Rapid transport of anthropogenic lead through soils in southeast Missouri

Applied Geochemistry 23 (2008) 2156–2170

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Rapid transport of anthropogenic lead through soils in southeast Missouri Panjai Prapaipong *, Colin W. Enssle 1, Julie D. Morris 2, Everett L. Shock 3, Rachel E. Lindvall 4 Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Dr., St. Louis, MO 63130, USA

a r t i c l e

i n f o

Article history: Received 29 November 2006 Accepted 9 May 2008 Available online 21 May 2008 Editorial handling by G. Filippelli

a b s t r a c t To investigate Pb transport and cycling, soils from the forest floor and cores from White Oaks (Quercus alba L.) were collected near a Pb smelter in SE Missouri at varying depths from the surface and varying distances. Lead concentrations in soil samples at the surface drop dramatically with distance from approximately 1500 mg/kg at less than 2 km from the smelter to around 100 mg/kg at localities greater than 2 km from the smelter. Lead contents in tree rings are below 0.5 mg/kg in samples dated prior to 1970, and rapidly increase in 1975–1990 samples. Isotopic compositions of soils and tree rings exhibit systematic variations of Pb isotopic compositions with depth and tree ring age. Distinguishable isotopic signatures for Pb sources allowed quantification of the contribution of smelter Pb to the soils. At depths where Pb concentrations decreased and approached constant values (10– 25 cm, 10–30 mg/kg), 50–90%, 40–50% and 10–50% of the Pb could be derived from the smelter for the samples at locations less than 2, 2–4 and over 4 km from the smelter, respectively. The remaining portion was attributable to automobile emission and bedrock sources. Because the smelter operated from 1963 to 2003 and samples were collected in 1999, it is estimated that smelter Pb infiltrates at rates of 1 cm/yr (30 cm in 30 yr). At distances less than 1.5 km from the smelter, even though Pb concentrations become asymptotic at a depth of 30 cm, isotopic evidence suggests that Pb has migrated below this depth, presumably through exchange with naturally occurring Pb in the soil matrix. This implies that soils heavily polluted by Pb can exceed their Pb carrying capacity, which could have potential impacts on shallow groundwater systems and risk further exposure to human and ecological receptors. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Lead is a toxic element that has been spread widely throughout the Earth’s surface, especially through combustion of leaded gasoline. Despite the phasing out of leaded * Corresponding author. Present address: School of Earth and Space Exploration, Arizona State University, Box 871404, Tempe, AZ 852871404, USA. Tel.: +1 480 7 965 9142; fax: +1 480 965 8102. E-mail address: [email protected] (P. Prapaipong). 1 Present address: Columbia Business School, 3022 Broadway, New York, NY 10027, USA; ARCADIS US, Inc., 8 South River Road, Cranbury, NJ 08512, USA. 2 Present address: Ocean Sciences Division, National Science Foundation, 4201 Wilson Blvd., Arlington, VA 22230, USA. 3 Present address: School of Earth and Space Exploration, Arizona State University, Box 871404, Tempe, AZ 85287-1404, USA. 4 Present address: Lawrence Livermore National Laboratory, Chemical Biology and Nuclear Chemistry Division, Livermore, CA 94550, USA. 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.05.002

gasoline, Pb from automobile exhaust has been traced in soils in numerous locations around the world. In addition, Pb poisoning still remains an environmental health hazard locally, from old leaded paint and in places near point sources of Pb like mines and smelters. Given the widespread distribution of anthropogenic Pb and its accumulation in many soils, questions persist about the long-term effects of anthropogenic Pb on the environment and ecosystems. This has motivated several studies that have quantified anthropogenic Pb in soils. Using isotopic techniques and adopting two-component mixing models, it was estimated that 85–90% of the Pb in surface soils near smelters and in urban areas has smelter and vehicular sources (Bacon and Dinev, 2005; Gallon et al., 2006; Mihaljevicˇ et al., 2006). In remote forests, a wide range of anthropogenic and atmospheric Pb proportions (2–100%) was reported in surface soils (Kanste, 2003; Bacon and

P. Prapaipong et al. / Applied Geochemistry 23 (2008) 2156–2170

Hewitt, 2005; Watmough and Hutchinson, 2004; Klaminder et al., 2006). When the lengths of time during which study areas were exposed to anthropogenic Pb were known, rates of Pb transport through soils, varying from 0.3 to 1.97 cm/yr, have been calculated (Puchelt et al., 1993; Erel et al., 1997; Whitehead and Ramsey, 1997; Ettler et al., 2004). Although these studies show that anthropogenic Pb has percolated through soils, a long residence time of Pb in soils was suggested by Klaminder et al. (2006). It was predicted that polluted Pb would release from soils to streams in a Swedish catchment in 200– 800 yr. Other studies have focused on distribution of anthropogenic Pb in various fractions of sequential extracts (Teutsch et al., 2001; Emmanuel and Erel, 2002; Négrel and Roy, 2002; Bäckström et al., 2004; Wong and Li, 2004; Bacon and Hewitt, 2005; Bacon et al., 2006) and in soil proportions with different particle sizes and densities (McGill et al., 2003; Graham et al., 2006). The results of these studies differ, as it has been shown that anthropogenic Pb is preferentially associated with the EDTAextractable fraction (Bacon and Hewitt, 2005), carbonates and Fe oxides (Teutsch et al., 2001), carbonates (Wong and Li, 2004), Fe and Mn oxides (Négrel and Roy, 2002) and the reducible fraction (Bacon et al., 2006). This suggests dynamic partitioning of anthropogenic Pb after it reaches the soil, and implies variations in solubility, bioavailability and mobility of anthropogenic Pb at different environmental settings. In this study, Pb elemental and isotopic measurements were used to identify Pb sources and to track Pb transport in soils and tree rings around a Pb smelter in a rural area in SE Missouri. This study benefits from the fact that the smelter provides a point source of Pb, and has been known to operate continuously between 1969 and 2003. Additionally, Pb concentrates used at the smelter are from Viburnum ores, which are isotopically distinct from Pb in local bedrocks and automobile emissions. This allows tracing the movement of Pb from the smelter through the atmosphere and soils to the biosphere during a known period of time.

2. Study area Soil and tree ring samples were collected from mineral soil horizons and white oaks (Quercus alba L.) from forests around the smelter. The smelter is located in Glover, Missouri, Midwestern USA, and situated in a north-south oriented valley with higher elevations at the north end as shown by the topographic map in Fig. 1. The elevations range from 200 m in the south to 500 m to the north of the smelter. The bedrock of the valley to the south is mostly Bonneterre dolomite, whereas St. Francois rhyolite forms the bedrock of the high elevation areas, such as Taum Sauk, Hogan and Ketcherside Mountains. These features are shown in Fig. 1 together with sampling locations, depicted by solid circles and squares. The smelter released Pb to the air in the amount of 30, 90–100, 150–170, and 10–20 metric tons/yr in 1988–1991, 1992–1994, 1995– 1996 and 1997–2003, respectively. From 1988 to 2003, releases of Pb from the smelter to the surface water and land

2157

were 1–80 kg/yr and 540–1600 metric ton/yr, respectively (Missouri Department of Natural Resources, 2002; The Doe Run Company, 2006a, b). Common trees in the sampling area include white oaks, northern red oaks, red cedars and shortleaf pines. Annual precipitation, mostly rain, is 109–112 cm/yr (Linsemier, 1997). Values of moist bulk density, pH and organic matter content are in the range of 1.00–1.90 g/cm3, 3.6–6.0% and 0.5–2%, respectively (Linsemier, 1997). Soil characterization is fully described in Prapaipong (2001).

3. Methods Sampling locations were chosen to provide broad azimuthal coverage at various distances, as well as to sample soils developed and trees located over both types of bedrock, within the constraint of being limited to public lands (Mark Twain National Forest) (Fig. 1). For soil, trenches of an approximate dimension of 30  100 cm were dug as deep as 60 cm. Soil samples from mineral horizons were collected from the trench walls at 10 cm depth interval using a HNO3-cleaned plastic spatula, and stored in HNO3-washed plastic zipper bags. The samples were airdried, crushed, sieved (2 mm), ground and oven-dried overnight at 60 °C. Tree cores were removed from white oaks of similar diameter using a 5 mm-diameter borer at a shoot height of 1.5 m, and stored in HNO3-washed polypropylene tubes. Multiple cores were collected from the same tree 10 vertical cm apart, with one used to establish a site chronology through cross dating with local tree ring indicies (Nash and Kincaid, 1990; De Visser, 1992; Eklund, 1995; Duvick, 2000). At 4 locations, cores from multiple trees were analyzed to evaluate within-site variability. Core wood was dissected into 3-yr increments, oven-dried and stored individually in HNO3-washed polyurethane vials. Approximately 1 g of each dried soil and tree ring sample weighed to the nearest 0.0001 g was digested with 1 mL of 48–51% w/w HF and 4 mL of 68–71% w/w HNO3 acids in a CEM MDS-2100 high pressure microwave oven. Total digestion, rather than extraction, was used in this study to account for all fractions of Pb in the soils. The digested samples were diluted with 1% HNO3, and analyzed for major and trace elements, and a subset were analyzed for isotopic ratios (208Pb/206Pb and 207Pb/206Pb) with a Thermo Electron Element 1 single-collector double-focusing magnetic sector inductively coupled plasma mass spectrometer (ICP-MS). Operating conditions of the microwave oven and ICP-MS are reported in Prapaipong (2001). To determine contamination that may have occurred during the analyses, control solutions (blanks) were treated and analyzed the same way as the samples. Analytical accuracy and precision for elemental and isotopic measurements were determined by analyses of NIST standard reference material 2711 (Montana soil), HPS Certified Reference Material Orchard Leaves Solution, and NIST 981 (common Pb isotopic standard). Isotopic ratio deviations due to mass bias were corrected for by using 205Tl/203Tl, nearby isotopes with a constant natural ratio of 2.3875.

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Fig. 1. A topographic map showing the smelter and sampling locations, together with contours showing distributions of Pb concentrations in the topsoil horizons in mg/kg. Jagged white lines depict roads. Black dashed curves encircle the areas underlain by rhyolite bedrock. Solid circles and squares represent locations on rhyolites and dolomites, respectively. White dashed curves are those where data are limited for contour constraint. Data for locations in italic are used as examples extensively in the text. Tree ring samples presented in this article include those from locations 991108B, 990525B and 991115A.

Precisions for Pb concentrations were within 1% (1r); measured and accepted values for standards agreed within the quoted errors. Isotopic ratio measurements were precise to

0.15% (1r, 3 runs  500 passes). This is a poorer precision than possible by thermal ionization mass spectrometry (TIMS), but it allows a wide range of isotopic ratios for

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the various Pb sources in soils to be distinguished and permits measurements of a large number of samples. Replicate measurements of NIST 981 (208Pb/206Pb of 2.1662 and 207Pb/206Pb of 0.91346) are in close agreement with the certified isotopic compositions of the NIST standard (208Pb/206Pb of 2.1681 and 207Pb/206Pb of 0.91464).

Lead concentrations in surface soils (A horizon) are depicted as white contours (in mg/kg) in Fig. 1. Lead concentrations in bulk soil samples at the surface decrease dramatically with distance from 1500 mg/kg in the immediate vicinity of the smelter to 100 mg/kg at localities > 2 km from the smelter. Preliminary sequential extraction results show that a majority of Pb in surface soils is found to associate with carbonate (15–50%), Mn oxides (15–50%) and amorphous Fe oxides (15–40%). With depth, Pb concentrations decrease drastically and reach constant values. Shown in Fig. 2 are examples of Pb variations with depth for six locations on dolomites and rhyolites at 1.2–10.2 km from the smelter. The concentrations at the surface depicted in Fig. 2 vary from 80 to 1200 mg/kg, depending on the distance from the smelter, as shown in Fig. 1. Typically, the decrease in concentration is rapid in the upper 10–15 cm, except for samples collected at less than 1.5 km from the smelter, where the average asymptotic depth is 20–25 cm. Below these depths, the concentrations mostly reach constant values of 10–30 mg/kg. Lead concentrations used for Figs. 1 and 2 and for other locations are summarized in Table 1. Lead concentrations in tree rings also show variations with location as well as age, examples of which are illustrated in Fig. 3. Across all the sites, samples dated prior to approximately 1970 show Pb concentrations generally below 0.5 mg/kg. From 1975 to 1990, rapid increases in Pb concentrations in more recent wood are observed from trees located less than 1.2 km from the smelter. After 1990, concentrations increase and do not reach constant values. Maximum concentrations vary from 1 to 10 mg/kg, correlating to the distance of the trees from the smelter. Intrasite variability was minimal as shown in tree rings from two oaks at location 991115A. 4.2. Isotopic composition The source of the Pb can be confirmed by measurement and consideration of the isotopic ratios 208Pb/206Pb and 207Pb/206Pb in the soil samples. Plotted in Fig. 4 are contours showing variation patterns of 207Pb/206Pb in soils from the surface (A horizon) around the smelter. The contours were generated from isotopic measurements of soils at locations shown as white squares in the figure. The Viburnum ores used at the smelter had a 207Pb/206Pb isotopic composition of 0.75, and the samples immediately around the smelter had 207Pb/206Pb isotopic compositions close to 0.75. Further from the smelter, the isotope ratios deviate from the value of the

depth, cm

4.1. Lead contents

10

20

30

40

50

60 0

50

100

150

200

250

300

[Pb], mg/kg 990924B, 4.2 km 990924A, 2.9 km 991003A, 1.2 km

(B) Soils on rhyolite bedrocks 0

10

depth, cm

4. Results

(A) Soils on dolomite bedrocks 0

20

30

40 990906A,10.2 km 990525A, 4.2 km 991108A, 3.2 km

50

60 0

50

100

150

200

250

300

[Pb], mg/kg Fig. 2. Examples of depth profiles of Pb concentrations in soils developed on (A) dolomites and (B) rhyolites at varying distance from the smelter. The profiles are similar in shape, but surface concentrations and asymptotic depths are variable.

Viburnum ores. The same type of pattern is found for 208 Pb/206Pb ratios. In addition to Viburnum ores processed through the smelter, other possible sources of Pb in the soil samples included bedrocks and automobile exhaust. Soils around the smelter were mostly products of direct weathering of local bedrocks, except for a small portion of loess origin (Linsemier, 1997). The sampling areas were from soils overlying the Bonneterre Formation (dolomite) in the south, as well as the Precambrian rhyolite of St. Francois Mountain in the north (Anderson, 1979). Although the smelter was situated in a rural area surrounded by forest, Pb from leaded gasoline emitted from automobile exhaust could be deposited through transport of small particles of Pb aerosols as well. Isotopic compositions for possible sources of Pb in

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Table 1 Lead concentrations and isotopic compositions of a subset of soil samples that were analyzed for both concentrations and isotopic compositions Location/samplea

Distance from smelter, km

Depth, cm

Soil horizon

[Pb], mg/kgb

207

Ratio

%rsd

SD

Ratio

%rsd

SD

991115D2c 991115D7c

12.1 12.1

3–7 26–30

A Bt1

131 15.1

0.7648 0.7787

0.65 0.65

0.0050 0.0051

1.9155 1.9543

0.65 0.59

0.0124 0.0116

990906A2d 990906A3d 990906A5d

10.2 10.2 10.2

1–3 9–11 39–43

A E Bt1–Bt2

107 40.4 13.6

0.7696 0.7853 0.8110

0.80 0.28 3.05

0.0061 0.0022 0.0247

1.9287 1.9646 2.0388

0.70 0.32 3.02

0.0135 0.0062 0.0615

991114B2c 991114B3c 991114B5c 991114B6c

5.9 5.9 5.9 5.9

4–7 9–14 19–24 24–28

A A–E BE BE

39.1 31.4 21.6 18.7

0.7930 0.8016 0.8138 0.8111

0.25 0.47 0.41 0.35

0.0020 0.0038 0.0033 0.0028

1.9703 1.9926 2.0198 2.0125

0.32 0.24 0.40 0.32

0.0063 0.0049 0.0080 0.0064

991115B2c 991115B3c 991115B5c 991115B7c

4.4 4.4 4.4 4.4

4–6 8–12 18–23 28–30

A E E E

77.7 15.4 11.7 12.9

0.7708 0.7824 0.7932 0.7940

0.14 0.65 0.80 0.29

0.0011 0.0051 0.0063 0.0023

1.9292 1.9629 1.9826 1.9870

0.19 0.60 0.68 0.29

0.0037 0.0118 0.0135 0.0058

990924B2c 990924B3c 990924B6c 990924B8c

4.2 4.2 4.2 4.2

6–8 12–16 37–41 51–53

A A E Bt

107 26.9 16.6 19.8

0.7667 0.7879 0.7973 0.7908

0.62 0.39 0.33 0.18

0.0048 0.0031 0.0027 0.0014

1.9202 1.9685 1.9890 1.9729

0.71 0.59 0.42 0.26

0.0136 0.0117 0.0083 0.0051

990525A1d 990525A2d 990525A3d

4.2 4.2 4.2

6–10 15–20 43–50

A E Bt

84 15 13.2

0.7737 0.8045 0.8091

0.31 0.91 0.85

0.0024 0.0073 0.0069

1.9377 2.0011 2.0134

0.28 0.97 0.87

0.0055 0.0194 0.0175

991108A2d 991108A3d 991108A4d 991108A5d 991108A8d 991108A10d

3.2 3.2 3.2 3.2 3.2 3.2

1–4 6–10 10–15 17–20 27–31 37–39

A A A A-E E E

271 65.7 41 29.2 13.6 25.7

0.7600 0.7729 0.7759 0.7847 0.7976 0.7941

0.74 0.23 0.89 0.66 0.42 0.06

0.0056 0.0018 0.0069 0.0052 0.0033 0.0004

1.9081 1.9355 1.9392 1.9646 1.9968 1.9806

0.67 0.26 0.76 0.65 0.64 0.31

0.0128 0.0050 0.0147 0.0127 0.0127 0.0062

990924A1.5c 990924A2c 990924A4c 990924A6c

2.9 2.9 2.9 2.9

5–6 10–14 35–40 49–51

A E E Bt1

260 42.4 13.5 11.2

0.7581 0.7752 0.7994 0.7907

0.16 0.40 0.45 0.30

0.0012 0.0031 0.0036 0.0023

1.9045 1.9336 1.9889 1.9718

0.36 0.45 0.36 0.39

0.0068 0.0088 0.0071 0.0077

990904A2c 990904A3c 990904A4c

1.9 1.9 1.9

2–4 16–25 32–40

A E E

403 36.5 14.5

0.7601 0.7650 0.7858

0.15 0.37 0.32

0.0011 0.0029 0.0025

1.9078 1.9167 1.9672

0.17 0.27 0.37

0.0033 0.0052 0.0072

990525B1c 990525B2c 990525B3c

1.2 1.2 1.2

6 20 50

A E Bt

623 28.1 115

0.7571 0.7740 0.7586

0.28 0.28 0.27

0.0022 0.0022 0.0020

1.8985 1.9378 1.9011

0.30 0.23 0.27

0.0056 0.0044 0.0051

991003A2c 991003A4c 991003A5c 991003A9c

1.2 1.2 1.2 1.2

3–5 17–21 24–30 55–57

A A–E E Bt

1120 56.1 26.4 27.7

0.7537 0.7587 0.7618 0.7482

0.81 0.87 0.23 0.65

0.0061 0.0066 0.0018 0.0049

1.8993 1.8981 1.9010 1.8471

0.86 0.78 0.29 0.61

0.0163 0.0149 0.0055 0.0113

991003B3c

1.1

6–10

A–E

1439

0.7544

0.31

0.0024

1.8876

0.29

0.0056

c

0.8 0.8 0.8

3–8 13–17 25–29

A A–E E

652 189 N/A

0.7541 0.7555 0.7554

0.57 8.47 0.77

0.0043 0.0640 0.0058

1.8944 1.8954 1.8958

0.59 8.46 0.63

0.0111 0.1604 0.0120

991108B2 991108B4c 991108B7c

Pb/206Pb

208

Pb/206Pb

a The first six digit numbers and the following letter of the sample names designate sampling locations, corresponding to those in Fig. 1. The last digit number indicates different depth. Samples in italic are used as examples extensively in the text. b Concentration measurements were precise to 1% (1r). c Dolomite bedrock. d Rhyolite bedrock.

the soils: smelter (Viburnum ores) (Goldhaber et al., 1995), bedrocks (Goldhaber et al., 1995) and automobile emissions (Rabinowitz and Wetherill, 1972; Rosman et al.,

1994; Hirao and Patterson, 1974; Chow et al., 1975) plotted in Fig. 5 were used to define 4 fields of possible sources of Pb in Fig. 6.

P. Prapaipong et al. / Applied Geochemistry 23 (2008) 2156–2170

1970, 7 yr after the smelter started its operation (Fig. 3). Isotopic compositions of soils and tree rings shown in Figs. 6 and 7 also exhibit systematic variations of Pb isotopic compositions with depth and tree ring age. This observation is consistent with atmospheric deposition of Pb derived from Viburnum ores via the smelter, and suggests as well that smelter-derived Pb is transported vertically through soils and taken up by oak trees. Comparison of results in Fig. 6 to those in Fig. 2 reveals that although Pb concentrations reach constant values at 10–20 cm depth, the vast majority of the Pb throughout the soil columns may have come from the smelter. To test this idea, the isotopic data have been used to estimate the proportion of smelter-derived Pb in these soil samples.

10 991108B, 0.8km 990525B, 1.2km 991115A, 4.2km, oak#1 991115A, 4.2km, oak#2

[Pb], mg/kg

8

6

4

2

0 1930

1940

1950

1960

1970

1980

1990

2000

year Fig. 3. Pb concentrations in tree rings at 3 locations at varying distance from the smelter. The arrows indicate the sapwood-heartwood boundary, determined by dendrochronology. Y-error bars show 1r analytical uncertainty. Note that the smelter operated from 1963 to 2003.

Also plotted in Fig. 6 are variations of 207Pb/206Pb and Pb/206Pb ratios for soil samples from 6 locations at various depths, distances and bedrock types, as examples of isotopic compositions in soil samples. Sample depths are labeled next to the data points. At locations > 1.5 km from the smelter, systematic variations with depth are observed. Samples at the surface have isotopic ratios close to the ores. Deeper samples plot further away from the ore compositions. At some locations, e.g., 990924A and 991108A, the shallowest samples plot in the ore field. Trends like these were also found for soils at 4 other locations > 1.5 km from the smelter despite differences in bedrock types, distances and directions from the smelter. In fact, isotopic compositions for all soil samples > 1.5 km from the smelter fall on a single line, with a linear correlation coefficient (r2) of 0.9934. In contrast, at locations < 1.5 km from the smelter (991003 A, Fig. 6), all of the samples plot in the field defined by the ores, regardless of sample depths. This observation holds for two other locations within 1.5 km from the smelter, 990525B and 991108B. Data used in these plots and other locations can be found in Table 1. Tree ring analysis also shows linear correlations between 207Pb/206Pb and 208Pb/206Pb ratios, as depicted in Fig. 7, with ages of the samples labeled next to the data points. It can be seen that in general younger tree rings plot near the ore area, whereas older samples plot further from the ore field. Slight differences within the same sites are observed; they nevertheless follow the same trend. 208

5. Discussion The bull’s-eye patterns of 207Pb/206Pb (Fig. 4) and Pb/206Pb, consistent with the pattern of concentration distribution in Fig. 1, confirm that Viburnum Pb emitted from the smelter has been atmospherically deposited in soils around the smelter. With increasing depth, Pb concentrations drop rapidly to constant values (Fig. 2). In tree rings, Pb concentrations begin to increase sharply around

208

2161

5.1. Isotopic interpretation Isotopic ratios, 207Pb/206Pb and 208Pb/206Pb, for soils and tree rings of various depths and ages at all locations fall on a straight line (Figs. 6 and 7, and Table 1). This suggests that Pb in the samples results from two-component mixing with very limited input from other sources, or from mixing of multiple components that happen to be co-linear in Figs. 6 and 7. These scenarios are investigated by plotting isotopic ratios against 1/Pb concentration, such as those in Figs. 8 and 9. While all of the data plotted together scatter (Fig. 9), a single straight line results for each location (Fig. 8). Plots of 208Pb/206Pb yield similar trends. Systematic variations with depth are observed in samples from both bedrock types and all locations. It follows that Pb in each soil profile results from mixing of two approximately homogeneous end-members, the compositions of which lie along the straight lines. To identify the two end-members, isotopic and concentration compositions of possible Pb sources are plotted in Fig. 10. All data were adopted from the literature (Rabinowitz and Wetherill, 1972; Hirao and Patterson, 1974; Chow et al., 1975; Rosman et al., 1994; Goldhaber et al., 1995). The compositions of the sources were used to define 4 fields of Pb sources corresponding to the 4 rectangles in Figs. 9 and 10, with a narrower range of the x-axis in Fig. 9. It can be seen that compositions of different sources are generally quite distinct with minor overlap between dolomitic and rhyolitic fields. Although literature data for rhyolite and dolomite bedrocks are for samples from locations 15 km from the soil sample locations in this study, dolomite compositions are similar to that of the 1943 tree ring samples obtained from trees adjacent to the soil location in this study (Fig. 10). Agreement between tree ring data from before smelting began and before widespread use of leaded gasoline in this region suggests that it is reasonable to adopt the bedrock compositions if Pb is taken up via roots. Isotopic data for US emissions overlap with dolomite, and Pb concentrations are not available, but will be constrained below. End-member compositions of the possible sources are constrained further by soil sample compositions. All sample data points fall within a ternary diagram, apexes of which are made up of compositions of possible sources of Pb (Fig. 9): ore Pb, US emissions from leaded gasoline and the bedrock appropriate to the sites. Given the

2162

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Fig. 4. Contour diagrams illustrating the geographic variations of 207Pb/206Pb in surface soils around the smelter. Note that the Viburnum ores is 0.75. Dashed curves are those where data are limited for contour constraint.

heterogeneity of these possible end-members, it is not possible to pinpoint a unique value; therefore calculations

207

Pb/206Pb ratio of

were made using both mean values and the most conservative values. For example, the Viburnum ore end-member

2163

P. Prapaipong et al. / Applied Geochemistry 23 (2008) 2156–2170 2.1

2.1 Bonneterre Dolomite

2.05

Bonneterre Dolomite

2.05 U.S. Emissions

2

Pb/

1988 1999 1997

1.9

1976 1976 1982 1988

1.95

208

1.9

U.S. Emissions

St. Francois Rhyolites

Pb

206

Pb

206

1.95

208

Pb/

2

St. Francois Rhyolites

1982 1988

Viburnum Ores

1999

Viburnum Ores

1991

1.85

0.74

0.76

0.78

0.8

207

0.82

0.84

0.86

206

Pb/

1.8 0.72

991115A, oak #1 991115A, oak #2 990924B, oak #1 990924B, oak #2

1997

1.85

1.8 0.72

1961

1982

0.74

0.76

0.78 207

Pb

0.8

Fig. 5. 207Pb/206Pb and 208Pb/206Pb values for possible Pb sources in the sampled soils, taken from the literature.

0.82

0.84

0.86

206

Pb/

Pb

Fig. 7. Systematic variations of Pb isotopic ratios for tree ring samples from 4 white oaks at two locations, 4.2 km from the smelter, with sample age labeled next to data points.

2.1 Bonneterre Dolomite

2.05

50 cm

2

1.95

U.S. Emissions

37.5 cm

35.5 cm

52 cm 14 cm

12 cm 5.5 cm

208

Pb/

206

Pb

St. Francois Rhyolites

7 cm

1.9

27 cm 9 cm

Viburnum Ores

(A) Soils on dolomites

4 cm

990924B, 4.2 km 990924A, 2.9 km 991003A, 1.2 km

1.85 56 cm

1.8 0.72

0.74

0.76

0.78 207

Pb/

0.8 206

0.82

0.84

0.86

Pb

2.1 Bonneterre Dolomite

2.05

1.95

208

Pb/

206

Pb

2

1.9

38 cm 29 cm 18.5 cm 12.5 cm 10 cm 8 cm 8 cm 2.5 cm 2 cm

St. Francois Rhyolites

U.S. Emissions 46 cm 18 cm

(B) Soils on rhyolites

Viburnum Ores

1.85

1.8 0.72

41 cm

990906A, 10.2 km 990525A, 4.2 km 991108A, 3.2 km

0.74

0.76

0.78 207

Pb/

0.8 206

0.82

0.84

0.86

Pb

Fig. 6. Variations of Pb isotopic ratios with depth, labeled next to data points, for soils at the same locations as those in Fig. 2. At locations > 1.5 km from the smelter, divergence from values defined by the ore Pb with increasing depth can be observed. Lead isotopic ratios for soil samples at a location <1.5 km from the smelter all fall in the range of Viburnum ores. Error bars represent measurement precisions (1r).

incorporated in the tree-ring and soil samples has to lie below and to the left of the samples. Possible 207Pb/206Pb, 208 Pb/206Pb and 1/[Pb] values are limited to around 0.752, 1.895, and 0, respectively. As for dolomite bedrock, the lowest 1/[Pb] possible (0.27) was chosen to be conservative by maximizing bedrock and minimizing ore contributions. Isotopic compositions, 207Pb/206Pb and 208Pb/206Pb, were calculated from the mean values (0.801, 2.007, respectively). Rhyolite bedrock compositions of 0.760 (207Pb/206Pb), 1.968 (208Pb/206Pb), 0.14 (1/[Pb]) were determined using a similar scheme. US emissions have a range of isotopic compositions (0.82–0.87 in Fig 9), and concentrations are not available. However, the sample data points have been bounded within a ternary diagram formed by the 3 end-members (ores, bedrocks and emissions), where the intersection of two trends which bound the rhyolite data set (from Viburnum ores to high 207Pb/206Pb values and from rhyolite to high Pb isotope composition) define an appropriate composition for US emissions in this region. Isotopic composition of the emission end-member must therefore lie above the highest sample (0.82). Similarly, the 1/[Pb] value has to be between 0.04 and 0.07. Again, as a conservative estimate, the composition closest to the sample ternary was chosen as the end-member. Values of 0.837, 2.077 and 0.058 for 207Pb/206Pb, 208Pb/206Pb and 1/ [Pb] were adopted for the emission end-member. Compositions of the three components provide a framework for interpreting Pb isotopic data from soil samples, shown as triangles in Figs. 8 and 9. Plots of isotopic ratios against 1/[Pb] for soil samples shown in Fig. 8 suggest that the two end-members are co-linear for each location, albeit somewhat distinct between locations. The linear regressions systematically point towards low 207Pb/206Pb values at high Pb (low 1/ Pb) at one end and towards high 207Pb/206Pb and lower Pb at the other end. For each location, position along the array correlates with soil depths. Extrapolating the welldefined linear trends to the boundaries of the ternary field, it is inferred that the compositions of the two

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P. Prapaipong et al. / Applied Geochemistry 23 (2008) 2156–2170

0.85

X1

US Emissions

X2

X3

Dolomite

Y2 0.8

Y3

207

/Pb

206

Y1

52 cm (50, 40, 10)

Pb

14 cm (55, 40, 5)

0.75

Smelter 4 cm (100, 0, 0)

27 cm (82, 5, 13) 19 cm (91, 4, 5) 56 cm

0

0.05

39 cm (40, 48, 12)

(A) Soils on dolomite 990924B, 4.2 km 990924A, 2.9 km 991003A, 1.2 km

7 cm (85, 15, 0)

0.1

0.15

0.2

0.25

1/[Pb] in μg/g

0.85

X4 29 cm (20, 48, 32)

Y4 0.8

38 cm (43, 49, 8)

Pb

207

/Pb

206

US Emissions

Rhyolites 0.75

Smelter

2.5 cm (92, 8, 0)

0

8 cm (77, 13, 0)

0.05

(B) Soils on rhyolites

18.5 cm (53, 38, 9)

12.5 cm (63, 26, 6)

0.1

0.15

990906A, 10.2 km 990525A, 4.2 km 991108A, 3.2 km 0.2

0.25

1/[Pb] in μg/g Fig. 8. Linear correlations between 207Pb/206Pb and 1/[Pb] for soils on (A) dolomites and (B) rhyolites, suggesting mixing of two homogeneous endmembers (X and Y). Variations of Pb compositions in samples and end-members are functions of depth. Each mixing line X–Y was used to determine smelter Pb proportions in soils at a corresponding location. Proportions of dolomite (or rhyolite): emission in X end-members are X1, 24:76; X2, 25:75; X3, 85:15; X4, 18:82, and smelter: emission in Y end-members are Y1, 83:17; Y2, 95:5; Y3, 100:0; Y4, 85:15. Numbers in parentheses are estimated percentages of smelter: emission: bedrock.

0.85

US Emissions

0.8

Pb

207

/Pb

206

Dolomite

Rhyolites 0.75

Viburnum Ores

0

0.05

0.1

0.15

0.2

0.25

1/[Pb] in μg/g Fig. 9. Ranges of 207Pb/206Pb and 1/[Pb] values for possible Pb sources in the sampled soils, taken from the literature, together with sample compositions shown as solid circles. Data points for the sources are plotted in Fig. 10.

end-members for each trend themselves lie along mixing lines between the US emissions and bedrock, and between the smelter and the US emissions (points X and Y in Fig. 8, respectively). In other words, the overall mixing process involves 3 components: US emissions, bedrock and the smelter, and can be thought of as occurring in two steps, in order to generate soil profiles that fall along a linear

trend. Before the commencement of smelter operations, mixing between US emissions and bedrock occurred, in proportions that were uniform with depth at a specific site, but different between sites. This observation implies that atmospherically derived Pb percolated through the sediment column to depths greater than those sampled here. After the smelter operations began, the emissions-bedrock

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P. Prapaipong et al. / Applied Geochemistry 23 (2008) 2156–2170

0.85

0.8

207

/Pb

206

US Emissions

Pb

Dolomite

Rhyolites

0.75

Viburnum Ores 0

1

2

3

1/[Pb] in μg/g

4

soil samples Viburnum ores US Emisssions Rhyolites 5 Dolomite 1943 tree rings

Fig. 10. Values of 207Pb/206Pb and 1/[Pb] for Pb sources, which were used to define end-member compositions in Figs. 8 and 9. Solid symbols are data for soil and tree ring samples obtained in this study, while open symbols are those from the literature. Note that the x-axis scale differs from Fig. 9.

end-member mixed with atmospheric fallout, a mixture of US emissions and smelter aerosols, in different proportions along the mixing line between the two end-members indicated by the X–Y points for each line in Fig. 8. For example, at location 990924B, before the smelter started operation, Pb from dolomite bedrock mixed with that from US emissions in a proportion of 24:76 (X1). After the smelter began its operation, aerosols were composed of 83% smelter and 17% emission (Y1), which then mixed with X1 in differing proportions with time. As a result, the sample at 14 cm depth is composed of 71% of X1 and 29% of Y1, which is equivalent to an overall distribution of Pb sources of 55% smelter Pb, 40% emission and 5% dolomite. At a single location, sample compositions change systematically as a function of depth, reflecting a fallout composition that is increasingly dominated by the smelter at shallow depths, reflecting more recent times. As an example, at 990924B, Pb in the deepest sample at 52 cm is composed of 50% smelter, 40% emission and 10% bedrock Pb. At shallower depth, fallout composition changes producing a mixing trend headed towards smelter-dominated compositions. This is revealed by elevated smelter Pb proportions of 55% and 85% at 14 and 7 cm depth, respectively. (Note that the sample at 39 cm does not follow this trend.) Increases in smelter Pb proportion in shallower soils are also observed at other locations on both dolomites and rhyolites in Fig. 8. Data labels for some locations are omitted in the plots because of limited space, but can be found in Table 2. The same approach was applied to 208Pb/206Pb data (Table 1); mixing proportions for all soil samples are in close agreement using each isotopic ratio independently (Table 2). Deviations from the proposed model are found at locations < 1.5 km from the smelter (991003A, 991108B, 990525B), where smelter input is greatest. As an example, in Fig. 8A, the 991003A sample at 56 cm plots outside the ternary diagram. An extreme case is found for 991108B (Tables 1 and 2), where all samples, regardless of depth, have the same isotopic composition as the smelter and do not form a line within the ternary diagram. Comparing isotopic compositions of the deepest samples to shallower

samples at these locations indicates that contributions of smelter Pb to these samples are very similar, at least around 90%. Changes in calculated Pb proportions from the three sources vary with depth (Table 2). The proportion of Pb from US emissions increases with depth, whereas absolute concentrations contributed by the emissions decrease with depth and reach constant values in the range of 3–10 mg/ kg. This is consistent with the fact that the emissions are deposited atmospherically and distributed evenly in this area. In contrast, percentages of Pb from bedrocks increase with depth, but absolute concentrations increase to 2– 5 mg/kg with depth. Depth profiles of calculated percentages of Pb from the smelter also vary with sample location. In general, the% smelter Pb decreases with depth, while absolute concentrations decrease sharply. Depth profiles of% smelter Pb at six locations are shown in Fig. 11. At locations > 10 km from the smelter (990906A), the proportion of smelter Pb reaches 10% or 1 mg/kg. At the same depth, smelter Pb accounts for 5–10 mg/kg or 40% of total Pb at locations 3–4 km from the smelter (990924B, 990924A, 991108 A). However, at location 991003A (<1.5 km from the smelter), the interpretation of the data indicates that most of the Pb originates from the smelter. In this case, asymptotic or ‘‘background” concentrations (Fig. 2) almost equal smelter Pb concentrations. 5.2. Rate of Pb transport Applying the same approach to all sampling locations reveals that at asymptotic depths where Pb concentrations become constant, 50–90%, 40–50% and 10–50% of the Pb may be derived from the smelter in samples at less than 2, 2–4 and over 4 km from the smelter, respectively. Because the samples were collected in 1999 and the smelter had been emitting Pb continuously since 1969, it follows that 10–90% of Pb at asymptotic depths must have been mobilized at an average rate of 1 cm/yr (30 cm in 30 yr). At location 990924B, an example from the 2–4 km range from the smelter, the surface Pb concentration is 100 mg/ kg, 80% of which is smelter derived; the asymptotic Pb

Location/ samplea

207

Pb/206Pb

208

Pb/206Pb ratios Calculated from

End-member composition

Sample composition

208

Pb/206Pb

Sample composition

%Bedrock Pb in emissionsbedrock end- member

%Smelter Pb in fallout end-member

%Fallout endmember

Pb Proportions in% %Smelter

%Emission

60

89

22

70

13

45

40

88

24

83

30

77

18

85

25

95

48

91

35

94

85

100

n/a

n/a

100 67 100 85 10 n/a 100 76 44 95 60 50 43 100 71 50 61 100 24 11 100 88 77 60 n/a 52 100 84 37 n/a 100 88 59 100 94 74 100 94 84 n/a n/a

89 58 82 60 9 56 42 25 22 80 58 40 42 85 55 40 50 77 18 8 92 77 68 53 49 43 95 73 35 40 94 82 50 97 92 70 100 91 82 n/a 100 97 91

11 22 18 38 68 48 58 72 68 20 24 36 38 15 40 48 40 23 60 64 8 23 26 28 51 49 5 24 47 35 6 10 30 3 6 22 0 4 5 n/a 0 2 0

n/a = Data do not fall on linear regression or ternary diagram. Sample locations are arranged in order of decreasing distance from the smelter, same as that in Table 1.

a

Pb/206Pb and

Pb proportions in mg/kg

Pb proportions in%

%Rock

From smelter

From emission

From rock

%Smelter

% Emission

%Rock

From smelter

Pb proportions in mg/kg From emission

From rock

0 20 0 2 23 0 0 3 10 0 8 24 20 0 5 12 10 0 22 28 0 0 6 9 30 8 0 3 18 25 0 8 20 0 2 8 0 5 13 n/a 0 1 9

116 9 88 24 1 22 13 5 4 62 9 5 5 91 15 7 10 65 3 1 249 51 28 15 7 11 247 31 5 4 379 30 7 604 26 81 1120 51 22 n/a 652 183 n/a

14 3 19 15 9 19 18 16 13 16 4 4 5 16 11 8 8 19 9 8 22 15 11 8 7 13 13 10 6 4 24 4 4 19 2 25 0 2 1 n/a 0 4 n/a

0 3 0 1 3 0 0 1 2 0 1 3 3 0 1 2 2 0 3 4 0 0 2 3 4 2 0 1 2 3 0 3 3 0 1 9 0 3 3 n/a 0 2 n/a

90 55 80 60 8 60 48 28 28 80 54 40 40 86 55 40 50 78 24 18 92 78 71 58 23 49 95 75 38 42 94 82 50 100 94 70 100 93 86 n/a 100 98 92

10 25 20 38 68 40 52 66 59 20 28 37 40 14 40 48 40 22 45 50 8 22 20 32 39 41 5 21 44 30 6 8 30 0 4 20 0 0 0 n/a 0 0 0

0 20 0 2 24 0 0 6 13 0 18 23 20 0 5 12 10 0 28 32 0 0 9 10 38 10 0 4 18 28 0 8 20 0 2 10 0 7 14 n/a 0 2 8

118 8 86 24 1 23 15 6 5 62 8 5 5 92 15 7 10 66 4 2 249 51 29 17 3 13 247 32 5 5 379 30 7 623 26 81 1120 52 23 n/a 652 185 n/a

13 4 21 15 9 16 16 14 11 16 4 4 5 15 11 8 8 18 7 7 22 14 8 9 5 11 13 9 6 3 24 3 4 0 1 23 0 0 0 n/a 0 0 n/a

0 3 0 1 3 0 0 1 2 0 3 3 3 0 1 2 2 0 4 4 0 0 4 3 5 3 0 2 2 3 0 3 3 0 1 12 0 4 4 n/a 0 4 n/a

P. Prapaipong et al. / Applied Geochemistry 23 (2008) 2156–2170

991115D2 991115D7 990906A2 990906A3 990906A5 991114B2 991114B3 991114B5 991114B6 991115B2 991115B3 991115B5 991115B7 990924B2 990924B3 990924B6 990924B8 990525A1 990525A2 990525A3 991108A2 991108A3 991108A4 991108A5 991108A8 991108A10 990924A1.5 990924A2 990924A4 990924A6 990904A2 990904A3 990904A4 990525B1 990525B2 990525B3 991003A2 991003A4 991003A5 991003A9 991108B2 991108B4 991108B7

Calculated from

207

2166

Table 2 Proportions of smelter Pb and Pb from other sources in end-members and soil samples in percentages and mg/kg, calculated from

P. Prapaipong et al. / Applied Geochemistry 23 (2008) 2156–2170

(A) Soils on dolomite bedrocks 0

10

depth, cm

20

30

40 990924B, 4.2 km 990924A, 2.9 km 991003A, 1.2 km

50

60 0

20

40

60

80

100

% smelter Pb (B) Soils on rhyolite bedrocks 0

10

depth, cm

20

30

40 990906A,10.2 km 990525A, 4.2 km 991108A, 3.2 km

50

60 0

20

40

60

80

100

% smelter Pb Fig. 11. Changes in calculated percentages of smelter Pb with depths in soils at the six locations. The profiles appear to depend on distance from the smelter, unlike the concentration depth profiles in Fig. 2.

concentration is 20 mg/kg, 40% of which is smelter Pb. This observation implies that 10 mg/kg of the surface Pb has traveled to 30 cm depth. At location 991003A, <1.5 km from the smelter, 1000 mg/kg of Pb in the top horizon is 100% smelter Pb; the Pb concentration drops to 30 mg/ kg, 80% of which is smelter Pb. This means 2.4% or 24 mg/kg of Pb from the surface has penetrated to 30 cm depth. The estimated rates of Pb mobility in soils from this study are close to those calculated by Miller and Friedland (1994), Whitehead and Ramsey (1997) and Puchelt et al. (1993), but higher than those from Erel et al. (1997) and Ettler et al. (2004). By measuring Pb accumulation at several depths and using a transport model, Miller and Friedland (1994) approximated transport rates of 0.82–1.97 cm/yr in forest floors in Vermont. From isotopic measurements, Whitehead and Ramsey (1997) calculated that Pb near a medieval smelter in the UK. migrated through fractured sandstone at a rate of 0.8 cm/yr. Similar velocities were estimated from isotopic variations with depth by Puchelt

2167

et al. (1993) who found that Pb from automobile emission penetrated through soils near parking lots in Germany at a rate of 0.8 cm/a as the worst case. A lower rate of 0.5 cm/a was estimated by Erel et al. (1997) for soils near roads in Israel. Ettler et al. (2004) obtained a rate of 0.3 cm/a for forest soils heavily polluted by a secondary Pb smelter in the Czech Republic. The model was limited to the soil profiles at 3–14 cm depth. This makes the rate of 0.3 cm/a a minimum estimate. Isotopic compositions of the secondary smelter Pb also changed with time, which made it difficult to identify compositions of the mixing end-members. Alternatively, the discrepancy may reflect differences in speciation of Pb input. For instance, dusts emitted from a secondary smelter are dominated by PbSO4 and Pb(OH)Cl (Ettler et al., 2005), while PbS and combined PbSO4 and PbSO4
PbO account for 60% and 20% of Pb in dusts from a smelter processing Pb ores (Sobanska et al., 1999). Other factors governing Pb mobility in soils, including soil acidity, moisture, mineralogy, climate and microbial activity, may also cause the differing rates. Depth profiles of% smelter Pb in Fig. 11 and concentration in Fig. 2 reveal that although Pb concentrations in all samples reach concentrations consistent with background values, isotopic evidence suggests that smelter-derived Pb has traveled below the asymptotic depths. The contradictory depth profiles for concentration and isotopic compositions are also observed in other studies. Lead concentrations of soils and fractured sandstone near a medieval smelting site decreased from 16,500 mg/kg at the surface to 270 mg/kg at 4 m depth, but 206Pb/207Pb of the fracture infill rocks was 1.1818, close to the 1.1821 of the surface soil (Whitehead and Ramsey, 1997). Values of 206Pb/207Pb in Scottish soils with farming and industrial activity also did not reach a constant value at 20– 30 cm depth (Bacon and Hewitt, 2005). Lead isotopic compositions in Mexican sediment cores remain constant throughout the 1900s (1–30 cm depth) although the concentrations dropped from 60 to 20 mg/kg (Soto-Jimenez et al., 2006). The authors believe that these seemingly contradictory depth profiles can be explained in two ways. First, changes in concentration are not as sensitive as those of isotopic composition to Pb input and may be difficult to detect in the given period of time. Alternatively, at locations with unusually high proportions of smelter Pb, it is possible that the soil components have a Pb carrying capacity that can become saturated. Smelter Pb may travel through the soil profile, but the saturated soils exhibit mechanisms that prohibit total Pb absorption, allowing smelter Pb to exchange with the existing Pb (from emissions or bedrock). 5.3. Implications for the biosphere Increases of Pb concentrations in the tree rings with time suggest elevated levels of bioavailable Pb in the surroundings, corresponding to the smelting activity (Fig. 3). It is evident that White Oaks (Quercus alba L.) can be used as Pb pollution archives. This supports myriad studies that have successfully used oaks for dendrochemistry and identified white oaks as a preferred species for pollution studies (Wickern and Breckle, 1983; McClenahen and

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Dochinger, 1985; Long and Davis, 1989; Cutter and Guyette, 1993; Hagemeyer, 1993, 2000; Eklund et al., 1996; Forget and Zayed, 1995; Smith and Shortle, 1996; Jonsson et al., 1997; Watmough, 1997; Nabais et al., 2001, cf. Szopa et al., 1973; Anderson et al., 2000; Cheng et al., 2007), but contradicts a recent study by Ruppert and Wischow (2006), where the reliability of tree rings as a pollution indicator was raised. It should be noted that the latter study, based on one tree of one species, supports the idea that many interrelated factors contribute to element compositions in tree rings and raises cautions when using dendrochemical methods. In fact, it has been well established that successful dendrochemical studies depend on species choices and environmental limiting factors that control tree growth, including temperature, light, water, nutrition, location, wind and soil chemistry (Wickern and Breckle, 1983; Smith and Shortle, 1996; Hagemeyer, 1993). Specifically, it was shown that certain species are not suitable for dendrochemical studies (Watmough and Hutchinson, 2002, 2003; Bindler et al., 2004). Therefore, while the study by Ruppert and Wischow (2006) advises caution in using dendroanalysis, we believe the study does not disprove the utility of dendrochemistry when applied systematically and to a larger sample population. In the present study, Pb in the tree rings systematically increases with time after the smelting operation began, indicating a relationship between bioavailable Pb and smelting activity. However, the exact causes of higher bioavailable Pb, which may include increases in Pb emission or increases in soil or precipitation acidity in the areas, need to be investigated further. Although factors contributing to Pb in the tree rings need to be examined further, it is evident that Pb is bioavailable, transported and incorporated in the white oak heartwood. Lead uptake can occur through several mechanisms including leaf deposition, bark deposition and root uptake. Several studies have indicated that Pb transport through bark is limited and slow (Lepp, 1975; Bellis et al., 2002). Leaf absorption via stomata occurs, but has not been proved conclusively as a major uptake pathway into xylem and wood (Ormrod, 1984; Treshow and Anderson, 1989; Hagemeyer, 1993; Eklund, 1995; Forget and Zayed, 1995; Tung and Temple, 1996). High Pb concentrations in leaf litter around the Glover smelter suggest that Pb transport from leaves to wood upon senescence is minimal (Crombie, 1997). Root uptake is believed to be a major and direct contributor of Pb in the heartwood (Woz´ny and Krezesłowska, 1993; Eklund, 1995; Trüby, 1995; Jonsson et al., 1997; Watmough, 1997; Fodor, 2002). This implies that deposited Pb has traveled through soils as deep as the oak root depth. If the roots are in contact with groundwater, Pb in tree rings may be used as indicators of historical groundwater Pb content and groundwater contamination. However, it has been reported that white oak vertical tap root systems rarely extend deeper than 1.5 m, with the majority of root biomass concentrated in lateral root system within around 50 cm of the ground surface (Danjon et al., 2007; USDA Forest Service, Northeastern Area, 2008). It is believed that in the present study, Pb in the tree rings originates from vadose zone soil water and does not necessarily indicate groundwater contamination.

Nevertheless, dendroanalysis may reflect historical groundwater chemistry in shallow groundwater areas using plant species with deep root systems. This study provides a ‘snapshot’ view of how far Pb has been transported in soils after 30 yr of continuous input of Pb from a smelter. Future studies can build on this foundation to test whether the rates determined here persist, or whether there are temporal variations in the behavior of Pb triggered by soil carrying capacity, changes in total load, weathering reactions, or even fluctuations in rainfall driven by climate change. This field area appears to be a prime location for following Pb as it is transferred from the geosphere to the biosphere, which follows pathways that are still not well understood. By demonstrating the usefulness of combining concentration and isotopic data to track Pb mobility, using ICP-MS techniques, it is hoped that this approach can be adapted for identifying pathways that Pb takes on its way to causing serious developmental problems in human health. Acknowledgements We appreciate technical assistance from Rosanna Ridings, Windy Bunn and Paul Giesting. Thanks are also due to Mikhail Zolotov, Andrey Plyasunov and Jan Amend for helpful discussions. This research was funded by NSF-EAR Grant (9807679) to Julie Morris, Jay Turner and Everett Shock. References Anderson, K.H., 1979. Geologic Map of Missouri. Missouri Geological Survey, Columbia, MO. Anderson, S., Chappelka, A.H., Flynn, K.M., Odom, J.W., 2000. Lead accumulation in Quercus nigra and Quercus Velutina near smelting facilities in Alabama,. USA: Water Air Soil Pollut. 118, 1–11. Bäckström, M., Karlsson, S., Allard, B., 2004. Metal leachability and anthropogenic signal in roadside soils estimated from sequential extraction and stable lead isotopes. Environ. monit. assess. 90, 135– 160. Bacon, J.R., Dinev, N.S., 2005. Isotopic characterization of lead in contaminated soils from the vicinity of a non-ferrous metal smelter near Plovdiv, Bulgaria. Environ. Pollut. 134, 247–255. Bacon, J.R., Hewitt, I.J., 2005. Heavy metals deposited from the atmosphere on upland Scottish soils: chemical and lead isotope studies of the association of metals with soil components. Geochim. Cosmochim. Acta 69, 19–33. Bacon, J.R., Farmer, J.G., Dunn, S.M., Graham, M.C., Vinogradoff, S.I., 2006. Sequential extraction combined with isotope analysis as a tool for the investigation of lead mobilization in soils: application to organic-rich soils in an upland catchment in Scotland. Environ. Pollut. 141, 469– 481. Bellis, D.J., Satake, K., Noda, M., Nishimura, N., McLeod, C.W., 2002. Evaluation of the historical records of lead pollution in the annual growth rings and bark pockets of a 250-year-old Quercus crispula in Nikko, Japan. Sci. Total Environ. 295, 91–100. Bindler, R., Renberg, I., Klaminder, J., Emteryd, O., 2004. Tree rings as Pb pollution archives?. A comparison of 206Pb/207Pb isotope ratios in pine and other environmental media. Sci. Total Environ. 319, 173–183. Cheng, Z., Buckley, B.M., Katz, B., Wright, W., Bailey, R., Smith, K.T., Li, J., Curtis, A., van Geen, A., 2007. Arsenic in tree rings at a highly contaminated site. Sci. Total Environ. 376, 324–334. Chow, T.J., Snyder, C.B., Earl, J.L., 1975. Isotope ratios of lead as pollutant source indicators. In: Symposium Isotope Ratios as Pollutant Source and Behaviour Indicators, 1974, Vienna, Austria. IAEA, Vienna, Austria, pp. 95–108. Crombie, M.K., 1997. Remote Sensing and Geochemical Investigations of Selected Surface Processes in Egypt and Missouri, Ph.D. dissertation, Washington Univ., Saint Louis, MO.

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