A multivariate assessment of metal distribution in inter-tidal sediments of the Blackwater Estuary, UK

The Blackwater Estuary, Essex, UK is the primary site for the investigation of Managed Retreat: a coastal management technique aimed at the restoration of salt marshes. The success of Managed Retreat trials will be judged by the development of healthy salt marsh communities. Accretion of sediments is a key part of this development. The metal content of accreting sediments may be significant in salt marsh development and has environmental importance. Therefore, information on estuarine sediment geochemistry in the Blackwater Estuary will be valuable in interpretation of the geochemical development of Managed Retreat sites. The spatial distribution of 15 metals in the sediments of the estuary has been assessed by the use of the multivariate statistical techniques: cluster analysis; multidimensional scaling; principal component analysis; and canonical variate discriminant analysis. The assessment indicated a general landward to seaward decrease in heavy metal concentrations in the estuary. This was the effect of elevated concentrations of heavy metals, particularly Pb, Zn, Cd and Cu, at the head of the estuary resulting from specific discharges associated with the congregation of anthropogenic activity in this zone. In comparison, sediments in the lower estuary, where Managed Retreat schemes have been instigated, were less contaminated. However, heavy metal concentrations were elevated in the lower estuary at a site of enhanced deposition, which was proximal to a localized source of metals. This has implications for concentrations of metals incorporated into sediments at Managed Retreat sites in the lower estuary where accretion is being actively promoted. A depletion of calcium was observed in salt marshes as compared to mudflats throughout the estuary.