Assessing trace metal concentration in urban soils: a study of urban parks and unforested urban areas

Urban soils act as long-term reservoirs for trace metals, yet how soil physicochemical properties and anthropogenic pressure interact over time remains poorly understood in medium-sized cities of the Global South. This study evaluates the spatiotemporal dynamics and ecological risks of Ba, Cu, Zn, Pb, Cr, Ni, Mo, Co, and Cd in urban topsoils, through a 33-week monitoring campaign. Linear Mixed Models (LMM) and Positive Matrix Factorization (PMF) were combined to identify retention controls and source contributions. No significant temporal variability was observed (p > 0.05), confirming soil as a stationary reservoir, whereas marked spatial heterogeneity occurred (p < 0.001), driven by localized soil properties. LMM results highlighted soil organic matter (SOM) as a key enhancer of Cu (β = 6.15) and Zn (β = 17.9) retention, while pH exerted a strong negative control on Pb mobility (β = -23.9 x 104). PMF identified three major contributors: geogenic sources (35.9%), industrial/traffic emissions (32.4%), and mixed anthropogenic-lithogenic inputs (30.7%). Although metal concentrations remained below intervention thresholds, localized Cd-driven ecological risks persist in densely populated areas. This study highlights that vegetation and SOM function as vital geochemical sinks for metal immobilization, offering effective pathways for sustainable urban risk mitigation.