Geochemical controls on trace element mobilization in contaminated estuarine sediments

Early diagenesis and its impact on Fe, Mn, Cr, Co, Pb, As, Cu, Cd, Ni, Zn and P behavior in Scheldt sediments were investigated in 2023 using DGT sampling and Visual MINTEQ modeling. Mobilization of trace elements was mainly related to the dissolution of iron and manganese oxyhydroxides under sub-oxic redox conditions (May and December) and a lower pH (December). Labile fractions of the various elements varied also with sediment depth and season. PCA analysis confirmed the mobilization behavior of the elements as observed in their vertical profiles and in relation to that of Fe and Mn, while the fractions of inorganic and organic element complexes, as well as the saturation index (SI) for specific minerals were calculated with Visual MINTEQ. The SI of CoFe₂O₄, Hematite, MnHPO₄(s) or Rhodochrosite was oversaturated. Upper estuarine sediments in the early 90's were partially anoxic with much higher sulfide and phosphate levels resulting in lower dissolved Fe and Mn levels than in 2023. In contrast, concentrations of all other trace elements in porewater were higher 30 years ago, due to much higher discharges of pollutants in that period, but this was not the case for the sediment solid phase with similar element concentrations found in both periods. Consequently, the high solid phase concentrations in upper estuarine sediments remain an important reservoir of trace elements and a potential threat to the local ecosystem, because, as we showed, under specific physico-chemical conditions, they can be released towards the water column. and assimilated by biota.