Bioavailability, (im)mobilization kinetics, and spatiotemporal patterns of arsenic and cadmium in surficial sediments of a river–estuary–coast system

Abstract

• High-resolution distributions of DGT-labile As, Cd, Fe, Mn and S are determined. • Fe cycling dominates As mobility, while aerobic OM oxidation controls Cd mobility. • Terrigenous input and redox condition trigger the contrasting seasonal availability of As and Cd. • Cd bioavailability evaluated by DGT clearly increases with salinity gradient. The mobility and bioavailability of toxic arsenic (As) and cadmium (Cd) in sediments are crucial to aquatic ecosystems, while their geochemical transitions and spatiotemporal patterns associated with manganese (Mn), iron (Fe), sulfur (S) and salinity in a river–estuary–coast system are yet largely unexplained. In this study, seasonal field investigation and salinity-dependent indoor experiments were combined using the diffusive gradients in thin films (DGT) technique to measure DGT-labile species from the simultaneous river-estuary-coast interface. The results revealed that As and Fe presented a strong coupling for both DGT species and solid phases, suggesting that As mobility was exclusively regulated under the redox transition of Fe. Unlike other chemicals, the relative enrichment of DGT-labile Cd in surface layers and the close affiliation between sediment Cd fractions and organic carbon indicated that Cd remobilization was largely associated with the aerobic decomposition of organic matter. In addition, the seasonal availability of As and Cd tended to be influenced by terrigenous input and redox condition. The indoor experiment results of an inconspicuous increase in DGT-labile As, and a steady increase in DGT-labile Cd with rising salinity confirm that the bioavailability of Cd evidently increased with salinity gradient, however, the effect of salinity on As remained further debatable. Nevertheless, a model simulation validated that this salinity-dependent Cd mobilization was suboptimal for sustaining a continuous Cd resupply from the sediment to porewater, particularly under limited labile Cd pools. Therefore, with expanding global marine hypoxia and sea level rising, this study highlights that As and Cd redox geochemistry in dynamic estuarine sediments should receive more attention, considering their distinctly contrasting (im)mobilization kinetics.