Source identification of lead in soils on the eastern slope of Gongga Mountain, Eastern Tibetan Plateau

Eight sites were selected along a high altitude gradient (2000-4500 m a.s.l.) on the eastern slope of Gongga Mountain, a typical high mountain on the eastern Tibetan Plateau. Soil samples, including organic layer (O-horizon), mineral layer (A-horizon) and parent material layer (C-horizon), were collected according to the soil developed level to investigate the spatial distribution of Pb, and to quantitatively identify its sources using statistical analysis and Pb isotope compositions. The results indicated that the concentrations of Pb (mean±SE) vertically decreased in the order of O-horizon ((41.73±4.38) mg/kg)>A-horizon ((30.47±2.17) mg/kg)>C-horizon ((23.04±1.50) mg/kg) (P<0.05). The concentrations of Pb in the O and A horizons were generally higher in the areas with middle altitude (approximately 3000 m a.s.l.) than at the low or high altitudes. However, the Pb concentrations in the C horizon did not show remarkable difference along with altitude, indicating the homogenous background of Pb in the soil of the eastern slope of Gongga Mountain. The ratios of ²⁰⁶Pb/²⁰⁷Pb increased vertically in the order of O-horizon<A-horizon<C-horizon, while the ratios of ²⁰⁸Pb/²⁰⁶Pb had an opposite trend. As indicated by isotope composition, Pb in the surface soils (O-horizon and A-horizon) was mainly influenced by fossil fuel combustion and Pb-containing ore smelting. In the O-horizon, Pb pollution reached 84.9%, 66.0% and 63.7%, estimated using factor-regression analysis, geochemical indicators and ternary mixing model respectively, and 56.6%, 27.8% and 44.9% respectively in the A-horizon. At the elevation lower than 2770 m (a.s.l.), the pollution Pb was mainly sourced from fossil fuel combustion, while above the timberline it was mainly from Pb-containing ore smelting.