Stable lead isotopic characterisation of the historical record of environmental lead contamination in dated freshwater lake sediment cores from northern and central Scotland

Abstract

Sediment cores from three Scottish freshwater lakes, Loch Ness in the remote north and Loch Lomond and the Lake of Menteith, much closer to the heavily populated and industrialised central belt, were analysed for 210Pb, 137Cs, Pb and stable Pb isotopic composition ( 206Pb/ 207Pb). The radionuclide data were used to establish chronologies for the Loch Ness and Loch Lomond cores, but a chronology could not be developed for the Lake of Menteith core, in which the surface sediment had been subject to intense mixing. Although Pb concentrations generally started increasing during the mid-17th Century, a small peak occurred for Loch Ness in the early 16th Century, perhaps attributable to the influence of medieval mining and smelting in mainland Europe. Temporal trends in the pattern of Pb accumulation were similar for Loch Ness and both sites in Loch Lomond, with 40–50% of the anthropogenic Pb deposited prior to the 20th Century. Fluxes of anthropogenic Pb to the lake sediments peaked during the 1950s at all locations where chronologies could be established. The 5-fold increase in anthropogenic Pb inventory for the southern basin of Loch Lomond relative to Loch Ness reflected geographical proximity to the main polluting sources. The 206Pb/ 207Pb data for anthropogenic Pb in the sediments from Loch Ness and Loch Lomond exhibited largely similar trends related to five different time periods. Pre-1820, the 206Pb/ 207Pb ratio was close to that for coal (1.181). From 1820 to 1900, a fairly constant 206Pb/ 207Pb ratio of ∼1.17 probably resulted from a combination of emissions from the smelting of indigenous Pb ore (1.170) and coal burning (1.181) in Scotland, and industrial activity to the south in England, where Australian Pb of characteristically low 206Pb/ 207Pb ratio (1.04) was already in use. From 1901 to 1930, the 206Pb/ 207Pb ratio declined by <0.01, due to the increasing influence of Australian Pb. From 1931 to 1975/1985, the 206Pb/ 207Pb ratio of anthropogenic Pb declined by a further 0.03 to 0.04, to minimum values from approximately 1975 to 1985, primarily a consequence of car-exhaust emissions of Pb arising from the introduction of alkyl Pb petrol additives ( 206Pb/ 207Pb ∼1.06–1.09). From 1975/1985 to the mid-1990s, the 206Pb/ 207Pb ratio of anthropogenic Pb increased by up to 0.015, a consequence of a reduction in car-exhaust emissions of Pb, resulting from reductions in the maximum permitted concentration of Pb in petrol, and the introduction and increasing uptake of unleaded petrol. Source apportionment calculations, on the basis of 206Pb/ 207Pb values in surface sediment, suggested that the contribution of Pb emissions from the use of leaded petrol was 27–40% of the atmospheric burden by the mid-1990s, in line with estimates from rainwater 206Pb/ 207Pb data.