River morphology redistributes potentially toxic elements in acid mine drainage-impacted river sediments: Evidence, causes, and implications

River morphology plays a vital role in the transport of substance within them. However, our understanding of how natural and artificial morphologies redistribute different potentially toxic elements in acid mine drainage (AMD)-contaminated rivers remains poor. In this study, we linked morphological river features and physicochemical sediment characteristics to trace the redistribution of various potentially toxic elements and elucidate their implications for remediating rivers prone to AMD pollution. A dense network of sediment/soil samples was collected from different river morphological units, such as channels, dam reservoirs, pools, floodplain sandbars and wetlands in an AMD-impacted river. The analyses showed that the contaminant levels in channel generally decreased downstream from the headwater mine site, however, local fluctuations in certain areas were observed due to the trapping effect of various dams along the river. The As and Pb concentrations were higher at floodplain sandbars, while river channels exhibited higher Cd and Zn contamination. The concentrations and geochemical fractions of As, Cd, Cu, Pb and Zn in sediment/soil cores from sandbar and river channel also varied. Additionally, structural equation modeling analysis indicated that spatial variations in contaminant distributions were directly affected by physicochemical properties (such as the soil/sediment Fe, Zn, and S concentrations, and pH), which are indirectly affected by river morphology. The diverse morphology of the river redistributed AMDderived contaminants and could be used to identify contamination hotspots. Our analyses suggested that the feasibility and efficiency of previously proposed countermeasures varied for contaminants in different geomorphological units. In river channels, As uptake from sediments by aquatic plants may be less efficient than Cd, Cu, and Zn uptake due to its lower bioavailability. Moreover, vegetation prevented contaminant enriched soil particle erosion more than it aided in the phytoremediation of As- and Pb-contaminated sandbars. Thus the finding of this study provide a theoretical foundation for further studies on the transport and storage of AMDderived contaminants in similar rivers, along with the development of targeted remediation methods.

Automated summary

River morphology is important in redistributing potentially toxic elements in AMD-contaminated rivers. The study provides insights into the implications of river morphology on remediating AMD pollution and identifies contamination hotspots.