New Peat Bog Record of Atmospheric Lead Pollution in Switzerland

The isotopic evolution of atmospheric Pb in central Ontario since AD 1800, and its impacts on the soils, waters, and sediments of a forested watershed...
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Environ. Sci. Technol. 2002, 36, 3893-3900

New Peat Bog Record of Atmospheric Lead Pollution in Switzerland: Pb Concentrations, Enrichment Factors, Isotopic Composition, and Organolead Species W . S H O T Y K , * ,†,‡ D . W E I S S , § M . H E I S T E R K A M P , |,⊥ A . K . C H E B U R K I N , ‡,∇ P. G. APPLEBY,O AND F. C. ADAMS| Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland, T. H. Huxley School of Environment, Earth Science and Engineering, Imperial College, RSM Building, Prince Consort Road, London, SW7 2BP England, Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium, EMMA Analytical, Elmvale, Ontario, L0L 1P0 Canada, and Environmental Radiometric Research Centre, Department of Applied Mathematics and Theoretical Physics, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX England

A peat core collected at Etang de la Grue` re, an ombrotrophic bog in the Jura Mountains of Switzerland, was analyzed for organolead species (DEL, TEL, DML, and TML) using GCMIP AES, Pb isotopes using TIMS, and total Pb using XRF and age-dated using 210Pb. The earliest occurrence of quantifiable alkyllead is found at a depth of 24 cm, which is dated at AD 1946 ( 3; this finding is consistent with the introduction of leaded gasoline in Switzerland in 1947. The maximum concentration of alkyllead (2.89 ng/g) is found at 5 cm, which is dated at AD 1988 ( 2. This same sample yielded 206Pb/207Pb ) 1.1254, which is the least radiogenic value in the entire 2K core and comparable to the isotopic composition of Pb in leaded gasoline. The highest concentrations of DML (906 ng/g) and DEL (1906 ng/ g) also were found in this sample. Total alkyllead never accounts for more than 0.02% of total Pb in any given sample. The spatial and temporal variations in organolead species is matched by the changes in the isotopic composition of Pb over the same interval. Despite this, the decline in anthropogenic Pb pre-dates the maximum in total alkyllead and minimum 206Pb/207 Pb, indicating that atmospheric Pb emissions had already begun their decline prior to the introduction of unleaded gasoline. In fact, the decline in anthropogenic Pb was probably in response to the introduction * Corresponding author telephone: +49(6221)54 4803; fax: +49(6221)54 5228; e-mail: [email protected]. † University of Berne. ‡ Present address: Institute of Environmental Geochemistry, University of Heidelberg, INF 236, D-69120 Heidelberg, Germany. § Imperial College. | University of Antwerp. ⊥ Present address: Mettler-Toledo GmbH, Ockerweg 3, D-35396 Giessen, Germany. ∇ EMMA Analytical. O University of Liverpool. 10.1021/es010196i CCC: $22.00 Published on Web 08/15/2002

 2002 American Chemical Society

of legislation, first in Germany and later in the European Union, establishing a maximum allowable concentration of Pb in gasoline.

Introduction The ombrotrophic peat bog Etang de la Grue`re (EGR) in the Jura Mountains of Switzerland has provided a continuous record of atmospheric Pb deposition of 12 370 14C yr BP (1, 2). The isotopic composition of Pb (summarized as the ratio 206 Pb/207Pb) combined with total Pb concentrations and 210Pb age dating [evaluated using radionuclide (241Am) and pollen chronostratigraphic markers, primarily Cannabis] was used to construct a geochemical mass balance that argued against significant post-depositional migration of Pb within the peat profile (3, 4). Very similar temporal trends in Pb enrichment factor (Pb EF) and Pb isotope ratios reported for EGR were observed in three other Swiss peat bogs north of the Alps (5), adding further strength to the view of peat bogs as reliable archives of atmospheric Pb deposition. As an independent check on this interpretation, the isotopic composition of Pb was measured in samples of Sphagnum moss that had been collected from peat bogs since AD 1867 and stored at the Herbarium of the University of Geneva. The chronology of changes in 206Pb/207Pb preserved by the moss samples (6) were remarkably similar to the variations preserved by the peat cores taken from the Swiss peat bogs (5). Here, we have undertaken a detailed investigation of a second peat core (2K) from EGR: this second core was prepared and analyzed in the same way as the previous core (2F) for total Pb and Pb isotopes and dated using both 210Pb and 14C. Moreover, as part of a Ph.D. thesis about organolead species in the environment (7), selected samples from the 2K core were also used to measure the following organolead compounds: dimethyllead (DML), trimethyllead (TML), diethyllead (DEL), and triethyllead (TEL). By including these analyses, we were able to evaluate independently the chronology of the introduction, growth, and subsequent decline in leaded gasoline consumption that has dominated the accumulation of atmospheric Pb since leaded gasoline was first introduced in Switzerland in 1947 (5, 6). Our main objectives are to (i) compare the chronologies of the changing concentrations and isotopic composition of atmospheric Pb obtained using the new age dates (210Pb and 14C) of this second peat core; (ii) compare the concentrations of organolead compounds (DML, DEL, TML, TEL) and their variation with time with the isotopic composition of Pb to evaluate the hypothesis that leaded gasoline has been the dominant source of atmospheric Pb, and (iii) compare the record of organolead compounds in the peat core with other archives of atmospheric deposition of organolead compounds.

Materials and Methods Sample Collection and Preparation. A monolith of peat ca. 10 × 10 × 100 cm was collected at EGR on 22.6.93 (core 2K) using a stainless steel Wardenaar peat coring device; the 2K core was taken approximately 4.5 m east of the 2F core, which was collected on August 26, 1991. Both cores were sectioned by hand into ca. 3 cm thick slices using a stainless steel knife with a serrated edge. Pore waters were extracted by pressing samples in plastic bags by hand, and the peat samples were dried and milled as described previously (8). VOL. 36, NO. 18, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Summary of Selected Physical and Chemical Parameters and Age Dates at Etang de la Grue` re, Core 2Ka depth (cm)

depth avg

ash (%)

bulk density (g/cm3)

210Pb

sample

age

( SD

2K1 2K2 2K3 2K4 2K5 2K6 2K7 2K8 2K9 2K10 2K11 2K12 2K13 2K14 2K15 2K16 2K17 2K18 2K19 2K20 2K21 2K22 2K23 2K24 2K25 2K26 2K27 2K28 2K29 2K30 2K31 2K32 2K33 2K34 2K35

+3-0 0-3 3-6 6-9 9-12 12-15 15-19 19-22 22-25 25-28 28-31 31-34 34-37 37-40 40-43 43-46 46-49 49-52 52-55 55-58 58-61 61-64 64-67 67-70 70-73 73-76 76-79 79-82 82-85 85-88 88-91 91-94 94-97 97-100 100-103

1.5 -1.5 -4.5 -7.5 -10.5 -13.5 -17.0 -20.5 -23.5 -26.5 -29.5 -32.5 -35.5 -38.5 -41.5 -44.5 -47.5 -50.5 -53.5 -56.5 -59.5 -62.5 -65.5 -68.5 -71.5 -74.5 -77.5 -80.5 -83.5 -86.5 -89.5 -92.5 -95.5 -98.5 -101.5

2.95 2.09 1.85 2.36 1.62 1.49 1.68 1.92 2.05 2.80 4.75 5.07 5.87 6.94 5.58 3.58 3.24 2.95 3.95 3.82 2.22 1.84 1.80 1.86 1.37 1.90 2.51 2.01 1.55 1.80 1.70 1.44 1.52 1.99 2.03

0.025 0.037 0.044 0.044 0.053 0.040 0.077 0.063 0.075 0.070 0.091 0.131 0.098 0.130 0.126 0.117 0.104 0.079 0.085 0.093 0.068 0.053 0.057 0.043 0.036 0.053 0.063 0.068 0.058 0.069 0.074 0.057 0.054 0.059 0.046

1993 1991 1988 1984 1979 1975 1968 1958 1946 1932 1914 1881 1837

0 2 2 2 2 2 2 2 3 3 5 9 31

a

14C age

( SD

700

60

1110

60

1310

60

1650

70

1600

70

1960

70

Al (%)

Ti (ppm)

Zr (ppm)

Sc (ppm)

Pb (ppm)

0.06 0.09 0.09 0.11 0.08 0.07 0.08 0.10 0.13 0.18 0.33 0.40 0.47 0.49 0.42 0.30 0.26 0.27 0.35 0.35 0.21 0.19 0.19 0.17 0.15 0.18 0.22 0.21 0.19 0.21 0.22 0.21 0.19 0.26 0.24

70.4 80.6 87.8 151.5 78.2 55.7

1.7 1.7 2.9 4.0 1.4 1.3 1.6 2.3 2.5 4.0 13.7 11.9 21.0 19.5 15.9 8.2 6.4 5.7 10.1 11.2 2.7 2.6 4.8 2.5 1.6 3.7 6.1 4.2 3.9 5.2 5.3 5.1 2.9 7.5 5.8

0.13 0.21 0.19 0.26 0.20 0.16 0.16 0.29 0.31 0.44 0.83 0.91 1.10 1.10 0.96 0.59 0.54 0.52 0.75 0.80 0.35 0.31 0.41 0.32 0.23 0.36 0.43 0.39 0.35 0.46

13.0 22.1 27.4 45.9 62.7 54.8 47.4 47.7 41.1 61.2 79.9 96.6 47.5 41.7 30.4 22.6 10.5 8.5 4.6 3.6 2.0 1.5 2.0 2.3 2.0 4.3 6.4 8.4 8.3 10.4 10.4 8.9 6.4 5.9 4.4

92.6 99.7 127.9 325.5 375.9 511.8 542.0 445.0 226.6 194.1 295.4 351.5 122.8 95.9 112.2 102.0 65.3 116.9 133.8 130.4 161.8 154.0 131.4 97.5 191.9 182.3

0.35 0.28 0.57 0.49

Al by wavelength dispersive XRF. Ti by NASTIA XRF. Zr and Pb by EMMA XRF. Sc by INAA.

Lead and Selected Trace Element Analyses of Solid Peat Samples. Lead and selected trace elements, including Zr, were measured at EMMA Analytical Inc. (Elmvale, ON, Canada) using the energy-dispersive miniprobe multielement analyzer (EMMA) with Mo Kβ (energy of 19.6 keV) as the exciting wavelength. Lead was previously measured in triplicate, in the 2K core, using Mo KR (9) as the exciting wavelength (17.44 keV), and the new values (obtained using Mo Kβ) are in excellent agreement with previous measurements (R ) 0.955β + 1.57; r2 ) 0.997, n ) 32). Lead was also measured in acid digests of the 2K samples using ICP-MS, and the values are very similar (ICP-MS ) 0.927 (β)XRF 0.66; r2 ) 0.996, n ) 33). Titanium was measured using a new analytical spectrometer for Ti (NASTIA) that consists of (a) X-ray monochromator with concave LiF (200) crystal and X-ray diffraction tube with Co target, (b) rotating sample holder (10 rpm), and (c) energy-dispersive X-ray spectrometer with ORTEC HPGe GLP 16195 X-ray detector. The X-ray tube is loaded with 25 kV at 10 mA. Acquisition time is 300 or 600 s, depending on Ti concentration. The counting rate is 400-600 counts per second for 1 g of peat sample in a 32 mm Spectro X-ray cap with 5 µm Prolene bottom. The instrument was calibrated in the low Ti concentration range using Ti liquid AAS standards, as was the practice previously for Zr (10). For higher Ti concentrations, the instrument was calibrated using NIST 1632b and NIST 1635 Standard Reference Materials. The lower limit of detection (LLD) for Ti using NASTIA is 5 µg/g (compared to 30 µg/g with EMMA), and precision is ca. 10% at concentrations 10× LLD. Complete details about the 3894

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design and construction of this instrument will be presented in a separate publication (Cheburkin and Shotyk, manuscript in preparation). Aluminum and Sc were measured in solid peat samples using wavelength-dispersive XRF (5) and instrumental neutron activation analysis (2), respectively. All elemental analyses are summarized in Table 1 along with the ash contents and bulk density of the peat samples. Lead Isotopes. Following microwave-assisted acid dissolution of peat samples (11), Pb isotope ratios were measured in slices 2K1-13 and slice 2K15 using solid-source thermal ionization mass spectrometry (TIMS) with a VG Sector mass spectrometer (5, 6). The Pb isotope data are summarized in Table 2. Alkyllead Species. A simplified derivatization method for the speciation analysis of organolead compounds followed by gas chromatography-microwave-induced plasma atomic emission detection (GC-MIP AES) was developed (7). An in-situ butylation using tetrabutylammonium tetrabutylborate in an acetate buffer medium of pH 4.0 with simultaneous extraction of the derivatized organolead species into hexane was performed; for the analysis of peat samples, the desorption of the different species from the matrix by acid leaching is also included in that step. This speciation analysis is capable of quantifying all relevant organolead species, namely, trimethyllead (TML (CH3)3Pb+), dimethyllead (DML (CH3)2Pb2+), triethyllead (TEL (C2H5)3Pb+), and diethyllead (DEL (C2H5)2Pb2+). Detection limits are at the subnanogram per liter range, and the accuracy of the method was confirmed by analysis of a standard reference material (BCR-CRM 605,

TABLE 2. Isotopic Composition of Pb at Etang de la Grue` re, Core 2Ka sample

depth (cm)

2K1 +3-0 2K2 0-3 2K3 3-6 2K4 6-9 2K5 9-12 2K6 12-15 2K7 15-19 2K8 19-22 2K9 22-25 2K10 25-28 2K11 28-31 2K12 31-34 2K13 34-37 2K14 37-40 2K15 40-43 a

average depth 1.5 -1.5 -4.5 -7.5 -10.5 -13.5 -17.0 -20.5 -23.5 -26.5 -29.5 -32.5 -35.5 -38.5 -41.5

206/204

( 2 SD ( 2 SD ( 2 SD (abs) 207/204 (abs) 208/204 (abs) 207/206

17.776 17.516 17.507 17.565 17.762 17.934 18.058 18.124 18.198 18.263 18.343 18.372 18.435

0.009 0.009 0.010 0.012 0.008 0.009 0.015 0.009 0.009 0.011 0.009 0.010 0.009

210Pb

age 1993 1991 1988 1984 1979 1975 1968 1958 1946 1932 1914 1881 1837

700 14 C 18.470 0.033

15.581 15.555 15.556 15.587 15.601 15.604 15.616 15.590 15.645 15.647 15.640 15.612 15.621

0.010 0.011 0.011 0.014 0.010 0.011 0.016 0.011 0.011 0.012 0.010 0.011 0.010

15.631 0.029

37.581 37.336 37.331 37.476 37.703 37.908 38.035 38.025 38.243 38.324 38.382 38.341 38.398

0.032 0.034 0.035 0.041 0.032 0.033 0.042 0.033 0.033 0.037 0.033 0.034 0.032

38.449 0.074

0.87655 0.88808 0.88857 0.88740 0.87832 0.87009 0.86480 0.86020 0.85971 0.85674 0.85266 0.84979 0.84735

( 2 SD (abs)

208/206

( 2 SD (abs)

206/207

( 2 SD

0.00019 0.00019 0.00022 0.00029 0.00019 0.00018 0.00031 0.00019 0.00018 0.00022 0.00019 0.00019 0.00018

2.11210 2.12944 2.13024 2.13140 2.12051 2.11164 2.10423 2.09593 2.09938 2.09632 2.09036 2.08481 2.08079

0.00087 0.00101 0.00098 0.00131 0.00086 0.00086 0.00090 0.00089 0.00085 0.00104 0.00090 0.00086 0.00084

1.14084 1.12602 1.12540 1.12689 1.13854 1.14931 1.15633 1.16252 1.16318 1.16721 1.17280 1.17677 1.18015

0.00019 0.00019 0.00022 0.00029 0.00019 0.00018 0.00031 0.00019 0.00018 0.00022 0.00019 0.00019 0.00018

0.84628 0.00023 2.07964 0.00101 1.18165 0.00023

Peat samples collected on June 22, 1993.

TABLE 3. Concentrations of Alkyllead Compounds, Etang de la Grue` re, Core 2K avg depth

210Pb

sample 2K1 2K2 2K3 2K4 2K5 2K6 2K7 2K8 2K9 2K10 2K11 2K12 2K13

1.5 -1.5 -4.5 -7.5 -10.5 -13.5 -17.0 -20.5 -23.5 -26.5 -29.5 -32.5 -35.5

1993 1991 1988 1984 1979 1975 1968 1958 1946 1932 1914 1881 1837

age

DEL (pg/g) 1536.4 1499.2 1906.1 1362.3 701.4 731.3 223.2 222.4 22.0

DML TML TEL total alkyl(pg/g) (pg/g) (pg/g) lead (ng/g) 539.7 100.9 773.6 50.3 906.1 50.0 518.6 54.6 429.0 49.7 116.1 51.8 87.5 41.7 92.0 41.0 9.9

7.4 35.4 29.3 57.3 31.8 56.5 20.1 22.5 23.4

2.18 2.36 2.89 1.99 1.21 0.96 0.37 0.38 0.06

road dust). A full description of the analytical method is given elsewhere (12). The organolead concentration data is summarized in Table 3. Age Dating. Age dating of the youngest peat samples (ca. the past 150 years) was obtained using 210Pb (13). Selected peat samples from deeper layers were dated using 14C decay counting (Radiocarbon Lab, Physics Institute, University of Berne) as described elsewhere (1). All age dates are summarized in Table 1.

Results Geochemical Description of the 2K Peat Profile. The trace elements measured using XRF in the peat core can be arranged into the following indicator groups: Ca, Fe, and Sr are indicators of the trophic status of the peat core; K, Rb, and Mn reveal the thickness of the zone of active plant bioaccumulation; and Al, Ti, and Zr reflect the variation in the amount of mineral matter in the peat. The concentration range and vertical distribution of ash, Ca, Fe, and Sr (Figure 1a) are typical of continental, ombrotrophic Sphagnum bogs that receive inputs of inorganic solids exclusively from atmospheric inputs (2). Given their vertical distribution, they indicate that the entire peat 2K core is ombrotrophic and that Pb and the other elements in this section of the peat bog have been supplied solely by atmospheric deposition. This interpretation is consistent with the detailed study of the chemical composition of the porewaters from this bog (14). Elements such as K, Rb, and Mn are taken up by living plants and concentrated in the uppermost layers; in the 2K core at EGR, the thickness of this zone is restricted to the first four or five samples (Figure 1b), meaning that this zone is confined to a depth of ca. 9-12 cm below the plant/peat interface.

Concentration Profiles Conservative, Lithogenic Elements: Al, Ti, Zr, and Sc. Aluminum, Ti, Zr, and Sc are conservative, lithogenic elements whose abundance and distribution reflects the variation in mineral matter concentrations in the peat core (2). Almost all of the mineral matter in ombrotrophic peat is supplied by airborne soil dust (15). The concentrations of Al, Ti, Zr, and Sc indicate a considerable variation in the amounts of mineral material within the peat core (Figure 2). Concentration Profiles of Total, Lithogenic, and Anthropogenic Pb. The concentrations of Pb and the conservative, lithogenic trace elements as well as the age dates (210Pb and 14C) are summarized in Table 1. The concentration profile reveals pronounced peaks in Pb concentration in samples 2K5 (dated at AD 1979 ( 2) and in sample 2K12 (dated at AD 1881 ( 9), repectively, with the older peak considerably greater than the younger one (Figure 2a). However, there are elevated concentrations of ash, Al, Ti, Zr, and Sc at similar depths to the older, deeper Pb peak (Figure 2a), suggesting that the abundance of Pb is due to some extent to elevated concentrations of mineral material. The higher concentrations of mineral material possibly reflects higher rates of soil dust input, slower rates of peat accumulation, or both. To take into account the variations in amount of mineral material (mainly soil dust) on the Pb concentration profiles, Pb can be separated into “lithogenic” and “anthropogenic” components. Taking Sc as an indicator of the concentration of lithogenic-derived aerosols supplied by rock weathering (2), the concentration of Pb that was supplied to the bog via atmospheric deposition of these aerosols can be estimated either as

[Pb]lithogenic ) [Sc]sample × (Pb/Sc)earth’s Upper Continental Crust or as

[Pb]lithogenic ) [Sc]sample × (Pb/Sc)atmospheric soil dust With respect to the former approach, the abundance of Pb and Sc in the earth’s Upper Continental Crust are 14.8 and 16 µg/g, respectively (16). However, the “natural background” of Pb and Sc in pre-anthropogenic peat from EGR, representing atmospheric soil dust (ASD) deposited between 8030 and 5320 14C yr BP, is 0.28 µg/g Pb and 0.07 µg/g Sc (1). This suggests that a Pb/Sc ratio of 4 is more appropriate for calculating lithogenic Pb than the ratio (0.93) of these two elements in the UCC. For comparison, lithogenic Pb has been calculated using both the values for the UCC (Figure 2b) and “pre-anthropogenic, atmospheric soil dust” (Figure 2c). Lithogenic Pb calculated using ASD is 4.3 times greater (4/ VOL. 36, NO. 18, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Major and trace element concentrations in the 2K core: (a) Ca, Sr, and Fe; (b) K, Rb, and Mn. Solid horizontal line at 0 cm indicates the interface between living plant layer and underlying peat core. 0.93) throughout the profile than the lithogenic Pb calculated using UCC (Figure 2b,c). Once the lithogenic Pb component has been estimated using either approach, anthropogenic Pb is calculated as

[Pb]anthropogenic ) [Pb]total - [Pb]lithogenic Notice that the two plots of anthropogenic Pb (Figure 2b,c) are virtually indistinguishable. A linear regression gives anthropogenic Pb, UCC ) 0.991 (anthropogenic Pb, ASD) 1.18, r2 ) 0.999, n ) 35). Either approach for calculating anthropogenic Pb is acceptable because both approaches provide estimates of lithogenic Pb concentrations that are far lower than the total Pb concentrations. Chronology of Pb Enrichments in Core 2K. In general, the chronology of Pb pollution recorded by the 2K profile (Figure 2b,c) is similar to that reported earlier for 2F (1). There is a pronounced anthropogenic Pb component during the Roman Period. Following the decline of Roman Pb pollution, the lowest concentrations of anthropogenic Pb seen during the past two millenia are found in peat dating from the Dark Ages; even here, however, our Pb concentration and isotope data (1) indicate that anthropogenic Pb was a significant, if not dominant, component of the total atmospheric Pb flux. The 2K core documents Pb contamination during the Medieval Period starting ca. 1100 14C yr BP, when silver was recovered by intensive mining of German, Austrian, and Swiss lead ores. With respect to modern Pb enrichments, these began in earnest with the onset of the Industrial Revolution, which reached its zenith either toward the end of the 19th century or during the early part of the 20th century. In particular, sample 2K12 (31-34 cm) dating from AD 1881 ( 31 contains the maximum concentration of anthropogenic Pb, reflecting atmospheric Pb contamination caused mainly by coal-burning and industrial production. A second peak 3896

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in modern Pb contamination is dated AD 1979 ( 2 (Figure 2b, c) and is due mainly to leaded gasoline (see below). Abundance of Organolead Compounds and Variation with Time. The first quantifiable occurrence of organolead compounds is in peat dated at 1946 ( 3 (Figure 3a), and this is consistent with the introduction of leaded gasoline in Switzerland in 1947. The lack of measurable concentrations of alkyllead compounds in peat samples pre-dating this probably indicates a lack of significant vertical downward migration of these compounds, implying efficient retention of these species by peat. The maximum concentration of total alkyllead is found in peats dating from 1988 (Figure 3a). At this point the maximum concentrations of DML (Figure 3b) and DEL (Figure 3c) also are achieved; the decline in DML, however, is much more pronounced than that of DEL. While total alkyllead has certainly gone into decline, probably as a result of the gradual phasing out of leaded gasoline, there remains significant concentrations of alkyllead compounds in the most recent (1993) peat samples. The enhanced lipid solubility and toxicity of organolead compounds as compared with inorganic Pb gives rise to justifiable concern for their dispersion and accumulation in the environment (17). The concentrations of alkyllead species are very low, with the sum of DML, TML, DEL, and TEL never exceeding 0.02% of total Pb in any given sample. It is well-known that, even though virtually all of the Pb in gasoline is present as alkyllead compounds, very little of these remain preserved in environmental archives such as snow, ice, and peat: during the combustion process, the majority of the Pb-C bonds are broken, and the organolead is converted into inorganic lead halides that leave the engine via the exhaust pipe (18). Depending on the motor type and the velocity, a small fraction of organolead (∼1%) survives because of incomplete combustion and escapes with the exhaust. Once released into

FIGURE 2. (a) Abundance of Pb, ash, Al, Ti, Zr, and Sc in the 2K core. (b) Lithogenic Pb and anthropogenic Pb in the 2K core calculated using Sc concentrations in the peat and the Pb/Sc ratio of the Upper Continental Crust (15). (c) Lithogenic Pb and anthropogenic Pb in the 2K core calculated using Sc concentrations in the peat and the “natural background” Pb/Sc ratio of pre-anthropogenic aerosols at EGR (2). Details are provided in the text. Age dates shown on the left-hand side of panel b were obtained using 210Pb (first 13 samples) and 14C (last 6 samples); the latter are given as conventional radiocarbon years Before Present (BP). the atmosphere, the alkyllead compounds can be transported over great distances as free molecules or adsorbed on particles. These species ultimately decompose via their ionic compounds (trialkyllead and dialkyllead) into inorganic lead (18). Isotopic Composition of Pb and Its Variation with Time. The variation in 206Pb/207Pb in the 2K core (Figure 3d) is very similar to that reported earlier for peat core 2F (4). For comparison with the ratios shown in Figure 3d, the preanthropogenic, natural background 206Pb/207Pb ratio is ca. 1.2 and is found in peats dating from ca. 8030 to 5320 14C yr BP (1, 2). In sample 2K13 dated 1837 (( 31), the value is 1.18015 ( 0.0002, which compared with the background value of ca. 1.2 reflects the importance of anthropogenic Pb from both industrial lead production and coal burning already at that time (5). The isotopic composition of Pb becomes progressively less radiogenic from the beginning of the Industrial Revolution onward, but the greatest change is seen after the introduction of leaded gasoline in Switzerland in 1947 because gasoline Pb is characterized by especially low 206Pb/207Pb ratios (5, 6). The lowest 206Pb/207Pb ratio is found in peat dating from 1988, and the most recent sample measured (corresponding to 1993) shows an increase, back toward more radiogenic values; this latest value, however, is far removed from natural, background values. The lowest 206Pb/207Pb ratio (Figure 3d) occurs at the same time as the maximum concentration of alkyllead (Figure 3a). Comparison of the 2F (1991) and 2K (1993) Core Chronologies. The most recent peak in anthropogenic Pb

concentrations pre-dates the minimum 206Pb/207Pb in both cores (Figure 4): in the 2K core, anthropogenic Pb reveals a peak at 1979 ((2), and minimum 206Pb/207Pb (1.1254 ( 0.0002) is dated at 1988 ((2); in the 2F core, the maximum concentration of anthropogenic Pb is 1967 ((2), and the lowest 206Pb/207Pb ratios are 1.1231 ( 0.0002 dated at 1985 ((2) and 1.1232 ( 0.0002 dated at 1989 ((2). Thus, both peat cores indicate that concentrations of anthropogenic Pb began to decline prior to the beginning of the gradual phasing out of leaded gasoline. This notable decline corresponds in a general way with the reduction of gasoline Pb emissions in Europe, beginning in Germany in 1971, when legislation was introduced banning gasoline with a Pb content greater than 0.4 g/L (19). From January 1, 1976, the maximum allowable Pb content of gasoline sold in Germany was further reduced to 0.15 g/L. Both of these changes and the reduction in gasoline Pb emissions that they effected (19) pre-date by more than one decade the introduction of unleaded gasoline in 1983 in Germany (and 1985 in Switzerland). While the general agreement between the 2K and 2F cores is very good, there are also some minor differences that probably are mainly the result of differential core compression during sample collection, the relative thick peat slices (3 cm) used to represent the individual samples, and the imprecision of the cutting technique used to section the cores.

Discussion Organolead Compounds in the Environment. The history of tetraethyllead and its introduction into American gasoline VOL. 36, NO. 18, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. EGR core 2K: (a) concentration of total alkyllead, no measurable alkyllead was found in samples pre-dating AD 1946; (b) concentration of dimethyllead; (c) concentration of diethyllead; (d) summary of Pb isotope measurements expressed as 206Pb/207Pb. in 1923 has been summarized elsewhere (20), as has the chemistry of organolead compounds in the environment (2124). Tetraalkyllead compounds are volatile (Henry’s law constant of 4.7 × 104 and 6.9 × 104 Pa m3 mol-1 for tetramethyl and tetraethyllead, respectively, according to ref 25). However, only a small fraction of the Pb leaving an automobile as exhaust is in this form (e.g., typically 0.1-10%; 23). In addition to the relatively low emission factor from leaded gasoline combustion, tetraalkyllead compounds are rapidly decomposed by homogeneous gas-phase reactions such as photolysis, reaction with ozone, triplet atomic oxygen, or hydroxyl radical (26, 27) with half-lives of less than 10 h in summer and 40 h in winter (28). According to Harrison and Allen (22), the chemical cycle of alkyllead compounds originating in gasoline lead additives can be summarized as

R4Pb f R3Pb+ f R2Pb2+f (RPb3+)* f Pb2+ where ( )* indicates an unstable species. On the basis of these observations, it is reasonable to expect therefore that organolead will represent a small percentage of total Pb in atmospheric samples and that ionic alkyllead species will dominate the inventory of organolead compounds; this is also what is observed in direct air measurements (27). Washout factors (concentration ratio in rainwater as compared to air) for Pb(II) is typically a factor of 10 greater than that for tetraalkyllead compounds that predict a relative enrichment of alkyllead in an aging air mass, as the alkyllead is less efficiently scavenged (22). Early work had suggested that the relative importance of organolead to inorganic lead in air from a remote maritime location (northwest Scotland) as compared to urban air (central England) may indicate an important natural source of organolead to the atmosphere (26). In fact, incubation studies of marine sediments employing 210Pb nitrate did document the formation of alkyllead compounds (29). However, subsequent studies of air samples from remote western Ireland showed that organolead compounds were mainly below the lower limit of detection (30). Given our 3898

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present understanding of the atmospheric chemistry of organolead compounds (23), it is no longer necessary to invoke natural emission sources to explain the occurrence of organolead compounds in air from remote locations. In fact, analyses of organolead compounds in snow samples from Greenland failed to detect organolead compounds snow pre-dating the introduction of leaded gasoline (31). Moreover, no methyllead was detected in these samples (32). While bacterially methylated Pb may be produced in the Arctic Ocean (33), this does not appear to be a significant source of atmospheric Pb as compared to alkyllead compounds from gasoline (23). Comparison of Organolead Recorded by the Jura Bog with Alpine Snow and Ice. A record of organolead pollution in the Alps (24) included analyses of snow and ice from Mont Blanc. In both the alpine snow/ice and the peat records, detectable concentrations of organolead compounds are restricted to those samples that post-date the introduction of leaded gasoline: the lack of organolead compounds in samples pre-dating the introduction and use of gasoline lead suggests that natural sources of these compounds are quantitatively insignificant. For example, the occurrence of methyllead appears in a Mont Blanc ice sample dating from 1962 (24) and in the peat core from the sample dating from 1958 ( 2 (Figure 3). In fact, methyllead was added to gasoline blends in Europe starting in 1960 (31). These results indicate a common emission sources in both cases, namely, European motor vehicle emissions. No methyllead compounds were found in snow/ice or peat samples pre-dating this time, again suggesting that natural sources of organolead compounds to the atmosphere are unimportant. However, the temporal changes seen in the two archives are rather different: while the concentrations increase since the beginning of the 1960s at both sites, the concentration increases in the peat profile are generally more gradual and reach a well-defined maximum in the late 1980s before they diminish again (Figure 3). In contrast, the snow/ice record shows a more rapid initial concentration change (probably because of the better temporal resolution of this archive and the greater accuracy

FIGURE 4. (a) 2K core: Pb, Sc, lithogenic Pb, anthropogenic Pb, and comparison with the 206Pb/207Pb. (b) 2F core: Pb, Sc, lithogenic Pb, anthropogenic Pb, and comparison with the 206Pb/207Pb. In both cases, lithogenic Pb and anthropogenic Pb were calculated using the “natural background” Pb/Sc ratio of pre-anthropogenic aerosols at EGR (2). Notice that in both cases the peaks in anthropogenic Pb indicated by the dark arrows were achieved prior to the minimum in 206Pb/207Pb (as indicated by the light arrows). of the age dating) followed by a more uniform range in concentrations from the early 1960s to the early 1990s (24). There is no obvious explanation for this difference between the two archives, but the most recent peat sample from EGR dates from 1993, and the most recent snow sample from Mont Blanc dates from 1994. Additional measurements of organolead compounds in new samples of EGR peat accumulated since 1993 and Mont Blanc snow since 1994 would be helpful to further compare these two archives. A second major difference between these two records of organolead pollution is the speciation pattern of lead: the alpine snow/ice is dominated by methyllead whereas in the peat ethyllead compounds are the most abundant species. There is no straightforward explanation why methyllead compounds are the most abundant species in the alpine snow and mostly ethyllead is present in the peat, but there are many possibilities. First, it may be that the differences reflect variation in the methyl/ethyl ratio of gasoline within Europe; however, this seems unlikely as many central European gasoline blends contain methyllead:ethyllead in the ratio ca. 1:1 (31). Second, it may be that the methyllead species are poorly retained by the peat core; this, too, seems unlikely, as the chronology of DML in the peat is consistent with the introduction of methyllead into European gasoline blends in 1960. Third, it may be possible that decomposition of methyllead species in the peat after deposition is significantly greater than that of ethyllead species. Again, this seems improbable because the degradation of ethyllead compounds in soils is rapid, with half-lives on the order of hours (34, 35): even in the dark, ethyllead species are much more sensitive

toward degradation than methyllead species (36). Thus, if decay of the organolead species in peat was important, the methyllead species should have become residually enriched. Finally, due consideration must be given to the difference in altitude of the two sites: 4250 m in the case of Mont Blanc as compared with 1005 m in the case of EGR: being above the planetary boundary layer (ca. 1500 m) means that snow and ice at Mont Blanc are more influenced by long-range transport processes. In contrast to the record of organolead at EGR, wine grown in France is dominated by methyllead species (37). However, it has since been suggested that this may be due to ethyllead species having decomposed during fermentation (38). Mikac et al. (36) have succinctly summarized the problems that confound any comparison of the relative importance of methyllead and ethyllead species in environmental samples. In particular, this paper reminds us that the ratio cannot be generalized because a great number of factors contribute to it, including the complexity of organolead sources, physical and chemical transformations, atmospheric transport, and deposition. Clearly, there is no simple correlation between kinetic stability of these compounds in experimental studies and their relative abundance in environmental samples. Relative Importance of Organolead to Total Pb in the Peat and Snow Archives. In the bog at EGR, not more than 0.02% of the total Pb is in an organic form. In contrast, in snow and ice from Mont Blanc, 0.1% of the total Pb is organic (24); in Greenland snow and ice, 1% of the total lead is in the form of organolead species (31, 32). Compared to the peat bog at EGR, the total Pb concentrations in alpine snow and VOL. 36, NO. 18, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ice are typically 3-4 orders of magnitude lower, and in Greenland snow and ice are 5-6 orders of magnitude lower. Thus, the relative importance of organolead in these archives increases with decreasing total Pb concentrations and decreasing proximity to the source; this most likely reflects the more efficient scavenging of ionic Pb species (greater washout), but other factors may be involved. In recent studies of Pb speciation in rainwater collected in central England, total Pb in rainwater has exhibited a strong decline since the 1980s because of the phasing out of gasoline Pb (39), but the decline in organolead in rainwater is much weaker. In fact, the proportion of organolead is actually increasing; a similar phenonmenon is seen in the bog at EGR: recent declines in total Pb are much more rapid than organolead (Figures 2 and 3). Given the enhanced lipid solubility and toxicity of organolead compounds as compared with inorganic Pb, this temporal trend and its ramifications certainly deserve further study.

Acknowledgments Many years of generous financial support from the Swiss National Science Foundation to W.S. is sincerely appreciated. W.S. would also like to thank Prof. Jan Kramers for kind support with the TIMS; Mr. Steve Reese, formerly of the Radiocarbon Lab, University of Berne, for the 14C age dates; Prof. R. M. Harrison for providing many valuable publications; and Prof. Heinz-Friedrich Scho¨ler for helpful discussions about organolead compounds in the environment. A research grant by the F.W.O.-Vlaanderen, Belgium, to M.H. is gratefully acknowledged. Our sincere thanks is also extended to the two anonymous reviewers who provided excellent comments and suggestions that helped us considerably in improving the text.

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Received for review July 26, 2001. Revised manuscript received July 8, 2002. Accepted July 10, 2002. ES010196I