Release of Nutrients and Metals Following Oxidation of Freshwater and Saline Sediment

Abstract

AbstractMississippi River deltaic sediments were collected from freshwater and adjacent saline environments along Louisiana's Gulf Coast to evaluate chemical changes that may develop when bottom sediment from different salinity regimes with contrasting levels of reduced S are exposed to anoxic environment. Chemical transformations of the dredged sediments were influenced by changes in the sediment‐water pH and oxidation‐reduction status. Sediment pH decreased as the redox potential (Eh) was increased in both the freshwater and saline sediment. Both sediments had near‐neutral pH when maintained under anoxic conditions and the minimum pH developed under oxic conditions was 5.1 and 3.0 for the freshwater and saline sediment, respectively. The acidity developed in the saline sediment resulted in the release of the potentially toxic metals Pb, Cu, Ni, Cr, Cd, and Sb into solution. There was also an increase in the solution concentration of Fe, Mn, Al, and Se. The solution concentration of these elements was inversely proportional Eh (P ≤ 0.05). The freshwater sediment when exposed to oxic conditions showed no increase in the solution concentrations of these elements. Results suggest that the concentration of reduced S in anoxic dredged sediment along other areas of the Gulf Coast should be closely evaluated in relation to potential acidity before subjected to upland disposal.