In situ measurements of labile Cu, Cd and Mn in river waters using DGT

The technique of diffusive gradients in thin-films (DGT) has been trialed in two river systems for in situ trace metal speciation measurements. This paper presents results for cadmium, copper and manganese concentrations in fresh and estuarine waters and demonstrates for the first time the effectiveness of using DGT to measure labile metal concentrations in such waters. This work has shown that even with very simple deployment systems the technique is sensitive and reproducible. The precision of in situ DGT measurements was typically 11% or better when the precision of subsequent analysis was good. The in-built metal pre-concentration procedure of DGT allowed Cd to be measured at concentrations below the detection limit of a direct determination by GF-AAS. The theoretically predicted linear increase in mass with time was obtained in river deployments of up to 72 h, confirming steady state river conditions and indicating no adverse effects due to biofouling over this period. Concentrations of DGT-labile Cu and Cd were equal to dissolved (0.45 micron) metal concentrations for the Ring and Stitt Rivers indicating the absence of tightly bound organic complexes or colloidal species. DGT-labile Cu and Cd were approximately 50% of the dissolved metal in the Que River and DGT-labile Cu was approximately 30% of dissolved Cu in the Savage River. A concentration-depth profile of labile manganese was obtained in a stratified estuary by deployment of a string of DGT devices at 0.3-m intervals across the redoxcline. Results revealed a large spike (maximum concentration = 1.4 mg/l) of DGT-labile Mn at the base of the redoxcline and demonstrate the utility of DGT to determine vertical changes in metal speciation across redox boundaries in stratified estuarine systems. It is in such dynamic systems as this that the in situ capabilities of DGT are likely to be most useful. DGT records multiple metal species as they exist in situ, overcoming the considerable problems of contamination often associated with sample collection and handling.