Dynamics of soil-to-plant radionuclide transfer in a tropical megacity: a longitudinal study and risk assessment in Ho Chi Minh City, Vietnam

The pollution of urban soils in tropical megacities with legacy radionuclides poses a significant environmental challenge, yet risk assessment is often hampered by a reliance on models from temperate climates. This study addresses this gap by presenting the first longitudinal (2022-2024) investigation into the dynamics of soil-to-plant transfer for both natural radionuclides (2³8U, 2³2Th, 40K), which define the geological baseline, and the primary anthropogenic contaminant, 1³7Cs in Ho Chi Minh City, Vietnam. Analysis of four ecologically significant plant species indicates that radionuclide bioavailability is not static. Instead, it exhibits significant fluctuations correlated with seasonal climatic shifts, a dynamic that challenges equilibrium-based assumptions and underscores the need for process-based risk assessment models. The results inform a "right plant, right place" management strategy for urban soil pollution. Araucaria columnaris was identified as an effective phytoextractor for 1³7Cs (TF up to 0.697), while Pinus kesiya was confirmed as a low-uptake species suitable for safe urban greening. Furthermore, risk assessment of the edible Moringa oleifera confirmed a negligible public health risk from its consumption (<3% of the public dose limit) and established its potential as a valuable sentinel species for long-term environmental monitoring. This research provides a foundational dataset on soil pollution in Southeast Asia and a transferable methodology for mitigating radiological risks in urban ecosystems.