Environ. Sci. Technol. 1996, 30, 3080-3083
Stable Lead Isotope Record of Lead Pollution in Loch Lomond Sediments since 1630 A.D. JOHN G. FARMER* AND LORNA J. EADES Department of Chemistry, The University of Edinburgh, Edinburgh EH9 3JJ, Scotland, U.K.
ANGUS B. MACKENZIE Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QF, Scotland, U.K.
ALEX KIRIKA AND TONY E. BAILEY-WATTS Natural Environment Research Council Institute of Freshwater Ecology, Bush Estate, Penicuik, Midlothian EH26 0QB, Scotland, U.K.
Stable lead isotope data can yield information on the geochemical origins of lead and on its relative contributions from sources such as coal burning, mining, smelting, and car-exhaust emissions. This extremely detailed 206Pb/207Pb profile for dated bottom sediments in Loch Lomond, Scotland, shows the trends clearly related to the varying nature and extent of anthropogenic lead inputs after 1630. In particular, a significant decline in the 206Pb/207Pb ratio of excess lead during 1929-1991 is attributable to the introduction and use of (206Pb-depleted) leaded petrol since the 1920s. This accounts, however, for just 2453% of the excess lead deposited since 1929 and e19% of the total excess lead inventory. Deposition of lead from industrial (and domestic) activities has predominated overall and, on an annual basis, until at least the mid-1950s.
Introduction Comparatively few of the many records of environmental lead pollution derived from dated aquatic sediments, peat bogs, ice cores, and corals have been accompanied by stable lead isotope data (1-13). Characteristic abundances of the four stable isotopes of lead (primordial 204Pb and radiogenic 206Pb, 207Pb, and 208Pb) in different materials (e.g., coal, petrol additives) afford the opportunity of source apportionment. For example, the 206Pb/207Pb isotopic ratio of leaded petrol in the U.K. today is 1.06-1.09, reflecting the mix of Australian ore (206Pb/207Pb ) 1.04) with British Columbian ore (206Pb/207Pb ) 1.16), whereas that for British lead ore used in the production of domestic water pipes during the 19th and early 20th Centuries is typically 1.16* Corresponding author e-mail address:
[email protected]; telephone: 0131 650 4757; fax: 0131 650 4743.
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FIGURE 1. Sediment core sampling site in the southern basin of Loch Lomond, Scotland, on the northwestern fringe of Europe.
1.18, similar to the 1.16-1.19 value for Scottish coal (14). In view of the finding in our preliminary work on sediment (7) and peat (6) in Scotland that the introduction of leaded petrol in the late 1920s could be detected through a reduction in the 206Pb/207Pb ratio of deposited anthropogenic lead from a late 19th/early 20th Century value of ∼1.17, we investigated the 206Pb/207Pb historical record in much greater detail. We chose sediments, for their vastly superior time resolution via feasible slicing of very thin sections (∼3 mm), from the southern basin of Loch Lomond (Figure 1), the site of much of our previous work (15) on environmental change.
Methods Sampling. An 80-cm-long mini-Mackereth core (LL-3E) and a 14.4-cm-long Jenkin core were collected on November 26, 1991, at a water depth of 18 m in Loch Lomond (56°56°19′ N, 4°30′-4°43′ W), 30 km to the northwest of Glasgow and situated on the fringe of the densely populated and formerly heavily industrialized west-central belt of Scotland (Figure 1). Both of these devices, a piston corer and a gravity corer, are designed to minimize disturbance of the sediment (16, 17). The sampling site shown is at OS grid ref. NS 391 884 in the southern basin of Loch Lomond, where the oxygen saturation of the water never falls below 78% (18). Core LL-3E was sliced into sections (0.3 cm thick to a depth
S0013-936X(96)00162-9 CCC: $12.00
1996 American Chemical Society
of 30 cm, then 2 cm to a depth of 50 cm, and 5 cm to a depth of 80 cm), which were dried at 35-40 °C and finely ground prior to chemical pretreatment. Analysis. Aliquots of the 116 dried sections from the core were digested for several hours in hot mixed acid (0.30.5 g of sediment, 20 cm3 of 8 M HNO3, 10 cm3 of 11.6 M HCl, filtration, further 5-10 cm3 aliquots of 11.6 M HCl until brown fumes of NO2 driven off) prior to preparation (in 1 M HCl) for determination of lead by flame atomic absorption spectrometry (Unicam SP-9 800) and in hot nitric acid (0.1 g of sediment, 30 cm3 of 8 M HNO3) prior to preparation (in 2% v/v HNO3) for analysis by inductively coupled plasma-mass spectrometry (VG Plasmaquad PQ1 or Plasmaquad PQ2) for 206Pb/207Pb (14). Precision ((1σ) was typically (3% for duplicate lead analysis and (0.004 for 206Pb/207Pb ratios in the range of 1.127-1.180. The mean 206Pb/207Pb ratio for the reference material IAEA SL-1, lake sediment, was 1.214 ( 0.006 (n ) 34), in agreement with the 1.214 ( 0.012 value of Viczian et al. (19). Radiocaesium from nuclear weapons testing in the 1950s and 1960s (137Cs) and from the Chernobyl nuclear reactor accident in 1986 (134Cs, 137Cs) was determined in the 48 0.3-cm-thick sections of the Jenkin core by γ-spectrometry (Canberra intrinsic Ge detector) (20, 21). The sediment samples were too small for satisfactory determination of the naturally occurring 210Pb radionuclide. Thus, we used the relative positions of the two main 137Cs peaks (taken to correspond to the major deposition from the atmosphere in 1963 and 1986) to derive a sedimentation rate of 30.9 ( 1.3 mg cm-2 yr-1. Detailed discussion of the radiocaesium data is presented elsewhere (21). The use of radiocaesium concentration profiles to establish sedimentation rates in freshwater lake sediments is well established, and the chronologies derived have been validated by repeated sampling over an extended time period and by close agreement between radiocaesium derived-chronologies and those obtained independently by 210Pb dating (22, 23). Such agreement was also observed in the present case with the accumulation rate derived from the radiocaesium distribution being in excellent agreement with the value, obtained from 210Pb dating of a core from a nearby site, of 32 ( 4 mg cm-2 yr-1 (20). The accumulation rate will be subject to an uncertainty arising from the finite thickness of the sampling increment and, potentially, broadening of the peaks in radiocaesium concentration as a consequence of bioturbation. While some broadening was observed, the peaks were nevertheless well resolved and the maxima were well defined, indicating that the effects of bioturbation on the chronology are probably negligible. An extrapolated sedimentation rate of 30.9 mg cm-2 yr-1 was used along with the cumulative weight (g cm-2) for each depth to date the sections of the mini-Mackereth core LL-3E and thus derive the chronology. Assuming a constant sedimentation rate, we estimate an uncertainty of no more than (5% in the age of each section.
Results and Discussion The lead concentration and 206Pb/207Pb ratio were constant below 28.5 cm, at 15 ( 4 mg kg-1 and 1.174 ( 0.002, respectively (Figure 2). Above 28.5 cm the lead concentration increased, especially above 18 cm, reaching a maximum of 158 mg kg-1 at 6.45 cm before declining to 65-81 mg kg-1 in the uppermost 2.4 cm. The 206P/207Pb ratio declined above 10.5 cm from values of ∼1.17 to a minimum of 1.127
FIGURE 2. Lead concentrations ()) and 206Pb/207Pb ratios (() in sections of the Loch Lomond sediment core LL-3E (mini-Mackereth) collected on November 26, 1991.
at 3.75 cm followed by an increase to 1.135-1.141 in the top 3.3 cm. For each of the 95 sections above 28.5 cm, the lead concentration and 206Pb/207Pb ratio were corrected with background values to derive an excess lead concentration with an associated 206Pb/207Pb ratio, representing anthropogenic input of lead. Using the value of 1.174 for the 206Pb/207Pb ratio below 28.5 cm, the 206Pb/207Pb ratio of the excess lead for each section was calculated from the formula: 206
Pb/207Pbexcess ) [(Pbmeas × 206Pb/207Pbmeas) (15 × 1.174)]/(Pbmeas - 15)
The excess lead concentration (mg kg-1) was then multiplied by the sedimentation rate of 0.309 kg m-2 yr-1 to yield the excess lead flux (mg m-2 yr-1), plotted along with the corresponding excess 206Pb/207Pb ratio against the calendar year in Figure 3. Three key periods can be identified for the excess 206Pb/ 207Pb ratio. (i) From the mid-17th to early 19th Century (1634-1817), it was fairly constant at 1.178 ( 0.006, during which time the flux slowly increased but never exceeded 8.7 mg m-2 yr-1. The total excess lead deposited during this period was 0.84 g m-2, 13% of the inventory for the core of 6.36 g m-2. (ii) From 1817-1929 the ratio was very steady at 1.169 ( 0.002 despite, as the Industrial Revolution flourished, a dramatic increase in flux, which was always g25 mg m-2 yr-1 after 1847 and peaked at 40.5 mg m-2 yr-1 in 1925. The total excess lead deposited during 1817-1929
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FIGURE 3. Excess (i.e., anthropogenic) lead fluxes ()) and associated 206Pb/207Pb ratios (() at Loch Lomond site LL-3E vs calendar year (1634-1991).
was 3.23 g m-2, 51% of the core inventory. (iii) From 1929, soon after the introduction of leaded petrol (24), the isotopic ratio decreased steadily to 1.122 by the mid-1970s, although the flux of lead remained at g35 mg m-2 yr-1, peaking at 44.2 mg m-2 yr-1 in 1960, and was still as high as 41.7 mg m-2 yr-1 in 1974. Thereafter, at the same time as steps were taken to reduce the lead content of petrol in the U.K. (25), the ratio increased slightly to 1.125-1.133 as the flux fell steeply to 15.4-18.8 mg m-2 yr-1 by 1985-1991. Overall, the excess lead deposited from 1929 to 1991 was 2.29 g m-2, 36% of the core inventory. Source Apportionment. We deduce that the shift in the 206Pb/207Pb ratio of excess lead occurring in 1929, after more than a century of stability, was due to the introduction of leaded petrol with a low 206Pb/207Pb value. Direct information on the 206Pb/207Pb ratio of petrol in the U.K. for the period since 1929 is not available, but our measurements since 1989 show a range of 1.06-1.09 with a mean of 1.071 ( 0.010 (n ) 18), in good agreement with Delves and Campbell (26). In view of the possible use at times of imported tetraalkyllead from the United States and contributions to atmospheric lead over the U.K. from carexhaust emissions in parts of continental Europe with higher 206Pb/207Pb ratios (27), we have taken 1.12 as a plausible upper limit for petrol-derived atmospheric lead. Figure 4 shows the outcome for relative contributions of “industrial” lead (206Pb/207Pb ) 1.169) and of “petrol” lead to the flux of excess lead for three scenarios based on petrol 206Pb/ 207Pb ) 1.06, 1.09, or 1.12 since 1929. The percentage of petrol contribution to the excess lead flux for each section is calculated using the formula:
Pbpetrol(%) ) [(1.169 - 206Pb/207Pbexcess) × 100]/ (1.169 - 206Pb/207Pbpetrol) At 1.06 (D), petrol lead accounts for 40% of the excess lead in 1991 (at 1.071, 45%) and 24% of that deposited from 1929 to 1991; at 1.09 (C + D), petrol lead accounts for 56% in 1991 and 33% from 1929 to 1991; at 1.12 (B + C + D), petrol lead accounts for 90% in 1991 and 53% from 1929 to 1991. For the latter scenario, petrol lead replaced industrial lead as the major contributor in 1955. The maxima for both relative and absolute contributions by petrol lead occurred in 1976, with 43% (17.7 mg m-2 yr-1), 59% (24.5 mg m-2 yr-1), and 96% (39.4 mg m-2 yr-1) for 206Pb/207Pb ratios of 1.06, 1.09, and 1.12, respectively. According to Department of Trade and Industry statistics (1975-1992) (28), estimated emissions of lead from petrol-
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FIGURE 4. Plot from 1850 to 1991 of the contributions of “industrial” lead and of “petrol” lead to the flux of excess lead at Loch Lomond site LL-3E for three scenarios based on petrol 206Pb/207Pb ) 1.06, 1.09, or 1.12 (see text). A represents the industrial lead flux, and B + C + D represents the petrol lead flux when the 206Pb/207Pb ratio of petrol is taken as 1.12. A + B represents the industrial lead flux and C + D represents the petrol lead flux when the 206Pb/207Pb ratio of petrol is 1.09. A + B + C represents the industrial lead flux and D represents the petrol lead flux when the 206Pb/207Pb ratio of petrol is 1.06.
engined road vehicles in the U.K. peaked in 1976. Overall, these trends are consistent with (i) the decline of heavy industry and major reduction in the use of coal as a fuel in the U.K. since the mid-20th Century and (ii) the corresponding increase over the same time period of the number of vehicles in use. Petrol lead has thus contributed up to 53% of the anthropogenic lead deposited in Loch Lomond sediments since its introduction in the 1920s, but this amounts to only 19% of the total excess lead inventory in the sediments. The apparent presence pre-1929 of petrol lead contributions can, in part, be attributed to the uncertainty in the chronology and the limited influence of bioturbation, but could also perhaps be due to the use or release of a very small amount of 206Pb-depleted lead in other activities. Industrial (and domestic) lead emission and deposition, especially at the high fluxes sustained in the 120 years prior to the onset of a steady decrease (in any of the three scenarios) in 1940 (Figure 4), has dominated the history of lead accumulation in Loch Lomond sediments. Just under 50% of the lead inventory in these sediments was deposited prior to 1900, long before the advent of increasingly strict U.K. environmental controls such as the Clean Air Acts in 1956 and 1968 and legislation in the 1980s to reduce and phase out the use of lead in petrol. This early deposition underlines the importance of taking past human activities into account in assessing the long-term burden, fate, and impact of lead in the wider environmental context (29, 30).
Acknowledgments We thank T. M. Shimmield and K. McKay (Scottish Universities Research and Reactor Centre) for their technical expertise in ICP-MS analysis; C. Robertson (SURRC) for help with γ-spectrometry analysis; and R. McMath, R. Tippett (Glasgow University Field Station), C. L. Bryant, and C. L. Sugden (University of Edinburgh) for assistance with fieldwork arrangements and sampling.
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(2) Shirahata, H.; Elias, R. W.; Patterson, C. C.; Koide, M. Geochim. Cosmochim. Acta 1980, 44, 149-162. (3) Ng, A.; Patterson, C. C. Geochim. Cosmochim. Acta 1982, 46, 2307-2321. (4) Petit, D.; Mennesier, J. P.; Lamberts, L. Atmos. Environ. 1984, 18, 1189-1193. (5) Shen, G. T.; Boyle, E. A. Earth Planet. Sci. Lett. 1987, 82, 289304. (6) Sugden, C. L.; Farmer, J. G.; MacKenzie, A. B. Proceedings of the Eighth International Conference on Heavy Metals in the Environment; CEP: Edinburgh, 1991; Vol. 1, pp 90-93. (7) Sugden, C. L.; Farmer, J. G.; MacKenzie, A. B. Proceedings of the Eighth International Conference on Heavy Metals in the Environment; CEP: Edinburgh, 1991; Vol. 1, pp 511-514. (8) Keinonen, M. Sci. Total Environ. 1992, 113, 251-268. (9) Rosman, K. J. R.; Chisholm, W.; Boutron, C. F.; Candelone, J. P.; Gorlach, U. Nature 1993, 362, 333-335. (10) Ritson, P. I.; Esser, B. K.; Niemeyer, S.; Flegal, A. R. Geochim. Cosmochim. Acta 1994, 58, 3297-3305. (11) Graney, J. R.; Halliday, A. N.; Keeler, G. J.; Nriagu, J. O.; Robbins, J. A.; Norton, S. A. Geochim. Cosmochim. Acta 1995, 59, 17151728. (12) Gobeil, C.; Johnson, W. K.; MacDonald, R. W.; Wong, C. S. Environ. Sci. Technol. 1995, 29, 193-201. (13) Croudace, I. W.; Cundy, A. B. Environ. Sci. Technol. 1995, 29, 1288-1296. (14) Sugden, C. L.; Farmer, J. G.; MacKenzie, A. B. Environ. Geochem. Health 1993, 15, 59-65. (15) Farmer, J. G. Hydrobiologia 1994, 290, 39-49. (16) Mackereth, F. J. H. Limnol. Oceanogr. 1969, 14, 145-151. (17) Ohnstad, F. R.; Jones, J. G. The Jenkin Surface-Mud Sampler User Manual; Freshwater Biological Association Occasional Publication 15, 1982.
(18) Maulood, B. K.; Boney, A. D. Hydrobiologia 1980, 71, 239-259. (19) Viczian, M.; Lasztity, A.; Barnes, R. M. J. Anal. At. Spectrom. 1990, 5, 293-300. (20) Bryant, C. L.; Farmer, J. G.; MacKenzie, A. B.; Bailey-Watts, A. E.; Kirika, A. Environ. Geochem. Health 1993, 15, 153-161. (21) Eades, L. J.; Farmer, J. G.; MacKenzie, A. B.; Kirika, A.; BaileyWatts, A. E. J. Environ. Radioact., in press. (22) Pennington, W.; Cambray, R. S.; Eakins, J. D.; Harkness, D. D. Freshwater Biol. 1976, 6, 317-331. (23) Bonnett, P. J. P.; Cambray, R. S. Hydrobiologia 1991, 214, 63-70. (24) Nriagu, J. O. Proceedings of the Seventh International Conference on Heavy Metals in the Environment; CEP: Edinburgh, 1989; Vol. 2, pp 361-366. (25) Lead in the Environment; Royal Commission on Environmental Pollution, Ninth Report; HMSO: London, 1983. (26) Delves, H. T.; Campbell, M. J. Environ. Geochem. Health 1993, 15, 75-84. (27) Hopper, J. F.; Ross, H. B.; Sturges, W. T.; Barrie, L. A. Tellus 1991, 43B, 45-60. (28) Digest of Environmental Protection and Water Statistics No. 15; Department of the Environment; HMSO: London, 1993. (29) Bacon, J. R.; Berrow, M. L.; Shand, C. A. Int. J. Environ. Anal. Chem. 1992, 46, 71-76. (30) Bacon, J. R.; Berrow, M. L.; Shand, C. A. Int. J. Environ. Anal. Chem. 1995, 59, 253-264.
Received for review February 22, 1996. Revised manuscript received May 29, 1996. Accepted May 31, 1996.X ES960162O X
Abstract published in Advance ACS Abstracts, August 1, 1996.
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