A pulse of mercury and major ions in snowmelt runoff from a small

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A pulse of mercury and major ions in snowmelt runoff from a small Arctic Alaska watershed Thomas Alexander Douglas, Matthew Sturm, Joel D. Blum, Christopher Polashenski, Sveta Stuefer, Christopher Hiemstra, Alexandra Steffen, Simon Filhol, and Romain Prevost Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03683 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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Environmental Science & Technology

Figure 1. A: a map of Alaska showing the general study location. B: a local map of the study area identifying Utqiaġvik, the Barrow Arctic Science Consortium (BASC), and the SnowNET site. C: a true color photo of the vicinity of the research area with the watershed area denoted in red. D: a digital elevation model of the watershed with the major flow path in blue and the location of the weir and water sample collection denoted by the black star. Ice wedge polygons, which affect the drainage, are clearly visible.

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Figure 2. Mercury, meteorology, and hydrology measurements in 2008. 1) gaseous elemental mercury in air; 2) snow pack depth and precipitation measured at the NOAA-GMD site; 3) air temperature from the NOAA-GMD site and discharge measured at the v-notch weir; 4) a series of repeat photographs representing different time periods during the melt. Note: time scales are different for top and bottom panels.


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Figure 3. Mercury, meteorology, and hydrology measurements in 2009. 1) gaseous elemental mercury in air; 2) snow pack depth and precipitation measured at the NOAA-GMD site; 3) air temperature from the NOAA-GMD site and discharge measured at the v-notch weir; 4) a series of repeat photographs representing different time periods during the melt. Note: time scales are different for top and bottom panels.



Figure 4. Stable oxygen isotope values of melt water and bulk snow pack from cores in 2008 (top) and 2009 (bottom). The horizontal solid lines on the y axis denote the initial snow pack value prior to the melt (-18.6 ‰ in 2008 and –21.6 ‰ in 2009).


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Figure 5. Major ion and mercury concentrations in snow melt water in 2008. Symbols on the vertical axes of the upper two panels denote the pre- melt total snow pack concentration. Vertical lines with bars denote standard deviation values for samples collected in triplicate.



Figure 6. Major ion and mercury concentrations in snow melt water in 2009. Symbols on the vertical axes of the upper two panels denote the pre- melt total snow pack concentration. Vertical lines with bars denote standard deviation values for samples collected in triplicate.


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1

A pulse of mercury and major ions in snowmelt runoff from a small Arctic Alaska watershed

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

THOMAS A. DOUGLAS1, * MATTHEW STURM2 JOEL D. BLUM3 CHRISTOPHER POLASHENSKI1,4 SVETLANA STUEFER5 CHRISTOPHER HIEMSTRA1 ALEXANDRA STEFFEN6 SIMON FILHOL1,2,7 ROMAIN PREVOST8 1,*

U.S. Army Cold Regions Research & Engineering Laboratory, PO Box 35170 Fort Wainwright, Alaska 99703; 907-361-9555; [email protected] 2 Geophysical Institute University of Alaska Fairbanks, Fairbanks, Alaska 99775; 3 Department of Earth & Environmental Sciences, University of Michigan 4 Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, New Hampshire 03755 5 College of Engineering and Mines, University of Alaska Fairbanks, Fairbanks, Alaska 99775 6 Environment and Climate Change Canada 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada 7 Now at University of Oslo Department of Geosciences Postboks 1047 Blindern 0316 Oslo, Norway 8 36 Rue Poissonaire 64100 Bayonne France


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Abstract

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Atmospheric mercury (Hg) is deposited to Polar Regions during springtime atmospheric mercury

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depletion events (AMDEs) that require halogens and snow or ice surfaces. The fate of this Hg

29

during and following snowmelt is largely unknown. We measured Hg, major ions, and stable

30

water isotopes from the snowpack through the entire spring melt runoff period for two years. Our

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small (2.5 ha) watershed is near Barrow (now Utqiaġvik), Alaska. We measured discharge, made

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10,000 snow depths, and collected over 100 samples for chemical analysis in 2008 and 2009 of

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snow and meltwater from the watershed snowpack and ephemeral stream channel. Results show

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an “ionic pulse” of mercury and major ions in runoff during both snowmelt seasons but major

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ion and Hg runoff concentrations were roughly 50% higher in 2008 than in 2009. Though total

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discharge as a percent of total watershed snowpack water equivalent prior to the melt was similar

37

in both years (36% in 2008 melt runoff and 34% in 2009) it is possible that record low

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precipitation in the summer of 2007 led to the higher major ion and Hg concentrations in 2008

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melt runoff. Total dissolved Hg meltwater runoff of 14.3 (+/- 0.7) mg/ha in 2008 and 8.1 (+/-

40

0.4) mg/ha in 2009 is five to seven times higher than reported from other arctic watersheds. We

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calculate 78% of snowpack Hg was exported with snowmelt runoff in 2008 and 41% in 2009.

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Our results suggest AMDE Hg complexed with Cl- or Br- may be less likely to be

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photochemically reduced and re-emitted to the atmosphere prior to snowmelt and we estimate

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that roughly 25% of the Hg in snowmelt is attributable to AMDEs. Projected Arctic warming,

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with more open sea ice leads providing halogen sources for AMDEs, may provide enhanced Hg

46

deposition, reduced Hg emission and, ultimately, an increase in snowpack and snowmelt runoff

47

Hg concentrations.

48



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Introduction Mercury is deposited to snow and ice surfaces in the Arctic and Antarctic during springtime

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atmospheric mercury depletion events (AMDEs) that require sunlight, frozen surfaces, and a

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reactive bromine source such as sea ice or halogen rich snow.1 During AMDEs, gaseous

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elemental mercury (Hg0; GEM) is oxidized to reactive gaseous mercury (HgII; RGM) which

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associates with particles into particulate Hg (PHg). PHg has been reported in snow and sea ice

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surfaces at concentrations above 1,000 ng/L, far higher than snow at lower latitudes2-4 despite

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limited Arctic Hg emission sources. During AMDEs mercury is deposited to snow and ice

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surfaces primarily as RGM and PHg.5 RGM Hg is photochemically reduced back to GEM and

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emitted to the atmosphere or is retained by the snowpack where it can contribute to spring melt

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runoff.6 Since the Arctic is snow-covered for up to three quarters of the year a majority of the

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surface runoff, including any AMDE-sourced Hg, occurs during the spring freshet.7 Snowpack

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Hg has a number of fates during spring melt- remaining in snowmelt water, sorbing to vegetation

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or soil surfaces,8 accumulating in soil by microbes,9 depositing to lakes,10 or exporting to the

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ocean.11

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Investigations of the fate of Hg in Arctic ecosystems during spring melt have observed

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photochemical reduction and emission of RGM from snowpacks as GEM but between 25 and

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75% of the AMDE Hg is believed to remain in the snowpack prior to snowmelt runoff.11-18 Few

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studies have discussed the impact of PHg on snowpack Hg concentrations, however, initial

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snowmelt Hg concentrations far greater than mean snowpack Hg concentrations have been

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reported.11, 12, 19 This “ionic pulse” of Hg is similar to what has been measured for sulfate and


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nitrate,20 chloride,21 and other major ions22 in initial snowmelt runoff.23-26 If solutes, particularly

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toxic bioaccumulative trace metals like Hg, are preferentially eluted from snowpacks during

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spring melt there may be physical, chemical, or biological processes promoting Hg retention and

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storage in terrestrial or nearshore environmental compartments.

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We undertook this study to ascertain whether there was a pulse of Hg and major ions in

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snowmelt runoff near Barrow, Alaska. Our study was located in a small (2.5 ha) watershed near

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the Arctic Ocean coast where AMDE chemistry and elevated Hg in snow and ice have been

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studied extensively.3, 27-31 In late winter prior to snowmelt (April) and during snowmelt runoff in

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May and June 2008 and 2009 we made over 10,000 snow depth measurements and 36 snow

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water equivalent (SWE) measurements in the watershed and surrounding areas to calculate the

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end of winter SWE and associated Hg loading. Gaseous elemental Hg concentrations in air and

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meteorology were measured continuously. We collected snow and water samples daily from melt

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initiation until the snowpack was gone. Snowpack, meltwater, and stream channel water were

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collected and analyzed for total dissolved Hg, major ions, and stable oxygen and hydrogen

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isotopes. Continuous discharge measurements allowed quantification of the water that left the

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watershed versus pre-melt snow water equivalent. We also calculated total snowpack and runoff

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water Hg fluxes. We know of no other study that presents measurements of Hg in air, snowpack

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and snowmelt water, and runoff through an entire melt season let alone two.

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Experimental Section

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Study location and hydrological setting. Our field site is on the Arctic coastal plain 6 km

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southeast of Barrow, Alaska (71.285°N, 156.575°W; Figure 1). The site is located in a pristine

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location within the Barrow Environmental Observatory upwind and far from any infrastructure



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for the prevailing wind direction.3 The 1981-2010 mean annual air temperature for Barrow was -

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11.2 °C with mean annual precipitation of 115 mm water equivalent and a total snowfall depth of

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826 mm.32

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We used a digital elevation model (DEM), and on site topographic land surveys to identify

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the watershed boundary and area (0.025 km2; 2.5 ha). The DEM was developed from airborne

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LiDAR with horizontal and vertical accuracy of 0.30 and 0.15 m, respectively.33 The watershed

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outline was ground-truthed with a high resolution (± 0.1 m) differential global positioning

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system (DGPS).

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The watershed drains directly into Elson Lagoon which is connected to the Beaufort Sea and

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is representative of Arctic Coastal Plain wetland watersheds with low hydraulic gradient and a

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deranged drainage network.34, 35 The watershed is underlain by continuous permafrost hundreds

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of meters thick and surface topography consists of a heterogeneous mix of high-centered, low-

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centered, and transitional ice wedge polygons.36 Soils are gelisols with an organic rich surface

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layer above silty clays and silty loams.37

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Typical end of season permafrost active layer thaw depths measured at the site in late fall

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2007 and 2008 were 30-50 cm. which is similar to measurements since 1994 at the nearby Cold

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Regions Research and Engineering Laboratory (CRREL) Circumpolar Active Layer monitoring

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site.38 The area develops a classic tundra snowpack comprised of 10-20 cm of basal depth hoar

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topped by a 10 to 20 cm layer of hard wind slabs with an occasional surface layer of recent

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snowfall or faceted crystals.39, 40 Late winter snowpack depths typically vary from 0-10 cm on

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windblown high centered polygon tops to 60-100 cm in low lying drifts. Bulk snow densities are

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generally 250-350 g/cm3 at the end of the winter.39


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The watershed acts as a "fill-and-spill" system generating surface runoff after exceeding

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storage threshold capacity.35, 41 As snow ablation begins, meltwater percolates to the bottom of

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the snowpack and “fills up” available watershed storage (small ponds, low-centered polygons,

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troughs, and organic layers). The storage controls connectivity within the watershed and the

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release (or "spill") of water needed to generate surface runoff. The partitioning of total snowpack

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water into storage, runoff, and vertical fluxes depends on antecedent storage conditions and the

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snow ablation rate. Vertical fluxes of evapotranspiration can be negligible during snowmelt

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season as both evaporation from snowpack and condensation onto the snow surface can occur

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depending on difference in vapor pressure between cold snow surface and overlaying air42. After

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ablation is completed storage recedes below the runoff-generating threshold and channel

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streamflow ceases. Occasionally, surface runoff may occur during major wet precipitation

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events.

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We installed a 2 m wide custom made V-notch weir across the base of the channel in the fall

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of 2007 (Supplementary Information Figure 1). Upstream water surface elevation (WSE) was

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measured during snowmelt with a sonic sounder (SR-50; Campbell Scientific, Utah) on a 2 m

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pole above the weir. The SR-50 measured distance between the base of the sounder and the

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water surface on a five minute interval during snowmelt in 2008 and 2009. A rating curve was

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developed by relating WSE to manual discharge measurements at the weir by recording the time

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needed to fill a container of known volume. Seventy-five concurrent manual discharge and WSE

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measurements representing a variety of streamflow conditions in 2008 and 2009 yielded a rating

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curve with an r2 of 0.94 between WSE and discharge. Supplementary Information Figure 2

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provides a summary of the discharge over time for both snowmelt runoff periods.

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Sample collection. Numerous pre-melt snowpack cores were collected in May and early

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June both years using a pre-cleaned, serrated, high density polyethylene (HDPE) 10 cm diameter

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tube inserted gently from the surface to the base of the snowpack ensuring no layers were

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preferentially collected or excluded. The snowpack core was poured into a pre-cleaned 500 mL

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HDPE sample bag. Snow was allowed to melt overnight and water was filtered through 0.45 µm

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HDPE filters into 60 mL pre-cleaned HDPE bottles for major ion and hydrogen and oxygen

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stable isotope measurements. A separate melted core was poured into precleaned PTFE (Teflon)

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wide mouth 1 L jars for Hg concentration measurements. Samples for Hg concentration were not

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filtered because Hg is very particle-reactive and we were interested in measuring dissolved Hg

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plus Hg sorbed to particles. During analysis Hg in solution and on the surfaces of particles was

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reduced to Hg(0) with SnCl2 and quantified following established methods.17, 39, 43, 44

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Meltwater was collected when it began to pool at the base of the snowpack. Initially, this

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consisted of water traveling downward through percolation columns. We used pre-cleaned tools

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to excavate a small trench 5 m upstream of the V-notch weir to sample this initial melt water.

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Once the meltwater started to move horizontally down the slope in the stream channel we

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collected flowing water at the same location. All water samples were filtered through pre-cleaned

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acid washed 0.45 µm HDPE filters in the field. Samples for major ion concentrations and

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hydrogen and oxygen stable isotope ratios were collected into pre-cleaned 60 mL HDPE bottles.

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Samples for total dissolved Hg plus Hg sorbed to particles were collected into 250 mL PTFE

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(Teflon) bottles and analyzed as described above for snow. Detailed information on sample

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collection and laboratory analytical procedures are provided in the Supplementary Information.

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Gaseous Elemental Mercury in Air and Speciated Atmospheric Mercury. Gaseous

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elemental mercury (GEM) was collected using a Tekran (Toronto, Ontario, Canada) 2537A

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analyzer at the Barrow Arctic Science Consortium (BASC) 6 km northwest of a long-term snow

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and meteorology measurement site called SnowNET. A mobile 2437A unit was also operated for

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three weeks on the sea ice 5 km from the BASC site. Based on similar GEM values from the

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instruments at both locations we are confident the BASC GEM measurements are a good proxy

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for the SnowNET site. GEM was collected continuously at 5 min intervals using two Hg traps

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following previously described methods.4, 30 Speciated atmospheric mercury in forms of GEM,

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RGM, and PHg was collected at the sea ice location using a Tekran 1130/1135 system.45 RGM

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and PHg are operationally defined as “what is collected using the Tekran 1130/1135 speciation

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system collected on a KCl denuder and quartz filter, respectively”. The data presented here were

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collected March 1-31, 2008 and March 14-26, 2009.

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Results and Discussion Snowpack, Snowmelt Runoff, and Meteorology. The snowpack in the area typically

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accumulates through a series of less than a dozen discrete snow fall events (Figures 2 and 3 show

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precipitation and snow ablation from February through June).46 The mean watershed snowpack

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depth on May 15 (N> 5,000 for each year) was 48.3 cm in 2008 and 46.6 cm in 2009, typical for

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a normal winter snowpack.39 Early June snow depths and SWEs were 34 cm and 141 mm in

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2008 (N=15) and 40 cm and 122 mm in 2009 (N=21), respectively. The 2009 SWE includes the

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sum total of end-of-winter SWE plus the water equivalent of a precipitation event May 28-31 (10

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cm of snow; 3.5 mm of water equivalent). The sun stays above the horizon for 24 h/day starting

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May 10th so there was continuous sunlight throughout both snowmelt periods.



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In 2008 air temperatures went above 0°C around May 18 and daily high temperatures

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exceeded 0°C almost daily during the snowmelt period. Meltwater started pooling at the base of

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the snowpack in the afternoon of May 28 and the first sample representing lateral flow from the

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channel was collected the afternoon of May 31. The snowpack was at 50% land coverage by

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June 8 and almost completely gone by June 12, leaving a waterlogged tundra surface. In 2009

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there was a weeklong warm period in late April which led to surface melt and snowpack settling.

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However, consistent daily maximum air temperatures above 0°C were not observed until May 18

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and no melt water made it to the base of the snowpack during the April warm period. Sufficient

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meltwater for sample collection pooled at the base of the snowpack in the afternoon of May 22

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and the first sample representing lateral flow from the channel was collected in the afternoon of

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May 28.

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The runoff lasted for 13 days in 2008 and 19 days in 2009 with peak discharge followed by a

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slow drying of the tundra surface both years. Based on the watershed area, SWE, and

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precipitation values, the total snowpack water volume prior to melt was 3.29 x 106 L in 2008 and

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3.95 x 106 L in 2009. Comparing these values with the total cumulative snowmelt discharge we

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calculate 36% of the 2008 SWE and 34% of the 2009 SWE left the watershed as snowmelt

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runoff. The similarity in runoff ratio for both years suggests the lasting effects of antecedent

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surface and subsurface storage deficit from the record dry summer and fall throughout the North

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Slope of Alaska in 2007.47 One third of the snowpack water volume left the watershed as runoff

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and two thirds remained in the watershed as surface and subsurface storage with a small fraction

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lost to evaporation and evapotranspiration during snowmelt. Evapotranspiration is negligible

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during the snowmelt period.48

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Atmospheric Mercury. Gaseous elemental mercury concentrations in air (Figures 2 and 3)

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exhibited heterogeneous values deviating strongly from the global northern hemisphere ambient

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background of 1.6 ng/m3.18 The GEM values were typically at or below 1 ng/m3 and even

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approached zero during a few periods in March and April 2008. These extremely low values are

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associated with AMDEs which deposit mercury to snow and ice surfaces.2, 3, 13, 17, 18, 27, 29, 30, 42, 49

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Following many AMDEs were periods of elevated GEM values (above 2 ng/m3) which are likely

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the result of GEM emission from photochemical reduction of RGM.14, 17 Some of the GEM

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values above 2 ng/m3 occurred when air temperatures approached or exceeded 0°C and are likely

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associated with snowpack surface melting, where AMDE Hg is predominantly deposited. Snow

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metamorphism has been found to lead to photochemical reduction of RGM to GEM and GEM

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emission.11 In March 2008, the RGM and PHg concentrations averaged 20.6 (range 1.3 to 245.5)

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and 128.5 (range 1.2 to 549.2) pg/m3, respectively (Figures S2 and S3). In March 2009, the RGM

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and PHg concentrations averaged 53.6 (range 2.4 to 167.1) and 74.9 (range 0 to 453.3) pg/m3,

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respectively. There was thus considerable speciated mercury available in the air for deposition to

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the snowpack and, in both years, PHg concentrations were significantly higher than RGM in

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March.

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Stable Water Isotopes in Snow and Snowmelt. Stable oxygen isotope values from

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snowpack cores were generally more negative than initial meltwater collected from the base of

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the isothermal snowpack (Figure 4). The δ18O values of melt water increased steadily over time

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and by the end of the snowmelt period surface water δ18O values were ~5‰ lower than initial

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bulk snowpack values. This suggests preferential fractionation of meltwaters with lighter isotope

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values into initial snowmelt water. Similar trends in the evolution of snowmelt stable isotope



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values have been observed in laboratory experiments tracking snow and meltwater δ18O values50

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and field based studies of interactions between downward percolating meltwater and snow (ice)

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crystals in melting snowpacks.51

234 235

Major Ion Composition of Snow and Snowmelt. In 2008 and 2009 initial snowmelt runoff

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major ion concentrations (except nitrate) were far greater than bulk snowpack values and they

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decreased as discharge increased toward peak runoff (Figures 5 and 6). Nitrate is likely sourced

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initially from the snowpack and, as the melt progresses, nitrate from vegetation and soil surfaces

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is added to melt waters. In 2008 we collected three melt water samples from the base of the

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snowpack before channelized flows started. These three samples yielded the highest values for

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all major ion concentrations measured. In 2009 the first five samples represent meltwater from

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the base of the snowpack and include some of the highest major ion concentrations for that melt

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year. The meltwater samples representing the end of the melt had similarly elevated major ion

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concentrations.

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The overall trend was for initially high concentrations decreasing as discharge increased

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followed by increasing concentrations of major ions as discharge decreased. In 2008 there was

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one large snow fall event during the melt (May 27, 6.3 cm of snow with 3.5 mm of water

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equivalent). In 2009 a major snow and rain event occurred from May 28-31 (10 cm of snow; 3.5

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mm of water equivalent). During this four day period major ion concentrations increased and

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then decreased once discharge climbed as the melt was reinvigorated by warm air temperatures.

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Snowmelt water interacted with vegetation and surface soil as it pooled onto, and eventually

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moved laterally across, the tundra surface. This interaction could have provided a source of ions

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to meltwater and the observed behavior of the chemical signal (increasing as the melt progressed


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and discharge decreased) is consistent with a strong interaction that varied with the pre-existing

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dryness of the tundra. For example, in 2008 major ion values in the snow were lower than in

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2009, yet the 2008 early meltwater major ion concentrations were two to three times greater than

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2009. It is possible that extremely dry soils under the 2008 snowpack experienced greater

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vertical saturation due to a lower presence of ice in surface soil pores and deeper percolation into

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surface vegetation and soils could have caused the higher runoff concentrations. Further, the

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extremely low precipitation summer 2007 conditions may have led to greater retention of Hg as

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part of dry deposition or litterfall in the watershed that was not exported by rain events.52 The

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same dry conditions, and the coastal location of the watershed, may have contributed to an

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increase in sea salt sourced major ions and this would have been particularly noticeable at the

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end of the melt runoff period when discharge waned and there was a longer time for runoff

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waters to interact with surface vegetation and a slowly thawing seasonal thaw front. This could

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explain the increase in major ions later in the melt runoff period. The extremely dry 2007

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summer conditions would have led to less runoff and less dilution of dissolved constituents such

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as sorbed major ions and mineral weathering products in vegetation or surface soils. This could

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be the reason for the markedly greater Hg and major ion concentrations in runoff throughout

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2008 compared to 2009. However we note the initial snowmelt water percolating to the base of

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the snowpack, which had little to no exposure to surface soils and vegetation, yielded the highest

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melt water major ion concentrations in both years.

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In total, these results suggest the preferential elution of major ions out of the snowpack

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during melt onset with Na+, K+, Ca2+, Cl-, and Br- representing the most abundant ions. This

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“ionic pulse” occurred in both years suggesting it is a perennial feature of the melt. The presence

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of Br- as a component in the enhanced meltwater signal is of significance because it has been



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hypothesized HgBr2, BrHgOBr, and BrOHgOBr are associated with RGM and complex

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atmospheric Hg during AMDEs.53, 54

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Mercury Concentrations in Snow and Snowmelt Water. Total dissolved mercury

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concentrations plus Hg sorbed to particles (hereafter total dissolved Hg) in pre-melt snowpack

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and stream channel runoff waters were greater in 2008 than 2009 (Figures 5 and 6), by a factor

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of about two. This is similar in magnitude to the difference in major ion concentrations in 2008

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compared to 2009. In both years initial meltwater Hg concentrations were twice or more what we

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measured in the snowpack prior to the melt. The highest Hg concentrations were measured in

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initial meltwater collected at the base of the snowpack. However, the concentrations we

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measured, mostly from triplicate samples collected at the same time on a given day, exhibited

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large heterogeneities in concentration and did not mimic the overall concentration-discharge

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relationship evident with major ion concentrations. The runoff Hg concentrations were always

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greater than core pre-melt snowpack Hg concentrations (13.9 ng/L in 2008; N=20 and 12.5 ng/L

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in 2009; N=31). It is possible that vegetation, soil, or aerosol particles provided sources of Hg

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and ions to the melt waters we collected. We made every effort possible to prevent the

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interaction between the melt waters and the soil and vegetation but could not prevent all

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interactions. We collected the melt waters in real time from the base of the snowpack to

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minimize the time of exposure to the underlying soil and vegetation and tried to avoid sampling

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standing water. Nonetheless, we cannot rule out that some of the Hg came from non-AMDE

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sources.

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Based on the overall snowpack depth and snow water equivalent in the watershed we

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calculated net end of winter snowpack Hg prior to the melt of 1826 ng/m2 in 2008 and 1975


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ng/m2 in 2009. These values are near the high end of the range of snowpack Hg reported in 2007

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spring melt from a fjord in Ny-Ålesund, Norway (200-2160 ng/m2).12 Unlike the major ions, Hg

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did not show a marked decrease in concentrations as discharge increased suggesting no dilution

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with increased discharge. We attribute this to the presence of multiple Hg sources in the

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watershed. These include AMDE and non-AMDE Hg present in the snowpack that became part

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of snowmelt, desorption of Hg from vegetation surfaces18, and/or interactions with surface soil

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material. Since snowmelt occurs at the end of winter when the ground is completely frozen and

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three is no ground water in the watershed percolation through the soil column is not a likely Hg

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source. However, the relative contribution of Hg from these different sources and their potential

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timing as Hg sources during the melt is unknown. We note that Hg deposited to the snowpack

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during springtime AMDEs would be comprised of particulate-associated RGM located in the top

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of the snowpack3, 13, 27, 39 while major ions were likely spread more evenly throughout the

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snowpack. As such, there are likely different physical and chemical elution processes, sources,

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and timing during the melt for the RGM than for the major ions. Finally, the timing, length, and

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chemical composition (i.e. presence of Hg-halide complexes) of Hg deposition during AMDEs

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could play a role in how much Hg is re-emitted versus remains in the snowpack to be a

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component of the melt.

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The Hg source for the early melt runoff is predominantly melting snow with minor

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contributions from the underlying tundra suggesting an elution process similar to the preferential

319

elution of major ions. The elevated atmospheric RGM and PHg concentrations we measured

320

indicate a strong likelihood of some atmospheric contribution to the snowpack during the AMDE

321

season. We note that during both of the spring melt events the runoff Hg concentrations were

322

consistently about four times the Hg concentration of the snowpack before and during the melt



Page 22 of 30

15 323

period and the runoff Hg concentrations do not decrease as the snowpack converts to melt water.

324

Based on this and the other measurements in this study our approximation is that 75% of the Hg

325

exported from the watershed during the snowmelt period came from non-AMDE sources while

326

25% is attributable to deposition from AMDEs. This percent would be lower if there were other

327

Hg sources to the snowpack and we assume there are no other Hg sources to the snowpack or

328

snowmelt runoff prior to and during the melt period.

329

For reference, in an earlier study, Hg concentrations in Barrow surface snow during AMDEs

330

yielded values from 30 to 200 ng/L while surface snow collected during non-AMDE conditions

331

yielded Hg concentrations from 10 to 70 ng/L.3, 17 We previously found that lower layers in the

332

Barrow snowpack, representing snow deposited from November to February when AMDEs were

333

not active, yield far lower Hg concentrations of 0.5-8 ng/L.3 Using mean meltwater Hg

334

concentrations of 30 ng/L in 2008 and 15 ng/L in 2009 and total discharge measured each year

335

the total dissolved Hg runoff from our 2.5 ha catchment was 14.3 mg/ha in 2008 and 8.1 mg/ha

336

in 2009. This is considerably higher than the 2.1 mg/ha reported for western Hudson Bay.14

337 338 339

An Ionic Pulse of Hg and Major Ions in Barrow Snowmelt Runoff. Snowmelt stable isotope fractionation, 50, 51 and the preferential elution of trace metals and

340

major ions has previously been reported to provide an “ionic pulse” of solutes to the base of

341

snowpacks in Arctic,55 continental,20, 26, 56 and mountain locations,55 and controlled laboratory

342

experiments.22 The enhanced major ion and Hg concentrations we measured in snowmelt waters

343

were likely caused by ion exclusion from the quasi-liquid water layer, a disordered micro-thin

344

layer of water on snow grain surfaces.58 In 2008, the initial snowmelt streamflowwater yielded

345

Hg concentrations ranging from ~40-70 ng/L while the 2009 initial snowmelt streamflow water


Page 23 of 30


16 346

Hg concentrations ranged from ~12-21 ng/L, despite similar snowpack Hg concentrations prior

347

to the melt in both seasons. The elevated Hg concentrations in 2008 compared to 2009 are

348

similar to elevated major ion concentrations in 2008 melt waters compared to 2009. This

349

suggests Hg behaves similarly to major ions in terms of the magnitude of the elevated

350

concentrations in the pulse of early melt waters. As snow and crystal temperatures approach 0°C

351

snow grains metamorphose into rounded melt grain clusters and water moves downward through

352

percolation columns to the base of the snowpack. Larger molecules or ions on the snow crystal

353

surfaces (like atmospherically deposited sulfate, lead, and mercury) likely undergo preferential

354

elution compared to smaller ions like chloride.21, 58 If Hg was present in the snowpack as Hg-Br

355

complexes, particularly if it was sorbed to the outer surface of aerosol particles or snow crystals,3

356

these molecules would have also been preferentially eluted from the snowpack.

357

Our results do not provide constraints on the percent of Hg deposited to snowpacks during

358

AMDEs that is photochemically reduced and emitted and is thus not part of snowmelt runoff

359

because our sampling occurred after most of the AMDE season had ended. However, the relative

360

contribution of PHg compared to RGM in the overlying atmosphere suggests a significant

361

portion of the deposited mercury is particulate-bound in the snowpack and is available for

362

snowmelt runoff6 and is not completely reemitted.59, 60 We can track the fate of the Hg in the

363

snowpack by calculating the quantity of mercury immediately prior to the melt, during the melt,

364

and following snowmelt. We calculated 78% of the 2008 snowpack Hg and 41% of the 2009

365

snowpack Hg left the watershed as snowmelt runoff and entered Elson Lagoon (see calculations

366

in Supplementary Information). This means 22% of the seasonal snowpack Hg was retained on

367

the tundra ecosystem or emitted during melt in 2008 and 59% of the Hg remained or was emitted



Page 24 of 30

17 368

in 2009. Presumably these remainders stayed behind in the watershed, sorbed to vegetation and

369

soil particles.

370

The ultimate fate of Hg retained in the watershed following the melt season is not well

371

known but elevated levels in peat deposited since the industrial revolution61 suggest much of it

372

remains in peat soil profiles. The results from this study cannot quantify the amount of the peat

373

Hg that is from AMDEs or other sources and we cannot identify what microbial or biological

374

processes control the retention of Hg from AMDEs or other Hg sources to the watershed. Little

375

is known about the consequences of a projected warmer future Arctic on Hg deposition, melt

376

runoff, and remobilization from the land surface. However, we postulate a warmer Arctic, with

377

increased first year sea ice and more open sea ice leads providing halogen sources for AMDEs,

378

greater Hg deposition will occur in the coastal zone.62-65. Current trends toward thinner sea ice, a

379

longer open water season, and increased ice dynamics are already being observed66, 67 and these

380

conditions are favorable for enhanced AMDE Hg deposition.31. The results from this study show

381

that between one quarter and half of the Hg concentration of pre-melt snow remains on land. As

382

a consequence, any increase in AMDE or other Hg depositional sources of Hg to Arctic coastal

383

locations could lead to enhanced Hg deposition to Arctic terrestrial and nearshore ecosystems.

384 385

Corresponding Author

386

*U.S. Army Cold Regions Research & Engineering Laboratory, PO Box 35170 Fort Wainwright,

387

Alaska 99703; 907-361-9555; [email protected]

388 389

Author Contributions


Page 25 of 30


18 390

All authors contributed to the field sampling, laboratory measurements, and manuscript

391

development and writing. All authors have given approval to the final version of the manuscript.

392

Funding Sources

393

This work was funded by the U.S. National Science Foundation Office of Polar Programs Grants

394

ARC-0435989 (U.S. Army Cold Regions Research and Engineering Laboratory), ARC-0435893

395

(University of Michigan), PLR-0632160 (University of Alaska Fairbanks) and the U.S. Army

396

Corps of Engineers Engineer Research and Development Center Army Basic (6.1 and 6.2)

397

Research Programs. The atmospheric work was funded by Environment and Climate Change

398

Canada and Canadian International Polar Year funding.

399 400

ACKNOWLEDGMENT

401

Extensive logistical support was provided by the Barrow Arctic Science Consortium and

402

their assistance is greatly appreciated. Chun-Mei Chiu, Amanda Grannas, Glen Liston, Anna K.

403

Liljedahl, and Glenn Rowland assisted with field sampling and surveying efforts.



Page 26 of 30

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A pulse of mercury and major ions in snowmelt runoff from a small

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