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 206/207Pb
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 µg g−1,
the maximum values being found near a ‘secondary’ Pb smelter.
The 206/207Pb 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 µg g−1 Pb and recorded
the lowest 206/207Pb ratios. Higher concentrations in the vicinity
of a coal-fired power station recorded a 206/207Pb ratio of
1.14, suggesting a significant contribution from fly-ash. The relative contribution
of lead from petrol (206/207Pb = 1.08)
and other sources such as coal (206/207Pb = 1.18)
were thus estimated using mass balance equations. Tree bark near the lead
smelter recorded an intermediate 206/207Pb ratio of 1.13 reflecting
the processing of material of mixed origin.
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