Reactivity and potential mobility of metals in human-impacted harbor sediments (port of Rotterdam, the Netherlands)

Purpose: Vast amounts of harbor sediments are being dredged every year globally. These sediments are often enriched in potentially toxic elements (PTEs), the abundance and potential mobility of which are important for sustainable harbor management practices. In this study, we investigate metal (especially PTE) distribution, abundance, and reactivity in sediments along a salinity gradient in the waterways of Europe’s largest harbor, the Port of Rotterdam. Materials and methods: Bulk surface sediments were analyzed for general physicochemical properties (e.g. grain size, total metal concentration). From selected locations covering the local salinity gradient, sediments were subjected to three independent chemical extractions to determine chemically reactive metal pools. Additionally, metal retention in two contrasting sediments (high versus low salinity) was further explored using pH-dependent leaching experiments in combination with a geochemical model. Results and discussion: The majority of the investigated sediments consisted predominantly of silt and were rich in organic matter. Concentrations of Cd, Pb, Zn, Cu decreased with increasing salinity. Concentrations of Al, Fe, V correlated negatively with grain size, because these geogenic metals are enriched in fine-grained silicates. Results from the chemical metal extractions showed clear differences in the reactivity and mobility potential of metals, that could be grouped into four clusters. The combined chemical and modeling results indicate that sorption onto metal (oxyhydr)oxides and organic matter as well as precipitation of metal sulfides and carbonates control metal retention. High reactivity and mobility potential were observed for Pb, Cd, Zn, particularly at low pH. Limited spatial variability in metal chemistry along the salinity gradient indicates that the highly variable depositional conditions have little impact on metal behaviors. Conclusions: Chemical extractions and pH-dependent leaching experiments revealed distinguishing metal reactivities from four clusters. Our results provide insight into metal distribution in the dynamic estuarine environment of the Port of Rotterdam and highlight the importance of understanding chemical speciation in addition to abundance for harbor sediment management.