Resuspension, reactivity and recycling of trace metals in the Mersey Estuary, UK

Distributions of dissolved and non-detrital (available to 1 M HCl) particulate Cd, Co, Cu, Ni, Pb and Zn have been determined in samples collected from the Mersey Estuary, UK, during four high-resolution axial transects. With the exception of Cu, dissolved metal concentrations displayed positive deviations from conservative behaviour. Cobalt and Ni appeared to have a common source, while dissolved Pb peaks coincided with maximum turbidities in the upper estuary. Concentrations of Cd, Co, Ni and Pb in the <63-μm grain size fraction of bed sediment were greater than those in suspended particulate matter (SPM). Magnitudes of dissolved metal peaks reflected the extent of metal enrichment in fine bed sediment compared with SPM, suggesting that addition of dissolved metal was effected primarily by desorption from resuspendable bed particles. An empirical sorption model, based on the mass balance of metal resuspended from the bed, was able to reproduce the axial distributions of dissolved metals that exhibited the most pronounced peaks (namely, Co and Pb) with reasonable success. More generally, model results indicated that bed sediment need only be moderately enriched in trace metal compared with SPM to cause measurable addition of dissolved metal to the overlying water column. Addition of dissolved metals on estuarine mixing is a common observation in macrotidal environments of western Europe, and it is proposed that resuspension of metal-enriched, fine bed sediment may be a significant agency for metal remobilisation in contaminated, turbid estuaries.