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Published online 11 May 2005
Published in J Environ Qual 34:1016-1025 (2005)
DOI: 10.2134/jeq2003.0421
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Heavy Metals in the Environment

Sources and Evolution of Anthropogenic Lead in Dated Sediments from Lake Clair, Québec, Canada

Sabary Omer Ndzangoua, Marc Richer-Laflèchea,* and Daniel Houleb

a Université du Québec, INRS-Eau-Terre-Environnement, 490 rue de la Couronne, Québec, Québec, Canada G1K 9A9
b Direction de la recherche forestière, Min. des Ressources Naturelles de la Faune et des Parcs du Québec, 2700 rue Einstein, Sainte-Foy, Québec, Canada G1P 3W8

* Corresponding author (mlafleche{at}inrs.uquebec.ca)

Received for publication December 1, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Two sediments cores were collected from the deepest part of Lake Clair (Québec, Canada) to assess the historical sources of Pb additions to the lake. The cores were collected by divers by carefully inserting a Plexiglas tube into the sediments. To determine the stratigraphic ages of the sediments, 210Pb and 137Cs activities were counted by {gamma}-ray spectroscopy. Lead concentrations and isotopic ratios were performed by inductively coupled plasma–mass spectrometry (ICP–MS), following digestion of the samples with a mixture of HF, HNO3, and HClO4 acids and Pb separation by anion-exchange chromatography. Starting at the middle of the 19th century, Pb content of the sediments increased until 1975. The maximum Pb enrichment factor of 35 times (relative to the natural background) was found in sediments deposited in 1975. At this time, excess Pb flux was estimated to be about 0.03 g m–2 yr–1. Before 1872, the Pb isotopic ratios were relatively stable (mean 206Pb/207Pb = 1.20 ± 0.01), reflecting the natural Pb background. Between 1872 and 1894, the source of anthropogenic Pb was highly radiogenic as shown by the Pb isotopic signatures of the sediments (mean 206Pb/207Pb = 1.22 ± 0.01), possibly reflecting deforestation and agricultural developments in the St.-Lawrence Valley. Between 1894 and 1937, widespread use of industrial and domestic charcoals may explain the isotopic composition of Pb accumulated in the sediments (mean 206Pb/207Pb = 1.19 ± 0.01). From 1937 to 1975, Pb isotopic compositions became less radiogenic (206Pb/207Pb from 1.18 to 1.17) even though elemental Pb abundance reached extremely high values (623 mg kg–1). This isotopic shift reflects increased use of alkyl-lead in gasoline. For sediments accumulated between 1967 and 1996, the U.S. contribution to anthropogenic Pb accumulated in Lake Clair sediments amounted to between 30 and 63%.

Abbreviations: CFCS, constant flux–constant sedimentation • LCW, Lake Clair watershed • WSI, water–sediment interface


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SINCE THE BEGINNING of the industrial era in North America, large amounts of Pb have been released to the atmosphere from activities such as metal refining; incineration of refuse, coal, and wood; and combustion of alkyl-leaded gasoline (Patterson and Settle, 1987; Sturges and Barrie, 1987; Carignan and Gariépy, 1995). Documented by early evidence from ice cores (Murozumi et al., 1969), anthropogenic release of Pb to the atmosphere has become increasingly well documented. For instance, records of Pb accumulation in the environment are well preserved in coral from the Atlantic and Pacific Oceans (Shen and Boyle, 1987), in snow and ice from polar and alpine regions (Murozumi et al., 1969; Rosman et al., 1994), and in estuary and marine sediments (Shirahata et al., 1980; Véron et al., 1987; Hamelin et al., 1990; Graney et al., 1995; Gobeil et al., 1995).

Lake sediments are also an important sink for anthropogenic trace metals and therefore, in watersheds remote from deforestation and agricultural and/or industrial activities, they should record the history of metals accumulation from natural and anthropogenic atmospheric sources. This interpretation may be supported by the fact that in undisturbed sediments, trace metal concentrations usually increase from the deep pre-anthropogenic core sediments toward the water–sediment interface (Nriagu et al., 1982; Gobeil and Silverberg, 1989). However, other processes occurring simultaneously in the watershed or in the lake itself can complicate the historical interpretation of such profiles.

For example, increasing erosion of the catchment due to deforestation (logging) or forest fires could increase the accumulation rates of inorganic and organic particles and cause the importation of large amounts of trace metals to the lake (Martin et al., 2000). In addition, diagenetic bioturbation and bioirrigation processes could have strong influences on the shape of trace metal profiles in sediments. In the same way, after their accumulation in sediments, trace metals can be mobilized and relocalized in the sediment column or they may diffuse to the water column (Rosenthal et al., 1995). All these processes may disturb the historical record of heavy metal accumulation in lakes. However, as shown in this study, when such secondary processes can be well constrained, reconstruction of historical atmospheric metal accumulation can give valuable information on the sources and relative contributions of diffuse contamination.

Although it is well known that elemental Pb concentrations have increased in North American and European lake sediments over the last 150 yr, the processes responsible for such contamination are still subject to debate. Because of both the very strong Pb anthropogenic signal in the environment and the possibility of using stable Pb isotopes as source indicators, Pb is an effective tracer of metal input and cycling in the atmosphere (Sturges and Barrie, 1987; Carignan and Gariépy, 1995). Determination of lead isotopes can give valuable quantitative source information in the study of atmospheric aerosol transport. These isotopic ratios may be utilized in the identification of atmospheric contamination sources (Settle and Patterson, 1982; Sturges and Barrie, 1987; Carignan and Gariépy, 1995). This method is based on the differences in isotopic ratios existing among different groups of materials, such as local bedrocks, gasoline additives, and industrial emissions.

In this study, we use elemental Pb concentrations and Pb isotope ratios as well as 210Pb and 137Cs sediment dating to (i) reconstruct the historical accumulation of atmospheric Pb in Lake Clair sediments (Québec, Canada) since the beginning of the industrial era, (ii) quantify the maximum Pb enrichment relative to natural background value, and (iii) trace back the principal sources of anthropogenic Pb and assess their relative historical contributions in this part of southern Québec.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
Located in the Duchesnay Forest Park, the Lake Clair watershed (LCW) is situated 50 km northwest of Québec City (Fig. 1a) . Basement rocks are made of Precambrian felsic gneiss of the Canadian Shield, which is in-filled in places by Quaternary tills of variable thickness. The LCW covers an area of 2.26 km2 and the forest stand is dominated by deciduous trees (83%) and conifers (17%). According to recent palynological and paleolimnological studies, the LCW has remained forested during the last 18 centuries (Richard et al., 2000), although locally, selective tree cutting occurred in the mid-1940s. No significant recent forest fires have been documented in the watershed. From 1988 to 1994, the average annual precipitation was 1300 mm (Houle et al., 1997).



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  		Fig. 1. (a) Map of the study area and (b) Lake Clair bathymetric map showing sampling site (modified from D'Arcy, 1993).

 
Lake Clair (Fig. 1b) is a small oligotrophic headwater lake with a surface area of 0.35 km2. The organic productivity in the lake is low, as attested by the chlorophyll a content (1.7 µg L–1) and the dissolved organic carbon content (1.9 mg L–1) (D'Arcy, 1993). The pH of the lake water has a mean value of 6.5 ± 0.4. During summer stratification (July–August), epilimnion depth is estimated to be approximately 6 m. The lack of tributaries implies that sediments are principally derived from local and atmospheric sources. For instance, fine, autochtonous organic particles (phyto and zooplankton) are produced from the lake itself via processes such as photosynthesis and allochtonous particles are imported from forest debris (pollen, spores, and leaf fragments) and erosion of the surrounding forest soils.

Sample Collection
Two sediment cores (50-cm length) were taken in 1996 from the deepest part of Lake Clair, Québec (approximately 26-m water depth; Fig. 1b). The cores were collected by divers by carefully inserting a Plexiglas tube (9.5-cm i.d.) into the sediments. Coring and sampling equipment were pretreated with nitric acid solution (Aristar 20%) and rinsed with ultrapure Milli-Q water (>18 M{Omega} cm; Millipore, Billerica, MA) to avoid trace-metals contamination. The thickness of the sampled sections varied according to depth (Table 1). The samples were then freeze-dried in the laboratory, ground with an agate mortar and pestle, and placed in a desiccator for later analysis. After sediment homogenization, each subsample was divided into two parts for isotopic and chemical analyses. Subsamples from the second core were used to evaluate possible effects of lateral or vertical variability in the geochemical data from site to site. As shown in Ndzangou (2003), both cores displayed nearly identical elemental and isotopic variations, suggesting extremely low spatial variability in the deepest part of the lake. Data from the second core are presented in detail in Ndzangou (2003) and for space considerations in this paper will not be presented.


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  		Table 1. Measured lead isotope ratios, elemental concentrations, and calculated anthropogenic lead values in Lake Clair sediments. All isotopic ratios were determined with a precision of 2{sigma} < 1%.

 
In October 1999, three sediment traps (1-m length and 12-cm i.d.) were deployed in the water column at a 15-m depth to collect settling particles. The traps were recovered after two weeks and particles were collected by decantation and filtration in the laboratory.

Chemical and Isotopic Analysis
To determine the stratigraphic ages of the lake sediments, 210Pb and 137Cs activities were counted by {gamma}-ray spectroscopy, incorporating a Compton-effect correction (Appleby and Oldfield, 1978; Carignan et al., 1994). Counting efficiencies were determined by labeling dry sediments samples (bulk densities of 0.5–1.6 g cm–3) with standard solutions (Amersham, Little Chalfont, UK) of mixed nuclides (QCY.48), 210Pb (RBZ.44), and 226Rn (RAY.44). Because of the short half-lives of these two radioisotopes (22.3 yr for 210Pb and 30 yr for 137Cs) only the top 9.5 cm of sediments was accurately dated.

Total Metal Analysis
All chemical preparations were performed in a Class 100 laminar-flow hood. Milli-Q water (>18 M{Omega} cm) and ultrapure acids (Seastar, Sidney, BC, Canada) were used during all chemical manipulations. A 200-mg aliquot of each sediment sample was digested in closed Teflon bombs (Savillex, Minnetonka, MN) using a mixture of ultrapure and concentrated HF–HNO3–HClO4 acids (Alfaro-De La Torre and Tessier, 2002). Aluminum, Fe, Na, and S concentrations were measured by inductively coupled plasma atomic emission spectrometry (ICP–AES) (Optima 3000; PerkinElmer, Wellesley, MA). Total Pb and Se concentrations were determined by inductively coupled plasma mass spectrometry (ICP–MS) (VG Turbo Plasma Quad PQ2+; Thermo Electron Corporation, Waltham, MA). Certified reference materials (Mess-2 and PACS-2; National Research Council of Canada, Ottawa, ON) were regularly submitted to the same digestion procedure as the Lake Clair sediments. Analytical precision was generally better than 1% for the major elements. For Pb, the detection limit was <6 µg L–1 and the determination of elemental concentrations was within 10% of the certified values.

Lead Isotope Analysis of the Sediments and Settling Particles
Lead separation was achieved by anion-exchange chromatography using the method described by Bacon et al. (1996). The columns were packed with AG1-X8 resin (Bio-Rad, Hercules, CA) in the chloride form. Briefly, 20 mL of the acid-digest solution of each sample were evaporated to dryness to remove any traces of HNO3. Then, the dry residues were dissolved in 5 mL of 1 M HBr and loaded manually in the columns. After washing with 10 mL of 2 M HCl, Pb was eluted by gravity using 10 mL of 6 M HCl. The collected fraction was evaporated to dryness and the final solutions were diluted in 5 mL of 0.8 M HNO3.

Measurements of Stable Lead Isotopic Ratios
Measurements of Pb isotopic ratios were performed by ICP–MS (VG Turbo Plasma Quad PQ2+). This instrument has a good sensitivity and produces a count rate of approximately 5.105 cps/µg L–1 for Pb. The NIST 981 Pb reference standard (National Institute of Standards and Technology, Gaithersburg, MD) was used to monitor accuracy. Data were taken in the scanning mode (for masses m/z between 200 and 210). Natural Tl isotopic ratio was used as an internal standard for calculation of the mass bias correction of the spectrometer (Ketterer et al., 1991). All isotopic ratios used in this study were determined with a precision of 2{sigma} < 1%. In ICP–MS measurements, the only notable interference is the 204Hg isobaric overlap on 204Pb. To correct this effect, we have monitored the 201Hg and subtracted corresponding 204Hg (Monna et al., 1997).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The principles underlying sediment chronology with excess 210Pb have been presented in detail by Robbins (1978). In the best conditions, this method allows the dating of the last 150 yr of sedimentation (Bollhofer et al., 1994). Sediments in modern lacustrine environments accumulate 210Pb in two ways. In part, 210Pb is derived from 238U decay series, due to the presence of the parent isotopes, specifically 226Ra within the sediment. The 210Pb from this source quickly reaches secular equilibrium with the 226Ra in the sediment, and remains constant at the time scale of interest to this type of study. Such 210Pb is termed "supported," as opposed to the second source, which is derived from the decay of atmospheric 222Rn. As 222Rn decays, newly formed 210Pb is rapidly removed from the atmosphere. Subsequently, the 210Pb adsorbs onto fine particles present in soils or in suspended sediments. This unsupported 210Pb, which decays at a rate proportional to its half-life of about 22.5 yr, is commonly termed excess 210Pb. It is the basis for dating sediments and determining sediment accumulation rates in modern marine and lacustrine environments. Differentiating supported and excess 210Pb is a matter of monitoring where 210Pb becomes constant with depth in a core. This constant rate of 210Pb activity is the supported 210Pb and the excess 210Pb is thus the difference between total and supported 210Pb activity.

Figure 2b shows the plot of unsupported 210Pb vs. cumulative mass of sediment (mc). The cumulative masses were plotted instead of core depth to eliminate the errors that may arise due to sediment compaction. Total 210Pb activity decreases progressively from 7.4 Bq g–1 at a depth of 0.25 cm below the water–sediment interface (WSI) to values close to those supported by 226Ra at depths below 9.5 cm. Some minor breaks are present in the ln 210Pbunsupported profile. These changes may result from bioturbation processes or may indicate changes in sediment accumulation rate. However, it is noteworthy that between the WSI and 5 cm (3 kg m–2) the distribution of natural logarithm values defines a straight line with a positive slope, which cannot be explained by bioturbation or mixing processes (Fig. 2b).



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  		Fig. 2. Radio-chronological data of the top 9.5 cm of Lake Clair sediments. (a) Variation of 137Cs activity vs. depth and (b) variation of unsupported 210Pb activity vs. cumulative mass of sediments. Error bars are counting error (±SE). (c) Approximate year of sedimentation vs. depth. The year of sedimentation was derived from the 210Pb constant flux–constant sedimentation (CFCS) model (Oldfield and Appleby, 1984; Robbins and Herche, 1993).

 
To establish the 210Pb chronology of the studied sediments, we first used the constant rate of supply model (Appleby and Oldfield, 1978). Calculated sediment accumulation rates varied between 10 and 80 g m–2 yr–1. Such a large variation is very unlikely for Lake Clair since there is no textural or compositional evidence suggesting significant modification of the sedimentary regime. Furthermore, knowing that the Lake Clair watershed remained forested and relatively undisturbed during the last 150 yr, it is not possible to explain an eightfold increase in the sedimentation rates.

Assuming a nearly constant sedimentation rate, we determined the sediment accumulation rate and the age of the Lake Clair sediments using the constant flux–constant sedimentation (CFCS) model (Oldfield and Appleby, 1984; Robbins and Herche, 1993). The sediment accumulation rate (R) was calculated from the mean slope of the profile using a least-squares fitting procedure. The layer between the 5- and 8-cm depth (equivalent to 3–5 kg m–2) was omitted in the regression line. Therefore, a single value of R = 50.9 g m–2 yr–1 was obtained. Sediment accumulation dates were obtained by dividing the cumulative mass (g m–2) by R (g m–2 yr–1) at mid-depth of a sediment interval. Figure 2c shows calculated years of sediment accumulation vs. depth. Accordingly, these ages range from 1872 to 1996, which was the year of sediment sampling. To establish a complete chronology, we assumed no changes in sedimentation rates below 9.5 cm. Accordingly, the ages of sediments below a 9.5-cm depth were estimated using the R value calculated for the top section of the sediment core.

Calculated dates do not agree with known discernible anthropogenic events in North America involving 137Cs. For example, initial 137Cs input in the atmosphere is about 1953 and maximum fallout from nuclear weapons testing is in 1963 (Davis et al., 1984; Anderson et al., 1987). In the Lake Clair sediments, the 137Cs peak is shallower and activity of this radionuclide occurred deeper than was predicted by the 210Pb model (Fig. 2a). The occurrence of 137Cs at much greater depth (approximately 9.5 cm) than expected (about 3.75 cm) is an indication of post-depositional mobility (Spezzano et al., 1993; Baskaran and Naidu, 1995). The occurrence of the 137Cs peak at a shallower depth has been commonly observed in lakes including those from the Canadian Shield and attributed to a focusing process. This process implies that winnowing of fine littoral sediments and their transport to deeper parts of lakes could provide a supply of 137Cs to the deeper site after 1963 (Davis et al., 1984; Anderson et al., 1987).

Elemental Pb can also be used as a stratigraphic marker to corroborate calculated 210Pb dates. In North America, anthropogenic Pb has been emitted to the atmosphere mainly by combustion of coal and leaded gasoline and by smelting activities (Shirahata et al., 1980; Graney et al., 1995). Although the occurrences of historical Pb events have not been as precisely dated as those of 137Cs, lead concentration profiles in North American lake sediments are often characterized by a well-defined peak attributed to the phasing out of leaded gasoline in the 1970s. In USA and Canada, suggested dates for this event vary between 1973 and 1975 (Ouellet and Jones, 1983; Graney et al., 1995). Figure 3 shows a peak in elemental Pb at a 2.25-cm depth, which would correspond to 1975 according to the CFCS age model. The consistent decrease in Pb concentration observed thereafter is also in agreement with the documented decrease in Pb emissions. The above agreements suggest that bioturbation effects are probably not important in the deep part of the lake since the history of Pb accumulation is fairly well preserved. Moreover, variation in the logarithm of the unsupported 210Pb against the cumulative mass of sediments defines a straight line, which is inconsistent with mixing process normally associated with bioturbation of sediments. Finally, as suggested by Gallon et al. (2004) and Ndzangou (2003), diagenetic processes, such as remobilization, molecular diffusion, bioturbation, and bioirrigation, have a negligible impact on solid-phase Pb profile observed in sediments from the Canadian Shield. Therefore, in the studied sediments, the Pb sedimentary record principally reflects the history of Pb accumulation within the watershed.



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  		Fig. 3. Variations of Pb and Se contents, Pb/Al, and Pb isotopic ratios in Lake Clair sediments vs. depth and approximate year of sedimentation. The vertical dashed line (S.P.) shows the isotopic composition of settling particles collected in sediment traps.

 
Sediment Lead Concentrations and Isotopic Ratios
The Pb contents and isotopic ratios in the sediments are relatively constant between 20- and 9.5-cm depths below the WSI and more variable upward (Fig. 3 and Table 1). Lead concentration increases progressively from a relatively constant background value (approximately 18 mg kg–1) below the 9.5-cm depth to a maximum value of 623 mg kg–1 at 2.25 cm. After this maximum peak, the Pb content decreases upward to about 399 mg kg–1 near the WSI.

The Pb isotopic ratios are relatively stable below 9.5 cm (206Pb/207Pb = 1.20 ± 0.01; 206Pb/204Pb = 18.4 ± 0.1) and increase to more radiogenic values until an 8-cm depth. Thereafter, between 8 and 2.25 cm, they decline to less radiogenic values and from 2.25 cm to the WSI, the isotopic ratios shift toward more radiogenic compositions (Fig. 3). Sediments near the WSI display elemental Pb and isotopic ratios similar to those measured in settling particles collected in sediment traps (Fig. 3). Isotopic variability observed in these settling particles is low, as shown by 206Pb/207Pb values bracketed between 1.17 and 1.18.

The near constancy of Pb isotopic ratios and elemental concentrations below 9.5 cm most probably represents the natural Pb background and thus the chemical signature of pre-anthropogenic sediments. It is noteworthy that elemental and isotopic compositions in the pre-anthropogenic section are similar to those reported for the mean composition of the upper continental crust (Pb = approximately 20 mg kg–1 and 206Pb/207Pb = approximately 1.21; Taylor and McLennan, 1995). The important enrichment in Pb concentrations, in the upper section of the sediment core (between 9.5 cm and the WSI), is accompanied by significant changes in Pb isotope ratios. This strong correlation implies the introduction of anthropogenic Pb in the sedimentary basin (Hamelin et al., 1990; Graney et al., 1995; Monna et al., 2000). Diagenetic remobilization of Pb, on the other hand, cannot explain such isotopic profiles.

Anthropogenic contamination in the Lake Clair sediments can also be demonstrated using appropriate elemental ratios such as Pb to Al (Schettler and Romer, 1998). Normalizing Pb abundances to a more conservative element such as Al, which is normally derived from natural geological sources in the watershed (e.g., silicated minerals), permits the estimation of anthropogenic Pb enrichment relative to natural background values. Accordingly, sediments located below 9.5 cm of the WSI have a mean Pb to Al ratio (g g–1) of 3.8 (Fig. 3), close to the value of 2.5 estimated for the mean upper continental crust (Schettler and Romer, 1998). This ratio increases upward to a mean value of about 91, which is much higher than the mean pre-anthropogenic value of 3.8. For recent sediments (between 0 and 9.5 cm), this suggests anthropogenic Pb enrichment factor ({Pb/Al}sediment/{Pb/Al}background) reaching a maximum value of 35 at 2.5 cm below the WSI. This value is similar to the enrichment factor of 34, which can be estimated by dividing the maximum Pb abundance (623 mg kg–1 at 2.25 cm below the WSI) by the average Pb background (18.3 mg kg–1).

The lack of tributaries or industrial and/or agricultural activities in the LCW implies that the anthropogenic Pb component comes essentially from the atmosphere. According to the 210Pb chronology of the sediments, anthropogenic Pb signatures in Lake Clair sediments can be detected from 1872. This age agrees well with the appearance of pollen from ambrosia (Ambrosia spp.) and other cultivated plants in Lake Clair sediments (Richard et al., 2000). Since the Lake Clair watershed has remained forested, the appearance of this pollen is believed to be associated with settlement and development of small villages (Ste-Catherine-de-Fossambault, 1824; St-Raymond-de-Portneuf, 1830; and Shannon, 1850) along the St.-Lawrence Valley, several kilometers around the Duchesnay park (Richard et al., 2000). Pollen was transported via the atmosphere and accumulated in Lake Clair sediments. Consequently, this section of the sediment core can be used as a marker horizon indicating the start of anthropogenic perturbation of the sediment record.

The isotopic signature of the anthropogenic Pb, added to the sediments, can be obtained by subtracting the natural (background) component from the total signature. In the following equation, we used the 206Pb/207Pb ratio as an example (Ng and Patterson, 1982; Graney et al., 1995):

[1]
where ax is the isotopic composition of the anthropogenic component at depth x, (206Pb/207Pb)x and {Pb}x are respectively Pb isotopic composition and concentration at any depth interval x, and (206Pb/207Pb)b and {Pb}b are background (pre-anthropogenic) values of sediments accumulated before 1872 (<9.5 cm). Calculated anthropogenic Pb component (Table 1) is the value needed to constrain anthropogenic Pb sources more accurately (Ng and Patterson, 1982; Graney et al., 1995).

Flux of Lead in Sediment Core
The fluxes of atmospheric (anthropogenic) Pb can be assessed as follows:

[2]
where FPbx is the flux of atmospheric Pb at any depth interval x (µg m–2 yr–1), R is the sediment accumulation rate (g m–2 yr–1), and {Pb}xa is the anthropogenic contribution (µg g–1) to the concentration of Pb at depth x. To minimize the effects of variations in Pb background values on these calculations, the anthropogenic contribution was estimated as follows (e.g., Norton et al., 1991):

[3]
where {Pb}x and {Al}x are respectively Pb and Al concentrations at any depth interval x, and {Pb}b and {Al}b are Pb and Al background values. In this formulation, the natural (geologic) component of total Pb is assumed to be contributed in constant proportion to Al. Background values were calculated as shown in Eq. [1]. Results are plotted with corresponding excess 206Pb/207Pb ratios against sediment ages in Fig. 4 . Calculated anthropogenic fluxes (0.001–0.03 g m–2 yr–1) fall within the range of those reported for other North American lakes (Norton et al., 1991).



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  		Fig. 4. Calculated excess Pb flux and corresponding 206Pb/207Pb ratios in Lake Clair sediments accumulated since 1872 vs. approximate year of sedimentation.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Sources of Anthropogenic Lead in Lake Clair Sediments
Figure 5 , derived from Graney et al. (1995), shows an estimate of Pb emissions to the atmosphere from wood and coal burning, ore production and consumption of gasoline for the entire United States as a function of time. From 1850 to 1880, the predominant anthropogenic source of lead emitted to the atmosphere was the burning of wood, from 1880 to 1930; there were approximately equal contributions from coal and ore smelting. After 1930, the combustion of leaded gasoline became the predominant source of Pb emissions.



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  		Fig. 5. Relative emissions of Pb to the atmosphere from different sources in the United States. Data are derived from Graney et al. (1995).

 
Distinct changes in isotopic ratios accompany changes in elemental Pb concentrations in Lake Clair sediments (Fig. 3). It is noteworthy that changes in the anthropogenic component of Pb in Lake Clair sediments follow fairly well the addition of Pb to the atmosphere predicted in Fig. 5. The most logical way to explain the Pb isotope characteristics of the anthropogenic component in sediments accumulated between 1872 and 1894 is to invoke deforestation in the regions located along the St.-Lawrence Valley. During this period, timbering played an important role in the local economies and major forest fires probably occurred in many areas of the valley. In Lake Clair sediments, this interpretation is supported by the appearance of ambrosia pollen and charcoal derived from wood burning (Richard et al., 2000). The importance of wood-burning processes at that time is supported by the selenium (Se) profile in studied sediments, which shows enrichment (Fig. 3; Wiersma and Lee, 1971; Cutter, 1991). On the other hand, it is more difficult to assess whether changes in Pb concentrations and isotopic ratios in sediments reflect burning of biomass or increased wind erosion due to land denudation and agricultural activities in the St.-Lawrence Valley.

From 1894 to 1937, Pb isotopic ratios probably reflect widespread and increasing combustion of coal for industrial and domestic uses. In the Lake Clair sediments, supporting evidence for increasing coal utilization include the presence of charcoal fragments (from coal burning) as well as systematic enrichment in Se concentrations. However, since Pb concentrations show large increase during this period compared with Se (Pbenrichment/Seenrichment = approximately 10), an additional source of Pb is needed. The production and consumption of Pb ore by industry (metal mining, smelting, and finishing) is a likely source for this additional source of anthropogenic lead. This is shown by a continuous decrease of Pb isotopes ratios during this period.

From 1937 to 1975, the anthropogenic component Pb isotopic ratios became less radiogenic (lessening of the 206Pb/204Pb ratio; Fig. 3). The least radiogenic values are observed at the time when Pb concentrations reach their maximum value (in sediments accumulated in 1975). Such variations have already been reported approximately at the same period in other North American sediments. They have been interpreted as evidence of ore smelting and increased use of alkyl Pb in gasoline (Shirahata et al., 1980; Graney et al., 1995). In the early 1980s, the isotopic ratios of the anthropogenic component shifted toward more radiogenic values simultaneously with a decrease in Pb concentrations. This is attributed to the phasing out of leaded gasoline in the United States in the middle 1970s and the beginning of the Canadian legislation prohibiting the use of Pb in gasoline (Gélinas et al., 2000). In Canada, the phasing out of leaded gasoline was completed in 1990. It is noteworthy that even after 6 yr of complete removal of alkyled Pb in U.S. and Canadian gasolines, Pb isotopic signatures of recent sediments remain less radiogenic than pre-anthropogenic sediments. This anthropogenic signature probably reflects continued emissions of industrial Pb from ore smelting.

Identifying Contributions of Different Anthropogenic Sources
We have related the increase in anthropogenic Pb since 1937 to the increased proportion of mixed industrial Pb from ore smelting and alkyl Pb from gasoline. When two sources of Pb have distinct isotopic signatures, the relative contribution of each one can be estimated using simple mixing considerations (Shirahata et al., 1980; Carignan and Gariépy, 1995; Monna et al., 2000).

In Canada, until June 1985, Pb additives have been produced exclusively by the Canadian divisions of the DuPont and Ethyl corporations (Sturges and Barrie, 1987). DuPont obtained galena Pb ores from New Brunswick mines, and Ethyl Corporation from British Columbia mines (with 206Pb/207Pb not higher more than 1.16). For the same period, less than 1% of Pb used as an additive in Canadian gasoline was imported (Sturges and Barrie, 1987). Isotopic ratios of lead used by the Canadian industry (for example, metallurgical smelters) are characterized by lower values. For example, atmospheric emissions of Pb from the Noranda Cu-smelter have a 206Pb/207Pb ratio of about 0.92 (Franklin et al., 1983). However, because Noranda is located many hundreds of kilometers from the studied region, such a contribution is likely to be negligible in Lake Clair sediments.

In a study of atmospheric particulate matter compositions in Canada, Sturges and Barrie (1987) concluded that the United States and Canada emit anthropogenic Pb that is mostly derived from distinct geological sources (United States: Pb from Mississippi Valley–type ore deposits; Canada: Pb from Pb–Zn ore deposits in British Columbia). Consequently, in the United States and Canada, industrial Pb has different isotopic compositions. Moreover, they estimated that between 24 and 43% of atmospheric Pb aerosols measured in Canada originated from the United States of America (depending on the prevailing wind trajectories). More recently, Carignan and Gariépy (1995) used the isotopic composition of epiphytic lichens sampled between 1990 and 1994 to trace the sources of atmospheric Pb emissions in southern Québec. These authors concluded that Pb recently accumulated in the St.-Lawrence Valley is mainly (around 60%) derived from U.S. sources.

Between 1967 and 1996, the 206Pb/207Pb ratios in Lake Clair sediments varied between 1.17 and 1.18. Similarly, Pb isotopic ratios of settling particles (collected in sediment traps in 1999) yield similar isotopic composition (206Pb/207Pb = 1.17 and 206Pb/204Pb = 18). These values are consistent with the mixing of Canadian (1.15–1.16) and American (1.19–1.23) aerosols. Considering the numerous indications suggesting that a great amount of Pb accumulated in the St.-Lawrence Valley is derived from U.S. sources and that anthropogenic Pb from both countries has distinct isotopic signatures, the relative contribution of U.S. and Canadian atmospheric Pb in Lake Clair sediments can be estimated using the binary mixing of Canadian and American end-members. If FCan and FAm are respectively the fractions of the Canadian and American Pb types responsible for the anthropogenic Pb isotopic composition, then the following algebraic mass balance constraints may apply:

[4]

[5]
where (206Pb/207Pb)Can and (206Pb/207Pb)Am are respectively the isotopic signatures of the Canadian and American end-members, and (206Pb/207Pb)anthropogenic is the isotopic signature of anthropogenic Pb derived from Eq. [1].

For the Canadian aerosols end-member, we have used the value of 1.15 reported by Carignan and Gariépy (1995). On the other hand, the isotopic composition of atmospheric Pb from the United States has been more variable and changed between 1967 and the present (Shirahata et al., 1980; Véron et al., 1992, 1993; Rosman et al., 1994). For instance, the atmospheric 206Pb/207Pb was 1.15 before 1967, but increased to 1.20 by 1974 and further increased to 1.21 to 1.23 by 1977 (Shirahata et al., 1980). The more radiogenic signature of the U.S. aerosols is mainly due to the use of Pb coming from sulfide ore extracted from Mississippi Valley mineral deposits (206Pb/207Pb = approximately 1.28–1.33; Shirahata et al., 1980). Since the mid-1960s, this radiogenic Pb has been used in gasoline additives. The phasing out of leaded gasoline in the United States resulted in changes in Pb isotopic signatures of atmospheric aerosols. For example, the average U.S. anthropogenic Pb component varied in 206Pb/207Pb from 1.21 to 1.23 in the mid-1980s to 1.19 to 1.20 after 1988 (Véron et al., 1992, 1993; Rosman et al., 1994). Therefore, for the estimation of historical U.S. contribution in Pb accumulation in Lake Clair sediments, we have used different isotopic values for U.S. aerosols.

Before 1967, we cannot discriminate between Canadian and American sources of Pb since both countries produced anthropogenic Pb with similar 206Pb/207Pb isotopic values (approximately 1.15). Depending on the selected value for the American end-member, our calculations suggest that from 1967 to 1996, between 30 to 63% of the anthropogenic Pb accumulated in Lake Clair sediments came from U.S. sources. These estimates fall in the range of proportions previously given by Sturges and Barrie (1987) and Carignan and Gariépy (1995) for American Pb contributions in Canadian aerosols. These observations confirm the suggestion of Ouellet and Jones (1983) that the Midwest Great Lakes region is a major source of atmospheric Pb along the St.-Lawrence Valley in southern Québec. Such signatures reflect the importance of cyclonic disturbances that move northeastward along the St.-Lawrence Valley in summer just as well in winter (Ouellet and Jones, 1983). These disturbances are submitted to orographic phenomena when they encounter some of the higher topographic features of the Canadian Precambrian Shield, giving rise to elevated precipitation (Ouellet and Jones, 1983; Carignan and Gariépy, 1995).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Geochemical and isotopic analyses of the Lake Clair sediments show a complex elemental and isotopic Pb record reflecting progressive contamination of the watershed by different atmospheric sources. Starting at the middle of the 19th century, Pb content of the sediments increased until 1975 and decreased thereafter. The maximum Pb enrichment factor of 35 times relative to the natural background is found in 1975. At this time, the excess Pb flux was also maximal with a value of about 0.03 g m–2 yr–1. Between 1872 and 1894, the source of anthropogenic Pb was highly radiogenic possibly reflecting deforestation and agricultural developments in the St.-Lawrence Valley. Between 1894 and 1937, widespread and increasing combustion of coal for industrial and domestic uses may explain the isotopic composition of Pb accumulated in the sediments. From 1937 to 1975, the Pb isotopic composition became less radiogenic while elemental Pb abundances reach extremely high values (623 mg kg–1). This isotopic shift principally reflects increased use of alkyl Pb in gasoline. For sediments accumulated between 1967 and 1996, we have calculated the relative contribution of anthropogenic U.S. and Canadian atmospheric Pb. Our calculations suggest that during this period, 30 to 63% of the accumulated anthropogenic Pb came from the United States.


   ACKNOWLEDGMENTS
 
We thank Stéphane Lorrain (Environnement illimité Inc.) and René Rodrigue (INRS-Eau) for their help during sampling of the sediment core and installation of dialyse cells in the Lake Clair. Special thanks are given to the Ministère des Ressources naturelles du Québec (MRNQ) for access to the Lake Clair experimental station. S.O. Ndzangou acknowledges financial support by a Gabon Government doctoral fellowship. M. Richer-Laflèche acknowledges financial support from the Fond de la Recherche Forestière (MRNQ) and from the Natural Sciences and Engineering Research Council of Canada (Grant OGP 0138413). Gratitude is also given to I. Bélanger, R. Gosselin, and C. De Blois for their generous help in analyzing the samples by ICP–AES and ICP–MS. This manuscript was kindly reviewed by A. Tessier, M. Thompson, and three anonymous reviewers.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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