Sphagnum Mosses from 21 Ombrotrophic Bogs in the Athabasca

Article pubs.acs.org/est

Sphagnum Mosses from 21 Ombrotrophic Bogs in the Athabasca Bituminous Sands Region Show No Significant Atmospheric Contamination of “Heavy Metals” William Shotyk,*,† Rene Belland,† John Duke,‡ Heike Kempter,§ Michael Krachler,∥ Tommy Noernberg,† Rick Pelletier,† Melanie A. Vile,⊥ Kelman Wieder,○ Claudio Zaccone,†,# and Shuangquan Zhang†,¶ †

Department of Renewable Resources, University of Alberta, 348B South Academic Building, Edmonton, Alberta T6G 2H1, Canada SLOWPOKE Nuclear Reactor Facility, University of Alberta, Edmonton, Alberta T6G 2N8, Canada § CEZ Curt Engelhorn Centre for Archaeometry, D-68159 Mannheim, Germany ∥ European Commission Joint Research Centre, Institute for Transuranium Elements, Post Office Box 2340, D-76125 Karlsruhe, Germany ⊥ Department of Geography & The Environment and ○Department of Biology, Villanova University, Villanova, Pennsylvania 19085, United States # Department of the Sciences of Agriculture, Food and Environment, University of Foggia, 71100 Foggia, Italy ‡

S Supporting Information *

ABSTRACT: Sphagnum moss was collected from 21 ombrotrophic (rain-fed) peat bogs surrounding open pit mines and upgrading facilities of Athabasca bituminous sands in Alberta (AB). In comparison to contemporary Sphagnum moss from four bogs in rural locations of southern Germany (DE), the AB mosses yielded lower concentrations of Ag, Cd, Ni, Pb, Sb, and Tl, similar concentrations of Mo, but greater concentrations of Ba, Th, and V. Except for V, in comparison to the “cleanest”, ancient peat samples ever tested from the northern hemisphere (ca. 6000−9000 years old), the concentrations of each of these metals in the AB mosses are within a factor of 3 of “natural, background” values. The concentrations of “heavy metals” in the mosses, however, are proportional to the concentration of Th (a conservative, lithophile element) and, therefore, contributed to the plants primarily in the form of mineral dust particles. Vanadium, the single most abundant trace metal in bitumen, is the only anomaly: in the AB mosses, V exceeds that of ancient peat by a factor of 6; it is therefore enriched in the mosses, relative to Th, by a factor of 2. In comparison to the surface layer of peat cores collected in recent years from across Canada, from British Columbia to New Brunswick, the Pb concentrations in the mosses from AB are far lower.

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INTRODUCTION

On the basis of a recent survey of heavy metals using snowpack sampling, it was claimed that open pit mining and upgrading of the Athabasca bituminous sands (ABS) in northeastern Alberta (AB) are also a significant source of atmospheric Pb, along with many other chalcophile elements, such as Ag, Cd, Sb, and Tl.5 To test the hypothesis that mining and upgrading are a significant source of atmospheric Pb and other heavy metals, Sphagnum moss was collected at three sites from each of 21 bogs in the vicinity of the ABS (Figure 1 and Table S1 of the Supporting Information) and total metal concentrations were measured. Sphagnum fuscum is the dominant hummock-forming moss in peat bogs, and as such, it receives metals exclusively from the air.6 This species is found

Human activities have had a greater impact on the geochemical cycle of Pb than that of any other element.1 A peat core from a Swiss bog2 and an ice core from the Arctic of Canada3 reveal unmistakable atmospheric Pb contamination beginning more than 3000 years ago, originating from Pb smelting in the Iberian Peninsula. Following periods of intense Pb emissions from the Roman Empire, during the Medieval Period of central Europe, the Industrial Revolution, and as a result of the introduction of leaded gasoline, atmospheric Pb emissions finally began to decline with the introduction of unleaded gasoline.2,3 While Pb deposition has certainly decreased during recent decades, the isotopic composition of Pb in the surface layers of an ombrotrophic bog in southern Ontario (ONT) showed that anthropogenic emissions continue to dominate, with smelting of metallic ores in northern ONT and Quebec (QUE) a significant source.4 © 2014 American Chemical Society

Received: Revised: Accepted: Published: 12603

August 2, 2014 September 20, 2014 September 26, 2014 September 26, 2014 dx.doi.org/10.1021/es503751v | Environ. Sci. Technol. 2014, 48, 12603−12611

Environmental Science & Technology

Article

Figure 1. Location of samples collected in relation to open pit mining and upgrading of ABS. Red circles represent mosses collected from the ABS region, while the samples collected from the Anzac bog (ANZ) are represented by the yellow circle. Gray shaded areas show the approximate extent of the open pit mines, with the black triangles showing the location of the upgraders (Syncrude, Suncorn, and CNRL). The exact sampling location of the mosses from the ABS region collected on July 20 and 21, 2013, by Melanie Vile are listed in Table S1 of the Supporting Information. In addition to these samples, S. fuscum was also collected by William Shotyk on August 7, 2013, from the JPH-4 bog, which is 12 km from the midpoint between the Syncrude and Suncor upgraders (see Table S1 of the Supporting Information ). S. fuscum was also collected by William Shotyk on August 6, 2013, from the Anzac bog, which is 69 km southeast of that midpoint (see Table S1 of the Supporting Information). The description of the JPH-4 and Anzac bogs and the reason for selecting this midpoint for reference are given elsewhere.45 In respect to the current study, the moss samples from JPH-4 (n = 3) are considered together with those collected by Melanie Vile (n = 60). However, the moss samples from the Anzac bog (n = 3) are considered separately, because of their distance from the mines and upgraders. In this respect, the samples from Anzac represent a tentative, contemporary, regional “background” signature until moss from other bogs further from the ABS can be collected.

the chamber was depressurized when the interior temperature was below 80 °C. A total of 12 mL of 18 MΩ cm water was added to the digestion vessels. For quality control purposes, the following certified standard reference materials were also dissolved and measured, along with each batch of samples: NIST 1575, NIST 1575a, NIST 1547, NIST 1515, BCR No. 62, BCR CRM 281, and IAEA V10, and the results are given in Table S2 of the Supporting Information. An In internal standard (Spex CertiPrep) was added to each of the sample solutions. All of the elements were measured using the iCAP Qc’s kinetic energy discrimination mode, with a 0.01 s dwell time and 20 sweeps: the element concentrations are the averages of the three runs during data acquisition. A detailed description of sample preparation, metal determination, and quality control as well as mapping is reported in the Supporting Information.

in bogs across North America and Europe and has been used for many decades, particularly in Europe, for monitoring atmospheric heavy metal deposition.7 A recent study of Sphagnum moss from four bogs in two regions of southern Germany (DE) showed that accumulation rates obtained from Pb analysis of Sphagnum were in good agreement with direct deposition measurements for the same regions.8

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MATERIALS AND METHODS

Three large handfuls of moss (almost exclusively S. fuscum) were collected by hand at each of 22 bogs using polyethylene gloves. In the lab, each Sphagnum bundle was trimmed to obtain a final sample consisting only of the green, living layer. Foreign plant material was removed prior to drying at 105 °C overnight in polypropylene jars. Dried samples were ground using an agate centrifugal ball mill. Trace metal analyses of the powders were carried out in the new, metal-free, ultraclean SWAMP lab, Department of Renewable Resources, University of Alberta. Sample aliquots of approximately 200 mg were weighed into polytetrafluoroethylene (PTFE) vessels and then dissolved in a mixture of 3 mL of concentrated nitric acid and 0.1 mL of tetrafluoroboric acid using a high-pressure microwave digestor pressurized to 50 bar using Ar. At the end of the microwave heating steps, the reaction chamber was cooled by circulated cooling water and

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RESULTS Pb. The average Pb concentration found in the moss samples from AB (0.71 ± 0.32 mg/kg, n = 63) is considerably lower than the Pb concentrations measured in Sphagnum moss collected recently from DE (Figure 2). For comparison to other sites in Canada, Sphagnum moss collected from 35 peatlands in eastern Canada in the early 1980s9 yielded an average of 23 ± 8 12604

dx.doi.org/10.1021/es503751v | Environ. Sci. Technol. 2014, 48, 12603−12611


Article

Figure 2. Average concentrations (and standard deviation) of Pb, V, Ni, Mo, Ba, and Th in mosses from the ABS region (AB; n = 63), the Anzac bog (ANZ; n = 3), and four bogs in DE. Wildseemoor (WI; n = 93) and Hohlohseemoor (HO; n = 55) are situated in the Northern Black Forest, and Gschwender Filz (GS; n = 53) and Kläperfilz (KL; n = 24) are situated in Upper Bavaria. The mosses from DE were harvested using stainless-steel scissors by harvesting 1 year of plant growth from plastic nets that were placed on the peat bog surface and fixed with plastic anchors. Each net represents a single sample, and up to 12 nets were installed at each sampling site; between 6 and 10 of these sites were established on the surface of each bog. In total, 225 samples, which had been exposed to atmospheric deposition during the period of 1 year (April 2007−2008) were harvested. All of the collected moss material was stored in a freezer (−18 °C) immediately upon arrival at the lab. Samples were prepared as described in detail elsewhere,8 and trace metals were measured using sector-field inductively coupled plasma−mass spectrometry (ICP−MS) in the ultraclean, metalfree lab in Heidelberg, Germany.

mg/kg of Pb (range of 11−43; n = 33). For example, Sphagnum moss collected from the Galbraith bog in an isolated valley in northern ONT in 1982 yielded an average Pb concentration of 37 ± 4.8 mg/kg (n = 4), and Sphagnum collected from 16 peatlands in the extremely remote James Bay Lowlands region of northern ONT (where there were effectively no roads at that time), during the same season, yielded 20 ± 5.1 mg/kg.10 During recent years, we have collected peat cores from ombrotrophic bogs across Canada (Table 1), and the living plant layers (top 1 cm) of these cores reveal Pb concentrations ranging from 2.0 mg/kg (Drizzle bog, Haida Gwai, west of the mainland of British Columbia) to 41.5 mg/kg (Point Escuminac, New Brunswick, on the Atlantic coast). In contrast, the Pb concentrations measured in the moss samples from AB range from 0.26 to 1.70 mg/kg (see Table S3 of the Supporting Information). In fact, the average Pb concentration in the

mosses from AB is only 3 times the lowest values ever recorded for ombrotrophic peat in the northern hemisphere,2 namely, the “natural background” concentration of Pb in peats from Switzerland dating from ca. 6000 to 9000 years ago (0.28 ± 0.05 mg/kg; n = 17). V, Ni, and Mo. It is well-known in geochemistry that the most highly enriched trace metals in bitumen are V and Ni, along with Mo. 11 For example, in the ABS, typical concentrations of V and Ni are 190 and 69 mg/kg, respectively.12 In contrast, the corresponding values for the Upper Continental Crust are 53 and 19 mg/kg, respectively.13 The enrichment of V in bitumen was first noted by Kyle;14 the role of marine organisms in its accumulation was discovered by Vernadsky;15 and Treibs16 recognized that this metal was present in the form of porphyrins. The ratio of V/Ni in the 12605


a

12606

Point Escuminac

Kuujjuarapik

Kuujjuarapik

Kuujjuarapik

Mer Bleue bog

Mer Bleue bog

Spruce bog

Luther bog

Sifton bog

53.92772′ N 132.10574′ W 54° 52′ 49″ N 101° 27′ 15″ E 55° 21′ 24.2″ N 102° 39′ 10.9″ E 42° 58′ 31″ N 81° 19′ 48″ W 43° 54′ 30″ N 80° 24′ 36″ W 45° 35′ 51″ N 78° 22′ 16″ W 45° 24′ 41.4″ N 75° 31′ 01.6″ W 45° 24′ 41.4″ N 75° 31′ 01.6″ W 55° 13′ 31.5″ N 77° 41′ 44.4″ W 55° 13′ 32.0″ N 77° 41′ 44.4″ W 55° 13′ 33.0″ N 77° 41′ 49.6″ W 46° 55′ N 65° 30′ W

coordinates

Nov 20−22, 1996

July 19, 2004

July 19, 2003

July 18, 2003

July 22, 2004

July 22, 2003

July 10, 2000

July 26, 2000

July 27, 2000

Sept 22, 2003

Sept 23, 2003

July 14, 2003

date of collection

41.5

6.6

3.9

6.4

6.1

4.9

10.1

4.5

9.3

3.9

3.7

2.0

Pb (mg kg−1)

The collection, handling, and sampling of the peat cores is described elsewhere.41

New Brunswick

Quebec

Ontario

Kotyk Lake

Manitoba

Sask Lake 4-1

Drizzle bog

name of bog

British Columbia

province

PEW-1

NQT-4

NQT-3

NQT-1

MBB-2

MBB-1

Algonquin Park

Luther Marsh Conservation Area

London

85 km from Flin Flon

27 km from Flin Flon

Graham Island, Haida Gwai (formerly Queen Charlotte Islands)

remarks

Heidelberg, EMMA XRF









Heidelberg, ICP−SMS (ultraclean lab)

Heidelberg, ICP−SMS (ultraclean lab)


lab and method

reference

44

Givelet (unpublished field report)





43

43

43

Rausch (unpublished field report)

Rausch (unpublished field report)

42

Table 1. Concentrations of Pb in the Living Plant Layer (Top 1 cm) of Peat Cores Collected from Ombrotrophic Bogs in Other Regions of Canadaa

Environmental Science & Technology Article



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Figure 3. Concentrations of Ag, Cd, Sb, and Tl in mosses from the ABS region, the Anzac bog, and four bogs in DE.

factor of 3 of “natural” values and well within the range for Sphagnum from other parts of Canada. Molybdenum concentrations in the mosses from AB range from 0.1 to 0.7 mg/kg (average of 0.28 ± 0.13). These Mo concentrations (see Table S3 of the Supporting Information), however, are no different from those found in the DE Sphagnum mosses (Figure 2). In fact, the mosses from AB yielded Mo values that are within a factor of 3 of the “natural background” value for Mo in peat from continental Europe20 and dating from ca. 6000 to 9000 years old (0.08 ± 0.02 mg/ kg; n = 17). Ba and Th. The average Ba concentration found in the moss samples from AB (21.4 ± 10.1 mg/kg; n = 63) is much greater than the Ba concentrations measured in the Sphagnum mosses collected from DE (Figure 2). The “natural background” value for Ba in ombrotrophic peat from continental Europe20 and dating from ca. 6000 to 9000 years old is 5.2 ± 2.6 mg/kg (n = 17). Barium was used by Patterson and co-workers as a reference element for Pb in his pioneering study of hemispheric Pb pollution using Greenland snow and ice21 because Ba concentrations in the atmosphere are unaffected by human activities but rather reflect the abundance of soil-derived mineral dust particles. As is true of polar snow and ice, Ba concentrations in ombrotrophic peat mirror the concentrations of other lithogenic elements and can be used as a surrogate for the abundance of mineral matter. 20 The elevated Ba concentrations in the moss samples from AB, therefore, in comparison to ancient ombrotrophic peat samples, reflect the elevated inputs of mineral dusts. The average Th concentration found in the moss samples from AB (0.26 ± 0.13 mg/kg; n = 63) is considerably greater than the Th concentrations measured in the Sphagnum mosses collected from DE (Figure 2). The “natural background” value for Th in ombrotrophic peat from continental Europe20 dating from ca. 6000 to 9000 years old is 0.07 ± 0.02 mg/kg (n = 17). Thorium is hosted in “heavy” minerals, such as zircon, a Zr

ABS is 2.7−2.9, and this has been used to help understand the genesis of petroleum in Alberta.17 The V concentrations in the mosses from AB (average of 3.3 ± 1.9 mg/kg) are only slightly greater than in the Sphagnum mosses collected recently from DE (Figure 2). Sphagnum moss collected from 35 peatlands in eastern Canada in the early 1980s9 yielded an average of 4 ± 2 mg/kg of V (range of 0.3− 9.5; n = 33). For example, Sphagnum moss collected from the Galbraith bog in northern ONT in 1982 yielded an average V concentration of 3.6 ± 1.4 mg/kg (n = 4), and Sphagnum collected from 16 peatlands in the remote James Bay Lowlands region of northern ONT, during the same season, yielded 3.4 ± 1.1 mg/kg.10 Thus, the V concentrations in the Sphagnum moss samples from AB, ranging from 0.8 to 8.6 mg/kg (see Table S3 of the Supporting Information), are within the range reported for Sphagnum from other locations in Canada and comparable to surface layers of contemporary peat from Scottish bogs.18 In fact, the mosses from AB yielded V values that on average are typically within a factor of 6 of the “natural background” value for V in peat from continental Europe19 and dating from ca. 6000 to 9000 years old (0.55 ± 0.13 mg/kg; n = 17). Nickel concentrations in the mosses from AB range from 0.5 to 3.4 mg/kg (average of 1.46 ± 0.70) and are notably lower than the mosses from DE (Figure 2). Sphagnum moss collected from 35 peatlands in eastern Canada in the early 1980s9 yielded an average of 2 ± 1 mg/kg of Ni (range of 1−4; n = 33). For example, Sphagnum moss collected from the Galbraith bog in northern ONT in 1982 yielded an average Ni concentration of 3 ± 0.4 mg/kg (n = 4), and Sphagnum collected from 16 peatlands in the remote James Bay Lowlands region of northern ONT, during the same season, yielded 2 ± 0.4 mg/kg.10 For comparison to these concentration data, the “natural background” value for Ni in peat from continental Europe and dating from 6000 to 9000 years old19 is 0.46 ± 0.09 mg/kg (n = 18). The Ni concentrations in the mosses from AB (see Table S3 of the Supporting Information), therefore, are within a 12607



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Figure 4. Linear regression of Pb, V, Ni, Mo, and Ba concentrations in the AB mosses versus Th.

abundance of mineral material, is V. From a geochemical perspective, this finding was largely expected, given that V is the single most enriched metal in the bitumen and that this phenomenon has been thoroughly investigated and welldocumented in the geological literature. To quote Goldschmidt: “some very remarkable concentrations of rare elements are met with in the hydrocarbons, as well as in bituminous rocks; this applies to the accumulation of vanadium, molybdenum, and nickel, which doubtless form organometallic compounds and migrate together with the hydrocarbons”.26 If any element should be enriched in terrestrial or aquatic samples of the ABS region, it should be V. Clearly, any study of the environmental significance of heavy metal emissions from the industrial development of bitumen resources in AB should begin with an understanding of their natural occurrence and geochemical cycles. V, Ni, and Mo are known to be enriched in bitumen, and these are the obvious starting point for any environmental impact assessment by “heavy metals”. Chalcophile elements, such as Ag, As, Cd, Cu, Pb, Sb, Tl, and Zn, on the other hand, are known to be highly enriched in sulfide minerals (hence, the appellation) and the massive ore deposits, which host them, along with coal, which is often rich in sulfide minerals.27 The enrichment of chalcophile elements in sulfides and the accumulation of V, Ni, and Mo in bitumen are the result of contrasting geochemical phenomena taking place in very different geological settings, namely, the

silicate that is one of the most stable minerals during the chemical weathering of crystalline rocks,22 and found in abundance in the ABS.23 A second important host mineral for Th is monazite, a rare earth element phosphate, which has recently been found in the ABS.24 Because of the extreme stability of its mineral hosts, Th provides a reliable guide to the abundance of mineral dust particles adhering to the mosses. Similar to Ba, the elevated concentrations of Th in the mosses from AB, relative to the mosses from Germany and the ancient peat from Switzerland, reflect the elevated abundance of mineral dust particles in the Sphagnum moss from the ABS region. Other “Heavy Metals”. Concentrations of Ag, Cd, Sb, and Tl are all lower in the mosses from AB versus DE (Figure 3).

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DISCUSSION Heavy Metals in Bituminous Sands. Considerable attention has been paid to the potential environmental impact of emissions of “heavy metals” from open pit mining and upgrading of bitumen in AB as revealed using snowpack sampling.5 That study5 generated considerable media attention, culminating in an editorial in the national newspaper of Canada, The Globe and Mail, that there are “elevated levels of cadmium, mercury, lead and other toxic elements in the Athabasca River” as a result of springtime snowmelt.25 However, of the list of trace elements reported here, the only metal that is enriched in the Sphagnum mosses, relative to the 12608



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Figure 5. Spatial distribution of Pb, V, Ni, Mo, Ba, and Th concentrations in the AB mosses. To allow for the metal concentrations to be shown for each bog, an average concentration was calculated using the three moss samples collected at each site.

the abundance of dusts. A map illustrating the spatial variation in metal concentrations shows that the distribution patterns of Pb, V, Ni, and Mo are similar to those of Ba and Th (Figure 5). Maps were also created for Ag, Cd, Sb, and Tl, along with wind rose diagrams (see Figure S1 of the Supporting Information). Given that the average Th concentration in the AB mosses (Figure 2) is 3 times that of the “pre-anthropogenic background” peat from the mid-Holocene,20 this explains why the concentrations of almost all of the heavy metal concentrations are also a factor of 3 greater in the mosses from AB than in the ancient peat samples. Considering the 6fold difference in V and 3-fold difference in Th, the mosses from AB, therefore, on average, contain twice the preanthropogenic background concentration of V. Thus, of the trace metals studied here, V is the only element that is enriched in the mosses, relative to the V contributed by the mineral particles. In fact, the average ratio of V/Th in the mosses (see Table S3 of the Supporting Information) is approximately 10:1, but this is also the ratio of V/Th in the Upper Continental Crust,13 indicating that, even in the case of V, the extent of metal enrichment in the mosses is rather limited. To emphasize this point, peat samples predating industrialization from the JPH-4 bog were measured for the same suite of trace metals, with little difference found between these peat samples and the

hydrothermal alteration of crystalline rocks versus the accumulation of organic matter in marine sediments. The key to understanding the concentrations of heavy metals in Sphagnum moss from the ABS region appears to lie in the abundance of mineral particles on the leaf surfaces of the plants. Plotting Pb concentrations against those of Th shows a strong linear correlation (Figure 4). However, even V, Ni, and Mo, which are enriched in the bitumen fraction of the ABS, increase in the mosses with increasing Th concentrations (Figure 4). In fact, all of the heavy metals, with the exception of Cd, are significantly correlated with Th (see Table S4 of the Supporting Information). The concentrations of heavy metals in the mosses, therefore, is mainly a reflection of the mineral dust particles that they contain. In this increasingly industrialized region, the sources of dust are by no means restricted to the open pit mines themselves but also include contributions from gravel roads and quarries as well as enormous piles of overburden (removed before mining) and coke (from upgrading of bitumen). The Sphagnum mosses growing in the bogs of this region integrate inputs from all of these sources, and at this stage of our investigation, no distinction is made about their relative strengths. However, the data clearly indicate that, although there are variations in metal concentrations, these tend to follow those of Th (Figure 4) and, therefore, also 12609



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mosses from Alberta and Germany, linear correlation coefficients, comparison of trace metals in pre-industrial peat from the JPH-4 bog versus living Sphagnum moss from the same site, independent evaluation of the V concentrations in AB mosses, and other studies of atmospheric deposition of trace metals in the ABS region. This material is available free of charge via the Internet at http://pubs.acs.org.

corresponding living moss layer sampled at the same time (see Figure S2 of the Supporting Information). Clearly, the key to understanding heavy metal emissions in this region of Canada lies in the chemistry and mineralogy of the ABS itself. The ABS are a combination of beach and dune sands.28 The sand is predominantly quartz,29 and 90% of the particles are larger than 100 μm.28 Atmospheric lifetimes of particles in this size range from minutes to hours,30 and because they deposit rapidly from the air, atmospheric transport is limited.31 In addition to the dominant quartz and heavy minerals, however, clays are abundant, primarily illite and kaolinite.32 In the ABS, V concentrations increase by a factor of 25 times as particle size decreases from ca. 150 to

Sphagnum Mosses from 21 Ombrotrophic Bogs in the Athabasca

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