Article pubs.acs.org/est
Pyrogenic Inputs of Anthropogenic Pb and Hg to Sediments of the Hood Canal, Washington, in the 20th Century: Source Evidence from Stable Pb Isotopes and PAH Signatures Patrick Louchouarn,*,†,‡ Li-Jung Kuo,§ Jill M. Brandenberger,§ Franco Marcantonio,∥ Charity Garland,§ Gary A. Gill,§ and Valerie Cullinan§ †
Department of Marine Science, Texas A&M University at Galveston, Galveston, Texas 77553, United States Department of Oceanography, Texas A&M University, College Station, Texas 77843, United States § Marine Science Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 98382, United States ∥ Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843, United States ‡
S Supporting Information *
ABSTRACT: Combustion-derived PAHs and stable Pb isotopic signatures (206Pb/207Pb) in sedimentary records assisted in reconstructing the sources of atmospheric inputs of anthropogenic Pb and Hg to the Hood Canal, Washington. The sediment-focusing corrected peak fluxes of total Pb and Hg (1960−70s) demonstrate that the watershed of Hood Canal has received greater atmospheric inputs of these metals than its mostly rural land use would predict. The tight relationships between the Pb, Hg, and organic markers in the cores indicate that these metals are derived from industrial combustion emissions. Multiple lines of evidence point to the Asarco smelter, located in the Main Basin of Puget Sound, as the major emission source of these metals to the watershed of the Hood Canal. The evidence includes (1) similar PAH isomer ratios in sediment cores from the two basins, (2) the correlations between Pb, Hg, and Cu in sediments and previously studied environmental samples including particulate matter emitted from the Asarco smelter’s main stack at the peak of production, and (3) Pb isotope ratios. The natural rate of recovery in Hood Canal since the 1970s, back to preindustrial metal concentrations, was linear and contrasts with recovery rates reported for the Main Basin which slowed post late 1980s.
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INTRODUCTION Combustion processes, whether natural (wildfires) or anthropogenic (industrial/urban, agricultural, and vehicular) are major sources of particulate matter, black carbon, volatile organic carbon, and metals to the atmosphere and aquatic systems.1−9 Evaluating the influence of these processes on the long-range redistribution of contaminants has often relied on complex approaches in environmental forensics including state-of-the-art analytical methods (trace level organic/inorganic analyses as well as molecular and isotopic signatures of analytes) and using natural “sentinel” archives (ice and sediment cores, mosses and lichens). Sediment cores, in particular, have often been used to reconstruct the historical impacts of various anthropogenic activities in watersheds influenced by various degrees of land use development.4,6,10−13 However, there are complications in using sediment cores for such reconstructions ranging from the influence of postdepositional transformations to the selected forensic indicators, uncertainties in geochronological dating, intersite variabilities, bioturbation, and sediment focusing.14 One approach to address, if not resolve, such difficulties is to © 2012 American Chemical Society
seek independent confirmation across multiple and various indicators (i.e., different classes of material that react differently in the environment) and sampling sites.4,6,13 The objective of this study was to use a multisite, multiindicator approach to estimate the input history and sources of lead (Pb) and mercury (Hg) to the Hood Canal, a sub-basin of the Puget Sound west of the metropolitan area of Seattle/ Tacoma. Hood Canal and Puget Sound Main Basin have been studied extensively in the recent past in the reconstructions of historical shifts in heavy metals and organic matter inputs, water column productivity, diagenetic indicators of deep water oxygen levels, and combustion emissions and sources in the Puget Sound region.4,6,13 Here we present new data from sediment cores for Hood Canal on Hg coupled with total Pb and its stable isotopic signatures relative to organic molecular Received: Revised: Accepted: Published: 5772
January 23, 2012 April 16, 2012 April 30, 2012 April 30, 2012 dx.doi.org/10.1021/es300269t | Environ. Sci. Technol. 2012, 46, 5772−5781
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with the relative error for PACS-2 (n = 17) of ≤15% and variability for duplicates (n = 17) ranged 0.2−18% difference. The detailed analytical procedure for PAHs extraction and quantification in these cores has been described previously by Kuo et al.6 Briefly, dry sediment samples were spiked with deuterated PAH surrogates (d10-phenanthrene and d10-chrysene) and extracted with a Dionex ASE 200 using dichloromethane. The extracts were purified using silica gel/alumina column chromatography. Internal standards (d10-fluorene and d12-benzo[a]pyrene) were added to the final concentrated extracts prior to gas chromatography−mass spectrometry (GCMS) analysis. Average recoveries of d10-phenanthrene and d10chrysene were 65 ± 9% and 79 ± 16%, respectively. The variability for duplicates was 0.90) in pre-1900 sediments confirm that prior to the major industrialization/urbanization of the Puget Sound region most pyrogenic inputs to the Hood Canal were derived from trace inputs of biomass combustion.6 The decreases in Ret/(Ret + Chry) ratios, starting in the early 20th century, coincide with the marked increases in both heavy metals and pyrogenic PAHs confirming a shift toward industrial combustion sources. One potential source of pyrogenic byproduct (e.g., black carbon and PAHs) and coemitted trace elements (e.g., Pb and Hg) to the environment is coal combustion, and strong relationships have been reported between these in environ-
mental archives tracing historical contaminant inputs from coal combustion.1,3,9,30,31 In the urban system of Seattle/Tacoma, Washington, the early 20th century reliance on coal combustion resulted in substantial emissions of pyrogenic PAHs to the environment, which is recorded in PAHs peaks in sediments of the Main Basin.6,16,17,26 However, starting in the 1930s, a decreased dependence on coal for domestic heating and a switch to other sources of energy6 resulted in a sharp decline in pyrogenic PAHs accumulation in Main Basin sediments deposited after the early 1940s.6,16,17,26 A predominant coal combustion source for Pb, Hg, and PAHs to the Hood Canal cores is thus unlikely since their peaks occur in a period when coal use had substantially declined in the region.6,26 Although the rise and fall of Pb concentrations in Hood Canal cores coincide with the historical use of tetraethyl lead (TEL) in the United States,32,33 the strong correlation between Pb and Hg argues against a gasoline source, because gasoline combustion is only a minor source of Hg to the atmosphere.34 Instead, since Hg is coemitted from industrial combustion processes (e.g., metal smelting),35 the strong co-occurrence of Hg with Pb in the sediments points toward a common industrial source. A major smelter was operated in Tacoma from the late 1800s to the mid-1980s (Asarco smelter), and was responsible for substantial emissions of Pb, Hg, Cu, and other trace metals to the Main Basin of Puget Sound.4,36 Environmental studies performed during the 1970s document both a substantial decrease in productivity since the late 1960s (∼50% decrease in ore processed from 1969 to 1980)37 and curtailment efforts that decreased particulate emissions from its main stack by ∼80% from 1970 until the early 1980s.37,38 This chronology of decreased smelter emissions since the late 1960s is consistent with the concentration trends in the Hood Canal cores. Similarly to the Hood Canal cores, excess Hg and excess Cu correlate positively with excess Pb in a sediment core near Tacoma (PS-1), which shows a strong record of metal inputs from the Asarco smelter during the 20th century4 (SI-Figure 6). The slopes of these relationships are not statistically different (homogeneity of slopes test, p > 0.05), suggesting that the contamination influence of the Asarco smelter extended beyond the immediate Main Basin and into the watershed of Hood Canal. The slopes of the Cu to Pb relationships calculated from smelter-impacted soil samples and pine needles (0.49− 5776
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Figure 2. Pb concentrations as well as modeled and measured
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Pb/207Pb values in Hood Canal cores HC-3 (left) and HC-5 (right).
signatures in deep sediments (1.219 ± 0.002 and 1.216 ± 0.0004 for HC-5 and HC-3, respectively), and a less-radiogenic anthropogenic input source, which peaked in the late 1960s− early 1970s. We calculated the signature of the anthropogenic endmember assuming that all excess Pb at peak input is derived from an anthropogenic source and using a binary mixing equation. The resulting estimated anthropogenic 206Pb/207Pb signatures are nearly identical in both cores (1.161 ± 0.003 and 1.161 ± 0.002 for HC-3 and HC-5, respectively), and much lower than average coal signatures in the U.S. (1.207 ± 0.017)44 and aerosols collected in Seattle in 1980 (1.226 ± 0.001),45 further confirming that coal combustion or gasoline lead were not significant sources of these metals to the Hood Canal watershed. We modeled the total sedimentary 206Pb/207Pb signatures using the estimated endmember signature for anthropogenic Pb (1.161) and the average 206Pb/207Pb value in deep sediments of each core (1.219 and 1.216 for HC-5 and HC-3, respectively) as the natural background endmember. The modeled and measured 206Pb/207Pb values (Figure 2) are strongly correlated in both cores (SI-Figure 8), confirming a predominant binary mixture between a radiogenic and less-radiogenic source. In HC-3 core, the agreement is very high with the exception of the surface interval, which predicts a more radiogenic signature than the one actually recorded. The calculated value for anthropogenic 206Pb/207Pb in surface sediments (1.145) is however very similar to aerosols values measured recently (1998−1999) in the city of Victoria in the southern part of Vancouver Island (1.145 ± 0.004),42 suggesting a potential new regional source. In HC-5, the agreement between measured and estimated 206Pb/207Pb ratio is not as tight, though the relationship still shows some agreement between both series (Figure 2). The most significant departure occurs in the first half of the 20th century, a period that also shows some discrepancy in PAH concentrations between both HC-3 and HC-5. The earlier peak in PAHs in HC-5 is consistent with prior studies,26,27 which point to local sources of combustionderived PAH (e.g., large pulp and paper mills and navy base in
0.59),36,39 collected in Vashon-Laury Island, ∼2−8 miles north of the smelter, fall within close range of those of the three sediment cores (0.62−0.68; SI-Figure 7). Similarly, Hg/Pb ratios in particulate matter emitted from the main stack between 1971 and 1973 (5.91 ± 0.06 mg/g)40 are comparable to the range of the slopes of the Hg vs Pb relationships in the three cores (5.22−5.68 mg/g; SI-Figure 6). Such comparable elemental ratios confirm a similar compositional signature of particulate matter emitted from the smelter. More importantly, the predominance of fine particulate matter in main stack emissions (50−80% by weight in 0.01) break in the linearity (changes of recovery rates) reported in the late 1980s in the Main Basin cores,4 the decreases of excess Pb and Hg in Hood Canal cores since peak inputs remain linear in Hood Canal (SI- Figure 9). The lack of significant slow down in recovery rates in Hood Canal in the last decades is consistent with the steady population and rural character in its subwatersheds during similar periods. Recovery projections, assuming that no major increases in watershed development or no new inputs occur in the coming decades, give estimated recovery ranges for reaching preanthropogenic conditions of 2040 ± 15 and 2054 ± 17 for Pb and Hg, respectively, in HC-3 and of 2030 ± 7 and 2033 ± 10 for Pb and Hg, respectively in HC-5 (SI-Figure 8). This is probably a best scenario estimate, particularly for Hg, considering the anticipated large increases in coal-generated energy production in Asia35 and the transport of combustion-derived particulate matter across the Pacific into North America.51 Decreasing PAH concentrations since time of peak inputs also point to a recovery trend during the 1960− 1980s, but in the last decades or so PAH concentrations in Hood Canal cores varied without significant decrease (HC-5) or have significantly increased (HC-3). These latter trends are entirely consistent with similar trends reported in recent sedimentary deposits in the Main Basin4,6 and an urban lake in Seattle.11,12 They point to the potential impact of increased vehicular traffic and total number of miles traveled on a net increased emissions of combustion byproduct.2,4,6,11,12 More studies are needed to confirm if the observed shifts in PAH concentrations in recent sediments of south Hood Canal are derived predominantly from new emission rather than the remobilization of legacy contaminants and an increase in inputs of nonpoint source contaminants (e.g., stormwater).
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and time since 1960 in Hood Canal cores (SI-Figure 9); and (3) a table with all raw data (SI-Table 1) and one with PAH diagnostic ratios (SI-Table 2). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (409) 740-4710; fax: (409) 740-4787; e-mail: loup@ tamug.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS P.L. would like to dedicate this paper to the memory of the late Dr. Rebecca Dickhut (Virginia Institute of Marine Science), whose seminal work on source reconstructions of PAHs in the environment had a strong influence on this study. This research was funded in part by the National Oceanic and Atmospheric Administration Coastal Hypoxia Research Program (grant NA05NOS4781203) and the U.S. Navy. We thank Dr. Wes Highfield (Department of Marine Sciences, Texas A&M University at Galveston) for his generous help with statistical tests and the map of the region. We also thank Danielle Aguirre, Lisa McDonald, Drs. Terry Wade, José Sericano, and Gilvan Yogui (GERG, Texas A&M University), and Dr. KeyYoung Choe (PNNL Marine Science Laboratory) for their technical support with chemical analyses. F.M. thanks Ken and Jane Williams for their generous support of the Radiogenic Geochemistry Laboratory at Texas A&M University. Finally, this manuscript benefited from the constructive reviews of three anonymous reviewers, and the assistance from Mr. Kevin Rochlin (EPA), Mrs. Karen Pickett, Mr. Greg Glass, and Mrs. Marian Abbett who provided valuable historical documents recording the operation and pollution abatements measures of the Asarco smelter in the early 1970s.
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ASSOCIATED CONTENT
S Supporting Information *
(1) Maps of the region of study and location of sediment cores (SI-Figure 1) and of loss of vegetation in Hood Canal subwatersheds from 1975 to 2000 (SI-Figure 4); (2) relationships between 206Pb/207Pb and 206Pb/208Pb signatures in Hood Canal core (SI-Figure 2); excess Hg and excess Pb in the two Hood Canal cores (SI-Figure 3), between retene/ (retene + chrysene) ratios and Hg, Pb, and PAHs (SI-Figure 5), between 206Pb/207Pb signatures and 1/Pb and 1/Hg (SI-Figure 7), between measured and predicted 206Pb/207Pb signatures (SI-Figure 8), between excess Hg, Cu, and Pb in Hood Canal and Main Basin cores (SI-Figure 6), between excess Pb−Hg 5779
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