Abstract
The current study evaluates the effect of seawater physico-chemical characteristics on the relationship between the concentration of metals measured by Diffusive Gradients in Thin films (DGT) passive samplers (i.e., DGT-labile concentration) and the concentrations measured in discrete water samples. Accordingly, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to measure the total dissolved metal concentrations in the discrete water samples and the labile metal concentrations obtained by DGT samplers; additionally, lead and cadmium conditional labile fractions were determined by Anodic Stripping Voltammetry (ASV) and total dissolved nickel was measured by Cathodic Stripping Voltammetry (CSV). It can be concluded that, in general, the median ratios of DGT/ICP and DGT/ASV(CSV) were lower than 1, except for Ni (median ratio close to 1) and Zn (higher than 1). This indicates the importance of speciation and time-integrated concentrations measured using passive sampling techniques, which is in line with the WFD suggestions for improving the chemical assessment of waterbodies. It is the variability in metal content in waters rather than environmental conditions to which the variability of the ratios can be attributed. The ratios were not significantly affected by the temperature, salinity, pH, oxygen, DOC or SPM, giving a great confidence for all the techniques used. Within a regulatory context such as the EU Water Framework Directive this is a great advantage, since the simplicity of not needing to use corrections to minimize the effects of environmental variables could help in implementing DGTs within monitoring networks.
Highlights
The EU Water Framework Directive (WFD, 2000/60/EC) defines good chemical status of water bodies as being achieved when the concentrations of priority substances do not exceed the relevant Environmental Quality Standards (EQS) established by Directive 2008/105/EC
The current study evaluates the effect of seawater physico-chemical characteristics on the relationship between the concentration of metals measured by Diffusive Gradients in Thin films (DGT) passive samplers (i.e., DGTlabile concentration) and the concentrations measured in discrete water samples
With respect to metals, the EQS refers to the dissolved concentration, i.e. the metal concentrations measured in a water sample previously filtered through a 0.45 μm filter and acidified or subjected to any equivalent pre-treatment (CIS, 2009)
Summary
The EU Water Framework Directive (WFD, 2000/60/EC) defines good chemical status of water bodies as being achieved when the concentrations of priority substances do not exceed the relevant Environmental Quality Standards (EQS) established by Directive 2008/105/EC (subsequently amended by Directive 2013/39/EU). Metals are present in different chemical forms; i.e., free ions, complexes with inorganic and organic ligands and/or adsorbed on the surface of particles or colloids (Hirose, 2006; Tercier-Waeber et al, 2012). This implies that different methodologies may measure different fractions (or chemical forms) of the total metal content. The most common monitoring approach used for the purpose of WFD compliance assessments for metals relies on water samples obtained by spot sampling, followed by filtration (dissolved metal), preconcentration and instrumental analysis (CIS, 2009). The inclusion of complementary methodologies, which integrate the environmental metal fluctuations and/or measure the metal speciation that can be more related to ecotoxicological effects, might improve the quality of the assessment (CIS, 2009)
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