Sediment resuspension under variable geochemical conditions and implications for contaminant release

Purpose Resuspension of sediment derived from natural and anthropogenic processes in surface waters may induce the release of metals, nutrients and other undesirable constituents from sediment to the water column. Characterizing the effects of sediment resuspension under variable geochemical conditions is important for predicting and mitigating contaminant releases during potential dredging operations, as metal(loid) transport may differ under oxidizing and reducing conditions. Materials and methods Lake sediment core samples containing elevated concentrations of mercury, other metal(loid)s and nutrients were physically and chemically characterized. Speciation analyses using sequential extraction and synchrotron radiation-based X-ray absorption spectroscopy methods were used to investigate the chemical form and distribution of mercury and other metal(loid)s. The sediment samples were subjected to resuspension under oxic and anoxic conditions to isolate the effects of air entrainment on contaminant release. Results and discussion Sequential extraction analyses suggested that the mercury was present primarily in moderately to strongly bound forms, such as mercury-organo chelates and mercury sulphides, whereas only trace amounts were found as less bound forms of mercury. Synchrotron-based micro-X-ray absorption near-edge structure (μ-XANES) and micro-X-ray fluorescence (μ-XRF) spectroscopy analyses suggested the presence of mercury oxide, mercury sulphide and copper sulphide phases. Higher aqueous metal(loid) and nutrient releases were observed under oxic resuspension conditions compared to anoxic conditions. Conclusions Higher releases of some constituents under oxic mixing conditions potentially were due to the oxidation of organic matter and/or sulphide mineral phases. These results suggest that resuspension tests performed under variably oxygenated conditions may provide a useful analytical tool for isolating the effect of air entrainment on contaminant release.