Isotopic studies of the environmental chemistry of lead
Sugden, Carol Louise
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Lead emitted into the environment, primarily from the combustion of leaded petrol and industrial activities, such as metal smelting and coal burning, retains the isotopic signature of the ores from which it is derived. Accordingly, it is possible, at least in principle, to distinguish sources of lead pollution via isotope ratio measurements. In this study, inductively coupled plasma mass spectrometry (ICP-MS) was used to characterise and investigate the extent of variation of lead isotope ratios in contemporary and historical environmental materials in Scotland. Reliable analytical methods were established for atomic absorption spec trometric measurement of lead, ICP-MS determination of stable lead isotopes, 208Pb, 207Pb and 206Pb, and gamma spectrometric assay of radioactive 210Pb in lake sediments and peat cores. Leaded petrol, atmospheric particulates and street dust sampled in central Edinburgh between February 1989 and December 1991 had mean 206Pb/207Pb ratios of 1.082±0.024, 1.092±0.011 and 1.109±0.016, respectively. These isotope ratios were found to be depleted in 206Pb compared with a mean of 1.160±0.012 for tap water in contact with lead pipes and typical ratios of 1.17-1.19 for British lead ore deposits and coal. Paint, with an observed wide range of isotopic compositions (206Pb/207Pb ratio range of 1.083-1.183 and 208Pb/207Pb ratio range of 2.363-2.592), appears to have significantly influenced house dust and some street dust isotopic signatures. Such overlaps and influences may hinder the quantitative apportionment of sources and routes of exposure in general population studies, especially for children. The identification of sources and the quantification of inputs of lead to the Scottish environment during the last 100-200 years was investigated using 210Pb-dated sediments from oligotrophic, eutrophic and acidified lochs and ombrotrophic peat cores. Pollutant lead fluxes to 210Pb-dated sediments from Round Loch of Glenhead tripled in magnitude from approximately 3mg/m2/y in the early 1800s to reach a maximum of 32mg/m2/y in the early 1940s. A similar pattern of deposition was observed in sediments from Loch Lomond and 210Pb-dated peat cores, which reveal a 10-20 fold enhancement in the fluxes of lead deposited from the atmosphere since 1800 both at rural sites near the heavily populated and industrialised central belt and on a remote island off the north-west coast of Scotland. This has been accompanied, largely during the last 50 years, by a significant reduction in the 206Pb/207Pb ratios from the 19th century values of 1.17-1.18 to 1.14-1.15, attributed to increasing contributions from car-exhaust emissions of particulate lead derived from alkyllead petrol additives manufactured from ores comparatively depleted in 206Pb. Trends in 206Pb/207Pb ratios of pollutant lead in Loch Lomond and Round Loch of Glenhead sediments were found to be consistent with atmospheric deposition records observed in peat, decreasing from 19th century values of 1.17-1.19 to 1.12-1.14. The calculated relative contribution of petrol lead to post-1950 sediments from Loch Lomond of 50-75% is considerably higher than that determined for the same period in sediment from Round Loch of Glenhead (5-30%) and for a peat core from Flanders Moss, a rural location in Scotland (27-43%), and indicates the importance of localised inputs, especially near industrial centres.