexico City Water-Energy-Food (WEF) Nexus environmental impacts using Organizational Life Cycle Assessment (OLCA), aiming to identify activities with the highest mitigation potential.Two methodological modifications to OLCA were proposed: considering WEF Nexus as the reporting organization and as the axis to define the system boundaries. These modifications allowed to evaluate of both local and federal government-controlled and private-sector activities. WEF Nexus direct activities (within Mexico City), food, water and energy supply indirect upstream activities, and indirect downstream activities, such as waste and wastewater treatment and disposal were evaluated (outside Mexico City).Indirect activities account for 13% to 60% of total impacts in the evaluated categories, proving the importance of using OLCA-focused methodologies to include indirect urban impacts when performing environmental assessments, to avoid underestimations regarding such impacts.Coupling OLCA with WEF Nexus approach allowed to broaden the scope of the analysis and identify critical points at local, regional, and federal levels. At a local scale, the highest environmental impacts are related to the transporting sector fuel consumption in Mexico City and for water sector due to the hydric stress currently faced by the city. At a regional level, food production had an impact mainly due to its high demand for water and due to excessive application of fertilizers. At the federal level, it was observed that as long as the high participation of fuels in the country's energy mix is maintained, it will continue to cause high associated environmental impacts.In the same manner, recommendations were identified to guide stakeholders in making informed decisions to mitigate the main environmental impacts. At the federal scale, it has been recommended that the share of renewable in electricity generation must increase to satisfy Mexico City electric energy needs and at the same time mitigate indirect impacts associated with fossil fuels. At a regional scale, it is recommended that the city influences the food systems and promotes a higher efficiency in irrigation methods and fertilizer use. At local scale, it has been identified as a key aspect the importance of promoting the use of net zero-emission transport and boosting the use of compost for food growing.The OLCA approach made it possible to identify the interactions between the WEF Nexus since as a common practice they are not considered in development plans. For Mexico City, there was identified a strong interaction between water-food sectors related to the high-water demand for food production, water-energy sectors interactions related to electricity for water management and negative impacts on water quality resulting from energy and food sector operations.This work displayed the type of information that needs to be provided to decision-makers for encouraging dialogue and joint decision-making between the WEF Nexus sectors, recognizing indirect impacts relevance, and promoting proper co-responsibility of the city with its environment and resources.For further researches, it is recommended to carry on an analysis with major detail bounded to the products with higher environmental impacts identified in this research; to propose more specific mitigation measurements.
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