MOBILISATION OF ARSENIC IN A SULPHIDE RICH REGION IN N. SWEDEN

The Västerbotten county in Northern Sweden hosts a considerable number of sulphide occurrences and gold deposits accompanied by considerable amounts of arsenopyrite. Outside the mineralised areas there exists about 4000 km² of metamorposed silt- and claystones with about 1 % sulphur and varying but clearly elevated amounts of arsenic.

About 70 groundwater samples have been collected from springs, dug wells and borewells. Arsenic was speciated into As(III and As(V) in the field. Major cations and trace metals have been determined on ICP-OES and anions on IC.

The results indicate two modes of mobilisation of arsenic, oxidation of sulphides and reduction of ferric oxyhydroxides with adsorbed arsenic. The former type has been predominantly detected in borewells with concentrations up to 0.350 mg/l. The arsenic contents are correlated to sulphate concentrations. Dug wells have generally low concentrations of arsenic. Springs and groundwater discharges in wetlands have shown up to 0.100 mg/l of arsenic and the arsenic is correlated to iron concentrations indicating recuction of ferric oxyhydroxides. Both in borewells and in wetland discharges the arsenic is predominantly As(III). Ferric precipitates in streams have been found to contain up to 5000 mg/kg of arsenic. The arsenic in lakes in the region is about three times the national average concentrations and correlated to TOC and iron. This indicates that ferric iron is picked up by humic matter and that arsenic is adsorbed onto the ferric iron.

About 10 % of the borewells show excess arsenic which warrants a check for arsenic which is not commonly analysed. The major ion chemistry, swlightly elevated sulphate and alkalinity can be used to prioritize the wells that are most likely to contain elevated arsenic. The springs discharging in wetlands does not seem to present a large problem. Only some Equisetum spp not palatable to grazing animals like mooses and reindeers are showing elevated arsenic contents. The fishes analysed, pike and brown trout, contain largely arsenobetaine, an arsenic sugar with low toxicity.