Application of Pb isotopic tracing technique to constraining the source of Pb in the West Lake Longjing tea

This paper dealt with the Pb contents and Pb isotopic composition of the West Lake Longjing tea. The results showed that in the tea leaves, from young leaf →old leaf →tea limb, the Pb contents tend to increase gradu-ally from 1.63 →4.84 →6.07×10-6, wich revealed that the Pb was accumulated gradually in the tea. After cleaned, the Pb contents of tea leaves were significantly reduced. This indicated that the deposits on the surface of tea leaves made a great contributuion to Pb contents. The survey results for soils in the relevant tea gardens showed that soil from the Longjing tea garden has higher Pb contents, with an average level of 49.6×10-6, two times those of common soils (24×10-6) in China. Results of the systematic analysis for tea, tea garden soil and the samples with the relevant background of the Pb isotopic composition displayed that the Pb isotopic ratio of tea is 206Pb/207Pb=1.164±0.005 (2σ). The ratio of 206Pb/207Pb for the soil gradually decreased from residue phase, soil dilute acid extract phase, and then to urban topsoil, i.e., 1.175 →1.171 →1.170. The 208Pb/(206Pb+207Pb) ratios also show a similar variation trend. 206Pb/207Pb ratios in the samples with the relevant background were: vehicle exhaust, 1.124; coal-combustion, 1.156; atmosphere, 1.168; and water, 1.166. Comparative studies have shown that Pb pollution is popular in the environ-mental media (soil, atmosphere, water) in Hangzhou. With the aggravation of Pb pollution, the Pb isotopic composi-tion gradually changed from the natural background (soil residues) to the direction of automobile exhaust. This phe-nomenon could illustrate that the pollution source was the vehicle exhaust, while the coal-combustion contributed little to environmental pollution in Hangzhou. The Pb of the Longjing tea came mostly from soluble phase Pb in the polluted soil. Moreover, secondary pollution was caused by vehicle exhaust.