Mobilization of porewater Pb and Zn in response to seasonal wetting and drying within contaminated floodplains

The mobility and bioavailability of metal contaminants such as lead (Pb) and zinc (Zn) is impacted by their interactions with other sediment constituents such as iron (Fe), sulfur (S), and organic matter, which depend on sediment redox conditions. Understanding the role that water level fluctuations have on redox conditions and subsequent impacts on metal mobility is critical for predicting impacts of increased wetting and drying cycles resulting from climate-related changes or management actions. This study measured the sediment-porewater partitioning of Pb and Zn in the Coeur d'Alene River basin downstream of the Bunker Hill Superfund Site under both flooded and seasonally dry conditions. The results show that both time of year and hydrology are important when considering metal exposure risks in contaminated floodplains. For Pb, seasonal spring flooding appears to mobilize dissolved Pb in both seasonally inundated and permanently inundated areas due to increases in sediment-derived Pb that undergo desorption processes from suspended Fe and DOC. For Zn, oxygenation of floodplain sediments in the fall drives elevated dissolved Zn year-round due to limited ZnS precipitation. Wetting-drying cycles had a significant impact on Zn mobility, which could be exacerbated by climate-driven hydrological changes or floodplain management actions. [Display omitted] •Dissolved Pb and Zn measured in floodplains with different wetting-drying cycles.•Spring flooding mobilizes Pb regardless of different wetting-drying cycles.•Floodplains that undergo wetting-drying have elevated dissolved Zn year-round.