Mercury release from thawing permafrost in the Arctic

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NTNU Norwegian University of Science and Technology Faculty of Natural Sciences Department of Chemistry

Morten HeistadMercury release from thawing permafrost in the Arctic

Morten Heistad

Mercury release from thawing permafrost in the Arctic

Master’s thesis in Environmental Chemistry Supervisor: Øyvind Mikkelsen

June 2021

Master ’s thesis

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Morten Heistad

Mercury release from thawing permafrost in the Arctic

Master’s thesis in Environmental Chemistry Supervisor: Øyvind Mikkelsen

June 2021

Norwegian University of Science and Technology Faculty of Natural Sciences

Department of Chemistry

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Abstract

In this master thesis, data for mercury (Hg) levels in soil and water ponds on the tundra in the Ny-Ålesund area are presented. The main aim was to investigate possible release of Hg from permafrost during summer thaw. Special focus was also dedicated a selection of trace metals of environmental concern associated with long-range transport to the Arctic, namely lead (Pb), cadmium (Cd) and zinc (Zn). Sampling of soil and water was conducted in July and August 2020, with supplement data for water samples from the area from 2018 and 2019. It was attempted to limit the influence from local anthropogenic sources.

The potential release of Hg was studied by comparing concentrations from July with August, to look for differences during the thawing period. Mean concentration of Hg in soil for July was 0.178 µg/g and 0.217 µg/g for August. The increase was not significant. For the water samples from 2020 there was observed a clear difference, because the majority of July samples were below detection limit, while the majority of August samples was clearly above. Additionally, the unfiltered water samples showed a significant increase with a concentration of 0.0179 µg/L in July and 0.0570 µg/L in August.

From PCA the differences between sampling periods were confirmed for water samples from 2018 and 2019, and Hg was positioned close to the area for the majority of August samples. From PC3, explaining 10% variation in the soil, Hg had a unique and strong impact. There was also generally low correlation with the other long-range transported metals, indicating that Hg may have a different impact from at least one other source than Pb, Cd and Zn. There is no clear evidence for permafrost being the source. However, the indications support the need for further research on Hg emissions from permafrost.

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Sammendrag

I denne masteroppgaven er det presentert data for nivå av kvikksølv i jord og vannpytter på tundraen for området rundt Ny-Ålesund. Hovedmålet for prosjektet var å undersøke mulig utslipp av kvikksølv fra permafrost som en konsekvens av tiningen om sommeren.

Spesielt fokus ble også rettet mot andre metaller som assosieres med langtransport til Arktis og som er en trussel i det Arktiske økosystem, nemlig bly (Pb), kadmium (Cd) og sink (Zn). Det ble utført prøvetaking av jord og vann i juli og august 2020. I tillegg ble det supplert med data for vannprøver fra det samme området tatt i 2018 og 2019. Det ble forsøkt å redusere påvirkningen fra menneskeskapte kilder.

Mulig utslipp av kvikksølv ble undersøkt ved å sammenligne konsentrasjoner av kvikksølv i juli mot august, med hensikt om å se etter endringer som følge av perioden med tining.

Gjennomsnittlig konsentrasjon for kvikksølv i jord for juli var 0.178 µg/g og 0.217 µg/g for august. Denne økningen var ikke statistisk signifikant.

For vannprøvene fra 2020 ble det observert en markant forskjell, da majoriteten av juli- prøvene var under deteksjonsgrensen, mens majoriteten av august-prøvene var klart over deteksjonsgrensen. I tillegg viste de ufiltrerte vannprøvene en signifikant økning i kvikksølvkonsentrasjonen fra 0.0179 µg/L i juli til 0.0570 µg/L i august.

Forskjellene mellom prøvetakingsperiodene ble bekreftet av PCA hvor vannprøvene fra 2018 og 2019 var inkludert. Kvikksølv var lokalisert i samme området som majoriteten av august-prøvene. For PC3, som forklarer 10% av variasjonen i jord, hadde kvikksølv en særegen og kraftig påvirkning. Videre var det liten grad av korrelasjon mellom kvikksølv og de andre langtransporterte metallene, som indikerer at kvikksølv kan ha en annerledes påvirkning fra minst én kilde enn det bly, kadmium og sink har. Det finnes ikke klare bevis for at dette skyldes tining av permafrost. Det finnes likevel indikasjoner for at permafrost kan være involvert og at det derfor er behov for mer forskning på utslipp av kvikksølv fra permafrost.

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Acknowledgements

I would like to thank my supervisor Øyvind Mikkelsen for his valuable guidance throughout this whole project. All the way from sampling in Ny-Ålesund, to discussing the final results.

I also have to thank Anica Simic for the ICP-MS analysis.

I am grateful for the financial support of the project from the Arctic Field Grant, and the logistic support from the Norwegian Polar institute during our stay in Ny-Ålesund. To be able to do fieldwork there was incredible. A thanks also to Kristyna Ruzickova for the help with soil sampling and sample preparation in July.

Lastly a special thanks to my fellow students, friends and family.

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Figures

Figure 2.1: Sources, fate, and transformation processes for mercury ... 19

Figure 2.2: Illustration of long-range transport mechanisms. ... 21

Figure 2.3: Bioaccumulation and biomagnification of methylmercury food web ... 24

Figure 2.4: Deposition and fate of mercury (Hg) in the Arctic. ... 26

Figure 2.5: Illustration of a DGT unit ... 29

Figure 3.1: Map of Svalbard ... 36

Figure 3.2: Water samples from July ... 38

Figure 3.3: Water samples from August ... 38

Figure 3.4: Soil samples from July. ... 40

Figure 3.5: Soil samples from August.. ... 41

Figure 4.1: Boxplot for Hg in organic soil. ... 47

Figure 4.2: Boxplot for Hg in organic soil ... 48

Figure 4.3: Boxplot for Cd in organic soil. ... 49

Figure 4.4: Boxplot for Pb in organic soil ... 49

Figure 4.5: Boxplot for Zn in organic soil ... 50

Figure 4.6: Boxplot for Hg in filtered water samples ... 54

Figure 4.7: Boxplot for Pb in filtered water samples ... 54

Figure 4.8: Boxplot for Cd in filtered water samples. ... 55

Figure 4.9: Boxplot for Zn in filtered water samples ... 55

Figure 4.10: Boxplot for Hg in unfiltered water samples ... 57

Figure 4.11: Boxplot for Pb in unfiltered water samples ... 57

Figure 4.12: Boxplot for Cd in unfiltered water samples. ... 58

Figure 4.13: Boxplot for Zn in unfiltered water samples ... 58

Figure 4.14: Boxplot for Ca in water samples. ... 60

Figure 4.15: Boxplot for Mg in water samples ... 60

Figure 4.16: Boxplot for K in water samples ... 61

Figure 4.17: Boxplot for pH for water samples ... 62

Figure 4.18: Boxplot for redox potential for water samples ... 62

Figure 5.1: PCA score-plot for soil samples for PC1 and PC2 with sample type ... 72

Figure 5.2: PCA score-plot for soil samples for PC1 and PC2 with PCA number. ... 72

Figure 5.3: PCA loading-plot for soil samples for PC1 and PC2. ... 73

Figure 5.4: PCA loading-plot for PC3 and PC4 for soil samples. ... 73

Figure 5.5: PCA score-plot for water samples from 2018 and 2019 for PC1 and PC2 ... 75

Figure 5.6: PCA loading-plot for water samples from 2018 and 2019 for PC1 and PC2. .. 75

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Tables

Table 2.1: Reactions and constants for mercury complexes ... 22

Table 3.1: Sampling points and location name for water samples. ... 39

Table 3.2: Sample points, location, and soil type from July. ... 40

Table 3.3: Sample points, location and soil type from August. ... 41

Table 4.1 Descriptive statistics for Hg in organic and mineral soil ... 46

Table 4.2: Descriptive statistics for selected elements in organic soil samples. ... 47

Table 4.3: Results of the Welch t-test between sampling periods for organic soil. ... 48

Table 4.4: Correlation coefficients from a Pearson correlation for soil. ... 51

Table 4.5: Descriptive statistics for filtered and unfiltered water samples ... 52

Table 4.6: Mann-Whitney U test of filtered and unfiltered samples. ... 53

Table 4.7: Results of Mann-Whitney U test for filtered sampples between periods. ... 53

Table 4.8: Test of means between sampling periods for unfiltered samples.. ... 56

Table 4.9: Test of mean concentration for Ca, Mg and K. ... 59

Table 4.10: Test of mean concentration for Ca, Mg and K between sampling periods .... 59

Table 4.11: Differences between sampling periods for pH and redox potential. ... 61

Table 4.12: Results from Pearson correlation for filtered water samples. ... 63

Table 4.13: Descriptive statistics from leaching experiment. ... 64

Table 4.14: Results from Pearson correlation for leaching experiment. ... 64

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Abbreviations

As AMAP ASGM Br C Ca Cd Cl CRM DGT DMHg DOC GEM GOM Hg

Arsenic

Arctic Monitoring and Assessment Program Artisanal small-scale gold mining

Bromine Carbon Calcium Cadmium Chlorine

Certified reference material Diffusive Gradient in Thin films Dimethylmercury

Dissolved organic carbon Gaseous elemental mercury Gaseous oxidized mercury Mercury

Hg(0) Hg(I) Hg(II) Hgp

ICP-MS K LOD LOQ LR(A)T MeHg Mg MWU m/z N Pb PCA POP PP ppb ppm Q1 Q3 QA/QC RGM (S)OM SRB TCD Zn

Elemental mercury Mercurous mercury Mercuric mercury Particulate mercury

Inductively coupled plasma mass spectrometry Potassium

Limit of detection Limit of quantification

Long-range (atmospheric) transport Methylmercury

Magnesium Mann-Whitney U Mass to charge ratio Nitrogen

Lead

Principal component analysis Persistent organic pollutant Polypropylene

Parts per billion Parts per million First quartile Third quartile

Quality assurance/Quality control Reactive gaseous mercury

(Soil) organic matter Sulfate-reducing bacteria Thermal conductivity detector Zinc

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The Arctic can often be thought of as a pristine and remote area, unaffected by human activity. That is however far from the truth. Arctic is one of the areas that experience the effects of climate change the most (1). The temperature increase has been most rapid in the northern latitudes and has critical effects on the cryosphere, as well as terrestrial and marine life. Glaciers is melting, permafrost is thawing and there is decreasing amounts of sea ice (2). This has numerous of impacts on ecosystems, positive feedback loops for climate change and destruction of infrastructure and houses.

In addition, it has been known for a while that considerable amounts of carbon (C) have been sequestered in the cryosphere for a long time, and that it may be released and take part in the C cycle again, increasing the load of C in the atmosphere (3). A similar event is also possible for another environmental threat, that is mercury (Hg) (4). Studies have estimated that the permafrost in the northern hemisphere stores substantial amounts of Hg (5, 6). Hg has been a concern in the Arctic for quite some time because it has the ability for long-range transport (LRT), due to its long residence time in the atmosphere (7). Hg is therefore a global issue, and measures have been taken globally to reduce Hg emissions, especially through the Minamata Convention (8). However, even before humans started to extract Hg, use it in products, and burn fossil fuel, Hg has been cycling in the environment (9). Hg has natural emission sources such as forest fires and volcanic eruptions. When Hg has reached the Arctic and deposited, some of it can become trapped in the frozen ground or ice. Human activity has altered the cycling of Hg in the environment and increased the atmospheric loading of Hg to the Arctic. Since the process has gone on for so many years, substantial amounts of Hg have accumulated, until now when the equilibriums may change (4).

Hg is of a special concern in the Arctic, since the methylated and most toxic form, also bioaccumulate and biomagnify in the food web (7). Increased Hg levels in biota applies additional stress to species, which simultaneously must adapt to a changing environment and are exposed to several other pollutants as well. For people living in the Arctic and rely on food resources from marine life, high in the food web, this is of great concern.

There are high uncertainties associated with the estimates for how much Hg the permafrost stores. Schuster et al. estimate 1656±962 Gg (6). However, even with the most conservative estimate, using the lower end of the range, implies that permafrost is a large reservoir of Hg in a global scale. How much of this that are likely to be released, how, and when is perhaps even more uncertain. However, some argue that there already is evidence of Hg being released from permafrost due to climate change (4, 10). It is observed increased levels in polar bears, and increased Hg export to lakes.

In this thesis, data for soil and water samples are presented with focus on Hg and a selection of other long-range transported trace metals, namely lead (Pb), cadmium (Cd) and zinc (Zn). The soil and water sampling were conducted in the beginning and towards the end of the thawing period. The aim is to investigate if there is an observable change in Hg concentration, as a consequence of permafrost thaw. This will be done by comparing data for the two sampling periods and compare with how other elements behave. In

1 Introduction

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addition, new data for trace metals in soil and freshwater is presented and viewed in relation to previous studies from the Arctic.

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2.1 Mercury

2.1.1 A global pollutant

Hg is unique in many ways and has therefore been a metal of interest for centuries. Hg is one of the few metals that is liquid in room temperature, has high surface tension, and a high vapor pressure, although it is both heavy and dense (9). It has been much attention to its toxic effects and viewed as an environmental threat in more recent years. However, Hg has had many applications for humans throughout history. It has been used as a pigment to color clothes and artifacts, to extract gold, as dental fillings, as medicine, and in thermometers to mention a few (11, 12). It is many of Hg’s unique properties that has made it an attractive element to use for humans, for instance the tendency to form amalgams with noble metals such as gold and platina (13). As the knowledge about the human and environmental threats have been exposed, the focus has shifted towards reduction in use and emissions. However, Hg is still used to a vast extent for instance in artisanal small-scale gold mining as a way to extract gold, and this is now one of the most important sources of global Hg emissions (14).

Hg has seven stable isotopes, where Hg²⁰²is the most abundant with 29.7% (13, 15). There are many minerals that contains Hg and the most abundant is HgS. HgS occurs in three different polymorphs, but the commonly known is cinnabar. This is also the main source for mined Hg (9, 12).

Hg is an ubiquitous element in nature and the overall amount of Hg on earth is constant (9). However, anthropogenic activities such as mining, fossil fuel, industry, and a wide variety of other uses, have resulted in a disturbance in the cycling of Hg in nature. The combination of high vapor pressure and long residence time in atmosphere, makes Hg subject to long-range atmospheric transport (LRAT). This ability makes Hg emissions a global problem, regardless of where on earth the emission source is located (16).

Hg is listed on WHO’s list of top ten chemicals of major public health concern and through United Nations, an international treaty called “The Minamata convention” has been established to manage this risk (11, 17). It is named after the horrible case in Minamata Bay, where many people suffered from Hg poisoning due to a chemical spill from a factory in the town (11). The aim of the convention is to “protect human health and the environment from the adverse effects of Hg.” Some key aspects of the convention are the ban of new Hg mines, phase-out of existing ones and a reduction in the use of Hg in products and industrial processes (8). It also sets focus on control and monitoring of emissions to the environment, whether it is to air, land, or water.

The convention was agreed upon in 2013, but did not enter into force before 2017 and has now 128 signatories.

2 Background

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2.1.2 Speciation, sources, and transport

Speciation

Hg has three oxidation states: 0 ,+1 and +2, which can be referred to as elemental mercury, mercurous mercury and mercuric mercury respectively (9, 13). Hg(I) is not stable under normal environmental conditions but exist as Hg(I) in many organic and inorganic compounds. Hg is volatile compared to its high atomic number with a vapor pressure of 0.27 Pa at 25°C (17). Especially Hg(0) has a has long residence time in the atmosphere, from 6 to 18 months. Of the total Hg present in atmosphere, 95% is Hg(0) (9). Some relevant species of Hg in atmosphere are the most abundant Hg(0) or gaseous elemental mercury (GEM), Hg(II) is often referred to as reactive gaseous mercury (RGM) or gaseous oxidized mercury (GOM), and particulate mercury (Hgp) which is Hg adsorbed on particles. RGM and Hgp have much shorter residence time in the atmosphere and are therefore more readily deposited. This can happen via wet deposition, where the RGM is for instance dissolved in rain droplets. It can also occur via dry deposition, where particulate matter with Hg adsorbed onto the surface are removed from the atmosphere by the force of gravity.

Hg(II) is the other stable form of Hg in the environment and is referred to as RGM in the atmosphere, which is only a small fraction of total atmospheric Hg under normal environmental conditions (18). However, it is the dominant form in water and can exist as an ion, in complexes, and various compounds. Hg(II), especially inorganic compounds is relevant in soil and minerals. Soil, sediments, and water are dominated by Hg(II) in an inorganic form (9). In biota it is the organic methylmercury (MeHg) that is the common specie and are of particular interest in respect to environmental risk assessment. Organic Hg substances are referred to as organomercurials, and some other examples of these are dimethylmercury (DMHg) and ethylmercury (19).

Toxicity and methylmercury

Hg has no known biological function and therefore not essential at all and is in fact one of the most toxic elements humans can be exposed to (12). Hg in all its chemical forms is therefore associated with some level of toxicity, but it is the organic compound MeHg (CH3Hg⁺) that is of highest concern (19). Unlike Hg(0) and Hg(II), MeHg are to some extent lipophilic, but more importantly it binds to the amino acid cysteine and mimic another amino acid, methionine. In that way it dispositions in the body and pass through strict membranes such as the blood-brain barrier and the blood-placenta barrier. It has a wide range of toxic mechanisms and is especially a dangerous neurotoxicant. It can cause adverse effects on fetuses such as delayed mental development and malformations. Both the effect on the nervous system and the fetus was brutally demonstrated in the Minamata incident. This was caused by discharges from the acetaldehyde plant owned by Chisso Company. They used Hg in the process and due to a change in catalyst it was formed MeHg that was discharged with the wastewater.

The MeHg was taken up by fish in Minamata Bay and was eaten by the inhabitants and developed severe symptoms of Hg poisoning and which would later be referred to as Minamata disease.

Hg may be transformed in the environment through natural processes to MeHg. This can occur via both biotic and abiotic methylation mechanisms (20). Biotic methylation is caused by methylating bacteria, and the most studied are sulfate-reducing bacteria (SRB) (21). It

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was previously thought that only SRB were responsible for biomethylation, however it is now known a wide variety of bacteria that can methylate Hg. It is thought that the gene pair hgcA and hgcB is the main factor for bacteria to be able to methylate Hg. Methylation can occur in the soil, in lakes, the water column in ocean and in sediments when the environmental factors favor net methylation (20, 21). There are also several processes that demethylate, for instance the abiotic pathway of photodegradation. Some of the effects that affect net methylation is speciation and bioavailability of Hg together with environmental factors such as temperature, pH, redox potential, biological activity, nutrients, and presence of complexing agents. Since these factors often affects each other as well, it creates a complex interplay and effect on net methylation can be difficult to predict. The complex cycling of Hg in water, wetlands and soil is presented in Figure 2.1.

It includes a selection of important transformation and transport mechanisms.

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Figure 2.1: Illustration of sources, fate, and transformation processes for mercury (Hg) in water, wetlands, and soil. The dotted arrows show transport, while the black arrows indicate transformation processes. The white circles have illustration of microbial cells and indicate

microbial mediated processes. From Beckers and Rinklebe, 2017 (9).

Sources

Sources of Hg emissions can be divided into primary and secondary sources (16). Primary sources are emissions that release Hg from a long-term reservoir in the lithosphere to the atmosphere. A primary source can be both natural and anthropogenic. Hg from the primary source will eventually deposit on a surface in the terrestrial or the aquatic environment.

Then it can be reemitted, and the process responsible for the re-emission can be classified as a secondary source and is only a redistribution between surface reservoirs. Hence, primary sources lead to larger total amounts of Hg in the surface reservoirs, while secondary sources will only distribute it from one surface reservoir to another, not changing the total amount (16).

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Primary natural sources of emission of Hg are mainly volcanic activity, weathering of rocks, and hydrothermal vents (22). They increase the amount of Hg to the atmosphere or the ocean. Large volcanic eruptions, in the scale of for example Krakatau 1883, are thought to have caused the release of massive amounts of Hg (9). A lot of the emissions from natural sources is also re-emission of previously deposited Hg. It may have been emitted from both natural and anthropogenic sources and is therefore a secondary source (22). Sources of re-emission can be burning of biomass, meteorological conditions and mechanisms related to the exchange of Hg between the atmosphere and surface interfaces such as water, soil, and snow. The most important source of natural and re-emission of Hg are the oceans, which are responsible for 36% of the emissions. Tundra and grassland are also among the most important sources with 6% (22).

There is however no doubt that human activity has disturbed the natural cycling of Hg in nature and emissions from primary anthropogenic sources is the main reason for increased Hg pool in surface inventories (16). Some important anthropogenic sources of Hg emissions are coal-combustion, artisanal small-scale gold mining (ASGM), waste disposal and waste incineration and non-ferrous metal production (22). Historically the chlor-alkali industry was a major source, but the emissions has decreased due to better control and replacement of Hg in the process with new technology (9). ASGM is now the most important source responsible for 37% of the emissions (22).

It is estimated that around 2000 tons of Hg per year, but there is high uncertainty to this number, so the range can be around 1000 to 4000 (16, 22). In comparison the annual emissions are estimated a range of 6500 to 8200 tons. As much as 4600 to 5300 tons of this is believed to come from natural sources, but this is when primary natural sources (80-600 tons) are combined with secondary emissions. Primary anthropogenic sources are therefore the dominant reason for why increasing amounts of Hg are included in the biogeochemical cycle (16). Amos et al. (2013) investigated anthropogenic influence on the biogeochemical cycle and argue that even if primary anthropogenic emissions are kept constant in the future, the Hg deposition will increase (23). This is due to the burden from legacy Hg that were released from anthropogenic sources over many years and has accumulated in the biogeochemical cycle.

Transport

Atmospheric transport of Hg is an incredibly important feature in the biogeochemical cycling of Hg. Due to the long residence time of Hg(0) in the atmosphere, it can be transported by wind currents (9). The global mixing of the troposphere is approximately 1 year, meaning that Hg can be transported all over the world (16). However, the mixing between the two hemisphere is limited, and it is observed 30% higher total Hg(0) in surface air in the northern hemisphere compared to the southern hemisphere. Within the hemisphere it is released, Hg(0) is readily transported long distances (16).

The ability for a pollutant to be transported far away from its emission source, is referred to as being subject to (LRT), and the most relevant in the case of Hg, LRAT (24, 25). This ability is one of the criteria for pollutants to be listed in the Stockholm convention and classified as a persistent organic pollutant (POP), and is seen for many other elements and compounds of environmental concern (26).

LRAT can be divided into two slightly different mechanisms. Global distillation or fractionation is the process of chemicals being transported from mid-latitudes towards the north or the south pole, depending on which hemisphere they were released, and that they

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are separated in the atmosphere based on their volatility (27). Semi-volatile compounds and elements will not travel as far as the most volatile. Another way is the chemicals that can evaporate, deposit and then re-evaporate again, thereby travel towards higher latitudes through a series of hops. This is called the “grasshopper effect” (27). The series of hops is due to for example seasonal variations, which changes that partitioning of the element or compound in the environment.

It is important to point out that LRT also can happen via ocean currents, but that is a slower process (7, 24). For Hg, transport to the Arctic is in the scale of days and weeks in the atmosphere by wind currents, but may take decades with ocean currents (7). Some of the most important mechanisms and pathways for long range transport, are illustrated in Figure 2.2.

Figure 2.2: Illustration of long-range transport, with main focus on atmospheric transport. Key mechanisms is illustrated, where global distillation is the transport towards higher latitudes based

on the mobility, and grasshopper effect is through multiple “hops”. From UNEP/GEF, 2003 (28).

2.1.3 Hg aquatic chemistry

All the three valence states of Hg (0, +1 and +2) may be present in the aquatic environment (20). The dominant dissolved species are Hg(II) as inorganic or organic complexes, organomercurials MeHg or DMHg, and also a dissolved fraction of Hg(0). In general, it is Hg(II) that is the main Hg specie in the aquatic environment, but MeHg can under certain conditions represent a high percentage of total Hg (21, 29). Due to the high volatility, Hg(0) is readily lost from the aquatic environment to the atmosphere, and most surface waters are therefore supersaturated in Hg(0) relative to atmosphere (20).

Redox potential and pH are two factors that affect speciation of Hg strongly in the aquatic environment (20). Another key factor is the availability of complexing ligands, both organic

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and inorganic. For inorganic forms there are mostly chloride and hydroxide complexes.

Due to the high concentrations of chloride in seawater, this is the dominant inorganic ligand while both hydroxide and chloride complexes can be present in freshwater. In freshwater it is typically Hg(OH)2, HgOHCl and HgCl2 that will be the most abundant inorganic complexes. However, in the presence of sulfide, Hg readily form complexes. Even more important is organic complexes, with especially humic matter and other organic matter (OM) that contains thiol groups (20). Some complex reactions involving Hg is presented in Table 2.1.

Table 2.1: Reactions and constants (for 25°C) presented by Hsu-Kim et al. for some relevant complex reaction involving mercury (Hg). From Hsu-Kim et al., 2013 (21).

Complexation reaction log K Hg2⁺ + RS22– ⇔ Hg(RS2) 28.7

Hg²⁺ + 2Cl– ⇔ Hg(Cl)2 14.0 Hg²⁺ + 2H2O ⇔ Hg(OH)⁰2 + 2H+ –6.2 Hg²⁺ + 2HS– ⇔ HgHS^-2 + H+ 31.5

With a dissolved fraction defined as the fraction smaller than 0.45 µm, organic colloids comprise a significant part of the dissolved Hg(20). As much as 90% of Hg in freshwater is believed to be complexed with OM. MeHg as well is mostly complexed by dissolved organic carbon (DOC). In seawater the OM has more competition with chloride and the fraction complexed by OM will be smaller. Redox and pH will also affect complexation, as will the presence of sulfide which will be a competitive ligand. In freshwater it is almost exclusively MeHg of organomercurials that is present. MeHg is very stable, but can decompose from microbial activity or photodegradation which is one of the most important demethylation processes (20, 30). DMHg can be the dominant organomercurial species in deep ocean water, but is otherwise readily lost from the aquatic systems, due to its high volatility.

2.1.4 Hg chemistry in soil

All soils are affected in some way by the minerals in the rocks that are forming the soil parent material (12). However, the amount of Hg in rocks with no OM is low. Atmospheric deposition of Hg originating from anthropogenic sources and transported long distances has proven to be a significant source to surface soils, and especially in hummus-rich areas (12, 17). Transformation of Hg in soil to the volatile species Hg(0) and DMHg are contributing to the evaporation from soil to atmosphere.

The main oxidation states in soil are Hg(0) and Hg(II) (12, 31). Redox conditions, soil characteristics, pH, and the presence of inorganic or organic complexing agents are some of the most important factors affecting the speciation (9). In addition, the pH and Cl concentrations are important factors (12). Speciation can also be affected by biotic transformations, and one of particular importance is methylation. The speciation of Hg in soil, or any other environmental compartment, is important to understand the dynamics of Hg in the environment, such as retention, mobilization, and re-evaporation. Although Hg(0) under certain conditions may be present in considerable amounts, the main Hg specie in soil is Hg(II) (31).

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Hg, and especially as Hg(II), has a high tendency to form complexes (12, 32). The concentration of free Hg(II) is therefore low. Hg can form complexes with many different ligands, such as Cl^- or OH^-, and which complexes that dominates are depending on several environmental factors. Hg forms stable complexes with OM, such as humic acids (17). The bonding is often to functional groups containing sulfur (S) on the organic ligand, such as thiol group. However, functional groups containing Cl^- or OH^-are also able to form stable complexes with Hg. Hg(II) have a tendency to form covalent bonds with S containing functional groups, because Hg(II) act as a soft acid, and the S-group as a soft base.

Carboxylic acid groups and amine groups on the other hand act as hard bases, and the rule is that soft acids form stronger bonds to soft bases than hard bases, due to the polarizability (17). However, much of the Hg is associated with other functional groups on OM, due to saturation of the S-containing groups.

There have been many documentations of the strong correlation between the amount of OM and Hg in soil. Hence, levels of Hg are found to be higher in top soil or the organic layer, than deeper down in the mineral soil. Soil organic matter (SOM) can differ significantly in weight and properties, such as hydrophilicity (17). Bonding to large humic acids is associated with immobilization of Hg, but smaller complexes can lead to higher solubility and mobilize Hg, such as fulvic acids. Formation of complexes together with adsorption on particles, for instance by ion exchange interactions, cause the retention of Hg in soil (12). However, adsorption is also dependent on several factors such as redox, soil pH and soil grain size distribution.

Hg in soil is in general bound to solids and only a small fraction is to be found in the aqueous phase (17). Leaching to groundwater is generally not significant (12, 17).

However, the mobility and transport can be affected by variations in speciation. For instance, there is major differences in solubility of different Hg compounds and the solubility is also affected by the soil conditions, as mentioned with the humic and fulvic acids. In addition, can Hg bound to solid phase colloidal particles be mobile, due to the small size of the particles. Colloidal particles in the pedosphere are defined within the size range of 0.005 to 5 µm in diameter. This overlaps with the range for dissolved fraction of 0.45 µm, and these particles may enhance the mobility of Hg (17).

Obrist et al. (2011) found several correlations with Hg and other parameters (33). The Hg content was positively correlated with the C content, soil clay content, precipitation, and latitude. Up to 94% of the variation in Hg could be explained by those factors. That study was done in forest sites at remote locations, with few local sources. It was also found that when the carbon/nitrogen (C/N) ratio was decreasing, the Hg/C ratio exponentially increased. A low C/N ratio is an indication of older and decomposed OM (9, 33). The suggested explanation was long-term exposure of atmospheric Hg through depositions, and that Hg could be retained in the OM during decomposition while C was removed. This would create a higher Hg/C ratio over time, as C was gradually removed while Hg was retained (9). However, Obrist et al. (2011) points out that it is not likely that the strong change in Hg levels can be entirely attributed to the accumulation internally, and that accumulation due to sorption could be a more important mechanism (33). Halbach et al.

(2017) found that Hg in Arctic soil accumulates in the top soil layer and that it strongly correlates with SOM (34). It was also concluded that atmospheric Hg deposition was the most important pathway for the accumulation in the terrestrial system and not as influenced by crustal variations as many other elements. For uncontaminated soils it is expected that Hg concentrations are below 0.1 ppm (9).

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2.2 Mercury cycling in the Arctic Environment

Arctic as a location has some unique features which affects the cycling of mercury (29).

Huge differences in seasons, with for example the changes in hours of sunlight and temperature, which again affects thaw and freeze of soil, waters, sea ice, and rivers. This impacts the deposition and accumulation process of Hg (29).

Although Hg is an element and ubiquitous, it is not natural to find the levels of Hg that have been documented in the Arctic throughout the years (7). The Arctic is a particular fragile ecosystem that in addition to experiencing a wide range of different pollutants, also is the area where the temperature increase is most rapid. The temperature change leads to massive changes in the environment, such as glaciers melting, permafrost thawing and less sea ice. This could have massive impact on the species living there (7). Pollutants, such as Hg, other toxic metals, and a wide range of persistent organic pollutants (POPs), are adding additional stress to species and that can be critical if they are subject to multiple stressors and are on their limit (7). Despite the few local sources of Hg, it is believed that several species experience levels above threshold. This is mainly because MeHg bioaccumulate in biota and biomagnify in the food web. For humans living in the Arctic, Hg is a serious concern. Human exposure is most typically related to MeHg which they are exposed through the food. The dietary habits of indigenous people in the Arctic are often relying of species high in the food web, which makes them especially exposed to toxic levels of MeHg (7). In Figure 2.3 the bioaccumulation and biomagnification in an exemplified Arctic food web is illustrated and how humans as an apex predator can be exposed to high concentrations of MeHg.

Figure 2.3: Bioaccumulation and biomagnification of methylmercury (MeHg) in the Arctic food web. From Beckers and Rinklebe, 2017 (9).

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2.2.1 Deposition mechanisms and fate

As mentioned, there are few local sources in the Arctic, yet there are found concentrations of Hg that can cause adverse effects on the fragile ecosystem, especially in species that are in a high trophic level (7). This Hg is mostly transported via LRAT, although ocean currents is also a possible pathway, but much slower. The AMAP report from 2011 stated that anthropogenic emissions, terrestrial emissions, and emissions from ocean contributed about one third each of total deposition of Hg to the Arctic (7). However, for both the terrestrial and the ocean Hg can be naturally re-volatilized. At that time, East Asia was the region that clearly contributed the most (7).

During the polar night, the concentrations of many pollutants in the Arctic atmosphere increases. When the sun returns, several reactions start happening and these pollutants can deposit. For Hg this has been a heavily debated topic called Arctic Mercury Depletion Events (AMDE) and has been an important mechanism for explaining the observed concentrations of Hg in the Arctic. However, in recent years there has been pointed out some limitations to this mechanism and other pathways have been suggested as more important (35).

AMDE was discovered as a mechanism, when Schroeder and Anlauf et al. observed a significant decrease in ozone in the atmosphere at the same time as Hg was depleted (36).

The relation of these two events was then explained by the oxidation of Hg from GEM to RGM caused by species formed from reaction with ozone (37). Halogen radicals formed by photolysis, especially bromine (Br) and chlorine (Cl), again reacts with ozone and creates for instance BrO which can oxidize GEM to RGM. Since the radicals are formed by photolysis, the event occurs in the period of polar sunrise.

In Figure 2.4 the deposition mechanisms and fate of atmospheric Hg are illustrated, and some of the important reactions involving halogens and ozone that are believed to be involved in AMDEs are presented (26).

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Figure 2.4: Deposition and fate of mercury (Hg) in the Arctic, and with important radicals formed by solar radiation and redox reactions for oxidation and following deposition of Hg. Possible fates after deposition, for instance sedimentation, methylation and re-emission are included. From AMAP

2002 (26).

Some of the critique against AMDEs being the most important deposition mechanism, is that most of the deposited Hg from depletion events to a vast extent are re-emitted to the atmosphere (29). Obrist et al. suggest based on their Hg-deposition mass-balance study that deposition of Hg(0) is a far greater source of Hg in the interior Arctic tundra than Hg(II) deposition from AMDE and or from wet deposition (35). They also pointed out that there are little evidence for AMDE occurs to a significant degree outside of coastal areas, which also limits its overall contribution. The deposition of Hg(0) occurs all year, but is found to be enhanced during summer, due to uptake by vegetation. The uptake of Hg by Arctic tundra result in high Hg concentrations in Arctic soil and is suggested as a global sink for Hg. They also suggest that this may explain the considerable amounts of Hg transported to the Arctic ocean by Arctic rivers every year, since the amounts exceeds what can be explained from direct atmospheric deposition (35).

2.3 Permafrost

2.3.1 Three layer model

Soils formed under permafrost conditions are classified as cryosols. Permafrost is: “earth material that remains continuously at or below 0 °C for at least two consecutive years”

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(38). It has classically been divided into two layers, the active layer, and the permafrost layer. However, a transition layer can also be defined. The active layer undergoes freeze- thaw processes every year, while the transition layer can experience a variation in the degree of freeze-thaw due to variations from year to year.

The thickness of the active layer depends on several factors such as air temperature, vegetation, drainage, type of soil and rock, texture, total water content, snow cover and degree and orientation of slope (39). In this layer of the permafrost, it is more movement of water, and the activity of both chemical and biological processes are highest.

The thickness of the active layer is often thin in high latitudes (39), but since the west- coast of Svalbard is affected by the gulf stream it has the warmest permafrost around that latitude (40). This also results in great variations in permafrost thickness, and can vary from a few meters to several hundred meters depending on distance to the sea and altitude (41). It is therefore natural that the active layer thickness at Svalbard varies as well and for Svalbard it is an interval of 0.8-2.5 meters (39). In 2016-2017 the observed active layer depth in Ny-Ålesund was around 2 meters (41).

2.3.2 Mercury in permafrost

Hg cycling is complex, and some key aspects have been described in this chapter. Since there is an interplay between natural sources, anthropogenic sources, and re-emission of legacy Hg, finding the original source is difficult. Hg emitted from a source to atmosphere can be deposited and stored in the terrestrial environment, be re-emitted, methylated and transferred to biota and the food web making it a threat to humans. However, whether it is natural processes or as a result of anthropogenic activity, there are increasing number of evidence that permafrost stores a huge amount of Hg. This should be worrying because estimates from models project that permafrost in Northern Hemisphere will experience 30- 99% reduction by 2100, if the emissions of greenhouse gases are not reduced (42).

Permafrost in the Arctic has been a sink for Hg for thousands of years. For many years it was only natural sources such as the geological sources by weathering of rocks from the soil parent material, atmospheric deposits of Hg originating from volcanic eruptions, and forest fires (43). Especially after industrialization, anthropogenic emissions have increased the amount of Hg in the atmosphere and depositions to permafrost regions. Hg, mainly Hg(0), transported to the Arctic in the atmosphere are taken up by vegetation and binds to OM on the surface of the soil and in the active layer (6, 35). Due to microbial decay this Hg may be released again, but sedimentation processes bury the OM and the Hg and when it reaches the permafrost depth it becomes trapped. However, in recent years the permafrost has begun to thaw because of climate change. The buried Hg bound to OM can then become available to microbial decay and be released. Since Hg has accumulated over a long period of time, it is a significant amount that is stored in the permafrost. Schuster et al. estimates that permafrost in the northern hemisphere contains 1,656 ± 962 Gg of Hg (6). Even when using the most restrictive estimate, it still makes it the largest reservoir of Hg on the planet. Over half of the mean value is estimated to be located in the active layer of the permafrost, making it more available to chemical and biological processes (6, 40). Another study estimated 136-274 Gg of Hg in the active layer in the northern hemisphere, which is much less than Schuster et al. (2018) estimated, but nonetheless a huge Hg pool (5).

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One example is the case of the Tibetan Plateau which is a permafrost region that has been degrading for the past decades and it was concluded that it holds a large Hg pool and that much of it is mobile (44).

Schaefer et al. (2020) points out possible release pathways for Hg, for example microbial decay, fire, and leaching into groundwater which could be followed by transport by rivers.

(45). The release of Hg from thawing permafrost is also hypothesized being responsible for the high fluxes of Hg in Arctic rivers, transporting Hg to the Arctic ocean (35). This has been indicated in other studies as well (46, 47). Massive amounts of freshwater are transported to the Arctic ocean every year and a lot of this is from areas that are underlain by permafrost (29). Hg and OM are often correlated in regards to riverine transport of Hg, and a majority of Hg is associated with particulates (46).

Another study looked at the release of Hg from palsa mires in Sweden with release of Hg from erosion and flooding of eroded soil and concluded that Hg mobilization from thawing palsa mires needs to be considered (4, 48). They especially express concern about peat areas flooded with water as a critical phenomenon as it can increase methylation and there is extensive pond formation in palsa mires systems in circumpolar areas.

Elevated levels of both total Hg and MeHg has been found downstream of thaw slumps (49). Due to rapid thaw a part of the permafrost can collapse and release large amount of material to streams and rivers. St.Pierre et al. measured record high concentrations for the area, and uncontaminated sites in Canada in general. It was a large difference upstream compared to downstream. In another study, some of the same scientist study the input and output of Hg and MeHg to a large lake in the Arctic, and observed increased levels there as well (50). Here as well, slumping from permafrost increased levels in streams and rivers.

2.4 Sampling and analytical methods 2.4.1 Diffusive Gradient in Thin films (DGT)

Diffusive gradient in thin films (DGT) is a relatively simple plastic device designed to accumulate dissolved species in an environment, either water, soil, or sediment (51). It is not an analytical method, but a sampling method.

DGTs are used to sample for a duration of time, which after analysis makes it possible to get an average concentration for this time period and not just a snapshot of the environmental state. The fundamental piece is a functionalized resin gel, which will accumulate the analyte by chelation of functional groups (51). Above this is the diffusive layer of hydrogel and on and a filter, usually a 0.45 µm to filter out particles to get the dissolved fraction. These layers are put on top of the plastic piston and then a plastic cap is put on after the three layers are in place. For accumulation of metals, there is used a Chelex gel. Metal free fishlines or similar can be used to secure the device when deployed into for example a lake or river. Ions will be transported through the filter, diffuse through the diffusive gel and the species with high enough affinity for the chosen resin gel will be accumulated (51). An illustration of the DGT unit is presented in Figure 2.5.

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After collection of devices, they need to be dismantled and the gel is retrieved and put in typically a 1M nitric acid solution overnight to elute the content (51). To calculate the concentration, it is necessary to have information of deployment time, thickness of the diffusion layers and diffusion constants for the analytes. The temperature must be known in order to know which diffusion constant to use. For analysis of the content accumulated in gel is used conventional analysis methods as AAS or ICP-MS and then the necessary calculations are conducted.

Figure 2.5: Illustration of a DGT unit, with the outer components to the left and a cross-sectional illustration with the different gel layers to the right (51).

The accumulated fraction is the labile dissolved fraction of the analyte (21). For Hg, it is as mentioned in earlier chapter a wide range of chemical species and it is required to make assumptions of properties such as diffusion coefficients, which greatly differ between low- molecular weight compounds and large, typically organic complexes.

2.4.2 Microwave acid digestion

If the analytical technique requires samples to be in a liquid state, solid sample needs to be decomposed (52). Microwave-assisted wet sample digestion is a technique where concentrated acids are used in order to decompose the sample matrix (53). For organic matrices, nitric acid is much used. It is a strong oxidizing agent, especially in a heated state.

One example is Milestone UltraCLAVE system, which combines microwave heating and high pressure, and the vessel is resistant to acid, which makes it possible to use for example nitric acid in the digestion. It is not a closed vessel system, but applies pressure instead with an inert gas (52). This is to avoid both boiling and to prevent cross-contamination of samples (53).

2.4.3 Total carbon and total nitrogen

To determine the total carbon (TC) and total nitrogen (TN) in a soil sample a Primacs SNC 100 instrument can be used. The instrument uses combustion of the sample with ultrapure oxygen at 1200°C. CO2 and N oxides are produced, in addition to other gases. Before detection, the gas mixture is carried through a splitter by a carrier gas, for example helium, and some of the gases are removed. Then the C in the sample is detected as CO2 with an IR-detector. After measurement of C, the gases are carried through a copper reduction oven where N oxides are reduced to N gas. After CO2 and water is removed by carry the gas mixture through a scrubber, the N can be detected by a thermal conductivity detector (TCD).

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2.4.4 ICP-MS

Inductively coupled plasma mass spectrometry (ICP-MS) is one of the most used techniques in elemental analysis, especially regarding trace elements. A quite unique feature with ICP-MS in elemental analysis, is its flexibility regarding it capability to detect at both very low concentration and relatively high concentrations (54). The span is from sub part per trillion (ppt) to around parts per million (ppm). In addition to detection limit, the main advantages are the speed of analysis, multielement characteristics and isotopic capability.

The samples usually need to be on a liquid form, so solid samples like soil and sediments will need pretreatments like extraction or digestion (55). The liquid samples are pumped with a peristaltic pump into a nebulizer where it is used argon gas to create a fine aerosol (54). Then a spray chamber separates the fine droplets from the larger ones and are transferred further into the plasma torch. Using an intense magnetic field induced by a coil and a radio frequency generator, it is created a plasma with very high temperature. It can be up to around 10 000K. When the sample reaches the high-temperature plasma, it becomes desolvated, vaporized, atomized, and ionized. The ions are extracted from the plasma and transferred into vacuum in what is called the interface region and by a couple of skimmer cones and then in the ion optics the ions are directed towards the mass separation device and to lose what remains of neural species, photons, and particulates.

The mass separation device, for example a quadrupole, separates the species in the ion beam based on their mass to charge ratio (m/z). Lastly the different ions are detected by converting the ions into an electrical signal (54).

Interferences in ICP-MS can be both spectroscopic and non-spectroscopic sources (55).

When an analyte and a non-analyte ion share the same m/z ratio, it is classified as a spectroscopic interference. Spectroscopic interferences can be divided into four main types.

The first one is isobaric interference and is simply that isotopes from different elements can have the same m/z and the peak occurring at that m/z will have contributions from more than one element. An example is nickel and iron, which both have isotopes that occur at 58 m/z (55).

The second one is that although it is mostly created ions with a single charge in the ICP system, some elements may occur as ions with double charge. This can then result in the same m/z ratio as an element with single charge, but twice the mass (55).

Polyatomic interference is the third spectroscopic interference and results from polyatomic ions that are created in the high temperature plasma and may have the same m/z ratio as some element of interest and therefore create a interference. They can form due to the composition of the sample matrix, from argon itself and entrained atmospheric gases (55).

The fourth and last one is tailing and is a result of overlap in the spectrum from elements that have m/z ratio close to each other. This is especially a concern when one of the analytes have much higher concentrations than the other and can lead to too high concentration of the element with adjacent m/z ratio (55).

The most important non-spectroscopic interference is matrix effects, which is when the signal of an analyte is either enhanced or suppressed as a result of the properties or constituents of the sample matrix (55). This can occur in several different ways and in

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different parts of the ICP-MS system. It can affect the sample introduction by for example disturb the size distribution of droplets in the aerosol or the transport efficiency. Another possible mechanism is plasma effects, where the matrix affect the ionization. It can be a reduction in the amount of analyte that is ionized leading to a lower signal, or some elements such as C can lead to higher amount of ionized analyte. One last mechanism to be mentioned is space-charge effects, where the ion beam is broadening due to repulsion between the positively charged ions and results in fewer ions reaching the detector (55).

ICP-MS technology is also improving and there has been many inventions to deal with these interferences. It can be a specific pre-treatment of the sample, matrix removal by for instance coupling ICP-MS to chromatography or use interference free isotopes. Even more advanced ways are reaction cells or collision cells, where either chemical reactions or collisions with the unwanted ions filter them out or change their location in the spectrum (55).

2.4.5 Quality assurance and Quality control (QA/QC)

Quality assurance (QA) and quality control (QC) is necessary in analytical and environmental chemistry to ensure that the results obtained in a study is accurate, precise and in line with the set quality standards (56). Both are under the whole concept of quality management

QA is a more overall process of ensuring that the quality standards are being met, while QC is more operational and are measures to ensure precision and accuracy in an analytical result (57). QA can involve the use of accredited laboratory and certified material that guarantee a certain standard (58). For this thesis it is mostly QC that is relevant to discuss, since the direct involvement in QA is limited. QC involves the use of blanks in the different steps of the process from sampling to the final analysis by for example ICP-MS or HPLC- MS.

In the process of environmental samples there are several steps. Sampling, transportation, storage, preparation and workup, extraction and clean up, concentration, identification, and quantification (56, 57). Although it is not always possible, due to limitations of time and resources, it is ideally to have as much control as possible of the effect each of these steps have on the sample. The steps can involve possibility for contamination from handling the sample, from the instrument, cross-contamination from other samples, loss of sample during extractions and other procedures, inaccuracy in the quantification and several other possible effects (58, 59). These can affect the accuracy and the precision of the data obtained and can therefore lead to wrong conclusions.

By using blanks, standards, replicates and validate the used instruments these effects can be accounted for in the analysis of obtained data (56, 58). Blanks can be used in different ways; a field blank is introduced from the beginning of the process and is brought out in the field and exposed to the environment during sampling. Method blank is introduced when the procedures in the lab begins and contain all components except for the analyte.

A reagent blank is not taken through all the preparation steps, but otherwise similar to a method blank.

Standards are also often use for QC (56, 58). It can be for the sake of instrument calibration, quantification, but also to correct for extraction efficiency, detection efficiency.

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A standard contain a known amount of a specific substance or substances (58). External and internal standards are typically to create a calibration curve to use for quantification, but internal standard can also correct for instrumentation variations during analysis.

Certified reference material (CRM) contains a known amount of the analyte in a similar matrix and is therefore often used to check the extraction efficiency or the efficiency of a similar preparation step. For the analysis, a recovery standard or spiked samples can be used, where known amounts of the analyte is added to check the response of the detected amount.

Establishing the limit of detection (LOD) and limit of quantification (LOQ) are also important to get reliable results (58). LOD is the smallest amount of analyte that can be distinguished from noise, and LOQ is the lowest concentration of analyte that can be reliably quantified and is often restrained by the lowest calibration point in the method (60).

Another important way of securing the quality of the work is to follow standard procedures, for example an ISO standard from the International Organization for Standardization.

However, in environmental science it is not always possible to follow exactly the procedure described. Particularly in the field can this be a challenge, but also in the lab due to available resources and time, which can cause some deviations from the standard. To base the sampling methodology on a standard will create more reliable data, because it will ensure that the samples are taken in a way that makes them homogenous, avoiding contamination and are representative for the purpose (59).

For the sampling activity, a number of standards describe the plan, strategies, the actual sampling, and storage and transport. For soil sampling it is ISO 18400-101,102,104 and 105 (60-62). In addition the part 106 of the 18400 series describes QA/QC (57). There are other standards that describe different factors to consider when sampling water. There is an own standard for sampling in lakes the 5667-4 and how to preserve 5667-3 (63, 64).

More general aspects related to QA/QC is described in 5667-14 (59). Especially measures to avoid contamination and to get representative sample is important to consider. For analysis of metals, it is important to not use any equipment containing metals because it will most likely contaminate the sample. In addition it is important to avoid cross- contamination, so the sampling equipment needs to be cleaned with water from the sampling location before taking a sample. To avoid the influence of chemical reactions and biological activity, the sample should be preserved as soon as possible, for example with nitric acid to achieve a pH value from 1-2. A list of common sources of errors related to sampling of water is listed in 5667-14 (59). To avoid confusion, it is important to properly mark the sample and to get a precise location (GPS-coordinated for example).

In advance it is important to select the appropriate sampling strategy (60). If it is random samples or some sort of system, how will the sample be taken and how many. It should also be considered if it is possible and useful to do measurements of other parameters in the field, and if so, it must be done without contaminating the samples.

2.4.6 Data treatment and Statistcal Analysis

To extract information from the data the analyzes have provided it is necessary to perform some statistics. Firstly, it is useful to obtain descriptive statistics such as the sample mean, median, range of the values and the standard deviation. The mean is the sum of all values divided by the number of samples (65). The median is the value in the middle of the dataset when it is organized with increasing or decreasing values. An important feature is

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the standard deviation, which is a measure of the spread in the dataset. The standard deviation is the deviation any observation is expected to have from the calculated mead value. A high standard deviation is synonymous with a huge spread of the values in the dataset (65).

Environmental samples are taken to represent the true environment that are investigated.

The mean value of the samples is not necessarily the same as the true mean. A larger number of samples will generally result in a better estimation of what the true mean (µ) is, but it in theory it is needed an infinite number of samples to get the true value. It is useful to include a confidence interval, often set at 95%. This is an interval of which it is 95% certain that contains the true value, for example the true mean, based on the dataset.

In other words it is a 5% chance it is not in the given interval. It is important to mention that the data obtained from the analyzes itself also comes with uncertainty. Even though an analysis can provide high precision in its measurements, it is no guarantee that it is high accuracy.

To test a hypothesis about a probable cause and effect relationship, it is necessary to do a statistical test. A statistical test aim to determine whether the differences that are observed in the data are random or a significant real differences, and gives a probability for this which is the p-value (66). The p-value is a measure of how likely it is that the null hypothesis is correct. When conducting a hypothesis test you formulate a null hypothesis and an alternative one. Typically, that the group means are equal and the group means are unequal.

In order to choose the correct test, it is important to know if the data complies with the underlying assumptions of the tests. The most important is if the data are normally distributed or not. A Shapiro-Wilk test can be used to test whether the data is significantly differently distributed than a normal distribution (67). If the data are normally distributed, parametric tests can be used. Some relevant tests in this regard are the Student’s T-test to test if two means are significantly different from each other (66). If the data is not normally distributed, a non-parametric test can be used instead. For the test of two means, the Mann-Whitney U (MWU-test) test is an option. The t-test tests the differences in means for two groups and include the two sample sizes and the sample standard deviations. It can differ whether it is one-tailed or two-tailed, depending on whether the hypothesis is to only check if the mean is larger or smaller, or if it tests both. The MWU-test work in a bit different way, where a random value from one group is tested against a random value from the other group. For more than one group ANOVA can be used for normally distributed data, or Kruskal-Wallis for non-normally distributed data (66, 68).

Correlations aim to quantify whether one variable can predict the value of another. It does not however say anything about cause and effect, just if there is some relationship in how the values for the two variables vary. The correlation coefficient is a number between -1 and 1, where -1 is perfectly negatively correlated and 1 is perfect positively correlated (69). If negatively correlated, the value of one variable increase when the other decrease, but for positively they both increase if one of them increase. One such correlation, which also test if it is significant or not, is the Pearson correlation.

There are a lot of available software that can be used to perform data treatment and statistics. SPSS is a statistical software from IBM and can be used for a wide variety of statistical analysis. From descriptive data, creating charts, perform hypothesis testing and correlations.

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Boxplots from SPSS display the median as a horizontal line in the box, the box around it is from the first to the third quartile and lines above and beneath the boxes are maximum and minimum (70). First quartile (Q1) is the median of the lower half of the data and the third quartile (Q3) is the median of the upper half of the data. The maximum and minimum is within a certain range based on Q1 and Q3, so outliers are plotted as single points.

Principal component analysis (PCA) is a technique that is used to make interpretation of large datasets easier. With many variables, samples, and parameters, it can be difficult to get a proper understanding of what affects what. PCA makes this easier, since it aims to reduce the number of dimensions of the data to a 2D plot (71). At the same time, it aims to preserve as much of the statistical information about variability in the data. It does this by project the data onto principal components (PCs) and aims to summarize this in an optimal way. The different PCs are not correlated with each other (71).

The result is usually a set of 2D plots, where the x- and y-axis represent principal components (PCs) that explains the variability in the data, to a different degree (71). The first plot is made up by PC1 and PC2, and PC1 explains the variability in a horizontal direction and PC2 in a vertical. In addition PC1 explains more of the variation than PC2, meaning that on unit of distance between two points along the x-axis implies a larger difference between the two observed data, than the same distance between two points along the y-axis. Data points that are similar to each other will group together in a cluster, and cluster that are different from each other will be separated in x and y direction depending on which of the PCs that explain their differences the most, and the difference in how much PC1 explains compared to PC2.

Mercury release from thawing permafrost in the Arctic

Morten Heistad

Mercury release from thawing permafrost in the Arctic

Master ’s thesis

Morten Heistad

Mercury release from thawing permafrost in the Arctic

Abstract

Sammendrag

Acknowledgements

Table of contents

Figures

Tables

Abbreviations

1 Introduction

2.1 Mercury

2.1.1 A global pollutant

2 Background

2.1.2 Speciation, sources, and transport

2.1.3 Hg aquatic chemistry

2.1.4 Hg chemistry in soil

2.2 Mercury cycling in the Arctic Environment

2.2.1 Deposition mechanisms and fate

2.3 Permafrost

2.3.1 Three layer model

2.3.2 Mercury in permafrost

2.4 Sampling and analytical methods 2.4.1 Diffusive Gradient in Thin films (DGT)

2.4.2 Microwave acid digestion

2.4.3 Total carbon and total nitrogen

2.4.4 ICP-MS

2.4.5 Quality assurance and Quality control (QA/QC)

2.4.6 Data treatment and Statistcal Analysis