ARSENIC IN SUB-ARCTIC LAKES INFLUENCED BY GOLD MINE EFFLUENT - THE OCCURRENCE OF ORGANOARSENICALS AND HIDDEN ARSENIC

A series of subArctic lakes near Yellowknife, Northwest Territories, C anada, are contaminated with arsenic released from a gold mine. The hi gh environmental arsenic levels afforded an opportunity to identify fa ctors that control methylarsenical production in freshwater sediments. A large variety of methylated arsenicals were observed in most pore w ater and water column samples from lakes contaminated by aqueous or at mospheric arsenic inputs, albeit at low concentrations relative to ino rganic arsenic (generally less than 10% of the total dissolved arsenic concentration). Biologically-mediated methylation/demethylation react ions, therefore, may influence the local arsenic cycle. Water column s amples exhibited a consistent methylarsenic composition in which the d ominant form was dimethylarsonic acid, followed by monomethylarsinic a cid. A much broader range of mono-, di- and trimethylated arsenicals w as found in sediment pore water, and the composition of methylarsenic species was highly variable both within and between cores. We provide indirect evidence that many samples contained mono-, di-, and trimethy lated arsenic(III)thiols; of the form (CH3)(n)As-III(SR)(3-n) (n = 1, 2, 3), the existence of which has been predicted but not previously de monstrated in the environment. The total dissolved methylarsenic conce ntration in cores consistently exhibited a subsurface maxima between 5 and 15 cm depth, which suggests - in conjunction with the presence of other metabolites in pore water - that methylation may be enhanced by sulfate-reducing bacteria. The role of sulfate-reducers in the produc tion of methylarsenicals observed in sediment pore water is also suppo rted by co-variations with depth between the concentration of methylar senic species and iron dissolution or other chemical distributions att ributed to metabolism by heterotrophic bacteria across different redox zones. Many samples contained appreciable concentrations of arsenical s the specific identity of which are presently unknown, and some of wh ich are 'hidden' to conventional hydride-generation analyses. At least one unidentified arsenical found in lake water and sediment pore wate r appears to have been produced directly or indirectly from atmospheri c emissions from the gold mine. The occurrence of methylated arsenical s and hidden arsenic at concentrations that comprise a substantial por tion of the total arsenic budget in some aquatic systems suggests that current models of lacustrine arsenic cycling that incorporate only in organic arsenicals need to be revisited.