engleski

Vertical distribution of trace metals in the Krka River estuary (Croatia) accessed DGT and voltammetry

Trace metal speciation in the highly stratified oligotrophic Krka River estuary (Croatia) has been studied by using diffusive gradients in thin films (DGT) and stripping voltammetry. Halocline in estuary is usually formed between 1.5 and 2.5 meters, and ranges from 30 cm to around 1 m. High degree of variability of physico-chemical parameters (S/T/pH/O2), as well as of major ion composition is spotted in the halocline, causing changes not only in metal concentrations but also in metal speciation. It is considered that increased biological activity in the freshwater- seawater interface (FSI) plays an important role in control of metal speciation. Two types of DGT devices (diffusive and restricted) were deployed for defined period (around 110 hours) at six depths. Each day, one (in winter) or two (in summer) discrete samples were taken at each depth, in order to track daily changes of metal concentrations. The same samples were used to make eighteen composite samples. Anodic (Zn, Cd, Pb and Cu) and adsorptive cathodic (Ni, Co) stripping voltammetry were used for determination of metal concentrations, as well as for copper speciation study. DGT-labile metal concentrations were determined by HR ICP-MS. High nautical traffic in the estuary during summer season generates significant increase of some trace metals in upper brackish layer (3-5 times for copper). Although concentration of Cu is increased in the upper layer (without pronounced change in DOC concentration), decrease in percentage of non-labile copper complexes in seawater layer suggests an existence of ligands which form stronger copper complexes in brackish layer. According to obtained metal speciation data, model projections show that in upper layer free copper concentrations could over-pass toxic level (10-11 M).

trace metal; speciation; DGT; stripping voltammetry

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