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A novel solid-phase preconcentration method is reported, using in-house gold-coated silica adsorbent packed in a microcolumn, for the determination of dissolved mercury in natural waters by atomic fluorescence spectrometry (AFS). The adsorbent was prepared by chemical reduction of a Au(III) solution with hydroxylamine in the presence of suspended silica particles. The resulting Au nanoparticles on the silica surface were highly efficient for adsorbing different mercury species from acidified waters without additional reagents. The acidified aqueous samples were passed over the microcolumn, either incorporated in a fully automated flow injection (FI) system directly coupled to the AFS or as part of a portable FI system for in situ preconcentration. After rinsing and drying of the column, Hg 0 was released by heating and directed to the AFS cell for quantification. The method offers significant advantages because no reagents are needed for species conversion, preconcentration, sample storage, or desorption and therefore the risk of contamination is minimized and blank values are lowered. This results in a low detection limit of 180 pg L -1 using a sample volume of only 7 mL and good reproducibility, with relative standard deviations <3.2% (n ) 10, [Hg] ) 5 ng L -1 ). Recoveries were all >90% in spiked river waters (spiked [Hg] ) 0, 1, 5, 10 ng L -1 ), and the experimental value for the certified reference material ORMS-4 (elevated mercury in river water) was 22.3 ( 2.6 ng Hg L -1 which was in good agreement with the certified value of 22.0 ( 1.6 ng Hg L -1 (recovery ) 101%). The method was successfully applied to seven different natural waters and wastewaters ([Hg] 0.5-4.6 ng L -1 ) from south west England.

1. Macroinvertebrate species and information on thirty-nine variables were recorded at forty-six sites on twelve Cornish streams affected to varying extents by past metalliferous mining. 2. Relationships between macroinvertebrate communities and environmental variables were examined using canonical correspondence analysis (CCA). Copper was the strongest correlate with Axis 1 of the analysis, suggesting that it may have a major role in determining community structure. There were also strong correlations between Axis 1 and aluminium, alkalinity, pH, dissolved organic matter and algal cover, and between Axis 2 and discharge.3. CCA distinguished four site groupings. In spite of seasonal changes in position on the ordination, as revealed through CCA and detrended correspondence analysis (DCA), sites retained their group membership. There were differences in the proportions of the major taxa within the four CCA-derived groups. In the group with the highest copper concentration, Ephemeroptera were absent, but triclads and chironomids were abundant, with Orthocladiinae the dominant group. 4. Changes in the occurrence of spedes along the metal gradient reflected changes in sensitivity. 'Tolerance' plots of selected species based on the CCA site ordination diagram were used to identify environmental thresholds. 5. As well as direct toxic effects of copper and aluminium on invertebrate communities, co-precipitation of these two metals could be an important physical factor. Further modifying influences resulting from interactions between toxic metals and pH, alkalinity, hardness and dissolved organic matter contributed to the complexity of conditions affecting the faunat community in metal-contaminated streams. 6. The most severely contaminated sites, with mean copper concentration exceeding 500(xgp', were all characterized by a reduced community dominated by the flatworm Phagocata vitta, the chironomids Chaetocladius melaleucus and Eukiefferielia claripennis and the net-spinning caddis Plectrocnemia conspersa. The necessity for spedes identification is demonstrated in relation to variation in metal tolerance.

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Arsenic is ubiquitous in nature appearing in various chemical forms.

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