Investigating the occurrence and accumulation of perfluoroalkylated substances and other persistent organic pollutants in snow and ice of the Earth’s Polar regions

Garnett, Jack and Halsall, Crispin and Wynn, Peter and Leeson, Amber (2021) Investigating the occurrence and accumulation of perfluoroalkylated substances and other persistent organic pollutants in snow and ice of the Earth’s Polar regions. PhD thesis, Lancaster University.

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Abstract

Persistent organic pollutants (POPs) are a highly diverse group of synthetic chemicals that are released into the environment from human activities and display a host of adverse effects in wildlife and humans. Perfluoroalkylated substances (PFASs) are one major group of industrial chemicals that are globally produced in vast quantities and are subject to various global regulations. However, PFASs are present in the Earth’s Polar Regions and yet little is known about their environmental fate and behaviour, particularly their accumulation and fate in snow and ice and the wider cryosphere. Moreover, climate change is altering the cryosphere, affecting sea ice and its properties for example, which in turn may affect the biogeochemical cycling of these pollutants and could lead to altered or enhanced biological exposure and uptake. This thesis examines the accumulation of PFASs in snow and firn as a useful recorder of environmental contamination in Antarctica, a region which lacks a multi-decadal time series of air monitoring data. The thesis also investigates in detail the processes that result in the uptake, distribution and accumulation of persistent organic pollutants in sea ice, particularly in the context of a warmer Arctic, where sea ice is now dominated by brine-rich single season ice. There is a lack of monitoring data on the levels of PFASs in Antarctica, and so a snow core was taken from Kohnen Station (East Antarctica plateau) to determine the historical deposition rates in the region. Results revealed low or non-detectable levels of perfluorosulfonic acids (PFSAs) but showed increasing trends of perfluorocarboxylates (PFCAs) between 1958 – 2017. Deposition rates for PFCAs in snow varied depending on chain length, with PFBA (C4) showing up to 2-orders of magnitude higher (1.3 and 280 ng m-2 yr-1) than PFOA (C8) (1.6 and 12.6 ng m-2 yr-1). Furthermore, correlations between PFCAs of varying chain length were strong (r2 > 0.7, p < 0.01). This information suggests that PFASs in the remote polar region, dominated in the main by the PFCAs, share similar transport pathways which can be largely attributed to the long-range atmospheric transport of volatile chemical precursors, which includes fluorotelomer alcohols (FTOHs) and other new CFC-replacement compounds. Together, this shows that global emissions of this group of chemicals is ongoing and have continued to rise following regulatory measures and also demonstrates the usefulness of a snow/firn core as a proxy for past atmospheric levels. In lieu of air monitoring infrastructure across Antarctica, then analysis of dated snow/ice cores provide a useful ‘barometer’ for global pollution and allow the effectiveness of international regulations in curbing the use of certain chemicals to be realized. Given the marked alterations to the cryosphere through climate change, there is a need to assess how the biogeochemical cycling of chemical contaminants may also be affected, particularly with respect to changing sea ice and altered pathways of exposure to organisms at the base of the marine food web. Hence, a series of laboratory experiments (in a dedicated sea ice chamber) were conducted in order to understand the chemical fate and behaviour of various ‘legacy’ and ‘emerging’ POPs in artificial sea ice. Results showed that POPs are incorporated into sea ice during its formation and growth with their distribution strongly influenced by salt (brine) dynamics. Enrichment factors (i.e. relative concentration) also demonstrate that some POPs (e.g., long-chain PFASs) behave differently with respect to other homologues, leading to their enrichment in bulk sea ice by up to 3-fold more (with respect to salt). This suggests that additional factors which are independent of physical processes also play a role in their environmental behaviour and chemical uptake during ice growth. Further results gathered from a controlled melt experiment indicates that this behaviour is related to the physicochemical properties of POPs, which alters the chemical partitioning between solid (i.e. ice) and liquid (i.e. brine) phases within sea ice. Importantly, concentrations of POPs were calculated for the in-situ brine environment, which show that POPs are present at significantly higher concentrations (e.g. up to 1-order of magnitude higher for PFOA) than in the initial underlying seawater and indicates the importance of brine for controlling chemical fate and biological exposure in young/single season sea ice, the dominant ice type in a warming Arctic. The presence of PFASs in the polar marine environment raises particular concern during summer when periods of high biological activity may be accompanied by intense chemical uptake and subsequent transfer through the marine food web. Moreover, the occurrence of icerafted snowpack along with the thinning of sea ice observed in recent decades risks the formation of more snow-ice, an ice type that is particularly susceptible to rapid melting through surface temperature fluctuations, which in turn could release its chemical burden into the underlying ocean. Various samples of sea ice, snow and under-ice seawater were collected during late summer in the Arctic. Based on previously determined differences in partitioning and environmental tendencies of PFASs with varying chain length, chemicals were grouped into short-chain (C4 − C7) and long-chain (C8 −C14) PFASs. Concentrations of ΣPFAS short-chain (around 3.0 ng L-1 ) were significantly higher in comparison to ΣPFAS long-chain (around 0.1 ng L-1) in snow. While in seawater, concentrations of ΣPFAS long-chain (around 1.0 ng L-1) were higher than ΣPFAS short-chain (around 0.4 ng L-1). This demonstrates that snow is still a major atmospheric pathway of contemporary PFASs (predominantly short-chain PFASs) to the Arctic marine environment and also shows seawater holds a large chemical burden of PFAS (historical and current) emissions. PFASs were also measured in sea ice and the levels and composition reflected inputs from snow and seawater. In particular, some surface layers (i.e. snow-ice) displayed especially high concentrations of ΣPFAS short-chain (up to 32 ng L-1) and ΣPFAS long-chain (up to 2.6 ng L-1). Furthermore, results showed that PFAS concentrations in the under-ice seawater were over 2-fold higher at one site which is suspected to have endured an early season melting episode. These results demonstrate that snow and sea ice are important dynamic reservoirs of these chemicals, and that rapid melting can dramatically influence the transfer and hence concentrations in adjacent compartments like surface seawater. In summary, the main findings from this thesis indicate that atmospheric transport of POPs and their chemical precursors from industrial regions to very remote regions like Antarctica (and well removed from local anthropogenic activities) is ongoing and continuing to rise. Secondly, the incorporation, distribution and enrichment of POPs in sea ice is strongly influenced by the presence and dynamics of ice brine; a major feature in fresh, single season ice which now dominates ice cover across the Arctic Ocean. Thirdly, snow-ice and hence the ice-rafted snowpack contains relatively high concentrations of PFAS. These findings in combination with changes to the physical aspects of the ice environment such as the transition from thicker multi-year sea ice a much thinner first-year sea ice, extreme temperature fluctuations and hence earlier and more intense seasonal ice melting, are all likely to influence the biogeochemistry and cycling of POPs in the Polar marine environment. In particular, sympagic organisms of the lower marine food web (e.g. ice algae, copepods, amphipods etc) whose niche includes brine channels on the underside of sea-ice may be subject to increased contaminant exposure in a warming Arctic