Short-term (24 h) effects of mild and severe hypoxia (20% and 5% dissolved oxygen) on metal partitioning in highly contaminated estuarine sediments

The effects of a short-term (24 h) reduction in bottom-water dissolved oxygen concentration (DO) on metal (Cd, Cu, Fe, Mn, Pb and Zn) partitioning within metal-contaminated sediments in replicated, undisturbed sediment/water chambers show that even very brief periods of hypoxia may significantly increase the dissolved fraction of these heavy metals within contaminated sediments, increasing their potential for ecological harm. This study used treatments consisting of three DO levels (75%, 20% and 5% saturation) representing ambient conditions, mild hypoxia and severe hypoxia. Although contaminant loads were very high in these sediments, pore-water concentrations were relatively low. Total sediment and dilute acid (1 M HCl) extracted metals were unaffected by low DO treatments but a diffusive gradient in thin-films samplers (DGTs), detected a two-fold increase in pore-water Cd and a five-fold increase in pore-water Cu in surface sediments (0–3 cm depth) under low oxygen conditions.