Temporal responses of hydrochemical variables and dissolved Fe(II) to flooding at a lake riparian wetland under different vegetation revealing by high resolution DGT.

The interplay between hydrological and biogeochemical processes in riparian wetland was recognized to lead directly to the temporal variations of surface water quality. However, the effects of flooding and vegetation on the release and entrapment of heavy metals and nutrients in riparian wetland remain poorly understood. The study aimed at investigating the influences of flooding and vegetation on the hydrochemical and Fe-redox change in the soil porewater and shallow groundwater, in Poyang lake riparian wetland through hydrochemical monitoring and diffusive gradient technology (DGT). The hydrochemical profiles and results of PCA analysis on the temporal datasets both demonstrated that vegetation had significant influences on the hydrochemistry of rhizosphere depth zone (RDZ) and shallow groundwater depth zone (SGZ). The Ca, K, Na, Mg, Mn and DOC at RDZ of both plants showed significant increasing trends from pre-to post-flooding while were observed minor change at the SGZ. The extracted PC1-PC3 from PCA analysis suggested that mineral dissolution and fermentation were dominating processes that explained 64.1% of the hydrochemical variability under the wetting-drying cycle. The synchronous changes of Fe(II), SO, DOC and ORP were found to occur at the SGZ of Carex, implying the likely occurrence of Fe- and S- redox reactions. The Fe(II) DGT profiles evidenced the temporal iron reduction and oxidation occurring at the rhizosphere following the wetting-drying cycle, as also reflected by the high opposite Fe and DO association through PCA analysis. The high resolution temporal-spatial Fe(II) distribution suggested also the interface between lake water and groundwater was relatively stable under flooding. These results highlight that the release of dissolved Fe(II) from the wetland rhizosphere driven by flood may result in the release of Fe-associated heavy metals from riparian wetland to surface water, and hence pose potential threats to the surface water quality. Thereby, the flow and flood should be properly controlled and vegetation effects need to be carefully considered in the water resources management of lake-floodplain system.