Mechanisms and Implications of Î±-HCH Enrichment in Melt Pond

Article pubs.acs.org/est

Mechanisms and Implications of α‑HCH Enrichment in Melt Pond Water on Arctic Sea Ice M. Pućko,†,‡,* G. A. Stern,†,‡ D. G. Barber,† R. W. Macdonald,†,§ K.-A. Warner,† and C. Fuchs‡ †

Centre for Earth Observation Science, University of Manitoba, 460 Wallace Building, 125 Dysart Road, Winnipeg, R3T 2N2, Canada Department of Fisheries and Oceans, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6, Canada § Department of Fisheries and Oceans, Institute of Ocean Sciences, 9860 West Saanich Road, Sidney, British Columbia, Canada, V8L 4B2 ‡

S Supporting Information *

ABSTRACT: During the summer of 2009, we sampled 14 partially refrozen melt ponds and the top 1 m of old ice in the pond vicinity for αhexachlorocyclohexane (α-HCH) concentrations and enantiomer fractions (EFs) in the Beaufort Sea. α-HCH concentrations were 3 - 9 times higher in melt ponds than in the old ice. We identify two routes of α-HCH enrichment in the ice over the summer. First, atmospheric gas deposition results in an increase of α-HCH concentration from 0.07 ± 0.02 ng/L (old ice) to 0.34 ± 0.08 ng/L, or ∼20% less than the atmosphere-water equilibrium partitioning concentration (0.43 ng/L). Second, late-season ice permeability and/or complete ice thawing at the bottom of ponds permit α-HCH rich seawater (∼0.88 ng/L) to replenish pond water, bringing concentrations up to 0.75 ± 0.06 ng/L. α-HCH pond enrichment may lead to substantial concentration patchiness in old ice floes, and changed exposures to biota as the surface meltwater eventually reaches the ocean through various drainage mechanisms. Melt pond concentrations of α-HCH were relatively high prior to the late 1980s, with a Melt pond Enrichment Factor >1 (MEF; a ratio of concentration in surface meltwater to surface seawater), providing for the potential of increased biological exposures.

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INTRODUCTION Melt ponds are common features of the summertime sea ice cover in the Arctic Ocean resulting from the positive net surface energy balance.1 In late May, the solar radiation in the central Arctic warms the snow cover sufficiently to initiate melt, which is manifested by meltwater accumulating in surface ice depressions usually above the seawater level.2 As the melt season progresses, the ponds deepen and shrink in diameter,3 eventually covering ∼20−50% of the ice area for about a month.2 In the latter half of the season, sea ice warms enough such that increased permeability allows surface meltwater to percolate into and under the ice until the hydraulic head (melt pond water level relative to sea level) is balanced to zero.4 In advanced stages of the melt sea ice allows for complete opening of the brine drainage network thereby allowing some surface melt ponds to become saline due to seawater intrusion up through the sea ice. Holes thawed through the ice appear toward the end of the melt season (July/August), but ponds eventually refreeze at the end of August.4 α-HCH is a legacy organochlorine pesticide (OCP) used globally in large quantities between the 1940s and 2000 as the major constituent of technical HCH.5 The peak of α-HCH emissions in the 1970s (∼200−250 kt annually) was followed by a rapid decrease in global usage and atmospheric concentrations in the 1980s.6,7 Water concentrations in the western Arctic Ocean increased until the early- to mid-1990s, when they started decreasing gradually.8 α-HCH is a chiral © 2012 American Chemical Society

compound consisting of two mirror-image enantiomers. The enantiomer fraction (EF) is defined as the concentration of the (+) enantiomer divided by the sum of concentrations of both enantiomers.9 Industrial α-HCH, which is racemic (enantiomer fraction = 0.5), undergoes enantiomer depletion by microbial breakdown, selective biological uptake, biotransformation and elimination in food webs.10,11 In the Beaufort Sea, atmospheric α-HCH EFs are racemic under ice-covered conditions with a mean ± SD value of 0.504 ± 0.008.12,13 Surface seawater is depleted in (+) enantiomer with a mean EF ± SD ranging from 0.436 ± 0.00314 to 0.442 ± 0.023;15 similarly, first-year sea ice (FYI) is 0.451 ± 0.022.15 HCH pathways within the snow-ice-brine system of FYI have been delineated (ref 15 (ice), ref 16 (snow), refs 17−20 (snowice), ref 21 (ice-brine)), but the limited measurements of HCH in perennial pack ice reveal only that concentrations are relatively high and more variable.15 Here, we describe mechanisms and implications of α-HCH enrichment in melt ponds during the ice summer survival. Received: Revised: Accepted: Published: 11862

July 31, 2012 October 1, 2012 October 5, 2012 October 5, 2012 dx.doi.org/10.1021/es303039f | Environ. Sci. Technol. 2012, 46, 11862−11869

Environmental Science & Technology

Article

Figure 1. Canadian Ice Service (CIS) sea ice concentration chart for the western Arctic Ocean (http://ice-glaces.ec.gc.ca) with melt pond sampling locations (thick first year ice, >120 cm, in green; and old ice in brown) (A) along with the NSIDC sea ice concentration chart for the circumpolar Arctic (http://nsidc.org/; B).

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(model 4000, Control Company, Friendswood, TX, ± 0.05 °C). Old ice samples and newly formed ice were collected with a 9 cm i.d., Mark II coring system (Kovacs Enterprises, Lebanon, NH). After collection, ice samples were melted in tightly closed containers. Melt pond water was collected from beneath the new ice deploying a Wildco Kemmerer water sampler (Ben Meadows Company, Yulee, FL), through the hole immediately after ice core extraction. Laboratory Analysis. Salinity of melted ice samples and melt pond water was calculated from conductivity and temperature using a HACH SENSION5 portable conductivity meter (Hach, Loveland, CO, ± 0.01). Samples (4−8 L) were subsequently spiked with d6-α-HCH surrogate (recovery standard) and pumped through a 0.7 μm Glass Fiber Filter (Whatman, Kent, UK, 42.5 mm i.d.), which was contained in a 47 mm in-line preassembled single stage filter holder (Savillex, Minnetonka, MN), followed by Oasis solid-phase extraction (SPE) cartridges with HLB 20 cc, 1 g (Waters, Mississauga, Canada). A Masterflex peristaltic pump (Cole-Parmer, Vernon

EXPERIMENTAL SECTION Samples were collected from three locations in the southern Beaufort Sea during the CCGS Amundsen ArcticNet/GeoTraces cruise between 31st of August and ninth of September 2009 (Figure 1). Field Sampling. Three old ice floes with partially refrozen melt ponds were sampled for α-HCH concentrations and EF in melt pond water, new ice on the pond (2 m. Melt ponds were measured for major and minor axis length, perimeter length, pond depth, and new ice thickness. Temperature of the surface of new ice, melt pond water and air were measured using a Traceable digital thermometer 11863

dx.doi.org/10.1021/es303039f | Environ. Sci. Technol. 2012, 46, 11862−11869


Article

Table 1. α-HCH Concentration, α-HCH Enantiomer Fraction (EF) and Salinity (S) of Melt Pond Water, New Ice (1 year) supplies the meltwater, initial melt pond α-HCH concentrations will be lower due to progressive loss of brine from the ice (desalination), reaching values as low as 0.07 ± 0.02 ng/L measured in this study, with α-HCH EF of 0.49 ± 0.02 and salinity of 1 ± 1. Most likely, α-HCH concentration in melt ponds in the Beaufort Sea region in 2009

at the melt onset was between 0.07 and 0.19 ng/L reflecting the origin of meltwater, e.g. snow-covered versus bare old ice. As the melt season progresses, melt ponds undergo α-HCH gas exchange with the atmosphere striving toward the equilibrium concentration. Using eqs 3 and 4, and a Beaufort Sea α-HCH air concentration of 16 pg/m3 measured in the spring of 2008 under ice-covered conditions,13 we calculated the melt pond α-HCH gas exchange equilibrium concentration at 0.43 ng/L, a value 20% higher than the average α-HCH concentration in the fresher melt pond group from this study (0.34 ng/L). Atmospheric concentration of α-HCH in the Beaufort Sea increases after ice break-up due to revolatilization from seawater; however, in permanent ice pack conditions (ice concentration >75% in the summer) in which we sampled, αHCH evasion from seawater has never been observed.12,13 Thus, in our calculations, we used α-HCH atmospheric concentration from before ice break-up measured over FYI, which is the closest spatially and temporally available value. The atmospheric origin of α-HCH increase in fresher group of melt ponds is further corroborated by significantly higher α-HCH EF in the melt pond water than in the old ice samples (p = 0.005, Figure 3B) or snow and upper 1 m of thick FYI meltwater in the spring of 2008 in the Beaufort Sea15 (p < 0.001), and relatively low salinity (Figure 3B). Melt ponds that have had brine channels completely opened or that thawed through the ice by the end of the melt season become replenished with seawater, with the average α-HCH concentration of 0.88 ± 0.03 ng/L and EF of 0.44 ± 0.01 as measured under thick FYI and old ice in the Beaufort Sea in April and May 2008.15 This replenishment will lead to an increased thaw hole α-HCH concentration and salinity, and decreased α-HCH EF (Figure 3B). Based on eq 5, the mixing ratio of seawater volume to melt pond water volume that would account for the observed average increase in salinity between 11866



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facilitate mixing because temperatures decrease with depth from a surface temperature of about 0.5 °C3 to near freezing (about 0 °C) at the bottom. The gas exchange model was run for melt pond areas and volumes measured in this study at the end of the summer; however, the initial surface melt in May is vast and shallow providing for A:V pond ratios exceeding the maximum sampled herein of 12.5.3 This circumstance, we speculate, should significantly advance the dry deposition process reducing the time required for surface meltwater to reach α-HCH gas exchange near equilibrium concentration possibly to several days. Implications of α-HCH Enrichment in Melt Ponds for Distribution in Old Ice. α-HCH enrichment in melt ponds through both atmospheric deposition and replenishment with seawater will lead to locally increased concentrations in the old ice. The first mechanism will result in vertical concentration patchiness, while the latter will lead to horizontal concentration inconsistency within the old ice floe. As the α-HCH atmospherically enriched melt pond water refreezes at the surface, newly forming ice will entrap α-HCH in a solvent depleting manner,8 leading to further increased concentrations in the underlying melt pond water as the ice thickens, and likely redistribution of α-HCH toward increasing concentrations downward in a completely refrozen pond. A similar refreezing scenario can take place at the bottom of the ice when fresher surface meltwater drains through the ice in the latter half of the melt season, forms an under-ice melt pond with a false bottom,29 and subsequently refreezes in an upward direction. Biological Implications of α-HCH Atmospheric Deposition to Melt Ponds. Using α-HCH historical concentration data for the Beaufort Sea8 and eqs 3 and 4, we calculated the αHCH concentrations in melt pond water that would be reached at near equilibrium gas partitioning for 1989, 1997 and used measured values for 2008−2009 (Figure 5A). In 1989, α-HCH concentrations were significantly higher in the surface melt pond water than in the under-ice seawater (7.55 versus 3.75 ng/L), while in 1997 and 2008/09 they were lower (1.01 versus 2.61 ng/L and 0.34 versus 0.90 ng/L, respectively). α-HCH deposited from the atmosphere into melt ponds have the potential to enter the ocean because a substantial portion of meltwater flows horizontally on the ice to eventually be discharged to the water beneath it through macroscopic flaws, seal breathing holes, and off the floe edges30 (Figure 5B). Additionally, a substantial portion of the surface meltwater remaining on the ice at the end of the melt season does not refreeze but, rather, percolates into and eventually drains and dissipates under the ice30 (Figure 5C). The former process will prevail in the first half of the melt season, while the latter will gain in significance as the season progresses.4 α-HCH in surface melt ponds and in the surface layer of the Arctic Ocean due to meltwater drainage have the potential to enter resident biota starting with exchange across membranes of primary producers and thence, through bioaccumulation or dilution (Figure 5B and C) passing into zooplankton, and higher trophic levels. There are at least five types of algal communities associated with the summer ice habitats that can be particularly affected by α-HCH enriched melt pond drainage: surface melt pond flora, bottom (interstitial) and subice algae, under-ice pond flora, and brackish halocline flora.31,32 Melt Pond Enrichment Factor (MEF) as a Measure of the Risk of Increased Exposures. We propose that the MEF, defined as a ratio of α-HCH concentration in the melt pond water at near gas exchange equilibrium to the

melt ponds and thaw holes in this study is 3.9. This degree of mixing would result in the α-HCH concentration in the thaw hole of 0.78 ng/L using eq 6, which matches the measured average α-HCH concentration in thaw holes (0.75 ± 0.07 ng/ L). Atmospheric versus oceanic origin of the α-HCH melt pond enrichment can be distinguished with EF analysis. α-HCH EF shifts from racemic to nonracemic with increasing fraction of open water as the melt season progresses,12,13 and thus, should not be used as a definite determinant of the α-HCH origin. However, EF can certainly provide additional information in regions that remain 80−90% ice-covered for the entire summer such as perennial ice pack. Melt Pond Coverage Duration As Limiting Factor for α-HCH Atmospheric Deposition. Melt ponds are shallow water reservoirs with relatively large surface area, which facilitates α-HCH atmospheric deposition. For the first 14− 21 days, melt pond surface water is well isolated from the underlying ice as the meltwater drainage rates are greatly reduced due to the formation of a low-salinity superimposed ice layer resulting from refreezing of fresh and warmer meltwater at the pond-ice interface.4 Only after about 30−50 days from the melt onset does the permeability of ice increase enough to allow melt pond water vertical drainage into and under the ice cover.4 Whether or not a melt pond reaches the α-HCH gas exchange near equilibrium concentration, defined as 80% complete saturation, depends on duration of pond coverage as well as area-to-volume (A:V) ratio for individual ponds. Using eqs 7−11 and α-HCH concentrations from 2008−2009, we modeled the α-HCH concentration in the melt pond water as a function of gas exchange duration for an average A:V, maximum A:V, and minimum A:V melt pond sampled in this study (Table 1S; Figure 4). Shallow melt ponds (high A:V)

Figure 4. α-HCH concentration in melt pond water (α-HCHMW) as a function of gas exchange duration for average area (A) and volume (V) melt pond with the range between the maximum and minimum areato-volume (A:V) ratio melt pond in 2008−2009; a, approximate pond coverage duration from ref 2.

approach α-HCH gas equilibrium within as little as 10 days of gas exchange at the average wind speed of 5 m/s, while deep ponds (low A:V) require as much as two months. The usual pond coverage duration in the Arctic of 35−58 days2 should be sufficient for the vast majority of ponds to approach atmosphere-water equilibrium under the assumption of complete daily mixing of meltwater. The complete mixing assumption is reasonable because (1) relative shallowness of ponds facilitates wind-driven mixing, and (2) density gradients 11867



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Figure 5. Conceptual schematic illustrating α-HCH concentration in melt pond water at the near gas exchange equilibrium with the atmosphere in relation to the surface seawater concentration in 1986, 1997, and 2008−2009 (A), and fate of surface meltwater in early (B) and late (C) summer in 1986 and 2008/09; water colors denote α-HCH concentration, FYI, first year ice; MYI, multiyear ice.

concentration in under-ice seawater, provides a simple index to assess the risk of increased (MEF > 1) versus decreased (MEF < 1) exposures as a result of melt pond contaminant enrichment. We calculated MEFs for α-HCH in 1989, 1993, 1997, 2004, and 2008−2009 at 2.0, 0.2, 0.4, 0.1, and 0.4, respectively. Until the late 1980s, when the use of HCH was greatly curtailed, α-HCH levels in the melt pond water would have been significantly higher than in the surface seawater providing for the potential of increased biological exposures. This phase was followed by a rapid increase in α-HCH seawater concentrations as a result of a lag between the peak in emissions and the peak in Arctic seawater concentrations,8 parallel to the decrease in melt pond water concentrations resulting from decreased atmospheric concentrations. Thus, since the late 1980s MEFs were below 1 suggesting that melt pond water then led to dilution of HCH exposures underneath sea ice.

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ACKNOWLEDGMENTS

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REFERENCES

We thank the crew of the CCGS Amundsen for the field work assistance without which this study could never have been accomplished. We thank Bruno Rosenberg for help and advice regarding laboratory analysis. We greatly appreciate valuable comments on the draft of the manuscript from Terry Bidleman, Liisa Jantunen, and Fiona Wong. Finally, we thank the Canadian program office of the International Polar Year, the Natural Sciences and Engineering Research Council (NSERC), Canada Foundation for Innovation (CFI), Canada Research Chairs (CRC), Canada Excellence Research Chairs (CERC), the Department of Fisheries and Oceans Canada, ArcticNet, Centre for Earth Observation Science, and the University of Manitoba for funding.

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ASSOCIATED CONTENT

S Supporting Information *

Table 1S with locations and physical characteristics of sampled melt ponds is available as Supporting Information (SI) to this manuscript. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 11868



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Mechanisms and Implications of Î±-HCH Enrichment in Melt Pond

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