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Isotopic composition of Pb in peat and porewaters from three contrasting ombrotrophic bogs in Finland: evidence of chemical diagenesis in response to acidification William Shotyk, Nicole Rausch, Tiina M. Nieminen, Liisa L. Ukonmaanaho, and Michael Krachler Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01076 • Publication Date (Web): 18 Aug 2016 Downloaded from http://pubs.acs.org on August 22, 2016
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Isotopic composition of Pb in peat and porewaters from three contrasting ombrotrophic bogs in Finland: evidence of chemical diagenesis in response to acidification
William Shotyk,*1 Nicole Rausch,§2 Tiina, M. Nieminen,3 Liisa Ukonmaanaho,3 Michael Krachler4
1 Department of Renewable Resources, University of Alberta, Edmonton, AB, T6G 2H1 CANADA 2 University of Heidelberg, Institute of Environmental Geochemistry, Im Neuenheimer Feld 236, D-69120 Heidelberg, GERMANY 3 Natural Resources Institute Finland (Luke), Box 18, FI-01301 Vantaa, FINLAND 4 European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, GERMANY
* Corresponding Author: Bocock Chair for Agriculture and the Environment Department of Renewable Resources, University of Alberta, 348B South Academic Building, Edmonton, AB CANADA T6G 2H1 Tel.: 780-492-7155 Fax: 780-492-4323
[email protected] §
Current address: European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, GERMANY
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ABSTRACT The isotopic composition of Pb was determined in Finnish peat bogs and their
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porewaters from Harjavalta (HAR, near a Cu-Ni smelter), Outokumpu (OUT, near a Cu-
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Ni mine) and Hietajärvi (HIJ, a background site). At HIJ and OUT, the porewaters
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yielded similar concentrations (0.1-0.7 µg/l) and isotopic composition (206Pb/207Pb =
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1.154-1.164). In contrast, the peat profile from HAR yielded greater concentrations of
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Pb in the porewaters (average 2.4 µg/l), and the Pb is less radiogenic (206Pb/207Pb =
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1.121-1.149). Acidification of the bog surface waters to pH 3.5 by SO2 emitted from
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smelting (compared to pH 4.0 at the control site) apparently promotes the dissolution of
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Pb-bearing aerosols as well as desorption of metals from the surfaces of these particles
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and from the peat matrix. Despite this, the chronology of anthropogenic, atmospheric
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deposition for the past millenium recorded by the isotopic composition of Pb in all three
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peat bogs is remarkably similar. While the immobility of Pb in the peat cores may
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appear inconsistent with the elevated porewater Pb concentrations, Pb concentrations
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in the aqueous phase never amount to more than 0.01 % of the total Pb at any given
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depth so that the potential for migration remains small. The low rates of vertical water
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movement in bogs generally combined with the size of the metal-containing particles in
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solution may be additional factors limiting Pb mobilization.
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INTRODUCTION
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Ombrotrophic (i.e rain-fed) peat bogs are excellent archives of atmospheric Pb1,2
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providing the first complete, long-term record (15,000 years) of atmospheric Pb
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deposition in Europe where anthropogenic inputs have dominated continuously for more
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than 3,000 years.3,4 Detailed histories of atmospheric Pb contamination have been
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reconstructed using peat cores from bogs across Europe by several groups working
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independently.5-13 With appropriate methods for sample collection, handling,
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preparation, and age dating14 peat cores can provide extremely detailed reconstructions
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of atmospheric Pb pollution histories.15 For example, precisely dated peat cores from
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Denmark and the Faroe Islands showed that atmospheric Pb contamination began its
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decline 25 years before the introduction of unleaded gasoline.16,17 Despite the number of published papers showing that peat cores from
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ombrotrophic bogs are faithful archives of atmospheric Pb deposition, there remains a
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long-standing concern regarding the possible importance of post-depositional migration
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of trace metals, including Pb.18-26 Regardless, there is very little empirical evidence to
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help understand the physical and chemical processes responsible for metal mobility.3,27-
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layers, the paper by Stepanova et al.,24 concludes by saying that Pb is being “leached
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from the peat column by downward penetrating fluids” (p.67) but provides no direct
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evidence of this. For trace metals to become mobilized subsequent to their deposition
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from the atmosphere, they must first be released from the solid phase (peat) to the
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aqueous phase (porewaters), for example by detachment and dissolution of the primary
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(host) particle, desorption of complexes from the peat surface, or by ion exchange.
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Porewater analyses provide a very sensitive indicator of the potential for metal
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mobilization in sedimentary environments, but remarkably little work has been done on
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porewaters from bogs, especially in respect to trace metals.33-35 In their study of peat
For example, based on elevated Pb/Al ratios in surface compared to deeper peat
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cores from three Finnish bogs, accurately dated using both 210Pb as well as the
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atmospheric bomb pulse curve of 14C, Rausch et.,al36 found that the peat bog record of
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atmospheric Cu deposition near Outokumpu (OUT, a Cu-Ni mine) was in good
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agreement with the known emission history and available monitoring data; this was not
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true however of Ni, Zn or Cd deposition. A careful study of the corresponding
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porewaters revealed elevated Ni and Cd concentrations in the aqueous phase from
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OUT compared to Hietajärvi (HIJ, a background location), providing direct evidence of
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partial dissolution of the metal-bearing particles deposited to the surface of the bog. At
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Harjavalta (HAR, near a Cu-Ni smelter), much higher concentrations were found in the
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porewaters for all of the metals: up to 2x greater for Zn, 10x for Cd, 20x for Co, and
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100x for Ni and Cu, relative to HIJ.37 The elevated metal concentrations in the pore
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fluids are clear evidence of mobilization from the solid phase. Solubility calculations help
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to explain the observed differences between the peat profile at OUT versus HAR: in the
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porewaters of the former, metal sulphide particles emitted from the mine as dusts are
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thermodynamically stable and therefore preserved, whereas in the latter case, metal
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oxide particles released to the atmosphere from smelting are thermodynamically
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unstable and dissolve; the very low pH of the porewaters at HAR (pH < 3.5) compared
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to the other sites (pH 4) tends to further promote dissolution.
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The dramatic differences in metal burdens between these three contrasting peat
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bogs, combined with the accuracy of the age dating and availability of porewater data
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renders these sites an excellent opportunity to examine their Pb records. Specifically,
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we wish to understand i) the extent to which metal smelting and refining may have
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impacted the chronology and intensity of atmospheric Pb deposition at the HAR site,
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and ii) the effect of surface water acidification on the potential to mobilize Pb from the
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solid to the aqueous phase. To do this, we present new data for the isotopic
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composition of Pb (206Pb/207Pb) for these peat profiles and their corresponding
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porewaters.
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The reliable determination of trace metal concentrations in natural freshwaters,
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however, poses two formidable challenges: 1) the concentrations of the elements of
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interest tend to be extremely low, requiring the most sensitive methods of analysis, and
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2) the risks posed by contamination during sample collection, preparation, and handling
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are extremely high.38-41 Here, we employ the protocols and procedures developed at the
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University of Heidelberg, Germany for the determination of Pb concentrations and
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stable Pb isotopes in peat42 and Arctic ice cores;43,44 these methods were also applied
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successfully to the determination of Pb and stable Pb isotopes in surface and ground
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waters from central Ontario.45,46
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MATERIALS AND METHODS
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Study sites and hydrological conditions
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Peat cores were collected from three ombrotrophic bogs in Finland using a
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Wardenaar-style peat monolith sampler:47 Hietajärvi (HIJ) is an ombrotrophic bog
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located in the Patvinsuo National Park in Eastern Finland. As there are no local sources
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of metal pollution, the main source of metal contaminants is long-term range transport.
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Outokumpu (OUT) is an ombrotrophic bog in the vicinity of a former Cu and Ni mining
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complex (mining operation from 1910 to the1980's; minor smelting activities from 1913
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to 1929), 8 km SW of the town Outokumpu in Eastern Finland. The Harjavalta peat bog
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(HAR) is 6 km NE to the city Harjavalta, where a Cu smelter has been operating since
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1945 and a Ni smelter since 1959. Further details on the sites and hydrological
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conditions are reported elsewhere35 and included in Table S1 (Supporting Information).
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Peat sampling, handling, preparation and age dating
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Peat cores were collected in September 2001. Detailed descriptions of sample
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handling, preparation, and age dating (210Pb and 14C) are presented elsewhere.35 The
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age dates obtained for the past 50 years using 210Pb were in excellent agreement with
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the age dates obtained using the calibrated 14C atmospheric bomb pulse curve.
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Porewater sampling
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In August 2003, bog porewater samples were collected in 5 cm-intervals up to a
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depth of 60 cm using a syringe-type Plexiglas sampler with polypropylene sample
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bottles. For trace metal analyses, three subsamples were taken at each depth using
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each time a new point of entry on the bog surface. Each subsample was filtered
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immediately after collection into a new sample bottle (0.45 µm GD/XP
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polyvinylidendifluoride syringe filter, Whatman, Brentford, United Kingdom), and stored
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in the dark at 6 to 13°C until arrival at the laboratory (maximum 9 days). Field blanks
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were taken at each site to account for potential sample contamination during the on-site
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filtration of the samples. The complete sampling protocol and quality control procedures
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are described elsewhere.36
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Separate porewater samples were collected for the determination of pH using a pH electrode (unfiltered samples), for anions (chloride and sulphate, without
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acidification), and for dissolved organic carbon (DOC, filtered into glass vessels
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containing diluted sulphuric acid), and these results were presented earlier.36
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Porewater sample handling and preparation
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Upon arrival at the laboratory, filtered porewater samples collected for Pb
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analyses were acidified inside a class 100 laminar flow bench using high purity HNO3
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(final concentration: 1% v/v HNO3) and immediately frozen at -18°C until Pb analyses.
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The humic acids (HA) which precipitated after acidification were separated by
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centrifugation (10 min at 3000 rpm), dissolved with 25 µl concentrated high purity HNO3,
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and diluted with 2.5 ml high purity water to obtain a final concentration of 1% v/v HNO3.
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At HIJ and OUT, the Pb content in the HA fraction was < LOD; at HAR, Pb in the HA
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fraction accounted for < 3% of total Pb. The supernatant was used for determining the
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Pb concentrations in the porewater (average of three subsamples per depth). Details
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are described elsewhere. 36
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Pb analysis
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Lead concentrations in the solid peat samples were determined using energy
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dispersive X-ray fluorescence spectroscopy52 whereas an ICP-sector field MS (ICP-
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SMS) was employed for porewater analysis (Element 2, Thermo Fisher Scientific,
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Bremen, Germany). The ICP-SMS was operated under clean room conditions to allow
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the reliable ultra-trace Pb concentration and isotopic analysis as described in detail
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earlier.37 High purity water from a Milli-Q-Element system (Millipore, Milford,
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Massachusetts, USA) and double sub-boiled nitric acid - obtained by purifying nitric acid
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(65%, analytical-reagent grade) using a quartz unit for sub-boiling of acids (MLS GmbH,
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Leutkirch, Germany) – were used throughout. Adequate clean room procedures, which
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have been developed for ultra-trace elemental analysis of polar ice,53 were adopted
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throughout the entire sampling and measurement procedure. Detailed information on
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the employed analytical protocol has been provided in our previous work.37,53
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Pb isotope ratios in peat digests and porewaters
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A procedure was developed to allow accurate and precise measurements of Pb
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isotope ratios (206Pb/207Pb) in acid digests of peat, and peat bog porewaters, directly
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and rapidly using ICP-SMS.42,43 The precision of 206Pb/207Pb ratios ranged from
