Urban wetlands regulate trace element dynamics at the sediment- water interface

Urban wetland systems play a crucial role in regulating hydrological processes and intercepting pollutants. However, the biogeochemical mechanisms governing trace element dynamics at the sediment-water interface (SWI) remain poorly understood. In this study, we investigated the vertical distribution of trace elements in sediments from Changguangxi (Wuxi, China) and explored their possible transformation processes. The results showed that urban wetlands could significantly modify the physicochemical conditions of the SWI and influence trace element dynamics through interactions involving iron and dissolved organic matter (DOM). The pollution loads of arsenic (As), cadmium (Cd), copper (Cu), lithium (Li), nickel (Ni), lead (Pb), antimony (Sb), and zinc (Zn) decreased from upstream to downstream sediments, and most elements exhibited higher concentrations in pore water than in overlying water. As, Co, and W showed persistent positive concentration gradients at the SWI in both upstream and downstream regions, whereas Sb, Ni, and Cd exhibited consistent negative concentration gradients, indicating that the dynamic migration of elements at the SWI may exhibit selectivity. Multivariate analysis indicated that element variability upstream was primarily associated with DOM, while wetland sediments showed combined associations with DOM and Fe²⁺/TFe. These results demonstrated a longitudinal shift toward DOM/Fe redox coupled control, suggesting that wetland conditions promote selective rather than uniform trace element release at the SWI. Our findings suggest that higher organic loads may increase the risk of releasing certain elements. Therefore, targeted hydrological control measures could be implemented to mitigate the potential pollution risks of urban wetlands.