Abstract
Metal behaviour and availability at a contaminated dredge material disposal site within UK waters has been investigated using Diffusive Gradient in Thin films (DGT) passive sampling technology. Three stations representing contrasting history and presence of maintenance dredge disposal, including a control station outside the disposal site, have been studied and depth profiles of fluxes of different metals (Fe, Mn, Pb, Cu, Cd, Cr, Ni, Zn) to the binding gel (Chelex 100) have been derived. Higher flux rates and shallower mobilisation of metals (Mn and Fe) to the binding gel were observed at the disposal stations compared to the control station. Here we describe metal mobilization at different depths, linking the remobilization of Fe 2+ and Mn 2+ to the sediment (re)supply of other heavy metals of interest with a focus on Cd, Ni and Pb and as they are on the Water Framework Directive (WFD) list of priority substances and OSPAR list of priority pollutants. Results showed that Cd, Pb and Ni exhibited signs of resupply at the sediment-water interface (SWI). There was a potential increased mobilisation and source to the water column of Pb and Ni at the disposal site stations, but there was no Cd source, despite higher total loadings. This information has the potential to improve our current understanding of metal cycles at disposal sites. This work can be used as an indication of likely metal bioavailability and also assist in determining whether the sites act as sources or sinks of heavy metals. This information could assist disposal site monitoring and dredge material licensing.
Highlights
There are approximately 150 sites designated for dredging disposal around the coast of England, with the majority of these located on the coast of the mainland, generally within a few miles of a major port or estuary entrance (Bolam et al, 2011)
Sampling approach: 0.1 m2 NIOZ box-cores were collected at 3 stations: (4 core replicates were collected from the Reference station, located south of the disposal site and 3 core replicates were taken from Station C and station S, both located within the disposal site (Figure 1)
The metal behaviour at each of the stations has illustrated the clear advantages of using suite of different techniques to characterise metal station and with depth behaviour rather than bulk analysis alone
Summary
There are approximately 150 sites designated for dredging disposal around the coast of England, with the majority of these located on the coast of the mainland, generally within a few miles of a major port or estuary entrance (Bolam et al, 2011). Approximately 40 million wet tones of dredged material are annually disposed of at these licensed sites, not all of the sites are annually used. Monitoring associated with these disposal sites is carried out annually and includes a number of different contaminant parameters, such as tributyltin, polyaromatic hydrocarbons, organohalogens and trace metals measured as total metal concentrations in bulk sediments. The monitoring is based on impact hypotheses, which are set up for addressing important questions These are: i) what is the fate of contaminants (including metals) imported to the site with the disposed material and what effect does this have on ecological components? The aim of this work was to apply DGT technology as a complementary tool to the routine monitoring conducted at Souter Point disposal site to provide an improved understanding of metal behaviour and fate
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