Heavy metals of the 20th century recorded in oak tree rings

Tree rings from 23 oak trees (Quercus robur L.) were used to to monitor historical heavy metal emissions from anthropogenic sources on a regional scale. The concentrations of Cd and Pb in the tree rings were analyzed using atomic absorption spectrophotometry. From the results, chronologies were constructed representing the heavy metal background load of the 20th century. The chronologies were compared with calculated estimates or the accumulation of these metals in soil. The method of tree-ring analysis proved to he useful for monitoring Cd and Pb emissions from diffuse sources when applied as a complement to other methods, in this case calculated accumulation. This study also indicates that no radial transport occurs in the inward direction, from sapwood to heartwood, but that outward diffusion to some extent might occur at least within the sapwood. The conspicuous difference between the soil/wood ratios for Cd and Pb implies that a larger proportion of the soil Cd is taken up by the tree than is the case for Ph.