Geochemical behaviors of metal elements during a single flood event in the natural Chishui River, Southwest China

The majority (up to 90%) of riverine materials is transported from the continent to the ocean mainly in flood events. It is thus crucial to characterize the geochemistry of elements and their flux in river system in order to better constrain their global biogeochemical cycling and impact on the oceanic ecosystem. However, the geochemical behavior including the distribution, migration and partitioning of typical metal elements amongst different phases, during hydrodynamic flood event remains still to be well explored. Here, we investigated the geochemical behaviors of typical metal elements in dissolved phase and suspended particulate matter collected from a single flood event in the natural Chishui River, Southwest China. The results showed clearly that the geochemistry of metal elements was largely controlled by the hydrodynamic effect, of which the different flowrates introduce a natural sorting of different mineral particles transported at different flood stages, depending on their shape, size and density. The maximum concentrations of alkaline and alkaline earth metals (Li, Mg, K, Rb and Sr) in SPM appeared before the flood peak, which was largely controlled by aluminosilicate minerals. However, transition metals (Cr, Mn, Fe, Ni and Cu) showed their abundance peaks lagging behind the flowrate summit, as a result of the late arrival of coarse particles or heavy minerals, evidenced by the mineralogical phase analysis. In addition, the distribution coefficient (K_d) between particulate and dissolved loads were lower and stable for soluble alkali/alkaline earth metals which could be affected by pH, while higher and fluctuant for transition metals that were largely influenced by SPM content. Overall, the present study reveals clear effects of hydrodynamic sorting on the geochemistry of metal elements during the flood event of the natural Chishui River, which should be taken into account when characterizing the riverine flux and their impact on marine ecosystem.