Nickel in the Humber Plume : Influences of Particle Dynamics and Reactivity

The concentrations of dissolved Ni (Nid) and particulate Ni (Nip) in suspended particulate material (SPM) and sediments have been determined in samples collected from the Humber coastal zone on four occasions during 1994 and 1995. At the mouth of the Humber Estuary, concentrations of Nid varied from 0·5–4·1 g l 1 and had a significant inverse relationship with salinity, whereas the concentrations of Nip (defined as available to 1M HCl) ranged from 3·5 to 45 g g 1 and had variable relationships with salinity. The combined flux of Nid and Nip from the Humber mouth to the North Sea was estimated to be 58 t a 1, with 50% of the annual load being discharged in approximately two months. Particulate Ni concentrations in the plume were lower, due to a combination of particle settling and mixing between SPM from the estuary and from ambient sources, including Ni-depleted particles from coastal erosion with mean a Nip concentration of 5·1 g g 1. The mixing ratio of estuarine to ambient particles was seasonally-variable and was dependent on the concentration of ambient SPM. The variability in the phase transport of Ni was determined from the combined Nip and Nid concentrations, which were bounded by partition coefficients (Kds) of 5 103 l kg 1 applying to estuarine SPM and 105 l kg 1 to ambient SPM. The results suggest that the Humber plume is the repository for significant fraction of the Nip discharged from the estuary and there is a substantial dilution of SPM from the Humber Estuary by SPM from coastal erosion. Thus, the impacts of Ni discharges from the Humber Estuary on the biologically-sensitive areas of the North Sea are mitigated by particle dynamics in the nearshore region.