Environ. Sci. Technol. 2002, 36, 152-157
Historical Trends in the Lead Isotopic Composition of Archival Sphagnum Mosses from Scotland (1838-2000)
of archival Sphagnum moss samples from the Herbarium Collection at the Royal Botanic Garden, Edinburgh, to investigate the inputs made by different, isotopically distinguishable, sources to the atmospheric lead burden in Scotland over the past 170 years. Archival moss has been used in previous studies in Norway (20) and Switzerland (21) but back to only 1974 in the former, while, in the latter, there were no samples between 1896 and 1951.
J O H N G . F A R M E R , * ,† L O R N A J . E A D E S , † HANNAH ATKINS,‡ AND DAVID F. CHAMBERLAIN‡ Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland, U.K., and The Royal Botanic Garden, Inverleith Row, Edinburgh EH3 5LR, Scotland, U.K.
In Scotland, such sources of lead have included the combustion of coal, the smelting of indigenous lead ores and the car-exhaust emissions from the use of leaded petrol, each with its characteristic lead isotopic composition, e.g. 206Pb/207Pb ratios of 1.17-1.19 (mean of 1.181 ( 0.011 for Scottish coal), 1.16-1.18 (1.170 ( 0.003 for the major Scottish lead ore deposit at Leadhills/Wanlockhead, 60 km southeast of Glasgow), and 1.06-1.09 (mean of 1.076 ( 0.011 for U.K. petrol, 1989-1998), respectively (22-25). The range of lower values for the 206Pb/207Pb ratio of leaded petrol in the U.K., prior to its withdrawal in 2000, reflected the major use of less radiogenic lead ores of characteristically low 206Pb/207Pb ratio, predominantly from Australia (e.g. Broken Hill, 206Pb/207Pb ∼ 1.04 (26, 27)), in the manufacture of alkyllead additives since the introduction of these anti-knock compounds ca. 1930 (25, 28). Even earlier than this, the effect of the importation and smelting of Australian lead ores in the U.K., especially England, during the second half of the 19th century is believed (7) to have affected the lead isotopic composition of atmospheric lead, as reflected in the decline from 1.170 to 1.145 in the 206Pb/207Pb ratio of archival herbage samples (1890-1930) collected and preserved at Rothamsted in the south of England (29). In contrast, the corresponding decline observed in the 206Pb/207Pb ratios of anthropogenic lead in sediment of Scottish lakes (e.g. Loch Lomond, Loch Ness) and peat bog cores (e.g. Flanders Moss) from a fairly constant 19th century value of ∼1.17 is typically less than 0.01 by the 1920s (2, 5, 30). Both the constant 206Pb/207Pb value in Scotland during the 19th century and its gentler decline relative to that in the south of England during the early years of the 20th century have been attributed to a combination of differing contributions from coal combustion, the smelting of indigenous and then, later on, Australian lead ores and the atmospheric transport of a proportion of lead emissions from the rest of the U.K. to Scotland (24). Thereafter, the influence of Australian lead of low 206Pb/207Pb ratio has been seen most clearly in the decline of the 206Pb/207Pb ratio of Scottish lake sediments and peat bogs by a further 0.02-0.04 to 1.12-1.14 by the 1970s/1980s and, after reductions in the maximum permitted concentration of lead in petrol and the introduction of unleaded petrol, a reversal in this decline since the mid-1980s (2, 5, 30).
The analysis of almost 200 Scottish Sphagnum moss samples collected over the past 170 years has revealed trends in the isotopic composition of lead similar to those previously established for dated Scottish lake sediments and peat bogs, lending credibility to these proxy records of atmospheric lead contamination and deposition. The effect of temporal variations in contributions from sources such as smelting of indigenous lead ores (206Pb/207Pb ∼ 1.161.18), coal combustion (206Pb/207Pb ∼ 1.17-1.19), and the use of imported Australian lead (206Pb/207Pb ∼ 1.04) was clearly seen in the Scottish moss 206Pb/207Pb record. This showed some differences from the corresponding archival herbage record for the south of England, where the initial influence of Australian lead occurred earlier, at the end of the 19th century. A significant decline from a 206Pb/ 207Pb value of ∼1.17 in the Scottish moss record began in the 1920s and continued until the 1980s (206Pb/207Pb ∼ 1.12). The success of measures to reduce lead emissions to the atmosphere over the past 20 years in the U.K., in particular from petrol-engined vehicles using alkyl lead additives manufactured primarily from Australian lead, is evident in both the increasing 206Pb/207Pb ratio and falling lead concentration data for Scottish moss.
Introduction The reconstruction of historical records of the deposition of heavy metal contaminants, such as lead, from the atmosphere is often based on the analysis of radiometrically dated sections of cores from freshwater lake sediments and ombrotrophic peat bogs (1-14). Unfortunately, the interpretation of such vertical profiles solely in terms of historical input from the atmosphere may be hindered by the presence of additional (nonatmospheric) inputs as well as by postdepositional redistribution processes, especially in sediments, and by uncertainty or insufficient time resolution in the dating, especially for slowly accumulating peat (3, 5, 8, 15-18). It is therefore of considerable value if independent archival material of known age (19) can be made available to test the validity of the derived historical records from the lake sediment and peat bog profiles. In this study, use is made * Corresponding author phone and fax: +44 131 650 4757; e-mail:
[email protected]. † University of Edinburgh. ‡ The Royal Botanic Garden. 152
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The objectives of this study were therefore (i) to determine the lead isotopic composition of Scottish Sphagnum moss samples selected from material collected over the past 170 years, (ii) to compare the data obtained with the apparent isotopic records of atmospheric lead deposition preserved in Scottish lake sediments and peat bogs, thereby assisting their deconvolution in terms of historical input and postdepositional perturbatory processes, (iii) to compare the data obtained with the isotopic record of atmospheric lead deposition preserved in archival herbage samples from the south of England, thereby assisting geographical resolution of the onset and extent of contributions from different sources, and (iv) to use the data in the quantitative source apportionment of atmospheric lead. 10.1021/es010156e CCC: $22.00
2002 American Chemical Society Published on Web 12/14/2001
37.7 ( 7.4 mg kg-1), and 44.7 ( 4.6 mg kg-1 (certified 45 ( 3 mg kg-1), respectively, were obtained (Table 1). For lead isotopic composition, the value of 1.217 ( 0.0031 obtained for the 206Pb/207Pb ratio of Lake Sediment (IAEA SL-1) (Table 1) agreed with the 1.214 ( 0.012 of Viczian et al. (31) and with values previously reported by Farmer et al. (2, 5). Replicate analyses (n ) 5-7) of three moss samples indicated that while overall analytical precision, which encompasses sample heterogeneity, was of the order of ( 10-30% (RSD) for mean lead concentrations, the corresponding figures for the 206Pb/207Pb, 208Pb/206Pb, and 208Pb/ 207Pb ratios were typically ( 0.1-0.35% (Table 1).
Results and Discussion
FIGURE 1. Map of Scotland with the Ordnance Survey National Grid reference system superimposed, showing the squares (100 km × 100 km), identified by letters, and the numbered grid lines from which the specific grid reference for each collected Sphagnum moss sample was derived.
Materials and Methods Sample Collection. The number of Sphagnum moss samples provided from the Herbarium Collection at the Royal Botanic Garden, Edinburgh, was 188, spanning the years 1838-1998, and a further 10 were collected in the field during 19992000. For each individual moss specimen, the sample identification code (including abbreviated scientific name of species, i.e., Sc: Sphagnum capillifolium (Ehrh.) Hedw.; Spal: Sphagnum palustre L.; Spap: Sphagnum papillosum Lind.), date of sample collection, location, and Ordnance Survey National Grid Reference are available as Supporting Information, which lists all 198 samples in chronological order. For the period 1838-1899 there were 34 samples and, thereafter, usually about 10-20 per decade with the exception of 30 for 1970-1979. The reference squares (100 km × 100 km, NT etc.) in which samples were collected are shown on the National Grid map in Figure 1. The samples can be further grouped into geographical zones, e.g. North [23; comprising HY(4), NC(4), ND(2), NH(10), NJ(2), NK(1)], West Coast [16; NF(2), NG(2), NM(5), NR(7)], Central Highlands [25; NN(25)], East [38; NO(38)], West-Central/Southwest [28; NS(14), NX(13), NY(1)], and East-Central/Southeast [68; NT (68)]. The historical samples (1838-1998) had been donated by collectors and stored carefully in the Herbarium Collection. Each of the specimens had been placed in an individual folded paper packet shortly after collection and the packets stored in closed boxes or mounted on herbarium sheets and stacked in cardboard folders placed in cabinets, those of the last 35 years being hermetically sealed. Sample Preparation and Analysis. Subsamples (typically 0.1 g) of the dried moss specimens were dry-ashed at 450 °C for 4 h, and the residues were digested with 10 mL of 8 M HNO3 (Aristar, BDH) in a CEM Mars 5 microwave digestion system. The samples were evaporated to ∼2 mL and then made up to 25 mL with 2% (v/v) HNO3 prior to lead determination by flame AAS (ATI Unicam Solaar 929) and, after appropriate dilution, stable lead isotopic measurement by ICP-MS (VG Elemental PlasmaQuad 3) in peak jumping mode over the range 203.6-209.4 amu (24, 25). The common lead isotopic standard NIST was used for calibration and correction for mass bias of ∼1% per amu. The method was validated for lead content via the analysis of three reference materials, Coal (BCR CRM No. 40), Lake Sediment (IAEA SL-1), and Orchard Leaves (NBS 1571), for which mean ( 1 SD values (n ) 26-28) of 22.7 ( 4.4 mg kg-1 (certified 24.2 ( 1.7 mg kg-1), 32.8 ( 4.3 mg kg-1 (certified
Lead Isotopic Ratios. The lead isotopic ratios (206Pb/207Pb, 208 Pb/206Pb, 208Pb/207Pb) of the 198 moss samples are listed individually in the Supporting Information. The 206Pb/207Pb ratio for each moss sample is plotted versus date of collection in both the appropriate individual and combined geographical zones in Figure 2. The results for three individual samples are excluded from further consideration: the comparatively low 206Pb/207Pb values of 1.143 ( 0.0008 and 1.132 ( 0.0028 for samples from Killin, Mid Perth (Central Highlands), in 1904 and 1918, respectively, were probably influenced by the temporarily rejuvenated smelting of the local ore deposits at Tyndrum (206Pb/207Pb ) 1.144 ( 0.004) and Ardtalanaig (206Pb/207Pb ) 1.130 ( 0.003), for which there is evidence in the dated sedimentary record of nearby Loch Tay (32), while the comparatively high lead concentration and 206Pb/207Pb ratio of 1.170 ( 0.0016 for a sample from Wanlockhead (WestCentral/Southwest) in 2000 reflects the location (NS286614 in Figure 1) of Scotland’s major lead ore deposit (22). The main features of the record are a rather constant 206Pb/207Pb ratio of 1.173 ( 0.004, irrespective of geographical zone (Table 2), for the 19th century, followed by the onset of a decline ca. 1920, with the ratio reaching a minimum ca. 1980, and a subsequent increase in more recent years (Figure 2). These trends are also reflected in the average 206Pb/207Pb ratios of Sphagnum moss lead by geographical zone and for Scotland as a whole in each decade of the 20th century (Table 2). Comparison of Environmental 206Pb/207Pb Records. Figure 3 compares the temporal trends in moss 206Pb/207Pb (averaged by decade, Table 3) from Scotland with those for anthropogenic lead in dated sections (post-1820) of a sediment core from the southern basin of Loch Lomond (2), lead in dated sections of four peat cores from Flanders Moss (5) and lead in archival herbage samples (averaged by decade from samples representing 5 years’ growth, Table 3) from Rothamsted in southern England (29). The values for the 206Pb/207Pb ratio of anthropogenic lead during the 19th century are remarkably similar, viz. 1.173 ( 0.004 for Scottish moss, 1.170 ( 0.002 for Scottish lake sediments, 1.169 ( 0.004 for Scottish peat, and 1.169 ( 0.002 for the herbage from southern England, although it should be noted that the value for the English samples relates only to the period 1860-1889 as a marked decline (to 1.156 ( 0.001) set in there during the final decade of that century (Table 3). For Scotland, the excellent agreement between the 206Pb/207Pb data for moss and those previously obtained for sediments and peat confirms the 19th century 206Pb/207Pb ratio of atmospheric lead in Scotland at ∼1.17. Farmer et al. (24) have quantitatively attributed this value to a combination of emissions arising from the smelting of the principal Scottish lead ore at Leadhills/Wanlockhead (206Pb/207Pb ) 1.170 ( 0.003), the burning of Scottish coal (206Pb/207Pb ) 1.181 ( 0.011), and industrial activities (e.g. coal burning, smelting) in the rest of the U.K., especially to the south in England, where supplies of indigenous lead ore (206Pb/207Pb ∼ 1.16-1.18) had largely expired by mid-19th century and been replaced by Australian lead (206Pb/207Pb ∼ 1.04) (33). VOL. 36, NO. 2, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Mean Lead Concentration and 206Pb/207Pb, 208Pb/206Pb, and 208Pb/207Pb Isotopic Ratios in Three Reference Materials (Coal, Lake Sediment, Orchard Leaves) and Three Scottish Sphagnum Moss Samples material
n
Pb (mg kg-1) (( 1 SD)
coal BCR CRM No. 40 lake sediment IAEA SL-1 orchard leaves NBS 1571 moss Sc 73964 (1872) moss Spal 73810 (1936) moss Spal 73823 (1971)
27 26 28 7 5 7
22.7 ( 4.4 32.8 ( 4.3 44.7 ( 4.6 52.2 ( 4.6 47.5 ( 14.3 24.9 ( 3.8
206Pb/207Pb
(( 1 SD)
1.178 ( 0.0027 1.217 ( 0.0031 1.172 ( 0.0031 1.173 ( 0.0015 1.155 ( 0.0041 1.124 ( 0.0027
208Pb/206Pb
(( 1 SD)
2.079 ( 0.0107 2.037 ( 0.0076 2.082 ( 0.0113 2.092 ( 0.0030 2.109 ( 0.0052 2.134 ( 0.0017
208Pb/207Pb
(( 1 SD)
2.448 ( 0.0120 2.479 ( 0.0071 2.440 ( 0.0116 2.453 ( 0.0017 2.432 ( 0.0031 2.400 ( 0.0048
the influence of Australian lead can be seen in the Scottish moss data, with a drop in the average 206Pb/207Pb ratio to 1.154 ( 0.010, although a wider range of values (1.139-1.165) than hitherto was observed. This decline was most evident in samples from southern Scotland (i.e. West-Central/ Southwest and East-Central/Southeast, Figure 2), where the average 206Pb/207Pb ratio of 1.145 ( 0.004 (Table 2), similar to the 1.148 ( 0.03 for southern England (Table 3) (29), was lower than the average of 1.157 ( 0.009 for the rest of Scotland (Table 2). That this decline in the 1920s is more readily seen in the moss data than in the lake sediment data (Figure 3) may reflect some delay in response and a damping of the sediment 206Pb/207Pb record due to continuing input of previously deposited lead from its catchment area. From the 1930s until the 1980s, there was a further fall in the 206Pb/207Pb ratio of Scottish moss (Figures 2 and 3) to average decadal minima of 1.127 ( 0.010 and 1.120 ( 0.018 by the 1970s and 1980s, respectively (Tables 2 and 3), very much in line with the magnitude and trends observed for the corresponding decline in the lake sediment and peat bog records, although the time resolution for the latter is inherently poorer (Figure 3). This decline (and perhaps the greater variability of moss 206Pb/207Pb data relative to those of the 19th century, Figure 3, Table 2) is attributable to the then growing influence of car-exhaust emissions of lead arising from the use of alkyl lead additives in petrol, for which the typical 206Pb/207Pb ratio (∼1.06-1.09) was dominated by the use of Australian lead (∼1.04) (25, 26, 28). Again the corresponding fall in the 206Pb/207Pb record provided by the archival herbage from southern England is greater, average decadal values of 1.112 ( 0.005 and 1.113 ( 0.018 being registered for the 1970s and 1980s, respectively (Table 3). Indeed, the average difference between the Scottish moss and English herbage 206Pb/207Pb decadal records for the period 1900 to 1989, with the exception of the 1930s (0.025) and 1940s (0.041), is 0.012 ( 0.0035.
FIGURE 2. Individual 206Pb/207Pb ratios (with a typical analytical precision, 1 SD, of ( 0.001-0.002) for the 198 Scottish Sphagnum moss samples, plotted against date of collection (1838-2000) by both geographical zone (see text) and for Scotland as a whole. The best-fit polynomial curve is shown for the combined data. The maximum permitted concentration of lead in U.K. petrol was reduced from 0.84 g L-1 (with a peak in estimated emissions of lead in 1976) to 0.40 g L-1 in 1981 and 0.15 g L-1 in 1986, with the introduction of unleaded petrol in 1986 and the prohibition of leaded petrol in 2000 (34). Confirmation of a later onset of decline in the 206Pb/207Pb ratio of atmospheric lead in Scotland relative to that for England, previously suggested by the lake sediment and peat data (2, 5, 24), is provided by the moss data (Figure 3). The average values of 1.172 ( 0.003 and 1.170 ( 0.004 for the 206Pb/207Pb ratio of Scottish moss for the decades 1900-1909 and 1910-1919, respectively, contrast markedly with the corresponding values of 1.161 ( 0.002 and 1.156 ( 0.001 for the English herbage (Table 3). During the 1920s, however, 154
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After the 1980s, the observed increase in the 206Pb/207Pb ratio of the Scottish moss from 1.120 ( 0.018 to 1.137 ( 0.010 for the 1990s (Table 3, Figure 3), based largely on data from southern Scotland (Table 2, Figure 2), reflects the reduction of the maximum permitted lead concentration of leaded petrol and the introduction and increasing uptake of unleaded petrol in the U.K. (34). After the ban on the use of leaded petrol in the U.K. at the start of 2000, the 206Pb/207Pb ratio of Scottish moss increased further to 1.151 ( 0.009 (Tables 2 and 3). These values and trend since the 1980s are in agreement with those found for rainwater in Scotland, for which the average 206Pb/207Pb ratio increased from 1.120 ( 0.016 in 1989-1991 to 1.144 ( 0.017 in 1997-1998 (25). Corresponding increases from 1.123 (1984) to 1.141 (1992) and from 1.131 (1987) to 1.154 (1996) have been found in moss samples from Switzerland (21) and Germany (35), respectively, and from 1.115 (1981-1989) to 1.143 (19921995) in urban aerosols in France (36). Similarly, in Norway, increases from 1990 values of 1.118, 1.127, 1.130, 1.134, and 1.143 to 1995 values of 1.125, 1.142, 1.146, 1.147, and 1.150, respectively, were found for moss samples from five different locations (20).
TABLE 2. Average 206Pb/207Pb Ratios of Lead in Scottish Sphagnum Moss Samples by Geographical Region during the 19th Century (1838-1899) and Each Decade of the 20th Century region (grid ref)
19th C
North (HY, NC, ND, NH, NJ, NK) (n) West Coast (NF, NG, NM, NR) (n) Central Highlands (NN) (n) East (NO) (n) West Central/Southwest (NS, NX, NY) (n) East Central/Southeast (NT) (n) All
1.173 (0.004 (2) 1.178 (0.002 (1) 1.172 (0.002 (9) 1.173 (0.004 (9) 1.174 (0.005 (10) 1.172 (0.001 (3) 1.173 (0.004 (34)
(n)
19001909
19101919
1.169 (0.001 (2)
1.172 1.164 (0.002 (0.001 (5) (1)
1.177 1.167 (0.002 (0.005 (1) (2) 1.171 1.172 (0.003 (0.003 (13) (7) 1.172 1.170 (0.003 (0.004 (21) (10)
19201929
19301939
19401949
19501959
19601969
19701979
19801989
19901999
2000
1.152 1.151 1.139 1.150 1.139 1.137 1.126 (0.003 (0.008 (0.012 (0.000 (0.006 (0.008 (0.026 (1) (2) (4) (1) (5) (2) (4) 1.160 1.151 1.133 1.132 1.135 (0.001 (0.002 (0.013 (0.001 (0.012 (1) (2) (8) (2) (2) 1.163 1.133 1.147 1.132 1.108 (0.001 (0.002 (0.004 (0.001 (0.001 (2) (2) (1) (1) (2) 1.156 1.162 1.151 1.148 1.138 1.135 (0.011 (0.016 (0.009 (0.013 (0.001 (0.005 (5) (6) (11) (2) (1) (4) 1.145 1.154 1.134 1.118 1.119 1.161 (0.006 (0.002 (0.007 (0.007 (0.015 (0.006 (2) (1) (2) (4) (3) (2) 1.144 1.155 1.139 1.140 1.126 1.127 1.137 1.147 (0.003 (0.004 (0.001 (0.009 (0.011 (0.001 (0.011 (0.006 (1) (1) (1) (5) (17) (1) (14) (5) 1.154 1.159 1.147 1.145 1.137 1.127 1.120 1.137 1.151 (0.010 (0.013 (0.011 (0.012 (0.010 (0.010 (0.018 (0.010 (0.009 (11) (10) (15) (9) (22) (30) (10) (16) (7)
TABLE 3. Average 206Pb/207Pb Ratios of Lead in Scottish Sphagnum Moss Samples by Decade (1830-2000) and Comparison with Archival Herbage Data from Rothamsted, Southern England (29)
FIGURE 3. Comparison of temporal trends in the decadal average 206Pb/207Pb ratios (( 1 SD) for Scottish Sphagnum moss samples (1830-2000) with the corresponding data for archival herbage samples (1860-1990) from Rothamsted, England (29) (Table 3), the 206Pb/207Pb ratios (( 1 SD) of anthropogenic lead in dated sections (1820-1991) of a sediment core from the southern basin of Loch Lomond (2, 5) and the 206Pb/207Pb ratios (( 1 SD) of dated sections (1820-1990) from four Flanders Moss peat cores (5). Source Apportionment of Lead. The influence of the less radiogenic Australian lead and, in particular, petrol lead upon the lead isotopic composition of Scottish moss samples is further demonstrated in the plot of 208Pb/206Pb versus 206Pb/ 207Pb for the moss along with values for various potential end member sources (Figure 4). Source apportionment calculations, say for the minimum average moss 206Pb/207Pb value of 1.120 ( 0.018 for the 1980s, using an average value of 1.076 ( 0.011 for the 206Pb/207Pb ratio of petrol and 1.17, 1.181 ( 0.011, and 1.19 for “industrial”, coal, and “geogenic” (i.e. average crustal) 206Pb/207Pb ratios (24, 25), respectively, yield an upper limit for the contribution of petrol lead of ∼70%. If, on the other hand, the average moss 206Pb/207Pb
decade
mean year
n
moss range
moss mean ( 1 SD
1830-1839 1840-1849 1850-1859 1860-1869 1870-1879 1880-1889 1890-1899 1900-1909 1910-1919 1920-1929 1930-1939 1940-1949 1950-1959 1960-1969 1970-1979 1980-1989 1990-1999 2000
1838 1846.8 1855.3 1865 1872.8 1884.5 1896.5 1904.3 1913.5 1922.9 1937.6 1944.6 1952.7 1966.1 1972.7 1985.6 1993.9 2000
1 4 3 1 5 8 12 21 10 11 10 15 9 22 30 10 16 7
1.171 1.170-1.178 1.174-1.179 1.169 1.171-1.178 1.168-1.179 1.160-1.177 1.167-1.177 1.163-1.176 1.139-1.165 1.134-1.170 1.123-1.175 1.118-1.157 1.118-1.150 1.100-1.146 1.087-1.143 1.125-1.163 1.137-1.165
1.171 ( 0.002 1.176 ( 0.004 1.176 ( 0.003 1.169 ( 0.003 1.174 ( 0.003 1.175 ( 0.003 1.171 ( 0.004 1.172 ( 0.003 1.170 ( 0.004 1.154 ( 0.010 1.159 ( 0.013 1.147 ( 0.011 1.145 ( 0.012 1.137 ( 0.010 1.127 ( 0.010 1.120 ( 0.018 1.137 ( 0.010 1.151 ( 0.009
herbage mean ( 1 SD
1.169 ( 0.002 1.170 ( 0.001 1.168 ( 0.002 1.156 ( 0.001 1.161 ( 0.002 1.156 ( 0.001 1.148 ( 0.003 1.134 ( 0.003 1.106 ( 0.021 1.133 ( 0.003 1.124 ( 0.002 1.112 ( 0.005 1.113 ( 0.018
value of 1.151 ( 0.009 for 2000 (Tables 2 and 3) is taken to represent the combined 206Pb/207Pb ratio for all other sources, the upper limit for the petrol lead contribution falls to 60% for the 1980s and, indeed, 35% for the 1990s. Lead Concentrations. The lead concentrations of the 198 moss samples are listed individually in the Supporting Information. The lead concentration data are summarized by concentration range for individual and combined geographical zones for six different time intervals, 1838-1899 (n ) 34), 1900-1926 (n ) 44), 1934-1959 (n ) 34), 19601979 (n ) 52), 1980-1999 (n ) 26), and 2000 (n ) 8), in Figure 5. There is a fall in the median concentration from > 50 mg kg-1 in the early part of the 20th century to < 2 mg kg-1 by 2000. The effectiveness of measures taken to reduce lead emissions to the U.K. environment over the past 20 years is reflected in a reduction of the median lead concentration for moss from 26 mg kg-1 to 10 mg kg-1 to 1.7 mg kg-1 in the periods 1960-1979, 1980-1999, and 2000, respectively (Figure 5). It is perhaps surprising that the periods when the median moss lead concentration was highest were 1838VOL. 36, NO. 2, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Plot of 208Pb/206Pb versus 206Pb/207Pb for Scottish Sphagnum moss samples (averaged from 1838 to 1899 for the 19th century and by decade for the 20th centuryssee Tables 2 and 3 for the specific 206 Pb/207Pb values), lead ore from Broken Hill, Australia (26, 27) and Leadhills/Wanlockhead, Scotland (22, 23), U.K. petrol (1989-1998) (25), and Scottish coal (24).
1973, while Weiss et al. (21) reported 48 mg kg-1 for an 1867 Sphagnum acutifolium var. fuscum moss from Tourbiere du Fonts Martel, Neuchatel, Switzerland, compared with 9.6 mg kg-1 for a 1992 Sphagnum fuscum (Schimp.) moss from Suruggen, Trogen AR, Switzerland. It is possible that the observed lead concentration trends for the archival moss (Figure 5) could to some extent reflect a different retentive capability of moss for the different types of particulate material emitted from the same source (e.g. coal combustion, lead smelting) with time as industrial emission technologies improved and also different, unknown time periods of growth and exposure in different locations prior to sample collection (38).
Acknowledgments This work was supported by a grant (GR9/04551) from the U.K. Natural Environment Research Council.
Supporting Information Available Table of lead concentration and isotopic ratios in Sphagnum moss samples from Scotland (1838-2000). This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited
FIGURE 5. Histograms of lead concentrations in Sphagnum moss samples from different geographical zones (see text) and for Scotland as a whole for five different concentration ranges [0-24 (0), 25-49 (///), 50-74 (t), 75-99 (\\\), and g 100 (9) mg kg-1] during six different time periods from 1838 to 2000. The median lead concentration for each period is shown for the combined data. 1899 (56 mg kg-1) and 1900-1926 (52 mg kg-1). That lead deposition was high prior to the introduction of leaded petrol, however, is reflected in the observations that 65% of the anthropogenic lead inventory in the bottom sediments from southern Loch Lomond was deposited prior to 1930 (49% prior to 1900), although annual lead fluxes to the sediments apparently peaked during the 1950s (2). It is noteworthy that Lee and Tallis (37) found a mean lead content of 28.6 mg kg-1 for 19th century Hypnum cupressiforme moss samples from North Scotland, compared with 18.6 mg kg-1 for 1972156
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Received for review June 1, 2001. Revised manuscript received October 15, 2001. Accepted October 18, 2001. ES010156E
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