High resolution dissolved heavy metals in sediment porewater of a small estuary: Distribution, mobilization and migration.

Identifying the distribution features, mobilization mechanisms and migration processes of heavy metals (HMs) in estuarine sediments is essential to predict their potential toxicity risk and for following contamination remediation. In this study, high-resolution dialysis (HR-Peeper) and a sequential extraction procedure were employed to determine the porewater dissolved iron (Fe), manganese (Mn), arsenic (As), chromium (Cr), vanadium (V), selenium (Se), molybdenum (Mo), nickel (Ni), zinc (Zn) and their geochemical species fractions in sediments of the Xixi River Estuary, Xiamen, China. The results showed that at estuarine sites with high TOC and TS content, sulfate reduction is the main diagenetic pathway of OC degradation and directly inhibits the reduction of Fe/Mn oxides. The mobility of most HMs in porewater profiles was influenced by multiple factors, such as the adsorption-desorption by Fe/Mn oxides, HM-sulfide co-precipitation, and the degradation of OM under different redox conditions. However, no environmental correlation and control factors of Ni and Zn have been found. In addition, the profile-averaged distribution of most HMs showed a seaward increasing trend, probably due to the severe industrial wastewater discharge and increasing salinity responsible for the competitive adsorption of HM ions. The overall positive fluxes of all HMs, together with the higher positive diffusion fluxes of some HMs such as Mn, Cr, V and Zn, suggest that the HMs mobility in small estuarine sediments should be seriously reconsidered due to its high contamination potential.