METAL METHYLATION IN ESTUARINE WATERS

Abstract

Metal methylation has been studied in estiiarine waters by directly coupled GC-AAS/llquid nitrogen trapped hydride generation systems and determined accurately. Surveys were carried out in the estuaries of the River Tamar, Camon and Beaulieu (S. England) over a seasonal cycle. Methylated forms of Fb were only observed in rain, drain and estuarine water samples collected in the Tamar. In estuarine waters (GH«)«Fb*'' (<2-10 ng Fb 1"^) was the predominant methylated Fb species. Monomethylarsenic and dimethylarsenic species were observed in the water column in a ll3 estuaries studied. In the Tamar, Beaulieu and Carnon concentrations of methylated As species were similar with concentrations ranging from <0.02-0.7 pg As 1"^. In Tamar porewaters concentrations of methylated As species ranged from< 0.02-0.7 As 1"-^. Methylated forms of Se {Se(CH^)^, 0.5ug Se l"""-) were detected on an occasion in the Carnon. Modelling studies on Pb methylation suggest that conversion of trimethyllead acetate to Ti-IL proceeds by a chemical sulphide-mediated pathway (maximum 2.8% conversion occurred for a biological sediment system). Methylation of inorganic Pb ( H ) salts was not reproducible (maximum conversion of 0,OZQ% for PbGl^) but when observed a biologically mediated methylation process was invoked. From laboratory modelling studies temporal variations in As speciation observed in the Tamar were attributed to seasonal contributions of methylated As species from 3 sources: (i) macro-algae; ( ii ) diatoms; ( iii ) porewaters. The predominance of dimethylarsenic species over monomethylarsenic species in summer reflects the importance of plankton sources. In winter a transition takes place where the predominance of methylated As species occur and concentrations of monomethylarsenic species exceeds that of dimethylarsenic species. This transition is attributed to the increased importance of the porewater source in winter. The significance of metal methylation in the cycling of Pb, As and Se in estuarine waters appears to follow the order Asj>Se>^Fb. The biomethylation of As being highly significant in the transportation of As in the environment.