Evidence for declining levels of heavy-metals in the Severn Estuary and Bristol Channel, U.K. and their spatial distribution in sediments

Levels of heavy-metals (Cd, Cu, Cr, Ni, Pb and Zn) in suspended particulate and in surface and subsurface sediments were determined at seven locations in the Severn Estuary and Bristol Channel. Sediment metal concentrations were highest at sites close to industrial centres but levels have decreased significantly over the last 30 years so that they are now close to, or meet, environmental quality guidelines. The greatest metal concentrations in deposited sediments were usually associated with the finest particulates at locations with muddy sediments, but this was not always true at sites with predominantly sandy sediments. The metals bound to suspended particulates at all sites were remarkably consistent, presumably reflecting the mixing capacity of this macro-tidal estuary. The re-exposure of older, more contaminated sediments could explain the observed differences between deposited and suspended material. Sediment redistribution due to strong seasonal currents might also explain the differences between winter and summer samples.