Peat bogs document decades of declining atmospheric contamination by trace metals in the Athabasca Bituminous Sands region

Peat cores were collected from five bogs in the vicinity of open pit mines and upgraders of the Athabasca Bituminous Sands, and from a control site (Utikuma) which is 264 km SW. Frozen cores were sectioned into 1 cm slices, and trace metals determined in the ultraclean SWAMP lab using ICP-QMS. The uppermost sections of the cores were age-dated with (210)Pb using ultralow background gamma spectrometry, and selected plant macrofossils dated using (14)C. At each site, trace metal concentrations as well as enrichment factors (calculated relative to the corresponding M/Th ratio of the Upper Continental Crust) reveal maximum values 10 to 40 cm below the surface which shows that the zenith of atmospheric contamination occurred in the past. The age-depth models show that atmospheric contamination by trace metals (Ag, Cd, Sb, Tl, but also V, Ni and Mo which are enriched in bitumen) have been declining in northern Alberta for decades. In fact, the greatest contemporary enrichments of Ag, Cd, Sb and Tl (in the top layers of the peat cores) are found at the most remote site (UTK) showing that long-range atmospheric transport from other sources must be duly considered in any source assessment.