Recent changes in stable lead isotope ratios in sediments of Lake Zug, Switzerland

In Switzerland, unleaded gasoline was introduced in 1985. In order to study the effects of decreasing lead emissions from gasoline on a lake sediment, profiles of lead concentrations and Pb-206/Pb-207 ratios have been determined in cores from Lake Zug. Sediment samples were digested and analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Accurate dating of sediment cores was achieved using four independent methods showing the results within a range of <10%. Pb emissions in Switzerland and Pb concentrations in the sediment correlated well. Background Pb concentrations in pre-industrial sediments were 16 mu g/g in Lake Zug and 13 mu g/g in Lake Baldegg (for comparison); background Pb-206/Pb-207 ratios were 1.20 in both lakes. In Lake Zug, lead concentrations reached a maximum of 110 mu g/g in a depth corresponding to ca. 1970, the time of highest lead emissions. The Pb-206/Pb-207 ratio profile provided additional information because this ratio of Pb used for gasoline additives in Europe ranges from 1.04 to 1.10, and the ratio of background Pb in central Europe is about 1.20. The ratios in the sediment had a minimum of 1.13 ca. 1980 and an increase up to 1.16 since ca. 1990. The isotope ratios reflected the fraction of Pb emissions from gasoline.