There are some differences for Hg, Pb, Cd and Zn in soil between July and August as are observable in the boxplots in Figure 4.2 to Figure 4.5. However, none of the differences are statistically significant. This could be explained by limited sample size. When excluding the mineral soil samples from August it is only 8 samples in that group. It would on the other hand be biased to include them, since there has been established that for example Hg is more abundant in organic soil. Hg shows an increase in concentration from July to August, while Cd, Pb and Zn show a decrease. For Cd it is especially high uncertainty, with a very large spread in the data for the August samples. This makes it more difficult to estimate the true tendency for Cd. This could indicate that there are some differences in sources between Hg and the other selected trace metals.
A up-concentration of Hg in organic soil during summer could have multiple explanations It is not necessarily only one factor affecting, and that they affect in the same direction. If Hg stored in the permafrost becomes available to microbial decay, Hg can be released (45). However, it is possible that Hg(0) released is not emitted to the atmosphere, but binds to SOM in the organic layer or is absorbed by vegetation (45). In fact, Schaefer et
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al. estimates that only 16% of Hg released from microbial decay escapes as Hg(0) to the atmosphere, while the majority is re-absorbed or bound by vegetation and SOM (45).
Another alternative is that Hg is emitted in the beginning of the thawing period but is re-deposited onto the surface and is therefore found in the organic layer. If some of the Hg is emitted to the atmosphere, this would make it more difficult to detect with the sampling method used in this project. An alternative is to measure Hg(0) gas in the soil pores as Obrist, Pokharel and Moore did (91). They also suggested Hg in mineral soil to be immobilized, due to oxidation, sorption, or dissolution processes.
Atmospheric deposition is one of the most important sources of Hg in surface soil, and this has also been indicated for the study area (34, 35). With the relatively new knowledge about tundra uptake by Hg(0) this strengthen this explanation (35). Obrist et al. also suggested this effect to be enhanced during summer, due to vegetation uptake. The vegetation around Ny-Ålesund however, is quite scarce in many locations, but for example Kiærstranda is greener, with more mosses and grass and a moister ground.
Differences between locations could create some noise in the analyses between periods.
There are more samples from some locations than other as well, which could amplify differences. Since the same locations were sampled for both periods it is still likely that a difference between the periods would be observable.
One last important aspect is that the majority of the sampling locations are within some hour walking distance from Ny-Ålesund. Although not a large settlement, there are some anthropogenic sources, such as a diesel power plant. Some locations could also be affected by the mining activity that lasted up to the 1960s. There is also a practice field for shooting, which could affect the Pb levels in the area, but it was not sampled close to this area.
5.2.2 Differences in water samples
Water was sampled from ponds found on the tundra with the underlying hypothesis that they will serve as gas traps for the Hg evaporating from the thawing permafrost below.
Firstly, it is important to point out, that for Hg, the majority of samples from July 2020 were below detection limit. This is affecting the strength of the statistical tests for Hg. The inclusion of data from 2018 and 2019 was done in order to get more data on Hg for water ponds. However, for 2020 there is an obvious difference for Hg in water. Only one sample from July was above the detection limit, while for August only two samples were below the limit. Those samples were from Storvatn, which is much larger than the other ponds and small waters that were sampled. It is also interesting that most of the samples from August are clearly above the detection limit, suggesting a significant difference between the two periods, although this cannot be tested statistically.
With 2018 and 2019 included, it is a significant difference for Hg in unfiltered samples, but not for the filtered ones. That could be because there is a difference between filtered and unfiltered samples but based on the samples from 2020 there is no obvious difference.
When inspecting the data, the 2020 data are generally higher in concentration than for 2018 and 2019. This was confirmed with a Kruskal-Wallis test, where 2020 levels are significantly higher. It is not obvious that this is due to some contamination either, since neither Pb, Cd or Zn show the same trend. Zn have significantly differences, but it arises from differences between 2018 and 2019.
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For the unfiltered samples, there is also a significant difference between the periods for lead. Pb shows a clear difference between filtered and unfiltered samples, indicating that large complexes or association with particles is more dominant than free ions. The other elements however have a slight increasing tendency between the periods. More abundant elements, such as Ca, Mg and K, show an increase, and for Ca and Mg the increase in concentration is significant. That all the elements show increasing tendencies could be explained by a more concentrated aquatic environment, due to evaporation of water. Since Hg is volatile, it might be expected that Hg as well would evaporate and escape the aquatic environment under such circumstances. Therefore it might be an input of Hg into the aquatic environment during this period as well. As for soil, a probable explanation could be atmospheric deposition. This could also explain why other long range transported elements show the same increasing tendency. The trend is not as obvious however, and Pb has some local sources due to use of weapons for polar bear protection and a practice field for shooting.
Hg can be released as Hg(0) from the thawed soil after microbial decomposition of OM bonded with Hg, and that some of the Hg vapor gets trapped in the thaw pond on the surface. Higher concentration in the thaw ponds in August than in July could support this hypothesis, since more of the soil beneath would have been thawed, the microbial activity is likely to be higher and more Hg would be available. On the other hand, it is the unfiltered samples that shows most difference, this would imply that Hg vapor have been oxidized and bound in larger complexes, or adsorbed on particles. This fits better with atmospheric deposition as main explanation. However, for the 2020 values, there was a clear difference for both filtered and unfiltered samples.
5.3 Leaching experiment
Leaching from thawing permafrost soil is listed as one of the release pathways by Schaefer et al. and that leaching of dissolved and particulate C facilitate transport of Hg(II) to rivers (45). Formation of thaw ponds on the surface of the tundra, is eroding on the permafrost, and could cause input of Hg to the pond. The large seasonal changes leads to snow and glacier melt in the spring, which could cause Hg to be washed away with the meltwater, and in that way be transported to nearby rivers, lakes, or the sea.
A leaching experiment was therefore conducted as a part of this project, to give some information about the mobility of Hg. The results are a bit difficult to interpret, since almost all the samples are below the detection limit, and those possible to quantify are only just above. The detection limit was 0.0110 µg/L, which is relatively high when comparing it to the levels for Hg in the water samples from Ny-Ålesund. This means that although the levels are not detected, do not mean that they are unsignificant in environmental context.
The samples used in the experiment was from Brøggerdalen and Kiærstranda. These areas were selected because many samples were from Brøggerdalen, and it was useful to have this area represented in the experiment. Kiærstranda was an area that was further away from local sources of pollution, and a bit different environment in regard to vegetation and moisture. The experiment was conducted with fixed pH values, but other environmental factors, such as redox potential, is likely to have an impact in the environment. There are for instance differences in solubility for Hg(0) and Hg(II). Since there were few samples above detection, it was not possible to establish a relationship with pH. However, pH is known to affect the adsorbed fraction of metals (12). Hg, is one of the metals that can be
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adsorbed even at moderately acidic conditions. The experiment used pH 4, 5 and 7 from the starting point, but perhaps pH 3 should have been included. However, it was considered less environmentally relevant for the area, since the lowest pH measurement for soil was around 4.