Depicting the historical pollution in a Pb-Zn mining/smelting site in Kabwe (Zambia) using tree rings

Abstract Historical Pb–Zn mining and smelting in Kabwe in Zambia have made it one of the most polluted cities in the world. Four soil profiles were collected around the smelter and in remote locations, and two pine trees (Pinus montezumae L.) were sampled for tree ring cores. These were analyzed for Pb, Zn and Cu contents and Pb isotopic ratio. Furthermore, the C isotopic composition of tree rings was also assessed. High concentrations of metals are present only in the top layers of soil and are higher in soils closer to the smelter (up to 16000 mg kg−1 Pb; 14000 mg kg−1 Zn; 600 mg kg−1 Cu) compared to remote soils (up to 194 mg kg−1 Pb; 438 mg kg−1 Zn; 46 mg kg−1 Cu). Metals are also present in trees and the one located furthest from the slag dump contains higher metal concentrations (up to 6.48 mg kg−1 Pb; 10.6 mg kg−1 Zn; 10.2 mg kg−1 Cu), possibly due to the deposition of wind-blown particles. Results of a sequential extraction procedure (SEP) showed that metal contaminants are not available for tree root uptake and the above-ground processes must be more important. The Pb isotopic ratios of slags, tree rings, and topsoils average at 206Pb/207Pb = 1.15, corresponding to the signature of local ores and smelting slags, thus confirming that the mine and smelter are the main sources of contamination in the area. The results were compared to the historical records of smelter production. Tree ring Pb and Zn concentrations, δ13C and 206Pb/207Pb reveal linear dependence on production with different statistical significance.