Exploring the critical zone of a tropical megacity: insights from environmental magnetism

Humanity is confronted with unprecedented challenges related to climate change, water scarcity, and food security, as underscored by the United Nations Sustainable Development Goals (SDGs). Overpopulated megacities, such as São Paulo, are particularly susceptible to these issues, which are anticipated to have significant environmental and social consequences. Tackling these emerging challenges necessitates a holistic approach to comprehending the increasing human influence on physical, biological, and social environments. This paper emphasizes the Critical Zone Observatory (CZO) initiative in a crucial area of São Paulo, a megacity that has undergone various urban transformations over the years. The São Paulo CZO seed site aims to address vital questions concerning the anthropogenic impacts on groundwater, soil, and vegetation. We present environmental magnetism techniques and geochemical analyses carried out to enhance our understanding of the structure and dynamics of the critical zone. The findings reveal notable discrepancies between soil analyses obtained through geochemical and environmental magnetic methods. Magnetic parameters have pinpointed a significant interface in the saturated zone, revealing transformations in mineral phases and grain sizes of iron-bearing minerals. At greater depths, magnetic properties distinguished samples closer to the Tietê River, which is heavily affected by sewage, potentially indicating geochemical changes caused by interactions with polluted waters. Environmental magnetism offers insights into the transformations occurring within both biotic and abiotic processes. These dynamic processes are frequently influenced by anthropogenic factors (e.g., soil contamination) and climatic events (e.g., droughts and floods). Therefore, environmental magnetism serves as a valuable tool for monitoring and understanding the resilience of the critical zone.