Natural enrichment of Cd and Tl in the bark of trees from a rural watershed devoid of point sources of metal contamination

To help understand the bioaccumulation of Cd and Tl in beaver tissue, we examined the enrichment of these metals in vegetation available to the animals. Bark was collected from 40 species of trees and shrubs, along with a complete soil weathering profile, within a small watershed devoid of trace metal contamination. Weathering resulted in a 5x enrichment of Cd in the soils relative to the underlying sediments, and a 6x Tl depletion: while Cd was lost from calcite and accumulates in the organic matter and oxyhydroxide fractions, Tl occurred only in the residual fraction. Soil processes alone, however, cannot explain the anomalous concentrations and enrichments of Cd in willow and poplar which contain up to 8.5 mg/kg Cd. The concentrations of Cd and Tl in the dissolved fraction (<0.45 mu m) of the Wye River are similar (1.2 +/- 0.4 and 1.6 +/- 0.1 ng/L, respectively), and are taken to estimate their bioavailability in soil solutions. Normalizing the Cd/metal ratios in bark to the corresponding ratios in water yields the Stream Enrichment Factor: this novel approach shows that all plant species are enriched in Cd relative to Ni; 33 relative to Cu, 13 relative to Zn, and 7 relative to Mn. Thus, many plants preferentially accumulate Cd, especially willow and poplar, over these essential micronutrients. Clearly, the enrichment of Cd over Tl in bark is not a reflection of differences in bioavailability, but rather on the preferential uptake of Cd by the plants. The profound natural bioaccumulation of Cd in the bark of willow and poplar, the two favourite foods of the beaver, has ramifications for the use of these aquatic mammals as biomonitors of environmental contamination, as well as for the direct and indirect consumption of bark for traditional food and medicine.