Trace Elements and the Lead Isotopic Record in Marula (Sclerocarya birrea) Tree Rings and Soils Near the Tsumeb Smelter, Namibia
Abstract
The contents of As, Cu, Cd, Pb, Mn, along with the Pb isotopic ratios Pb-206/Pb-207 and Pb-208/Pb-206 were studied in both soils and tree rings of the marula tree (Sclerocarya birrea) in the vicinity of the Tsumeb deposit (Namibia). Amounts of all the studied metals and As are higher in the immediate vicinity of the Tsumeb Cu-Pb smelter in the soil.
The tree rings also have their maximum content of all the studied substances in the vicinity of the smelter (with the exception of Pb). At a more distant site, the maximum concentration of Pb in the soils was 29.8 mg/kg, while the content in the soil in the vicinity of the smelter was as much as 8,174 mg/kg.
In the vicinity of the smelter, the maximum Pb content in the tree rings reaches a value of 5.7 mg/kg, compared to a more distant site, where the contents are as high as 9.2 mg/kg. The lower Pb content in the trees on contaminated soil indicates that the composition of the xylem determines the above-ground uptake, rather than the root uptake.
Similarly, the above-ground uptake is documented by the isotopic composition of Pb at the distant location, where the tree rings have different contents of Pb isotopes compared to in the soil. The As, Cd, Cu, Pb, and Zn contents are highest in the tree rings from the 1950s (and older), along with those from the 1990s, while the Mn contents were highest in those from the 1960s and 1990s.
The contaminant peaks in the 1950s and 1960s could be associated with the roasting of sulfidic ores, while the peak values in the 1990s could have been caused by the start of Cu slag reprocessing in the late 1980s, and culmination of works at the smelter prior to the closing of the mine. The tree rings of the marula tree were found to be a suitable archive for above-ground pollution close to Cu and Pb smelters.