Metallhalter i vattendrag längs Umeälven : Avvikelser mot uppskattade bakgrundshalter

The aim of this study was to investigate metal concentrations in unaffected streams located in Västerbotten, Sweden, with the hypothesis that even though unaffected by human activities, some streams will show high metal concentrations due to bedrock geology. Based on previous hypothesis the aim was also to evaluate regional background levels set by the Swedish Environmental Protection Agency, to emphasise the difficulty in using said background levels when evaluating metal loading. Twenty four streams were investigated and plotted according to bedrock, stream order and distance from area of mining explorations, TOC and pH. The results showed that metal concentrations differed from estimated background levels, especially in regard to arsenic and lead. Concentrations of arsenic was in many locations higher than estimated background levels, in Barsele area, very high concentrations was noted. There were also a distinct difference between locations in mountainous areas (mostly mafic and sedimentary rock), that had lower concentrations, and locations in the Baltic shield (mostly felsic and pyrite rock), with higher concentrations than estimated background levels. Concentrations of lead was in all cases lower than estimated background levels, and lowest in the mountainous area. Locations with stream order one often had higher metal concentrations than streams with higher order, indicating a negative correlation between background levels and stream order, the opposite of what had been estimated by Swedish Environmental Protection Agency – with lower metal concentration in smaller streams.