A lead isotopic assessment of tree bark as a biomonitor of contemporary atmospheric lead

The outermost bark layer of trees, predominantly Scots pine (Pinus sylvestris), was sampled at 82 non-urban locations from six arbitrarily designated areas (Northwest, Northeast, Central Highlands, Central and East, Central and Southwest, Southeast), 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 Pb-206/Pb-207 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), Northeast (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 Southeast (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 mgkg(-1))and an overall mean Pb-206/(207) Pb ratio of 1.122 +/- 0.014 (range 1.089-1.168, median 1.122). The overall mean Pb-206/Pb-207 ratio for bark was therefore significantly lower (p < 0.01, t test) than the mean atmospheric Pb-206/Pb-207 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 Pb-206/Pb-207 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 Pb-216/Pb-207 ratio = 1.076 +/- 0.011) 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 Pb-206/Pb-207 ratio = 1, 122 +/- 0.016) and surface (0-2 cm) soil (mean Pb-206/Pb-207 ratio = 1.145 +/- 0.022) samples, in only six cases (i.e. 22%) of which did the corresponding Pb-206/Pb-207 ratios agree within 2 SD. Likewise, bark Pb-216/Pb-207 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 Wantockhead/Leadhills lead ores (Pb-206/Ph-207 = 1.170 +/- 0.003) could be seen, however,. in the sycamore bark lead concentrations of up to 3050 mg kg(-1) and Pb-206/Ph-207 ratios (1.168-1.171) observed at a distance of 0.5 km away. (c) 2007 Elsevier B.V. All rights reserved.