| Total metal concentrations
in surface sediments and historically contaminated sediments were determined in
sediment cores collected from three estuaries (Thames,
Medway and Blackwater) in south-east England. The partitioning behaviour of
metals in these sediments was also determined using a
sequential extraction scheme. These data were then compared with sediment
quality values (SQVs) to determine the potential
ecotoxicological risk to sediment dwelling organisms. When total metal
concentrations in surface sediments are examined, no risk to
biota in any of the estuaries is indicated. However, when historically
contaminated sediments at depth are also considered, risks to biota are
apparent and are greatest for the Thames, followed by the Medway and then the
Blackwater. This suggests that regulatory authorities should
examine vertical metal profiles, particularly in estuaries that are experiencing
low sediment accumulation rates where historically contaminated
sediments are in the shallow sub-surface zone and where erosion or dredging
activities may take place. When metal partitioning characteristics
are also considered, the risk to biota is comparable for the Medway and the
Blackwater with the potentially bioavailable fraction
presenting no ecotoxicological risk. Conversely, over 70% of metals are labile
in the Thames Estuary sediments and toxic effects are probable.
This suggests that the application of SQVs using total sediment metal
concentrations may over- or under-estimate the risk to biota in
geochemically dissimilar estuarine sediments.
Keywords: sediment quality values, estuarine sediments,
metal contamination, partitioning, sequential extraction |