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Published online 31 August 2007
Published in J Environ Qual 36:1488-1494 (2007)
DOI: 10.2134/jeq2006.0367
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Heavy Metals in the Environment

Dendrochemical Record of Historical Lead Contamination Sources, Wells G&H Superfund Site, Woburn, Massachusetts

Aaron Burnetta, Andrew C. Kurtza,*, Daniel Brabanderb and Mark Shailerc

a Dep. of Earth Sciences, Boston Univ., Boston, MA 02215
b Geosciences Dep., Wellesley College, Wellesley, MA 02481
c Dep. of Environmental, Earth and Ocean Sciences, Univ. of Massachusetts Boston, Boston, MA 02125

* Corresponding author (kurtz{at}bu.edu).

Received for publication September 13, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
Laser-ablation inductively coupled-plasma mass-spectrometry analysis of red oak (Quercus rubra) from a well documented heavy metal contaminated United States Environmental Protection Agency superfund site in Woburn, Massachusetts reveals decade-long trends in Pb contaminant sources. Lead isotope ratios (207Pb/206Pb and 208Pb/206Pb) in tree rings plot along a linear trend bracketed by several local and regional contamination sources. Statistically significant interannual variations in 207Pb/206Pb suggest that atmospheric Pb is rapidly incorporated into wood, with minimal mobility subsequent to deposition in annual growth rings. We interpret the decadal trends in our record as a changing mixture of local pollution sources and gasoline-derived Pb. Between 1940 and 1970, Pb was predominantly derived from remobilization of local industrial Pb sources. An abrupt shift in 207Pb/206Pb may indicate that local Pb sources were overwhelmed by gasoline-derived Pb during the peak of leaded gasoline emissions in the late 1960s and early 1970s.

Abbreviations: LA-ICP–MS, laser ablation inductively coupled plasma mass spectrometry • SRM, standard reference material • USGS, United States Geological Survey


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
TREES growing in temperate regions produce annual growth rings that can reveal information about environmental change. Because pollutants are incorporated into these growth rings, dendrochemical records can be used to reconstruct regional contaminant histories on decadal time scales. Typically, studies that have used the concentrations or isotope ratios of pollutants in wood as a biomonitor of metals have used multi-year growth increments to look for long-term trends in contaminant loading patterns (Marcantonio et al., 1998; McClenahen et al., 1989). With the advent of in situ analytical approaches such as secondary ion mass spectrometry (Brabander et al., 1999; Martin et al., 1994), particle-induced X-ray emission (Lovestam et al., 1990; McClenahen et al., 1989), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP–MS) (Hoffman et al., 1994; Watmough et al., 1997; Watmough et al., 1998) and the combination of isotope ratio analysis (for pollutants such as Pb), it is possible to investigate changing pollutant source characteristics on an annual or even sub-annual time frame.

We have developed a LA-ICP–MS methodology that can measure Pb isotope variations in wood with annual resolution. The method is analytically simple, rapid, and precise enough to allow discrimination of Pb contaminant sources in an urban watershed that has been subjected to several sources of Pb ranging from early industrialization to atmospheric fallout of Pb associated with the combustion of leaded gasoline.

Lead Dendrochemistry
Although there are many pitfalls associated with applying tree-ring records toward reconstruction of contaminant histories (Brabander et al., 1999; Watmough, 1999), the approach used here is relatively robust. We measure isotope ratios rather than metal concentrations in wood. Metal concentrations may vary significantly between adjacent trees or even within annual rings in the same tree (Bondietti et al., 1989). Concentrations may also be influenced by the age of the tree or by environmental parameters other than metal loading (e.g., climate, changes in soil chemistry, health of the tree). Because Pb isotope ratios are not fractionated biogeochemically, interpreting isotope ratios in terms of Pb sources involves fewer sources of uncertainty. In contrast to nutrients like Ca, which may be selectively transported from the nonliving portion (heartwood) to the living portion of the tree (sapwood), toxic metals such as Pb and Cr are much less subject to later movement (Brabander et al., 1999; Prohaska et al., 1998; Watmough et al., 1999a). The low lateral mobility of Pb suggests that once incorporated into the wood of the tree, the Pb is effectively immobile. Thus, annual growth rings provide a record of Pb incorporated by the tree during the year that ring was growing and, by inference, record the isotope ratio of average bioavailable Pb each year.

The precise mechanism of Pb uptake by plants is not entirely understood, and at least three pathways are viable under some circumstances (Watmough, 1999). Lead uptake from soil by roots, an important pathway for uptake of many metals, is generally inefficient for Pb due to the low bioavailability of soil Pb. Another potential pathway, uptake by leaves, has been shown experimentally to be inefficient in spruce (Watmough et al., 1999b) but may be more important in other species. A third potential pathway for Pb delivery to sapwood is uptake through bark. Early experiments with Pb isotopic tracers (Lepp and Dollard, 1974) showed that Pb applied to bark was efficiently incorporated into the outer (growing) tree ring. However, more recent experimental results (Watmough and Hutchinson, 2003) question the importance of this pathway, demonstrating inefficient transfer of applied Pb from bark to wood. A recent study of Pb uptake using natural Pb isotopes as a tracer (Klaminder et al., 2005) showed that Pb budgets in Scots pine in a remote Swedish boreal forest are dominated by Pb derived from atmospheric pollution. The authors argued that approximately 40 to 50% of this Pb was derived directly from the atmosphere. The remaining Pb was sourced largely from atmospheric Pb that had accumulated in organic-rich shallow (0–3 cm) soils, with only minor contributions from natural Pb in mineral soils. Direct incorporation of Pb deposited on leaves or bark would allow dendrochemical records to closely track the timing and magnitude of environmental changes in Pb loading. In contrast, unless cycling of Pb through soil, into roots, and subsequent transport and deposition into wood is very rapid, this pathway would be expected to produce a significant time lag and damping in tree ring records, hampering the utility of dendrochemical pollution archives. The relative importance of the known Pb delivery pathways depends on an array of factors including tree species, relative Pb concentrations in soil and rainfall, and soil chemistry.

Wells G&H Study Site
Our study site is within the Wells G&H Superfund site, which is located within the densely populated Aberjona River watershed in Woburn, Massachusetts, 20 km north of Boston (Fig. 1 ). The site has a long and well documented history of heavy metal contamination (Aurilio et al., 1995; Spliethoff and Hemond, 1996). Beginning in the late 19th century, industrialization contributed to severe contamination of the watershed by a variety of volatile organic compounds and heavy metals, including Cr, Pb, and As (Spliethoff and Hemond, 1996). Important industries were leather tanneries and chemical manufacturing, including sulfuric acid and lead-arsenate pesticide production (Aurilio et al., 1995; Durant et al., 1990).


Figure 1
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  		Fig. 1. Location map indicating the Wells G&H and Industriplex United States Environmental Protection Agency Superfund sites. The tree core described in this paper was sampled at the wetland upland boundary adjacent to Well G. Figure modified from (Aurilio et al., 1994).

 
Prior work in the watershed has reconstructed contaminant histories from lake sediment cores (Rauch and Hemond, 2003; Rauch et al., 2006; Rauch et al., 2004; Spliethoff and Hemond, 1996). These studies indicate that the history of metal contamination extends back to the mid-19th century, when coal combustion by early industry first caused sediment Pb concentrations to rise above geologic background levels. Several decades later, severe metal contamination ensued, resulting from a combination of sulfuric acid production (1888–1929), arsenical pesticide production (1899–1927), and chrome-based hide tanning (~1900–1940). Early to mid-20th century sediments from Upper Mystic Lake, within the Aberjona watershed, have Pb concentrations 30 to 60 times higher than sediments from the mid-19th century (Spliethoff and Hemond, 1996). Local industries were largely phased out by 1931 as chemical producing plants in the watershed were shut down. Sedimentary metal concentrations consequently decreased from peak concentrations but never returned to preindustrial values. Redevelopment of the upper portion of the watershed in the 1950s, 1960s, and 1970s remobilized previously deposited contaminants, resulting in secondary peaks in lake sediment metal concentrations (D. Brabander, unpublished data; Spliethoff and Hemond, 1996).

Lead contaminant sources in the Aberjona watershed can be distinguished on the basis of Pb isotope ratios of regional endmember sources. Contaminant Pb measured in soils from the "arsenic pit" at the Industriplex site (Fig. 1) likely reflects the isotopic composition of Pb used in the production of lead-arsenate pesticide (Fig. 2 ). Pyrite used in the production of sulfuric acid was derived from the isotopically distinctive Iberian Pyrite Belt. Lead contamination in Aberjona watershed sediments (D. Brabander, unpublished data) is largely explained as mixtures of these two sources, with tanneries rarely contributing significantly to Pb pollution.


Figure 2
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  		Fig. 2. Lead isotope ratios of Aberjona red oak core annual rings, soils (this study), and Upper Mystic lake sediment and endmember Pb contamination sources (D. Brabander, unpublished secondary ion mass spectrometry data). Typical error bars for our laser ablation inductively coupled plasma mass spectrometry tree ring measurements are plotted in upper left for reference.

 
An additional source of Pb contamination that must be considered is gasoline emissions. Beginning in 1923, Pb was used as an anti-knock gasoline additive in the USA. Its use increased steadily from the start of our period of record in 1940, peaked around 1970, and dropped abruptly during the late 1970s and 1980s (Weiss et al., 2003). Lead used in gasoline in the USA was derived from several different ore sources; therefore, it is not possible to assign a unique end-member isotopic composition to a gasoline Pb. Tailpipe emissions and urban snow collected in Boston in 1981 (Rabinowitz, 1986) provided some constraints on the isotope ratio of leaded gasoline used in the region (Fig. 2).

Our dendrochemical method can address regional Pb contamination at annual resolution covering the 60-yr history recovered in our sampling. Based on the high levels of trace metal contamination at our site, we assume that natural background Pb is not a significant contributor to our dendrochemical record. Most of what is known about the contamination history of this site comes from historical records and lake sediment archives. Lead recorded dendrochemically may not be quantitatively the same as Pb recorded in lake sediments because the pathways of Pb delivery are distinct. Sediments provide relatively long records that reflect the watershed-scale Pb flux to sediments related to pollution point sources, runoff, and erosion. Dendrochemical records, although generally shorter, offer higher temporal resolution than is typically obtainable from sediments and reflect changes in atmospheric Pb delivery, soil contamination, and mobilization of bioavailable Pb.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 Conclusions
 REFERENCES
 
Sample Collection
Red oak (Quercus rubra) was selected for this study based on results from our previous work (Brabander et al., 1999) and because successful dendrochemical studies using oak trees have been reported in many experimental studies (Brabander et al., 1999; Eklund, 1995; Jonnson et al., 1997; Majumdar et al., 1991). These studies have indicated that ring-porous species tend to minimize lateral transport within xylem tissue. Ring-porous species also experience over 90% of the water uptake in the outermost growth ring, allowing for the detection of high-frequency temporal changes in isotope ratios (Ellmore and Ewers, 1986). The tree sampled for this study is located adjacent to the Aberjona River at the wetland/upland boundary near Municipal Well G (Fig. 1). Sampling was performed with a standard forestry service 5-mm increment borer at breast height. The core was transferred to a polyethylene bag, which was placed in a cooler for transport and later dehydrated by freeze drying at –80°C. After dehydration, the core was sectioned into 2-cm segments using a stainless steel knife. Each segment contained 5 to 7 annual rings and was labeled to ensure chronological continuity. The circular core was sanded perpendicular to the grain of the wood using 60-grit silicon-carbide sandpaper to produce a flat surface approximately 3 mm wide. Core segments were mounted in modeling clay for laser ablation analysis. This method proved superior to tested epoxies, which were found on test cores to infiltrate wood pore spaces and contribute a significant Pb blank.

LA-ICP–MS Analysis and Data Reduction
Samples were analyzed on a VG PlasmaQuad ExCell ICP–MS (VG Elemental, Cheshire, England) (inductively coupled plasma–mass spectrometer) equipped with a 213-nm laser ablation microprobe (Microprobe II; New Wave Instruments, Bozeman, Montana). Each annual ring was analyzed by ablating six 50-s spots with a 300-µm spot size at 100% beam intensity. To minimize potential surface contamination effects, we discarded the first 10 s of data on each spot. Data were collected at masses 206, 207, and 208 using a peak-jumping routine consisting of five 100-sweep runs per ablation spot using five points per peak. Mass 204 produced an insufficient count rate for quantification. Wood Pb concentrations were not quantified because absolute count rates vary as a function of wood Pb concentration and ablation efficiency.

We measured two laser ablation standard reference materials (SRM) to calculate instrumental mass bias and to determine external reproducibility of Pb isotope ratios. NIST 612 (trace elements in glass) was analyzed at the start of each of our three data acquisition sessions. United States Geological Survey (USGS) SRM BCR-2 (basaltic glass) was analyzed once each hour during data acquisition (every five to six samples; 12 analyses total). Each SRM analysis consisted of five 50-s ablation spots with a 50-µm spot size at 50% beam intensity. Additional ICP–MS settings during SRM analysis were identical to analysis of unknowns.

Because mass bias varies as a function of instrument tuning, we used the daily average of measured isotope ratios of BCR-2 to calculate a daily mass bias correction factor for each isotope ratio:

Formula 1A[1a]
and

Formula 1B[1b]
where the subscript A signifies the accepted isotope ratios for BCR-2 (Collerson et al., 2002), and M refers to the measured ratio. Mass bias was observed to be significantly smaller and less variable for 207Pb/206Pb (+0.001 to –0.004) than for 208Pb/206Pb (–0.008 to –0.01). The negative values of mass bias indicate preferential transmission of the lighter isotope (206Pb), in contrast with our experience measuring lighter isotope ratios (e.g., 74Ge/70Ge) on this instrument (Scribner et al., 2006).

Daily values of the mass bias correction factors are then applied to measured isotope ratios of SRM NIST 612 and to unknowns to calculate mass bias corrected ratios:

Formula 2A[2a]
and

Formula 2B[2b]

Soil Samples
Soils were collected in the immediate vicinity of our study tree to evaluate the degree and nature of soil Pb contamination. Soils were sampled at 2- to 5-cm increments from the surface to 60 cm depth, where standing water was encountered. Soils were dried at 80°C and packaged into polyethylene cups with mylar film for determination of Pb and other elemental concentrations by X-ray fluorescence (XRF). XRF analysis was performed using a SPECTRO XEPOS bench top XRF spectrometer, with NIST-2709 as the primary SRM. Concentrations of soil organic matter were determined as "loss on ignition" (Soil and Plant Analysis Council, 2000). One to two grams of soil were heated for 4 h at 105°C, weighed, heated again for 4 h at 400°C, and reweighed to determine mass change due to combustion of organic matter.

Powdered soil samples were digested in a concentrated HF-HCl-HNO3 mixture in Teflon vessels in a Milestone ETHOS Plus (Milestone, Inc., Shelton, CT) microwave digestion system. Digested samples were dried down and then diluted in 3% HNO3 to a solution Pb concentration of approximately 2 ppb for bulk soil Pb isotope ratio determinations by ICP–MS. Data collection and reduction routines were similar to those described previously for LA analyses. Count rates were collected at masses 206, 207, 208 using a peak-jumping routine. USGS SRM BHVO-2 (digested and diluted to 2 ppb Pb) was run after every third unknown as a monitor of instrumental mass bias and as a measure of external reproducibility. Lead blanks were measured (0.015–0.035 ppb) and determined to have no significant effect on measured isotope ratios of unknowns.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
Discussion
 Conclusions
 REFERENCES
 
Establishing analytical precision and accuracy is important in terms of understanding the significance of trends apparent in the dataset and for attributing observed Pb to known contaminant sources. Analytical accuracy can be evaluated based on the mass-bias corrected analyses of NIST 612. The three five-spot corrected analyses averaged 0.9059 ± 0.003 (2 sigma SD) for 207Pb/206Pb and 2.158 ± 0.018 for 208Pb/206Pb. Accepted values for NIST 612 are 207Pb/206Pb = 0.9073 and 208Pb/206Pb = 2.1645, well within the error bounds of our analyses. These two-sigma SDs provide an estimate of external precision of 0.4% for 207Pb/206Pb and 0.8% for 208Pb/206Pb. Two-sigma internal precision is defined as two times the SD of the mean of 25 to 30 measurements (five runs each of five or six spots) of 207Pb/206Pb and 208Pb/206Pb ratios for each analysis of an SRM or an annual growth ring. Two-sigma internal precision was typically 0.3 to 0.5% for 207Pb/206Pb and 208Pb/206Pb. Error estimates are based on the larger of the defined external precision (0.4% for 207Pb/206Pb and 0.8% for 208Pb/206Pb) or the internal precision for the individual analysis (Goldstein et al., 2003).

Pb isotope ratios for annual rings from the Aberjona red oak core range from 0.828 to 0.853 for 207Pb/206Pb and 2.028 to 2.078 for 208Pb/206Pb (Table 1). This range significantly exceeds our uncertainty, allowing us to attribute variations in the core dataset to Pb source variability. The data lie on a linear trend bracketed by suspected Pb contaminant sources (Fig. 2). One interpretation of this trend is that our dataset reflects simple two-component mixing between the two dominant historical industrial Pb contaminant sources, Pb-arsenate pesticide and Iberian pyrite Pb used in sulfuric acid manufacture with no significant contribution from Tannery Pb. However, as we argue below, this explanation may be an oversimplification.


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  		Table 1. Lead isotope ratios in Aberjona Oak core.

 
Temporal changes in Pb isotopes provide important additional information. Given the larger range of measured values and the greater precision of the 207Pb/206Pb ratio vs. the 208Pb/206Pb ratio, we examine temporal trends in terms of changes in 207Pb/206Pb only (Fig. 3 ). Trends in 208Pb/206Pb (not plotted) are exactly parallel but subject to larger uncertainty. The most prominent feature of the record is a pronounced shift in 207Pb/206Pb occurring in the early 1970s from values >0.840 before 1972 to values <0.840 after 1972. This shift is superimposed on high frequency (multi-year to decadal) variability.


Figure 3
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  		Fig. 3. Annual values of 207Pb/206Pb for Aberjona red oak core. Error envelope indicates 2 sigma (95%) confidence range. Important features of this record are a sharp decline in of 207Pb/206Pb around 1972 and statistically significant interannual changes in 207Pb/206Pb. Horizontal lines indicate average 207Pb/206Pb for identified contaminant Pb sources.

 
Precision and accuracy of Pb isotope data for soils run as solutions were similar to laser ablation data described previously. Instrumental mass bias correction factors for the solution run were 0.003 for 207Pb/206Pb and 0.0097 for 208Pb/206Pb based on accepted BHVO-2 values determined by multi-collector inductively coupled plasma mass spectrometry (Li and Nui, 2003). Two-sigma external precision, defined as 2 times the SD of 16 analyses of BHVO-2, was 0.8% for 207Pb/206Pb and 0.5% for 208Pb/206Pb. As a check on accuracy, mass bias corrected Pb isotope measurements of USGS SRMs AGV-1 and BIR-1 agree with accepted values (Li and Nui, 2003) within analytical precision. Measured soil Pb isotope ratios lie along the array defined by historical Pb sources and the tree-ring record (Fig. 2). Shallow (0–5 cm) soils cluster around 207Pb/206Pb = 0.828; 208Pb/206Pb = 2.041. Deeper soils generally have higher 207Pb/206Pb and 208Pb/206Pb ratios but are difficult to distinguish within the limits of our analytical precision. Two soils collected at 20 to 25 cm depth have clearly distinct Pb isotopes, with 207Pb/206Pb averaging 0.843 and 208Pb/206Pb averaging 2.068 (Table 2).


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  		Table 2. Lead isotopes in Wells G&H site soils.

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
Conclusions
 REFERENCES
 
Interannual changes in 207Pb/206Pb frequently exceed experimental uncertainty. Sharp jumps in Pb isotope ratios between individual growth rings support the assertion that our record is minimally homogenized by inter-ring lateral mobility of Pb in red oak. These abrupt changes further suggest that Pb incorporated by the tree was not drawn solely from a large, homogenous Pb pool.

Bulk soil Pb concentrations range from 18 to 295 ppm and are highest in shallow, organic-rich soils (Table 2). Organic-poor soils from 5 to 20 cm and deeper than 30 cm have Pb concentrations less than 30 ppm. Lead isotope ratios in these soils probably reflect predominantly background geologic Pb sources. The most contaminated samples (48–297 ppm Pb), all organic-rich soils from the top 5 cm, have Pb isotope ratios that plot at the low 207Pb/206Pb, 208Pb/206Pb end of the tree ring array, with isotope ratios similar to or slightly lower than the youngest (post-1970) tree rings. The pattern of a surface Pb concentration maximum and sharp decrease with depth is consistent with an atmospheric fallout source, with accumulation of Pb on shallow soil organic matter (Kaste et al., 2006). Soils from 20 to 25 cm, the location of a secondary peak in soil organic matter and Pb concentrations (Table 2; 47–74 ppm), have quite distinct 207Pb/206Pb and 208Pb/206Pb, plotting with the population of tree rings older than 1970. The secondary peak at 20 to 25 cm depth may represent an older buried A-horizon or sediment transported from the adjacent wetland during a past flood event.

The simplest interpretation of our dendrochemical record would call on a two-component mixing model based on local historical Pb contaminant sources. The abrupt decrease in 207Pb/206Pb (Fig. 3) could be attributed to an increased pesticide-Pb (or decreased Iberian-Pb) contribution beginning around 1970. This model, although appealing in its simplicity, is difficult to reconcile with the land use history in the Aberjona watershed and data from lake sediment records. Production of sulfuric acid and Pb-arsenate pesticide ended well before the start of our dendrochemical record, so although both contaminants may have been present in large quantities, it is difficult to envision how the relative proportions of these as received by our tree would vary on an interannual basis or change abruptly in the 1970s. A change in land use, such as excavation of a pre-existing Pb contaminated site within the watershed, might cause such a shift, but this interpretation is not supported by lake sediment Pb archives. Contaminant Pb delivered to Upper Mystic Lake sediments dating to around 1970 has high 207Pb/206Pb and 208Pb/206Pb ratios, similar to Iberian pyrites (D. Brabander, unpublished data). Therefore, if remobilization of old industrial Pb were responsible for the shift in 207Pb/206Pb in our record, we would expect 207Pb/206Pb to shift in the opposite direction from that observed.

Given the combination of a pronounced shift in the early 1970s and significant inter-annual variability in 207Pb/206Pb (Fig. 3), our preferred interpretation is that our record reflects a complex mixture of the local industrial Pb sources described previously with deposition of gasoline-sourced Pb. We have no direct data on the isotope ratio of local gasoline-derived Pb for much of our period of record. Leaded gasoline produced in the USA relied increasingly on isotopically anomalous (low 207Pb/206Pb {approx} 0.769) Mississippi-Valley-Type Pb ores through the 1970s and 1980s. Data from California (Hurst, 2000) show a decrease in 207Pb/206Pb of leaded gasoline to below 0.840 in the early 1970s, decreasing further to below 0.820 as leaded gasoline production declined in the 1980s. Direct constraints are available on the isotopic ratio of gasoline Pb used in Boston in the early 1980s. Tailpipe emissions and urban snow collected in Boston in 1981 had 207Pb/206Pb between 0.826 and 0.831 (Rabinowitz, 1986); these values were virtually identical to the surface soil samples (Fig. 2) and consistent with the shift toward lower 207Pb/206Pb in the later part of our record (Fig. 3).

Pre-1970 tree rings (and 20–25 cm soil) Pb isotope ratios might record the Pb isotope ratio of gasoline-derived Pb (with higher 207Pb/206Pb as in California) during the early part of the leaded gasoline era. Alternatively, it is possible that before the peak in historical leaded gasoline consumption (i.e., before 1970), contaminant Pb was at least partly derived from the abundant local sources. Tree ring Pb isotope ratios in this interval (ca. 1940–1970) are also consistent with a mixture of pesticide and Iberian Pb producing a 207Pb/206Pb that averages 0.846, roughly 70% Iberian-derived Pb based on isotope mass balance. It is not possible to precisely constrain the relative importance of all of these sources over the entire 60-yr dendrochemical record, but gasoline-derived Pb, which seems to dominate the shallow soil Pb pool at present, also seems to have dominated bioavailable Pb at our site since the peak of the leaded gasoline era in the early 1970s.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
REFERENCES
 
Our data confirm that LA-ICP–MS can be an efficient analytical approach to measuring Pb isotopes in tree cores, producing annually resolved contamination histories. Analytical precision is adequate (~0.4% in 207Pb/206Pb) to resolve inter-annual changes and long-term trends in Pb sources in this study. Inter-annual variations in Pb isotopes suggest minimal post-depositional mobility of Pb in red oak. These variations also indicate an important role for incorporation of atmospheric Pb into trees, although the uptake mechanism is uncertain. Our study suggests that bioavailable Pb in the Aberjona watershed is more complicated than a simple two-component mixture of historical industrial Pb sources. Instead, these remnant industrial sources contribute a background Pb contamination that was overwhelmed by leaded gasoline emissions during the peak of USA leaded gasoline consumption.


   ACKNOWLEDGMENTS
 
Funding for this project was provided by Boston University's Undergraduate Research Opportunities Program (UROP). We thank Terry Plank (BU) for advice and assistance with LA-ICP-MS analyses. Nathan Phillips (BU) and Richard W. Hurst (Cal State U., Los Angeles) provided help with interpretations. The paper was greatly improved thanks to reviews by G. Filippelli and two anonymous reviewers.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 





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