Heavy Metal Fate from Water into Sediment and Its Risk Assessment: A Lesson from Cadmium

Anthropogenic heavy metal emissions can severely impact lake ecosystems. However, the fate processes of heavy metals and their impact on the aquatic environment during acute input events are poorly understood. To address this, our study selected cadmium (Cd), the predominant metal contaminant in Baiyangdian lake sediments, as a model pollutant to simulate the fate processes within sediment microcosms. We examined the total Cd content and its fraction distribution within the sediment profile, revealing a substantial increase in the Cd content within the surface layer while also identifying its predominant presence in the exchangeable fraction and quantifying its mobilization index. Notably, the fated Cd resulted in a pronounced increase in the ratio of simultaneously extracted metals to acid-volatile sulfide in the 0 to -4 cm depth range, exceeding toxic thresholds for benthic organisms. By deploying diffusive gradients in thin films, we characterized variations in bioavailable Cd with depth. Results indicated that the surge in bioavailable Cd concentrations in the surface layer was facilitated by the simultaneous rise in the concentration of dissolved Cd in porewater and the exchangeable fraction in sediments. This study sheds light on the fate of heavy metals in the aquatic environment and their ecological consequences during acute input events.