Environ. Sci. Technol. 2001, 35, 4180-4188
Estimating Distributions of Endogenous and Exogenous Pb in Soils by Using Pb Isotopic Ratios R EÄ D A M . S E M L A L I , † F O L K E R T V A N O O R T , * ,† LAURENCE DENAIX,‡ AND MICHEL LOUBET§ INRA, Unite´ de Science du Sol, RD 10, 78026 Versailles Cedex, France, INRA, Unite´ d’Agronomie, Domaine de la Grande Ferrade, BP 81, 33883 Villenave d’Ornon Cedex, France, and Universite´ Paul Sabatier, UMR 5563, 38 rue des 36 ponts, 31400 Toulouse, France
Metal contamination of soils by diffuse atmospheric deposition is a worldwide phenomenon. The assessment of incorporation of exogenous metal contaminants in soils is of major environmental importance. Once entering in the soil’s biogeochemical cycling, specific pedogenetic soil processes govern metal distribution patterns with depth. In this paper, we attempt to estimate the distribution of endogenous and exogenous Pb in two soils with contrasting pedogenesis, both representative of undisturbed ecosystems. Pb isotope analyses were performed using highprecision thermal ionization mass spectrometry. Endogenous and exogenous Pb concentrations and exogenous 206Pb/ 207Pb ratios of the samples were calculated using bulk Pb and Sc concentrations and bulk 206Pb/207Pb ratios. Endogenous Pb distribution was in agreement with dominant soil characteristics, almost constant in the young Andosol and with a clear minimum and maximum in the eluvial and illuvial horizons of the Podzol. The distribution of exogenous Pb was closely related to that of organic C in both soils. Exogenous Pb was evidenced in significant amounts at depth. Using moderate dispersive particle-size fractionation allowed us to evidence the presence of exogenous Pb in functional soil compartments and to highlight preferential distributions of Pb, according to pedology.
Introduction Lead is a trace metal naturally present in rocks and soils. High Pb concentrations are frequently reported at the soil’s surface and ascribed to contamination from anthropogenic sources such as coal ash, petroleum Pb, metal mining and smelting, or agricultural inputs. Since the beginning of the industrial era, Pb concentrations in soils have continuously increased as a result of human activities. Although atmospheric Pb pollution in Europe started in antiquity, the greatest Pb atmospheric fluxes occurred at the end of the 1970s (1). In the 1980s, anthropogenic Pb pollution was estimated to account for more than 95% of total atmospheric * Corresponding author phone: + 33 130 833 251; fax: + 33 130 833 259; e-mail:
[email protected]. † INRA, Unite ´ de Science du Sol. ‡ INRA, Unite ´ d’Agronomie. § Universite ´ Paul Sabatier. 4180
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Pb deposition (2). Atmospheric Pb deposits in forest soils were found to preferentially accumulate in the surface layer because of high organic matter content, but a portion of this Pb could be transferred in deeper soil horizons (3). Pb pollution is of significant concern because Pb is toxic to most plants and animals (4), and Pb that migrates through soils may impact groundwater. To identify sustainable land-use practices under increasing environmental health constraints, it is necessary to estimate the cycling rates of trace elements in soils giving special attention to specific soil-forming processes. In this context, it is essential to quantify the proportion of endogenous and exogenous metals (i.e. respectively originating and not originating from the parent material) in soils. The measurement of Pb isotopic compositions provides a useful tool to identify the source of Pb in soils and to determine the contribution of different Pb sources in soil samples using mixing models (5). Pb has four stable isotopes: 204Pb, 206Pb, 207Pb, and 208Pb. The radiogenic 206Pb, 207Pb, and 208Pb are produced by radioactive decay of 238U, 235U, and 232Th, respectively. The isotopic composition of Pb varies because of the radioactive decay of long-lived radioactive parents, where Pb ores display isotopic signatures that depend mainly on the original composition and the age of the ore bodies. Pb emissions from anthropogenic sources reflect the isotopic composition of the ores from which the Pb originated. Previous studies demonstrated the usefulness of this for seawater (6), freshwater (7-9), atmospheric (10), sediment (11-15), and peat bog (1, 15-17) samples. Several authors have examined isotopic composition of Pb in soils (5, 16, 18), but few data exist about the localization of exogenous Pb in soils as related to actual pedogenesis. Trace metals in soils, either exogenous or endogenous are redistributed by biogeochemical cycling processes. The identification of preferential sites of redistribution and metal binding is aided by the separation of different size and density fractions that are generated by specific pedological processes. The use of moderate dispersive treatments enhances the distinction of different classes of soil constituents and strongly aggregated soil fabrics, which can be considered as functional soil compartments regarding metal-binding (19). Such soil compartments include the soil’s clay fraction, coarse light fractions composed of particulate organic matter (20), and Fe-Mn concretions (21). Identifying the location of metals in such compartments provides insight about the relationship between metal distributions and pedogenesis. In addition, the application of the Pb isotopic ratio method allows the study of distributions of endogenous and exogenous Pb in soils. Consequently, the present work aims to (1) propose a method for quantifying exogenous Pb at the scale of soil horizons and at the scale of soil constituents and (2) relate these distributions to specific soil characteristics.
Methodology Description and General Characteristics of Study Sites. Two soils were chosen because of their contrasting geochemistry and pedological processes. The location of both sites in France is illustrated in Figure 1. The sites were representative of undisturbed ecosystems, located far from pollution point sources, such as cities, main roads, and industries. The first site consisted of an Andosolic soil, located on the east slope of the Puy de la Vache volcano (Massif Central, France), under forest dominated by Pinus sylvestris at an elevation of 1000 m. The slope of the site is 2%. Annual precipitation ranges 10.1021/es0002621 CCC: $20.00
2001 American Chemical Society Published on Web 10/02/2001
TABLE 2. Selected Physical and Chemical Characteristics of the Podzol
horizon Oa A E Bh Bs C a
FIGURE 1. Location of the study sites, Marque` ze and Puy de la Vache, on a map of France.
TABLE 1. Selected Physical and Chemical Characteristics of the Andosol
horizon Oi Oa Ah1 Ah2 Bw C R
depth, cm 7.5-2.5 2.5-0 0-14 14-40 40-55 55-60 60->110
texture
allobulk pH phane density, carbon, % kg m-3 g kg-1 (water)
521 278 loam 68 loam 59 loamy-sand 32 sand 7.5 gravel 0.8
5.5 5.9 6.3 6.7 8.5
12 15 25 23 1
6.9 19.8 520 590 600 630 700
from 700 to 1000 mm, and the average annual temperature is 10 °C. The soil developed on 8 m of basaltic scoria originating from the last volcanic eruption, 7650 YBP (22). The soil shows a succession of Ah1, Ah2, Bw, C, and R horizons (according to FAO, 23), overlain by an O horizon composed of undecomposed (Oi) and partly decomposed litter (Oa). Soil carbon content was high, progressively decreasing with soil depth from 68 to 7.5 g kg-1 (Table 1). The pH varied from 5.5 in the A horizon to 6.7 at 60 cm but reached 8.5 in the R horizon. Allophane content ranged from 12% in the Ah1 horizon and 25% in the Bw horizon (24). The second site consisted of a Podzolic soil, located in Marque`ze in Les Landes (southwest of France) under forest dominated by Pinus pinaster. The slope of the site is less than 2%; the altitude is 75 m. The climate is oceanic, with an average annual precipitation of 1100 mm, and an average annual temperature 13 °C. The soil developed on Quaternary eolian sand containing more than 90% of quartz and approximately 5% of feldspar. Textural and mineralogical continuity along the soil profile was assessed in thin sections. The soil shows a succession of A, E, Bh, Bs, and C-horizons. Bh and Bs horizons are indurated and form a hardpan (alios) (25). The soil is overlain by a 7 cm layer of humified organic material (Oa). The pH varies from 3.8 to 4.3 (Table 2). Carbon concentration was about 400 g kg-1 in the Oa, decreasing to 2.7 g kg-1 in the E horizon, and reaching 30 g kg-1 in the illuvial Bh horizon. Chemical Analyses. Bulk samples of each horizon of the two soils were collected. To limit redistribution of metal elements, grain size fractionations were performed using deionized water without destruction of the organic matter nor use of chemical dispersive agents. The procedure was a modification after that presented by Ducaroir and Lamy (26).
depth, cm 7.5-0 0-45 45-100 100-155 155-210 >210
texture
carbon, g kg-1
pH (water)
bulk density, kg m-3
sand sand sand sand sand
393.8 33.2 2.7 28.7 8.1 1.2
3.8 4.2 4.1 4.3 n.d.a
6 1213 1579 1780 1803 1560
Not determined.
Approximately 100 g of each homogenized sample was dispersed mechanically by slow agitation in 250 mL of deionized water for 12 h. The samples were wet sieved to separate six size fractions: >2 mm, 2000-200 µm, 200-100 µm, 100-50 µm, 50-20 µm, and 200 µm light E 200 µm dense E >200 µm light Bh 200 µm Bs 200 µm C 200 µm
393.8 33.2 2.7 28.7 8.1 1.2 444.2 103.2 86.6 9.1 380.9 1.6 388.9 0.3 449.2 286.7 21.4 14.3 11.9 1.4 0.2 400.2 300.7 274.7 284.4 217.7 6.8 3.5 227.3 169.8 111.0 31.9 5.6 3.8 224.4 1.0 0.5
25.5 4.7 4.0 9.2 8.5 5.7 47.0 34.4 31.2 30.5 41.4 6.8 27.1 1.6 18.5 44.6 28.6 28.7 30.2 9.8 2.6 8.1 72.6 67.0 66.6 54.2 12.9 6.4 65.9 69.5 48.7 22.6 15.0 5.6 116.9 14.6 4.1
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sample
206Pb/204Pb
2σ
206Pb/207Pb
2σ
2.1 1.4 0.7 0.8 0.1 7.1 0.1 87.4 0.3 0.6 0.6 0.4 0.4 6.3 91.6 0.1 2.6 0.6 0.7 0.7 6.8 88.6 1.7 0.2 0.2 0.4 8.9 88.7 0.2 1.1 98.7
17.995 18.520 18.497 18.409 18.432 18.492 18.356 18.346 18.289 18.269 18.277 18.354 18.360 18.765 18.378 18.413 18.367 18.310 18.281 18.352 18.580 18.250 18.396 18.402 18.411 18.405 18.375 18.418 18.400 18.407 18.401 18.452 18.406 18.436 18.278 18.542 18.542
0.007 0.007 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006
1.1516 1.1837 1.1818 1.1767 1.1777 1.1805 1.1747 1.1734 1.1691 1.1684 1.1694 1.1732 1.1743 1.1977 1.1754 1.1753 1.1742 1.1702 1.1676 1.1738 1.1846 1.1675 1.1753 1.1747 1.1750 1.1760 1.1743 1.1772 1.1762 1.1762 1.1764 1.1792 1.1737 1.1780 1.1695 1.1816 1.1841
0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002
In the A horizon, Pb isotopic ratio values are clustered in three groups (Figure 3b). Light and dense 20-100 µm fractions 4184
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weight of fractions/ horizon, %
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form the first group, with lowest 206Pb/207Pb ratios. The second group, including 200 µm fractions, shows significantly higher 206 Pb/207Pb ratio values. The highest 206Pb/207Pb ratio is observed for the dense >200 µm fraction. In the E horizon, the values of the 206Pb/207Pb ratios are more regularly distributed (Figure 3c). The lowest value is in the light >200 µm fraction (1.1675), followed by the silt fractions (1.16761.1742), and the clay fraction (1.1753). The dense >200 µm fraction has the highest 206Pb/207Pb ratio (1.1846), although slightly lower than the same fraction in the A horizon. This is probably due to a sporadic occurrence of thin humus coatings containing exogenous Pb on coarse skeleton grains in the E horizon. In the Bh and Bs horizons, isotope ratios are lowest in the 100-200 µm fractions (Figure 3d,e), followed by the clay, the silt, and the >200 µm fractions. These values are grouped more closely than in the overlying A and E horizons. The 206Pb/207Pb ratio of the clay fraction of the C-horizon is among the lowest for this soil (Figure 3f), suggesting the presence of exogenous Pb in this fraction. All intermediate fractions of the C horizon were grouped in one 2-200 µm fraction because they represented less than 0.1% in weight. The 206Pb/207Pb ratios of the 2-200 and >200 µm fractions are high (1.182-1.184) but still significantly lower than 1.198 observed for the >200 µm dense fraction of the A horizon. These data can be explained by the presence of
thin isotropic coatings on mineral grains in the C-horizon, as can be observed in thin sections. These coatings would be carriers of exogenous Pb. Total Pb and C concentrations are not correlated among soil horizons. At the scale of the size fractions, they are strongly correlated in the Bh (r ) 0.997, R ) 0.005) and Bs horizons (r ) 0.96, R ) 0.005), but no correlation is found for fractions in the A, E, and C horizons. Endogenous and Exogenous Pb. The Pb/Sc background value calculated from the mean Pb and Sc concentrations of cleaned, coarse mineral fractions of A, E, B, and C-horizons is 12. In comparison to the Andosol, this much higher value is mainly due to the high proportion of quartz skeleton grains in the Podzol. Using this ratio at the scale of the soil profile, endogenous Pb concentrations vary from 1.5 mg kg-1 (E horizon) to 5.9 mg kg-1 (Bh horizon). These variations clearly illustrate strong weathering and leaching conditions in the surface horizons of the Podzol and accumulation of endogenous Pb in the illuvial B-horizons. Endogenous and exogenous Pb concentrations are of the same order in the soil horizons. Calculated exogenous 206Pb/207Pb ratios vary from 1.140 to 1.173. The lowest exogenous Pb isotopic ratio is in the Bh, followed by the Oa, Bs, C, A, and E horizons. At the scale of particle size fractions, exogenous Pb concentrations VOL. 35, NO. 21, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 6: Podzol: Sc, Endogenous and Exogenous Pb Concentrations, and Exogenous Pb Isotopic Ratios sample
Sc, mg kg-1
Pb/Sc
endogenous Pb, mg kg-1
exogenous Pb, mg kg-1
Oa horizon A horizon E horizon Bh horizon Bs horizon C horizon A 200 µm dense A >200 µm light E 200 µm dense E >200 µm light Bh 200 µm Bs 200 µm C 200 µm
0.3 0.2 0.1 0.5 0.3 0.2 3.8 1.5 0.7 0.9 1.7 0.3 1.2 0.4 1.1 4.7 1.3 0.5 1.0 0.4 0.1 0.4 3.4 3.3 3.1 3.1 1.0 0.2 4.3 4.2 3.0 3.6 1.1 0.2 4.8 2.5 0.2
76.6 25.5 32.5 18.7 25.1 24.3 12.4 22.7 45.4 32.8 24.3 25.0 22.6 4.1 16.8 9.5 22.9 58.1 30.5 23.7 40.5 20.3 21.3 20.6 21.5 17.8 12.8 39.3 15.4 16.7 16.1 6.2 13.3 25.5 24.5 5.9 24.8
4.0 2.2 1.5 5.9 4.0 2.8 45.7 18.2 8.3 11.2 20.4 3.2 14.4 1.6 13.3 44.6 15.0 5.9 11.9 5.0 0.8 4.8 40.9 39.0 37.2 36.6 12.1 2.0 51.3 50.1 36.2 22.6 13.5 2.6 57.4 14.6 2.0
21.5 2.5 2.5 3.3 4.4 2.9 1.3 16.2 23.0 19.3 20.9 3.6 12.7
1.143 1.172 1.173 1.140 1.160 1.165
(0.002 (0.004 (0.002 (0.015 (0.005 (0.005
1.147 1.159 1.152 1.142 1.151 1.149
(0.009 (0.002 (0.004 (0.008 (0.006 (0.008
5.3
1.121
(0.028
13.6 22.8 18.3 4.8 1.8 3.3 31.6 28.0 29.3 17.6 0.8 4.4 14.6 19.5 12.5
1.149 1.163 1.149 1.150 1.179 1.125 1.147 1.144 1.147 1.132
(0.008 (0.002 (0.005 (0.007 (0.001 (0.015 (0.010 (0.011 (0.010 (0.020
1.168 1.103 1.123 1.117
(0.002 (0.048 (0.028 (0.034
1.5 3.0 59.6
1.161 1.143
(0.005 (0.008
2.1
1.172
(0.004
reach up to 59.6 mg kg-1 in the clay fraction of the C-horizon (Table 6). Obviously, not all exogenous Pb is immobilized in illuvial horizons, and a significant portion migrates deeper in the soil and, ultimately into groundwater. The lowest calculated exogenous Pb isotopic ratios are found in the Oa, Bh, and Bs horizons. This result seems to indicate that modern, anthropic Pb with low isotopic ratios is likely to bypass the eluvial A and E horizons. For fractions with Pb/Sc ratios very close to the chosen Pb/Sc background value our calculations indicate a lack of exogenous Pb. For the 200 µm fraction of the A horizon (1.1977) became assigned as the value of the parent material. Calculated exogenous 206Pb/ 207Pb ratios vary from 1.103 in the 200 µm dense fraction of the E horizon. In the A horizon, the calculated exogenous 206Pb/ 207Pb ratios vary from 1.121 to 1.159. The value of the exogenous 206Pb/207Pb ratio of the bulk A horizon is 1.172, higher than the values of the particle size fractions. This underestimation in the isotopic balance is most likely due to our assignment of the 206Pb/207Pb ratio of the >200 µm dense fraction as the value of the parent material. By doing so, we hypothesize that this particle size fraction is free of exogenous Pb and therefore, cannot calculate its exogenous 206Pb/207Pb ratio. The real isotopic signal of the parent material is certainly slightly higher than that of the >200 µm dense fraction of the A horizon. In fact, the exogenous 206Pb/207Pb 4186
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exogenous 206Pb/207Pb
ratio of the dense >200 µm fraction of the A horizon can also be estimated by subtracting the weighted exogenous Pb isotopic ratios of the other particle-size fractions of the A horizon from the total exogenous 206Pb/207Pb ratio of the bulk A horizon. If we do so, the calculated value reaches 1.179, identical to the calculated Pb isotopic ratio of the >200 µm dense fraction of the E horizon. Another limit of our method concerns fractions where exogenous Pb concentrations are very low. For these cases, the errors of the calculated exogenous 206Pb/207Pb ratios are maximal, so these values have little validity. For instance, no data are given for the dense 100-200 µm fractions of Bh and Bs horizons (Table 6). At the scale of the entire soil profile, no correlation is observed between the soil C and endogenous or exogenous Pb concentrations. At the scale of the size fractions, endogenous Pb and C concentrations are only correlated in the Bh horizon (r ) 0.972, R ) 0.005), whereas exogenous Pb and C concentrations in the particle size fractions are strongly correlated in the Bh (r ) 0.982, R ) 0.005) and B horizons (r ) 0.898, R ) 0.025) but not in the A and C horizons. General Discussion. The use of Pb isotopic ratios in soils to study distribution of endogenous and exogenous Pb is rather novel. Determined values in soil horizons and grain size fractions of the two soils were consistent with soil forming processes. The data were used to distinguish the distributions of endogenous and exogenous Pb, based on the use of a constant ratio of Pb and an endogenous reference element, in our case Sc. In the case of low weathering rates (Andosol), the use of the Pb/Sc ratio of the parent material for the whole soil produces reasonable values and distributions of endog-
FIGURE 4. Total, exogenous, and endogenous Pb concentrations in the Andosol.
FIGURE 5. Total, exogenous, and endogenous Pb concentrations in the Podzol.
enous and exogenous Pb. However, in strongly weathered materials (Podzol), differential weathering of feldspars in surface horizons (containing a dominant part of endogenous Pb) versus quartz (containing predominantly Sc), the Pb/Sc of the parent material cannot be applied to the whole soil profile. In that case a Pb/Sc ratio calculated from the mean values of Pb and Sc concentrations of cleaned, primary minerals of the A, E, B, and C horizons is used. While still an approximation, the calculated values and distributions appears reasonable and in agreement with specific podzolization processes. Despite these assumptions, our approach offers new insight in cycling dynamics of metals in soils and particularly emphasizes the role of soil functioning. Nature of the Exogenous Pb. In France, recent values of the 206Pb/207Pb ratios of gasoline range from 1.069 to 1.094, while 206Pb/207Pb ratios varied from 1.143 to 1.155 for industrially derived Pb (36). These isotopic signatures are less radiogenic than the signature of natural Pb present in French rocks and soils because anthropogenic Pb originates, to a large degree, from remote Pb ore deposits formed in the first half of the history of Earth. In Europe, historical pollution Pb isotopic ratios were of about 1.16-1.18 (1). The observed isotopic ratios of the soil horizons were closer to the values of the historical Pb pollution. The decrease of the Pb isotopic ratios of the soil samples is attributed to the presence of exogenous Pb. Variations among the total Pb concentrations and the total Pb isotopic ratios clearly demonstrate the incorporation of exogenous Pb in the two soil profiles. Decreases of the total Pb isotopic ratios, although significant, were small. Except for the litter layers, bulk 206Pb/207Pb ratios only reach 1.1844 and 1.1767 in the case of the Andosol and the Podzol, respectively. These values are rather high compared to literature data for Pb isotopic ratios of leaded gasoline or industrially derived Pb (36). Impact of Pedogenesis on the Distribution of Exogenous Pb. Andosol pedogenesis on basaltic rocks is characterized by hydrolysis of volcanic material and neoformation of shortrange order minerals (allophane, imogolite), intimately associated with organic matter (33, 37-38). These secondary minerals preferentially occur in the finest fractions and as coatings in coarse fractions (24). Organic matter and shortrange-order minerals contribute to high Pb retention (3941). Exogenous Pb is present in all particle-size fractions, but its concentration generally increases with decreasing size of the particle fractions. In addition, the incorporation of exogenous Pb in the soil profile decreases with the depth (Figure 4). All these data suggest that exogenous Pb is strongly associated with organic matter and secondary soil constituents such as allophanes. The distribution of exogenous Pb in the Andosol reflects cumulative soil formation as well as current soil processes. Recently Semlali (42) showed that actual gravitational transfer of Pb is very slow in this Andosol. Little variation in endogenous Pb concentration is observed
for bulk horizons (about 4.5 mg kg-1). No correlations were observed between endogenous Pb and C, neither at the scale of the soil profile or at the scale of soil constituents. Therefore, endogenous Pb still is mainly localized in unweathered primary minerals. Andosols are frequently described as metaltrapping materials, which was one of our criteria for choosing the soil for this study. The demonstrated preferential occurrence of exogenous Pb in the finest fractions, its progressive decreasing incorporation with depth, the endogenous Pb exclusively representing the total Pb concentration in deeper soil horizons, fully corroborate this high metal-sorbing character of Andosols. Podzols are strongly weathered, highly acidified soils frequently developed on well-drained, coarse textured mineral substrates. The organic acids produced in the litter layer act as strong weathering and metal-complexing agents, leaching the E horizon and accumulating in the B horizons. The distribution of Pb, in particular endogenous Pb (Figure 5), illustrates this process well. Exogenous Pb is dominant in litter horizons. In the A horizon, it is mainly located in particulate organic matter and to a lesser extend in stable soil aggregates, mostly fecal pellets. The coarse sand fraction is almost free of exogenous Pb (Figure 3b). In this horizon, organic and mineral constituents are simply adjacent, rather than intimately complexed. In B-horizons, exogenous Pb is preferentially located in humus coatings on mineral grains and in the finest fraction. Considering its high C-content (Table 5), this clay fraction includes dispersed fragments of these coatings. Wang and Benoit (43) explain the mechanism of Pb migration in Podzols by either migration of Pb associated with colloidal organic matter or by migration of Pb2+. According to their model, the precipitation of Pb in B horizons is due to adsorption of Pb2+ on amorphous organomineral coatings or to accumulated colloidal organic matter. In the studied Podzol, exogenous Pb is well correlated to organic C only in the B horizons, and we assume that its migration and accumulation in the B horizons follow Wang and Benoit’s model. Significant accumulation of exogenous Pb in B-horizons was demonstrated by Bindler et al. (16). When considering the amounts of exogenous Pb and 206Pb/207Pb ratios in the finest fractions of the different soil horizons, the data suggest that after mobilization by organic substances in the litter horizon, exogenous Pb migrates directly toward B horizons. A portion is intercepted from the soil solution by biological activity in the A horizon and immobilized in plants residues. Under the B horizon, exogenous Pb migrates to deeper horizons, concomitant with the migration of fine organic and organo-mineral constituents, as suggested by the high C and Pb content in the
