The potential of elemental and isotopic analysis of tree bark for discriminating sources of airborne lead contamination in the UK

Samples of tree bark, which accumulate airborne material, were collected from seven locations in the UK to provide an indication of the magnitude and source of lead pollution. Measurement of the Pb content and Pb-206/207 stable isotope ratio by inductively coupled plasma mass spectrometry revealed significant differences between the sites. The concentration of Pb varied over almost four orders of magnitude from 7.2 to 9,600 mug g(-1), the maximum values being found near a 'secondary' Pb smelter. The Pb-206/207 isotope ratios varied from 1.108 +/- 0.002 to 1.169 +/- 0.001. The lowest Pb concentrations and highest isotope ratios were detected in bark samples from the Scilly Isles, reflecting the low-level of industry and road traffic. In contrast, samples obtained from a city centre (Sheffield) and near a motorway (M1) contained 25-46 mug g(-1) Pb and recorded the lowest Pb-206/207 ratios. Higher concentrations in the vicinity of a coal-fired power station recorded a Pb-206/207 ratio of 1.14, suggesting a significant contribution from fly-ash. The relative contribution of lead from petrol (Pb-206/207 = 1.08) and other sources such as coal (Pb-206/207 = 1.18) were thus estimated using mass balance equations. Tree bark near the lead smelter recorded an intermediate Pb-206/207 ratio of 1.13 reflecting the processing of material of mixed origin.