Post-flood soil geochemistry and arsenic mobilization following the 2024 YAGI storm in a transboundary river basin of northern Thailand.

This study investigates the impacts of the 2024 YAGI storm on floodplain soil geochemistry along the transboundary Sai River between Thailand and Myanmar. A total of 30 soil sampling sites, including 5 pre-flood (December 2023) and 25 post-flood (October 2024) locations, were analyzed to assess flood-induced alterations in trace metal and nutrient concentrations across stratified depths (0-30 cm and 30-60 cm). Multivariate analyses, including PCA and hierarchical clustering, revealed distinct geochemical signatures separating floodway, floodplain, and pre-flood soils, primarily driven by enrichment in As₂O₃, PbO, Au, and nutrient oxides such as K₂O and P₂O₅. Ternary diagrams and bivariate plots highlighted a strong positive correlation between As₂O₃ and Fe₂O₃, confirming the dominant role of Fe(III) oxides in arsenic retention, while As₂O₃/Fe₂O₃ ratios rose with pH (r = 0.76, p < 0.01), indicating that under the slightly alkaline, sub-oxic floodplain conditions arsenic is preferentially mobilized relative to iron. Spatial mapping and longitudinal analyses revealed downstream hotspots of arsenic and nutrient accumulation, particularly in low-energy floodplain zones where depositional processes dominated during the YAGI storm. These findings emphasize the dual nature of flood impacts, simultaneously enhancing nutrient enrichment while mobilizing potentially toxic elements. This work provides a comprehensive framework for assessing post-flood soil geochemistry under extreme hydrometeorological conditions and underscores the urgent need for integrated soil monitoring and floodplain management strategies in vulnerable transboundary watersheds.