The outermost bark layer of trees, predominantly Scots pine (
Pinus sylvestris), was sampled at 82
non-urban locations from six arbitrarily designated areas (Northwest, North
east, Central Highlands, Central and East, Central and Southwest, South
east), throughout Scotland during 2002–2003 and analysed for lead concentration and stable lead isotopes by flame atomic absorption spectrometry (AAS) and inductively coupled plasma-mass spectrometry (ICP-MS), respectively. The mean lead concentration and mean
206Pb/
207Pb ratio (± 1 standard deviation, SD) for bark samples from the areas were as follows: Northwest (8.0 mg kg
− 1, 1.121 ± 0.014,
n = 17), North
east (8.9 mg kg
− 1, 1.
117 ± 0.012,
n = 12), Central Highlands (
11.3 mg kg
− 1, 1.130 ± 0.010,
n =
11), Central and East (35.3 mg kg
− 1, 1.120 ± 0.007,
n = 10), Central and Southwest (20.6 mg kg
− 1, 1.125 ± 0.018,
n = 22) and South
east (34.4 mg kg
− 1, 1.120 ± 0.005,
n = 10), with an overall mean lead concentration of 18.5 mg kg
− 1 (range 0.6–146 mg kg
− 1, median 8.4 mg kg
− 1) and an overall mean
206Pb/
207Pb ratio of 1.122 ± 0.014 (range 1.089–1.168, median 1.122). The overall mean
206Pb/
207Pb ratio for bark was therefore significantly lower (
p < 0.01,
t test) than the mean atmospheric
206Pb/
207Pb ratio of 1.154 ± 0.006 (range 1.144–1.167,
n = 50) and 1.154 ± 0.010 (range 1.134–1.171,
n = 26) as determined in rainwater collected routinely at Glensaugh, Central Highlands, during 2002 and 2003, respectively. The bark
206Pb/
207Pb values, 90 % of which lay between 1.10 and 1.14, were more akin to those recorded for the atmosphere (via rainwater, atmospheric particulates, moss, etc.) at various locations throughout Scotland during the 1990s, a decade over which the use of leaded petrol (mean
206Pb/
207Pb ratio = 1.076 ± 0.0
11) declined markedly before its complete withdrawal in 2000. This strongly suggests that the lead content and isotopic composition of tree bark from Scots pine, which reputedly sheds its outer layers every couple of years or so, reflect exposure to atmospherically deposited lead (in the atmosphere or soil) over a much longer time period than just the previous 2–3 years of exposure to contemporary atmospheric lead.
The possible influence of soil lead upon tree bark through external attachment was not observed in a comparative sub-set study of 27 paired bark (mean 206Pb/207Pb ratio = 1.122 ± 0.016) and surface (0–2 cm) soil (mean 206Pb/207Pb ratio = 1.145 ± 0.022) samples, in only six cases (i.e. 22 % ) of which did the corresponding 206Pb/207Pb ratios agree within ± 2 SD. Likewise, bark 206Pb/207Pb values exhibited no discernible trend with distance from the nearest road, with similar average values for 0–20 m (1.123 ± 0.015, n = 34) and 20–700 m (1.122 ± 0.012, n = 48), although the corresponding mean (and median) lead concentration of 23.5 (10) mg kg− 1 vs. 14.9 (7.5) mg kg− 1 was slightly higher for the former. The influence of airborne dusts from waste deposits related to former mining/smelting of Wanlockhead/Leadhills lead ores (206Pb/207Pb = 1.170 ± 0.003) could be seen, however, in the sycamore bark lead concentrations of up to 3050 mg kg− 1 and 206Pb/207Pb ratios (1.168–1.171) observed at a distance of 0.5 km away.
