Particulate metals and their settling velocities in the Humber Estuary, UK

Concentrations of particulate Fe (FeP), Mn (MnP), Zn (ZnP) and Ni (NiP) have been determined in suspended particulate matter (SPM) from the mouth of the Humber Estuary and its coastal waters during five cruises in 1994–1995. The particulate metal concentrations were inversely related to salinity, although the relationship was less pronounced at times of high fluvial input and increased tidal amplitude. During each tidal cycle, water samples were collected using a quasi in situ Settling Velocity Tube (SVT). Five settling fractions of SPM were retrieved from the tube as a function of time and the median settling velocities (ws50) determined for SPM and for each particulate metal. At the mouth of the Humber ws50 for SPM was in the range 1 to 200 μm s−1, whereas the SPM from the coastal waters had a value of 40 μm s−1. Concurrent measurements of in situ particle diameters of SPM, using a laser reflectance particle sizer, gave values varying between 44 and 166 μm, with the largest particles being detected on flood tides in winter 1995 and summer 1995. The values of ws50 for the particulate metals covered two orders of magnitude, with the highest values being for MnP (270 μm s−1), NiP (110 μm s−1) and ZnP (130 μm s−1) during dynamic conditions in spring 1995, whereas in summer 1995 all the values of ws50 were in the range 1–3 μm s−1. In some cases, particulate NiP and ZnP had lower values of ws50 than the bulk SPM. The results suggest a differential transport of metals based on their settling velocities and their preference to associate with various components of a particle assemblage. The results are discussed in the context of the implications for the refinement of coupled hydrodynamical–geochemical models for SPM and particulate metals.