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Published online 4 August 2005
Published in Soil Sci Soc Am J 69:1399-1404 (2005)
DOI: 10.2136/sssaj2004.0095
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Soil Chemistry

Quantification of Pollutant Lead in Forest Soils

Eiliv Steinnesa,*, Torill Eidhammer Sjøbakka, Carmen Donisab and Maja-Lena Brännvallc

a Dep. of Chemistry, Norwegian Univ. of Science and Technology, NO-7491 Trondheim, Norway
b Dep. of Chemistry, Univ. of Montreal, C.P. 6128 Succursale Centre-Ville, Montréal, QC, Canada, H3C 3J7
c Dep. of Ecology and Environmental Science, Umeå Univ., S-901 83 Umeå, Sweden

* Corrresponding author (Eiliv.Steinnes{at}chem.ntnu.no)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Fifteen podzolic forest soils in Norway covering sites with a wide range of atmospheric deposition rates were assayed for their contents of pollutant Pb. Samples from the Of, Oh, E, B, and C horizons were studied. Nitric acid soluble contents of Pb and the corresponding stable Pb isotope ratios were determined by sector field inductively coupled plasma–mass spectrometry (ICP–MS). On the basis of existing knowledge on 206Pb/207Pb ratios in atmospheric deposition over Norway across time, the percentage of the Pb supplied by air pollution was calculated for the various samples and soil horizons, assuming that the C horizon was undisturbed. More than 90% of O horizon Pb was from pollution, even at remote sites in the far north. Significant fractions of Pb were pollution-derived also in the E and B horizons at most sites. In the south, more than half of the Pb derived from air pollution has now moved to the upper mineral horizons. Stable Pb isotope ratios are a very precise tool for revealing Pb pollution in terrestrial ecosystems. The present work suggests that similar studies should be done in other parts of the world to objectively assess the anthropogenic contribution to surface soil Pb.

Abbreviations: ICP–MS, inductively coupled plasma–mass spectrometry • TIMS, thermal ionization mass spectrometry


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
THE RECOGNITION of long-range atmospheric transport as a significant source of Pb in terrestrial ecosystems happened independently in the early 1970s in Scandinavia (Tyler, 1972) and in North America (Reiners et al., 1975). Since then, considerable evidence has been presented on both sides of the Atlantic (Allen and Steinnes, 1980; Smith and Siccama, 1981; Johnson et al., 1982; Friedland et al., 1984; Steinnes et al., 1989, 1997a; Page and Steinnes, 1990; Tyler, 1992; Njåstad et al., 1994; Miller and Friedland, 1994; Johansson et al., 1995) on the character and magnitude of the problem. Strong arguments have been presented in favor of atmospheric deposition as a major source of Pb in surface soils in different parts of the world. However, a generally accepted tool that would unmistakably prove the importance of air pollution relative to local geogenic sources has not been available, and some skepticism to the above findings has been presented by geochemists, most explicitly in a paper by Rasmussen (1998) using a limited data set of Scandinavian soils. Another example is the study by Reimann et al. (2001) in northwestern Russia, showing gradually increasing contents of Pb in the O-horizon with southern latitude. This was explained by the authors as being a result of "plant pumping and organic binding" and not related to atmospheric deposition.

Lead has four stable isotopes 204Pb, 206Pb, 207Pb, and 208Pb, but unlike most other elements, Pb shows rather large variability in its stable isotope composition in nature. The reason for this variability is the radioactive decay of long-lived isotopes of U and Th to form stable isotopes of Pb. Although the feasibility of using stable isotope ratios to trace Pb pollution was demonstrated more than 30 yr ago (Chow and Johnstone, 1965; Rabinowitz and Wetherhill, 1972), this approach was not generally adopted before nearly 20 yr later, with applications on samples such as natural waters (Flegal et al., 1987), aerosols (Maring et al., 1987), lichens (Keinonen, 1992), peat (Sugden et al., 1993), snow (Rosman et al., 1993), sediment (Graney et al., 1995), and herbage (Bacon et al., 1996). Numerous papers have been added in recent years on the use of stable Pb isotopes in these areas.

In spite of the fact that the use of stable Pb isotopes was introduced rather early in studies related to soil pollution (Gulson et al., 1981), the applications to soil studies have been relatively few, especially in North America. The papers published have been mainly related to local pollution studies (Bacon et al., 1992, 1995; Walraven et al., 1997; Marcantonio et al., 2000; Hansmann and Köppel, 2000; Kaste et al., 2003; Haack et al., 2003) or processes in the soil (Erel et al., 1990, 1997; Harlavan et al., 1998; Emmanuel and Erel, 2002). In Sweden, a group at Umeå University has shown convincingly that air pollution is the main source of Pb in boreal forest soils all over the country (Bindler et al., 1999; Brännvall et al., 2001a), and that the pollution-derived Pb is being transferred down the soil profile (Brännvall et al., 2001b). In the present paper, their findings are tested by applying the Pb isotope technique to study the contribution from air pollution to 15 podzolic soils from different parts of Norway experiencing a large range in atmospheric deposition of Pb (Steinnes et al., 1994). Podzol is the dominating soil type not only in Scandinavia, but also in the vast territory of boreal forest covering large parts of North America and Russia. It is characterized by a distinct layered structure: A 5- to 15-cm-thick humic horizon (O: 60–95% organic matter), a well-developed bleached horizon (E), a dark brown illuvial horizon with Al and Fe sesquioxides (B), and a gradual transition into unaltered parent material (C).

A dominating influence from atmospheric pollution to the Pb content in natural surface soils in Norway was strongly suggested by previous studies (Allen and Steinnes, 1980; Steinnes et al., 1989, 1997a; Page and Steinnes, 1990; Njåstad et al., 1994). The main purpose of this paper is to demonstrate the substantial contribution from anthropogenic Pb even in pristine areas of Norway by the use of stable Pb isotopes.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
The sites selected for this investigation were 14 forested areas (A–L) extending from 58°N to almost 70°N (cfr. map in Fig. 1). Complete podzol profiles were sampled at these locations in 2000 (D–L) and 2002 (A–C) in stands of Norway spruce [Picea abies (L.) H. Karst.], Scots pine (Pinus sylvestris L.), or birch (Betula pubescens Ehrh.). In area A, sampling was performed in two different forest stands, increasing the total number of sites to 15. The geographical coordinates of the sampling sites and the forest types in question are shown in Table 1.



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  		Fig. 1. Map of Norway showing the sampling sites.

 

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  		Table 1. Basic data for the different sampling sites, lead concentrations and 206Pb/207Pb ratios down the profiles, and calculated percentage of anthropogenic lead in each soil horizon.

 
The sampling was performed by digging a pit in the forest floor with a spade and collecting soil samples from the sidewalls of the pit with a stainless steel spoon. The sampling started from the bottom of the soil profile to avoid contamination from above layers. Samples were collected from the Of, Oh, E, B, and C horizons. In the case of the B horizon, two samples were collected in some cases, one (B1) in the uppermost, dark brown layer, and another (B2) 10 to 15 cm below in the lighter brown colored zone. In other cases, the B2 sample was replaced or supplemented by a sample taken in the B/C transition zone. The various horizons were identified according to their characteristic colors.

The samples were dried at 35°C and sieved (0.2-mm nylon sieve). Weighed aliquots of about 0.4 g were decomposed with 4 mL 14 M HNO3 in a microwave oven. Reagent blanks were run simultaneously. After dilution to 25 mL with H2O, the solutions were analyzed for their Pb concentration and isotope composition by sector field ICP–MS, using a Thermo Finnigan Element (Thermo Electron Corporation, San Jose, CA). The Pb concentration values were checked against soil humus reference samples (Steinnes et al., 1997b). A solution made from NIST SRM 981 was used as the reference sample for Pb isotopic composition. The 206Pb/207Pb isotope ratio was typically determined with a precision of 0.5 to 0.7%.

The concentration of pollutant Pb (Pbpoll) in the various layers of each soil profile was calculated according to the following equation, assuming that all HNO3–soluble Pb in the C horizon is natural and all additional Pb in the higher horizons is from air pollution:

where 206Pb/207Pbsample is the isotope ratio of a given sample, 206Pb/207Pbbackground is the isotope ratio in the C-horizon of the profile in question, 206Pb/207Pbpollution is the ratio representative of air pollution at the site, and Pbtotal is the Pb concentration in the sample.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
In Norway, the development of the 206Pb/207Pb ratio in airborne Pb has been well known since the mid 1970s, from analysis by thermal ionization mass spectrometry (TIMS) of moss samples used for monitoring atmospheric metal deposition (Rosman et al., 1998; Steinnes et al., 2005). A complete set of moss data at 22 sites from five different times is available (E. Steinnes, G. Åberg, and T.E. Sjøbakk, 2004, unpublished data). The mean value in moss during the period 1975 to 2000 at the site most closely corresponding to the location of soil sampling was used in the calculations to represent the 206Pb/207Pbpollution at each of the present sites. These values were as follows: A: 1.143, B: 1.140, C: 1.140, D: 1141, E: 1.142, F: 1.142, G: 1.142, H: 1.141, I: 1.140, J: 1.145, K: 1.145, and L: 1.145.

It is known, however, from investigations of dated peat cores from ombrotrophic bogs (Dunlap et al., 1999; Steinnes et al., 2005) that considerable atmospheric Pb was deposited in Norway before 1975, with a 206Pb/207Pb ratio higher than 1.14. Available data from bogs in different parts of the country indicate that the amount of Pb deposited before the advent of leaded petrol in Europe was similar to that which was later supplied from the use of petrol Pb. The 206Pb/207Pb ratio of that Pb, determined in the peat samples by TIMS, shows some temporal variation, but the weighted average is close to 1.165. If all Pb deposited in Norway since the outset of industrial activity in Europe was retained in the surface soils, and 50% of it arrived before the leaded petrol period, the average 206Pb/207Pb ratio for the present sampling sites should be of the order of 1.152 to 1.155, rather than the above figures derived from the moss analyses. On the other hand the values calculated from the moss data are mean values for the five deposition surveys performed since 1977, not taking into account the fact that the Pb deposition rates at most places in Norway during the first part of this period were 5 to 10 times greater than at the end of the period (E. Steinnes, G. Åberg, and T.E. Sjøbakk, 2004, unpublished data). Considering only the two first moss surveys (1977 and 1985), the mean values for the present sites were within the range 1.133 to 1.141, with the lowest values appearing in the south. This is distinctly below the interval 1.140 to 1.145 calculated for the whole period 1977 to 2000, and tends to pull the total estimates for the mean anthropogenic contribution across time in the opposite direction.

Since the residence time of Pb in the surface soil is not known, it is not possible to calculate the contribution from different time periods to the present concentration levels in different soil horizons. On the basis of the above considerations, it was therefore decided to use the mean 206Pb/207Pb ratios estimated for the Pb deposition since 1975 in the calculation of Pbpoll.

Results from the determination of Pb concentrations and 206Pb/207Pb ratios in the 15 podzol profiles are shown in Table 1. The sites are ranked according to latitude, starting in the south. The observed Pb concentrations in the O horizon confirm a well-known geographic pattern (Allen and Steinnes, 1980; Page and Steinnes, 1990, Steinnes et al., 1997a) with generally high levels in the south, and about 10 times lower at some sites in the north. The levels in the C horizon are rather uniform (2.1–9.4 µg/g), with no significant difference between south and north, as also shown previously (Njåstad et al., 1994). Examples of the development of Pb concentration and the corresponding 206Pb/207Pb ratio as a function of depth are shown for three soils in Fig. 2.



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  		Fig. 2. Lead concentration (µg/g) and 206Pb/207Pb ratio as a function of depth in three podzol profiles located in different parts of Norway (cf. Fig. 1).

 
The 206Pb/207Pb ratio in the surface soil is quite uniform all over the country, with values ranging from 1.138 to 1.149 in that part of the humus layer (Of or Oh) showing the lowest value, except for three cases: Sites K and L in the far northeast are at 1.159, and site H at 1.201. The 206Pb/207Pb ratio in the C horizon, which is assumed to represent the natural undisturbed Pb, varies between 1.205 and 1.737 among the sites, depending on the geological history at the site.

Results from the calculation of percentage contribution of pollutant Pb are listed in Table 1. In some cases, the Of horizon sample shows a higher 206Pb/207Pb value than the corresponding Oh sample. This may be explained by the fact the recently deposited Pb has had a higher 206Pb/207Pb ratio than that supplied 15 to 25 yr ago (Steinnes et al., 2005). This assumption is supported in some cases by the fact that the Pb concentration values in the Of are much lower than in the Oh horizon, also in accordance with recent trends in atmospheric deposition. The real Ppoll values for such samples may therefore be as high as for the corresponding Oh sample. Supply of windblown mineral dust to the surface soil from recent human activity is another factor that could possibly contribute to higher 206Pb/207Pb ratio in the Of horizon, particularly at sites K and L, which are both situated within a 15-km distance from a site for large-scale surface mining of iron ore.

The uncertainty in the estimated 206Pb/207Pb values together with the analytical error in the ICP–MS analyses is probably what mainly limits the accuracy of the calculated values of Pbpoll. The estimated 206Pb/207Pbpollution value could be as much as 1.5% off in a worst case. Considering this number and the analytical uncertainty in the determination of the 206Pb/207Pb ratio (0.5-0.7%), it may be safely assumed that the calculated Pbpoll values in the surface soil are accurate to 5% or better, at least in cases where there is a great difference between the 206Pb/207Pb values in the horizon in question and that of the geological parent material. In cases where that difference is smaller, the analytical error in the 206Pb/207Pb ratio will play a relatively greater role, and the relative error in the Pbpoll value may be proportionally higher.

If the real 206Pb/207Pb ratio of the integrated pollution Pb in the soil was higher than that derived from the moss data, the real numbers for Pbpoll would be higher than the present calculated figures. This could easily be the case for the subsurface horizons, where a greater part of the atmospherically derived Pb is likely to originate from the prepetrol period when the 206Pb/207Pb ratio in atmospheric deposition was higher than the values used in the present calculations.

From Table 1 it is evident that not only is the surface layer Pb near 100% pollution-derived all over the country, including the far north, but the pollutant Pb is also predominant in the E horizon in most of the soils, and in the far south (sites A–C) also in the upper B horizon. Best protected against local air pollution as well as long-range atmospheric transport is apparently site H, remotely located in the north. As illustrated in Fig. 1, this soil has a very uniform Pb concentration in all horizons, and it might have been tempting to conclude from the Pb concentration data that this soil had nothing but natural Pb in the humus layer. The results, however, show that even in this case, about 75% of the humus layer Pb is derived from pollution. This trend is consistent with the conclusions from the stable Pb isotope work of Brännvall et al. (2001b) in Sweden, and confirms that there is a recent enrichment of Pb in the humus layer of natural soils in Scandinavia caused by pollutant Pb. This is not the case only in the south where deposition has been high, but even in remote areas in the north.

It is apparent from the present data that the vertical transport of Pb differs considerably between the sites. Whereas some sites show little or no pollution-derived Pb in the B-horizon, around 70% of the Pb in the upper B horizon at the three southernmost sites comes from atmospheric deposition. This means that, in the south of Norway, a major part of the pollutant Pb is no longer in the O horizon but has moved down to the E and B horizons. Similar conclusions were reached in the corresponding Swedish study (Brännvall et al., 2001b). Possible reasons for the more extensive leaching in the south include higher total input of Pb and possibly a greater turnover of humic matter at higher mean annual temperatures. The most important factor, however, is likely to be the significantly higher atmospheric deposition of acidifying substances in the south, which may, to a great extent, release Pb from exchange sites in the humus layer and promote downward leaching.

This study confirms that, in most parts of Norway, an overwhelming part of Pb in the humus layer of natural soils is derived from air pollution, as strongly suspected from previous studies. The results from the present work should call for a reassessment of conclusions on sources of Pb in natural surface soils from some previous studies in northern Europe, such as that of Reimann et al. (2001). Moreover, it suggests that similar studies should be done in other parts of the world to objectively assess the anthropogenic contribution to surface soil Pb.

Received for publication March 8, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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