AD-A270 430E~~o]' mlllllll_*02

Elution of Ions from Melting SnowChromatographic Versus Metamorphic MechanismsJames H. Cragin, Alan D. Hewitt and Samuel C. Colbeck July 1993

DTICELECTEOCT 129W3' A

aCold Ac

Water Vapor Flux

a Warm

ti t2 t3

93 10 8 103 93-23896

Columns of natural and laboratory-aged snow grains and frozen water dropletswere washed with deionized distilled water and with synthetic precipitationsolutions to Investigate both snowpock chemical froctionalton and preferentialIon elution. Concentrations of CI-, NO- and SO,2- in sequential fractions of thecolumn's eluate showed no chromatographic effects, indicating that ice crystalsdo not possess selective affintly for Inorganic anions. Consequently, preferentialchemical elution previously observed In melting snow Is not caused by snowacting as a chromatographic column. Additional column experiments involvingelution from frozen solution drops and from aged snow showed that bothfractionation and preferential elution were strongly influenced by Ion exclusionduring the snow crystal growth.

Cover- Theoretical depiction of chemical changes occurring during snow

For conversion of SI metric units to U.SiBrlish customary units of measurementconsult Standard Pracice for Use of the Internotional System of Units (S), ASTMStandard E380-89a, published by the American Society for Testing and Maler-ials, 1916 Race St., Philadelphia, Pa. 19103.

CRREL Report 93-8

US Army Corpsof EngineersCold Regions Research &Engineering Laboratory

Elution of Ions from Melting SnowChromatographic Versus Metamorphic MechanismsJames H. Cragin, Alan D. Hewitt and Samuel C. Colbeck July 1993

Pcpslae foroNTIS cRA&I {UD 1,C TAE83ý'i'U-,a-)"oul,•ed [

Just •if~icto ........ ...................

Availability Codies

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Prepared forOFFICE OF THE (-HIr- r)F ENG'NEERS

Approved for public release; distribution is unlimited.

PREFACE

This report was prepared by James H. Cragin, Research Chemist, Snow and Ice Branch,and Alan D. Hewitt, Research Physical Scientist, Geological Sciences Branch, Research Divi-sion, and by Dr. Samuel C. Colbeck, Senior Research Scientist, of the US. Army Cold Re-gions Research and Engineering Laboratory. Funding for this study was provided underDA Project 4A161102AT24, Research in Snow, Ice and Frozen Ground, Task SS, Work Unit 002,Chemistry of Snow Covers.

Earlier versions of this work were presented at the 46th and 48th Eastern Snow Confer-ences. The authors appreciate the helpful review comments received as a result of theseconferences, especially those of Professor H.G. Jones of the University of Quebec. Dr. RE.Davis and Dr. D. Meese of CRREL also provided helpful comments on the manuscript.

The contents of this report are not to be used for advertising or promotional purposes.Citation of brand names does not constitute an official endorsement or approval of the useof such commercial products.

ii

CONTENTS Page

Preface .......................................................................................................................... iiIntroduction ................................................................................................................. 1M ethods ....................................................................................................................... 2

Chem ical analysis ................................................................................................... 2Snow collection ....................................................................................................... 2Natural and synthetic snow grains--chromatographic experiments ................ 2Laboratory grain growth-metamorphism experiments .................................... 2C olum n elution ..................................................................................................... 4

Results and discussion ................................................................................................ 4Chromatographic experiments ............................................................................ 4Metamorphism experiments .................................................................................. 8

C onclusions ............................................................................................................... 12Literature cited .......................................................................................................... 12A bstract ........................................................................................................................ 15

ILLUSTRATIONS

Figure1. Schematic of experimental arrangement used for aging snow ....................... 32. Schematic of apparatus used for chemical elution experiments ..................... 43. Normalized cation concentrations in sequential eluate aliquots for natural

snow elution ................................................................................................ 54. Normalized anion concentrations in sequential eluate aliquots for natural

snow elution ............................................................................................... 55. SO -/C 1- and N03/C 1- ratios in sequential eluate aliquots for natural

snow elution ................................................................................................ 56. Ion balance for natural snow elution ................................................................ 67. Normalized anion concentrations in sequential eluate aliquots for elution

from frozen solution droplets .................................................................. 68. Normalized SO4-/CI- and NO/C 1- ratios in sequential eluate aliquots

from frozen solution drops ....................................................................... 79. Normalized anion concentrations in sequential eluate aliquots for

chromatographic experiments using frozen deionized water drops ...... 710. Normalized SO94-/C1- and N0-/C17 ratios in sequential eluate aliquots

for chromatographic experiment where synthetic snow solutionsolution was allowed to flow over frozen distilled water drops ............. 7

11. Original snow used for metamorphosis/elution experiments consisting of0.2- to 0.3-mm rounded crystals ............................................................... 8

12. Snow crystals aged for one week with an impcxzd temperature gradient or36°C /im; sample taken from bottom third of column 1 ........................... 8

13. Snow crystals aged for two weeks; sample taken from bottom third ofcolum n 2 ..................................................................................................... 8

iii

Figure Page14. Snow crystals aged for four weeks; sample taken from bottom third of

colum n 3 ...................................................................................................... 815. Snow crystals aged for eight weeks; sample taken from bottom third of

colum n 4 ..................................................................................................... 816. Snow crystals aged for eight weeks; sample taken from top two-thirds of

colum n 4 ..................................................................................................... 817. Concentrations of SO04-, H-, N03 and C1- in initial aliquots of eluate from

snow aged for 7,14,28 and 56 days ...................................................... 1018. Theoretical depiction of chemical changes occurring during snow meta-

m orphosis ................................................................................................. 1019. Hypothetical distribution of SO- and C1- within a mature, aged snow

grain that has grown by vapor deposition at the bottom ..................... 1120. Individual normalized SO-/ NO3 ratio for initial elution aliquots ........... 11

TABLES

Table1. Bulk ionic composition of homogenized natural snow used for chemical

elution experim ent .................................................................................... 32. Composition of solution used to prepare synthetic snow grains ................. 33. Composition of eluant solution used for anion chromatographic experi-

m ent .......................................................................................................... 34. Average normalized ionic ratios for initial meltwater eluate aliquots ........ 11

iv

Elution of Ions from Melting SnowChromatographic Versus Metamorphic Mechanisms

JAMES H. CRAGIN, ALAN D. HEW1TF AND SAMUEL C. COLBECK

INTRODUCTION While this model is useful, it does not describe thephysical or chemical mechanisms that produce such

When snow melts, the concentration of solutes in a two-phase system.the meltwater is not constant but varies with time: The exact cause of preferential elution is still poor-the initial 30% of meltwater contains 50-80% of the ly understood. One proposed mechanism is thattotal solutes in the snow (ohannesssen et al. 1975, there are chromatographic effects when meltwaterJohannessen and Henriksen 1978). This phenome- percolates through a snowpack (Tranter et al. 1986).non, now technically referred to as "fractionation" In other words the snowpack itself acts as an ionor the "ionic pulse," can contribute to the episodic chromatograph, with ice grains selectively retain-spring acidification of streamwater, colloquially ing and retarding the elution of certain ions (e.g.,called "acid flush." The highly concentrated ionic Cl-) over others (e.g., NO and S024-). The extentpulse has been the focus of several laboratory (Col- and strength of this sorption depends upon the phys-beck 1981, Bales et al. 1989) and field (Williams and ical and chemical properties of both the sorbed spe-Melack 1991) studies which have shown enhanced cies and the substrate. As a substrate, ice can sorbionic concentrations in meltwater from snow that has molecular vapors such as SO 2 and NO) from the at-been subjected to melt-freeze cycles. mosphere (Valdez et al. 1987), but whether ice can

A more recently observed phenomenon is "pref- sorb inorganic ions from meltwater is unknown.erential elution" within the highly fractionated ini- Another mechanism proposed by Hewitt et al.tial snowpack meltwater; that is, not all ions are en- (1989) is that preferential chemical elution in melt-riched to the same extent and some ions elute or are water is a result of preferential ion exclusion fromremoved sooner than others. This process was first snow grains during snow metamorphosis. Whenreported by Davies et al. (1982) and has been exten- snow undergoes metamorphosis, some individualsively investigated experimentally by Brimblecombe, ice crystals grow larger at the expense of others (Col-Davies, Tranter, and co-workers at the University of beck 1987). While these grains are growing, chemi-East Anglia and University of Southampton, United cal constituents are being incorporated into and ex-Kingdom. Although the specific order of elution var- cluded from the ice crystals with different efficien-ies with the age of the snow and its chemical cies. This produces mature snow grains with acomposition, NO and SOý4- generally appear in nonhomogeneous distribution of ions. When snowmeltwater fractions before Cl-. The composition of containing these mature grains melts, ions are re-meltwater has been modeled in terms of the mixing leased or eluted sequentially with time, and the re-of two different liquid solutions: a highly concentrat- suiting elution order is related to the exclusion effi-ed surface brine and a more dilute solution derived ciency of different ions imposed by the recrystalli-from snow-grain interiors (Brimblecombe et al. 1988). zation process.

In this report we present results of experiments trode was calibrated with low ionic strength buffersconducted to evaluate the importance of both chro- (Orion).matographic and metamorphic processes upon Analytical standards for the determination of Cl,snowpack elution chemistry. Columns of naturally NO3 and SO2 were prepared from NaCI, KNO 3and laboratory-aged snow and artificial ice pellets and (NH4)2SO4 (Baker, reagent grade). The concen-were washed with solutions of deionized disdin-ed tration of the working standards and the determin-water and the eluate collected in sequential fractions. ation of pH were verified daily by the analysis ofThis process simulates the removal of ions from the National Institute of Science and Technologymetamorphosed snow grains by rainwater percola- simulated rain water reference standards, SRM-2694tion through a snowpack, and has been shown 1&11.(Tranter et al. 1992) to be analogous to the percola-tion of snow meltwater itself. Snow collection

Snow used in these experiments was collected onMETHODS two separate occasions from a rural location in Han-

over, N.H.: one fresh snowfall in January after anAll plastic, glass, and stainless steel laboratory overnight precipitation event and another in March

utensils used for snow collection, grain growth, sam- from a two-month-old snowpack. The fresh snowple storage and analysis were thoroughly cleaned was composed of a mixture of submillimeter nee-as described previously (Hewitt et al. 1989,1991). dies and irregular crystals, while the aged snow con-

sisted of rounded 1- to 2-mm grains. The fresh snowChemical analysis was used for metamorphosis experiments while the

Analyte concentrations were determined using a aged grains were used for the chromatographic ex-flame (FAA) and graphite furnace (GFAA) atomic periments.absorption spectrophotometer, an ion chromato-graph and a pH electrode. The small (1-mL) aliquot Natural and synthetic snow grains-restricted analysis volumes to 200 pL per instrumen- chromatographic experimentstal technique. Bulk compositions of the naturally aged snow

Sodium concentrations were determined by FAA samples and the solution used for the frozen dropswith a Perkin-Elmer model 403 spectrophotometer. are given in Tables 1 and 2, respectively. ArtificialTo reduce sample volumes, 100-gL aliquots were ice beads were made by freezing 10-jil drops of ei-injected into a conical vial before aspiration into the ther Mili-Q (Millipore Corp) deionized water or thisburner chamber. Potassium, calcium and magne- water spiked with nitric acid, ammonium chloridesium were determined by GFAA with the same spec- and ammonium sulfate to produce the concentra-trophotometer and a Perkin-Elmer model 2200 heat- tions shown in Table 2. The water droplets were fro-ed graphite atomizer. Precision of FAA and GFAA zen on polyethylene sheets in a Class 100 clean airmeasurements was ±10% or better. station located inside a coldroom at -12°C. The fro-

Anions were determined with a Dionex model zen hemispherical drops weighed an average of 6.42010i ion chromatograph, equipped with an HPICE- mg, with the loss of mass being attributed to subli-AS3 separator column, AFS 1-2 anion fiber suppres- mation. Deionized water or deionized water spikedsor and 50-mL sample loop. The eluant was 3-mM with nitric acid and ammonium salts representativeNaHCO 3/2.4-mM Na2CO3 and the regenerate was of precipitation were employed as column eluants12-5-mM H2SO4, flowing at 2.9 mL/min and 3.0 mL/ (composition given in Table 3).mim, respectively. The determination of ammoniumion was also performed by ion chromatography. For Laboratory grain growth-this analysis a HPIC CS-1 separator column, CFS metamorphism experiments1-2 cation fiber suppressor, and 100-PL loop were Fresh snow was aged with an imposed tempera-employed. The eluant was 7.5-mM HCI, with a flow ture gradient in 1-mm-thick butyrate plastic cylin-rate of 2.3 mL/min, and the regenerate was 0.04 M drical columns (3.7-cm O.D. x 55-cm length) fittedBa(OH)2, with a flow rate of 3.0 mL/min. Precision with polyethylene end caps. These columns wereof ion chromatographic analyses was ±+5%. filled after mechanically mixing the snow and pass-

Hydrogen ion concentrations were determined ing it through an 18-mesh (1.3 mm) polypropyleneto within ±0.02 pH unit using an M41-410 micro-com- sieve. The desegregated snow crystals (= 0.3 to 1.3bination pH electrode (Microelectrodes, Inc.). Qui- mm) were poured through a large funnel into theescent 100-jL aliquots were measured after the elec- growth columns, which were agitated while filling

2

Table 1. Bulk ioniccomposition of ho- Table 3. Composi-mogenized natural Table 2. Composition tion of eluant solu-snow used for chemi- of solution used to pre- tion used for anioncal elution experi- pare synthetic snow chromatographic ex-ment. grains. periment.

Concentration Concentration ConcentrationIon (leg/L) Ion (jLeq/L) Ion (peg/L)

H+ 22 (pH = 4.7) H+ 100 (pH = 4.0) H' 10 (pH=5.0)Na + 13.5 NHZ 114 NH• 28.7

K+ 0.4 CI" 14.3 Cr- 2.8Ca2÷ 2.9 N03 100 NO,- 16.1Mg2 + 0.8 1;+=40 SO42- 104 SO" 20.8

C17 14.8

NOý 17.7SO2- 10.6 Z- = 43-4

to facilitate uniform packing. The snow transfer op- grain growth columns had been fitted into the cavi-eration was performed at the opening of a clean air ties of the grain growth apparatus, a plastic bag wasstation housed in a -2.2°C coldroom. Four columns put over the top of the Sono tube to protect againstwere sequentially filled as described above to a 50- particle contamination, and additional fiberglasscm depth, resulting in densities of 226 ±7 kg/m 3. insulation (9 cm thick x 56 cm wide) was wrappedAdditionally, three subsamples of the sieved snow three times around the vertical walls. The grainwere collected for determination of the bulk snow growth apparatus was positioned on an aluminumchemical composition. plate, 0.6 cm thick x 20 cm O.D. Centered below the

The filled grain-growth columns were taken to a aluminum plate and on top of a large rectangular 5--20'C coldroom and placed in an insulated temper- cm-thick Styrofoam board was a thin (= 1.0-mm thickature gradient apparatus (Fig. 1). This apparatus x 1.8-cm O.D.) heating pad. Current to the heatingpositioned the columns at equal distances from the pad was controlled with a rheostat. The tempera-walls and from one another inside a 20-cm-I.D. card- ture of the aluminum plate was monitored with a 9-board Sono tube. The spaces between the four cy- cm-long, 3-mm-diameter needle probe thermocou-lindrical cavities were filled with urethane foam in- ple that fitted snugly into a hole drilled radially intosulation and covered at both ends with rigid Styro- the aluminum plate. Throughout the grain growthfoam (Dow Chemical Co.) insulation. Once all four experiment, the rheostat was set to maintain a tem-

perature of-2+± IC within the aluminum plate, pro-Plastic Columns Containing Snow ducing a temperature gradient between the top and

bottom of the growth columns of approximately36°C/m. (Such a temperature gradient can be foundin snowpacks in the western U.S. and Canada.) Tem-perature readings were taken at least daily. Individ-ual growth columns were removed after periods of

Insulation 7,14,28, and 56 days, and similar placebo columnsfilled with loosely packed fiberglass insulation wereinserted into the vacant cavities to maintain thermal

Rheostat Thermocouple balance.Probe Once removed, columns were placed on a dean

polyethylene sheet inside a clean air station within

120 V 0.5-cm Aluminum Plate Heating Pad the -20cC coldroom. Taking precautions not, to dis-turb the fused snow grains, we cut the column into

Figure 1. Schematic (not to scale) of experimental ar- two sections with a razor knife, resulting in a lowerrangement usedforaging snow. The baseofthe columns was 15-cm portion (bottom 1/3) that had been adjacent tomaintained at -2 C with a heating pad while the top of the the aluminum plate and an upper 35-cm (top 2/3)columns was at the temperature of the coldroom, -20 0C. portion. The snow from each section was then trans-

3

ferred to polyethylene bottles. The bottle contain- umn was metered with a syringe pump (Sage In-ing snow from the bottom 15 cm was shaken to dis- strument Co.). The eluant (deionized water or deion-aggregate the fused grains. Subsamples of the dis- ized water spiked with dilute acids and salts) ap-persed snow grains were taken for microscopic peared to wet the surfaces of all snow grains.crystal identification and for bulk snow chemical de- Column eluate was removed by suction created byterminations. Most of the remainder was used for the syringe. Volumes ranging between 100 and 300elution experiments. pL were drawn off the column through the three-

way stopcock into the section of tubing attached toColumn elution the syringe. The three-way stopcock was then turned

The flow of meltwater or rainfall through snow so that this aliquot could be injected into a collec-was simulated using a 1.8-cm-diameter x 30-cm-long tion vial. Once the vial contained I mL it was re-glass column (Fig. 2) filled with natural or labora- moved and emptied into a 7.5-mL CPE polyethyl-tory-grown snow grains, and frozen solutions of ene bottle. The collection vial was rinsed withdeionized water drops. The column was made by deionized water between samples. After eluant ap-replacing the stopcock of a Pyrex 100-mL burette plication onto the column was stopped, only a sin-with a plastic three-way stopcock. A 5-cm 3 plastic gle additional aliquot could be obtained. The totalLuer-Lok (Becton Dickinson) syringe was connect- surface areas for the natural snow grains and theed to the horizontal port of the three-way valve. Te- frozen droplets contained within the column wereflon tubing attached the stopcock to the glass col- approximately 0.14 m 2 and 0.10 M2

, respectively.umn and to the syringe. The vertical path of thethree-way stopcock transferred aliquots of the col- RESULTS AND DISCUSSIONumn eluant to a 1.5-mL collection vial (Perkin-Elm-er sample cup). Chromatographic experiments

Column elution experiments were conducted Three different types of column elution experi-within a Class 100 clean air station in a 0°C cold- ments were conducted for this portion of the studyroom. At the start of an experiment the aqueous so- using 1) natural snow gratis eluted with deionizedlutions passed onto the columns were at 1P + 1°C. water, 2) frozen solution drops eluted with deion-Eluant solution flow of 0.57 mL/min onto the col- ized water and 3) frozen deionized water drops elut-

ed with synthetic salt solutions.For the first experiment the elution column was

fi8led wit homogenized, aged snow (- to 2-mmalt , •grains). The bulk chemical composition of this snow,

Pumpi as determined by triplicate analysis, is given in Ta-ble 1. Deionized water was applied at a rate of 0.57

e mL/min to the top of the column and 1-mL aliquotsof eluate were collected at the bottom. Concentra-

30 Column with tio- s of cations (Fig. 3) and anions (Fig. 4) were asIcemuch as 4.5 times as high in the initial eluate as in

Eluant the bulk snow itself. Divalent ions (calcium, mag-Reservoir nesium and sulfate) were fractionated to a greater

extent than univalent ions. The high initial concen-trations decrease exponentially and appear to stabi-

___lTubing lize after the eighth aliquot. The concentration of theW S least fractionated ion, chloride, shows the smallest

teaoy change and was always less than the bulk snow con-centration, most likely because of dilution by theapplied deionized water. Other ions were also di-luted by the deionized water, reducing their con-centrations in the eluate so that the actual enrich-

Polyethylene ment (fractionation) would have been appreciablyVial higher if the snow grains had been allowed to melt

naturally. Using deionized water for elution reducedFigure 2. Schematic (not to scale) of apparatus used for the time of the experiment from an estimated 10-12chemical elution experiments, hours to about 30 minutes. Of the five cations stud-

4

400 0 Hydroweo Sodkum

U) a Potassium300- a Magnesium

o \& Calcium

j 200

Figure 3. Normalized cation concentrations in sequen- E 100

tial eluate aliquots for natural snow elution. Concen- 9trations have been normalized to bulk snow concentrations. o f

0 2 4 6 8 10Aliquot

500 I

"* ChlorideCo 400 - Nitrate

* Sulfate

9300-

c 200

a100-C

Figure 4. Normalized anion concentrations in sequen- 0' ____

tial eluate aliquots for natural snow elution. 02na0a 2 6 8 10

Aliquot

ied, hydrogen ion shows the least relative enrich- 1.1 for the first aliquot to about 0.8 for the tenth ali-ment (Fig. 3); other constituents, especially divalent quot. While some of the deviation could be attribut-ions, are fractionated to a greater degree. On a mo- ed to experimental error (±5% for each ion) this ef-lar basis hydrogen ion is the major ion in fractionat- fect should be random and is not expected toed meltwater because it is the dominant ion in snow produce an appazent trend. Alternatively, since theitself (see Table 1). ratio decreases with aliquui, an unanalyzed cation

The effects of chemical fractionation are clearly may be present later in the elution sequence. Anshown in Figures 3 and 4 and, while preferential elu-tion is apparent, it is more clearly seen in Figure 5 6 1 i 1 1 1where ionic ratios show the early elution of sulfate,and to a lesser degree nitrate, relative to .horide.Notice that the ratios plotted in Figure 5 have been . 4-anormalized to the corresponding ratios in the bulk a:

snow so that even with the high enrichments (frac-tionation) of initial aliquots, sulfate and nitrate ap- E

pear earlier (elute earlier or preferentially) than chlor- 0 Nitrate/Chlo2deide.

As an indirect check on the accuracy of our ana-lytical methods and to determine if we measured 0 4 6 8 1all major constituents present in the snow, anion/ 2 4 6 8 10

cation ratios were calculated and are plotted in Fig- Aliquot

ure 6. Ideally, this ratio should be unity for ionic Figure5. S02-/C-andNOO/C- ratios in sequentialbalance and electroneutrality. Generally, the ratio is eluate aliquots for natural snow elution. These ratios arenear unity but with a slightly decreasing trend from normalized to the corresponding ratios in bulk snow.

5

Figure 6. Ion balance for natural snow elution. Anion/cationratios were calculated from summed concentrations in peqlL.

1. I I i I I Ia:0.8

. 2 4. 6--1

"Uro Sulfate

Su iýe IonI I

C 1575 150- Hydrogen Ion

2 100

.50C Figure 7. Normalized anion concentrations in se-

0. quential eluate aliquots for elution from frozenOo 2 1 solution drops.

0 2 4 6 a 10 12 14Aliquot

obvious choice of cation would be N I-It but the con- Again, fractionation (Fig.?7) and preferential elutioncentrations we determined were too low to explain (Fig. 8) are occurring but to a smaller extent than inthe observed differences. This decreasing anion/cat- natural snow grains. For example, sulfate shows only"ion ratio has also been observed in natural snow a two-fold enrichment over chloride for the frozensamples by others and has been tentatively attrlbut- solution drops (Fig. 8) instead of the six-fold enrich-"ed to dissolved forms of organic carbon.* ment for natural snow.

To simulate the process of fractionation and pref- From the previous two experiments it was noterential elution in the laboratory, we prepared synt- possible to distinguish if the observed preferentialthetic snow grains by freezing droplets of a solution elution was caused by an actual chromatographiccontaining ions at the concentrations shown in Ta- sorption mechanism or if it was due to the fact thatble 2. These concentration levels are typical cf acid the initial meltwater from ice grain surfaces had asnow for the northeastern United States. The column different composition than the later meltwater fromwas filled with approximately 5000 frozen hemni- inner grain material, as recently proposed by Brim-spherical droplets of this solution and, as with the blecombe et al. (1987, 1988). The third type of elu-natural snow grains in the previous experiment, elut- tion experiment was designed specifically to deter-ed by slowly passing deionized water downward. mine whether or not snow has ion chromatograph-Concentrations in the eluate (Fig. 7) are initially high ic properties. This experiment was essentially the(2-fold for S0O2-) and decrease rather rapidly as elu- reverse of the second experiment. instead of perco-tion progresses. The fact that the relative chloride latinig distilled water through frozen solution drops,"concentration in the first aliquot is 100% (i.e., the a synthetic solution (Table 3) representative of nat-same as the bulk snow) is fortuitous and simply in- ural precipitation was allowed to flow through adicates that any fractionation present was offset by column of frozen distilled water drops.dilution with the deionized water used for elution. Results of this experiment are shown in Figure 9

__________for the three anions studied. As in any chromato-*M. Tranter, Univ. of Southampton, U.K., pers. comm., 1999. graphic analysis, the column was first conditioned

6

2.0 1

0 1.6a:

. 1.2

E 0.8 Nitrate/Chloride0z

Figure 8. Normalized S04 2CV- and N03/CI- ratios 0.4in sequential eluate aliquots from frozen solution drops. 0 __ I_,_____I________

2 4 6 8 10 12

Aliquot

e Chloride

e80 o Nitrate"- Sulfate

60 -

Figure 9. Normalized anion concentrations in sequential Ci 40)eluate aliquots for chromatographic experiments usingfrozen deionized water drops. Arrows on the abscissa indi- 20cate the beginning and the end of application of synthetic snowsolution. Prior flow was due to application of deionized water. 0

4 6 8 10 12 16 18

Aliquot

1.0

.o 0.8

: -Nitrate/ND 0.6 Chloride

Figure 10. Normalized S04-/Cl- and N03/C0- c cratios in sequential eluate aliquots for chromatograph- 0ic experiment where synthetic snow solution was al- Z -. 7lowed to flow over frozen distilled water drops. Arrows 2 Son the abscissa are the same asfor Figure 9. 0

2 4 6 8 10 12 14 16 18

Aliquot

with a carrier fluid, in this case deionized water. The first aliquot was due to a small but finite amount ofsynthetic snow solution was treated as a "sample" impurity (2.2 gg Cl-/L)in the deionized water usedin standard chromatographic analysis, "injected" and to prepare the frozen droplets used in the column.then allowed to flow downward through the column. Chloride in the deionized water was excluded as theSample flow into the column started between col- drop froze and was forced to the surface, making itlection of the fourth and fifth aliquots as indicated available for rapid initial release from the elutionby the arrow on the abscissa. After a short period of column.sample application the eluant was then switched Shortly after the synthetic precipitation solutionback to deionized water (second arrow on abscissa was applied, concentrations of all three anions in theof Fig. 9). The high concentration of chloride in the eluate rose simultaneously to that of the bulk solu-

7

-- e

Figure 11. Original snow used for metamorphosislelu- Figure 12. Snow crystals aged for one week with an im-tion experiments consisting of 0.1- to 0.3-mm rounded posed temperature gradient of36°C/m. Sample taken fromcrystals. Rounding offresh snouflake arms and angularities bottom third of column 1. Crystals are larger (0.2-0.4 mhm)occurred during isothermal storage at -20Cfor several days. than original snow and have developed some angularity.

w

Figure 13. Snow crystals aged for two weeks. Sample tak- Figure 14. Snow crystals aged forfour weeks. Sample tak-enfrom bottom third of column 2. Crystal size ranges from enfrom bottom third ofcolumn 3. Crystals are larger (up to0.5 to 0.7 amm. 2 lrm), sintered and faceted.

Figure 15. Snow crystals aged for eight weeks. Sample Figure 16. Snow crystals aged for eight weeks. Crystalstaken from bottom third of column 4. Crystals are as large are from top two-thirds of column 4. Crystals are 0.3-1as 3 mm, sintered and highl /faceted. Many hexagonal, cup- mm in size and some faceting (see hexagonal plate in lowershaped dept h-hoar crystals are present, like the one in the low- right) is apparent, although most crystals are rounded due toer center of photo. the slower growth rates because of the lower temperature in the

upper section of the column. This material is similar inl appear-ance to snow aged for one week in the bottomn third of colhmtn I(see Fig. 12).

8

tion and remained at this level until deionized wa- tal growth was more extensive (05 to 0.7 mm) andter was reapplied. Throughout the experiment all the crystals are angular with hexagonal plates be-normalized concentrations paralleled each other ginning to appear. After four weeks (Fig. 14) the crys-very closely and were even identical for many of tals are much larger (= 1 mm), faceted, and begin-the aliquots. Normalized ionic ratios (Fig. 10) were ning to sinter. The final sampling, taken after eightconstant and essentially unity in the eluate of the weeks (Fig. 15), contains large (up to 3 mm), sintered,synthetic solution. All three ions eluted simulta- highly faceted crystals, many of which are hexago-neously (i.e., no preferential elution), indicating that nal and cup-shaped, similar to depth-hoar.sorptive chromatographic effects are absent for these To show the effect of temperature upon crystalthree anions. growth, the top two-thirds of column 4 was sam-

pled (Fig. 16). Although this material was under theMetamorphism experiments same temperature gradient as that in the bottom

If it is not chromatographic, what, then, is the third of the column, the average temperature wascause of preferential chemical elution in snowpack -14'C as compared to -5°C for the bottom third. Themeltwater? Previously, Tsiouris et al. (1985) ob- crystals in the upper section of the column are small-served preferential chemical elution from snow er (0.3 to I mm) than those in the bottom section (3grains that had been aged for one year in a cold- mm) and, while some faceting is apparent, most areroom. But the same snow sample showed no pref- still rounded because of the slower growth rates aterential chemical elution when it was originally col- the lower temperature in the upper section of thelected. This indicates to us that preferential elution column. This material is similar in appearance to thatis related to processes occurring during snow meta- present in the bottom third of column I (see Fig. 11).morphosis. We hypothesize that selective exclusion While the snow crystals are undergoing meta-of ions during grain growth, whether occurring in a morphic physical changes, concurrent chemicalcoldroom or natural snowpack, produces mature changes are taking place as well. Figure 17 showssnow grains that have enriched chemical concentra- sulfate, hydrogen, nitrate and chloride ion concen-tions on the surface compared to the interior. Al- trations in sequential elution aliquots for the snowthough growth of grains in dry snow occurs from crystals aged in columns. The aging times of 1, 2,4,the vapor (no melting is required), the exclusion of and 8 weeks are the same as the crystals in the pho-ionic salts is analogous to the rejection of ions from tographs of Figures 12 through 15.a slowly freezing salt solution. Because different ions Chemical fractionation (higher ionic concentra-are excluded with different efficiencies, the ionic ra- tions in initial meltwater) and preferential elutiontio in the surface portion of growing grains is differ- (some ions are enriched more than others) are readilyent from that within the grain interior and the bulk apparent in Figure 17. Our explanation is that bothsnow itself. This preferential exclusion may be the these phenomena result from ion exclusion duringresult of steric incompatibility of ions within the ice snow grain metamorphosis. Figure 18 shows thislattice itself. Both fractionation and the order of pref- concept schematically. Assume that all crystals haveerential chemical elution are consistent with this hy- similar initial (time tI) solute distributions. As thepothesis. To test this hypothesis, we aged fresh snow snowpack ages and vapor is transferred from warm-under an imposed temperature gradient for various er to colder crystals (Colbeck 1987), the concentra-lengths of time and then conducted elution experi- tion of impurities increases for the shrinking grainsments to determine the effect of grain growth upon (time t2) until they have completely disappearedthe degree of fractionation and preferential elution. (time t3), leaving their impurities upon the surfaces

Figures 11-15 show the effect of aging (metamor- of the mature grains. At the same time, the largerphosis) upon the snow crystals. The original start- grains are growing, selectively rejecting chemicaling material (Fig. 11) for the experiment consisted species within the newly developed layers of ice lat-of small (0.1- to 0.3-mm) crystals and crystal frag- tice. This rejection is energetically favorable becausements whose apexes became slightly rounded dur- impurities located on ice grain surfaces or at disor-ing the time between sample collection and experi- dered grain boundaries cause less strain than if theyment initiation due to the higher vapor pressures at are located within the ice matrix itself. Of course,sharp curvatures (Kelvin effect). After one week of not all the small grains disappear completely and ataging with an imposed 36°C/m temperature gradi- any given time the snowpack or column has a dis-ent (Fig. 12), crystals in the bottom of the column tribution of grain sizes and impurity concentrations.had developed some angularity and had grown to But the overall trend is an exclusion of impurities02 to 0.4 mm, while after two weeks (Fig. 13) crys- from snow grain interiors to their surfaces.

9

140 ....

200 I I ' 20

a 7 Days-5 160 141.0 0 7Days 1 100 o 28

"\14 560 28

2120 *56 8.2 8

800S80 • 60

C4o_400

I 20 I ,I t I

2 4 6 8 10 20 2 4 6 10Aliquot Aliquot

a. SO4 b. H+100 I I ' l I 100 I I l ' I ' I

80 a 7 Days 80 a 7 Days75 s 1 14''14

60 028-• \ o28 28

S0 660 6

C -

o 0

m 40 40

C C:

0 0

O 20 0 20

2 4 6 8 10 2 4 6 a 10

Aliquot Aliquot

c. NO d. Ct-

Figure 17. Concentrations of SO4 2 H+, NO 3 and Ct- in initial aliquots of eluatefrtom snow aged for 7, 14,28, and 56days.

Cold

er Vaor kIux

Warm • .,@

tl 1 t2 3111 t3

Figure 18. Theoretical depiction of chemical changes occurring during snowmetamorphosis. Assume initially (time tj) that all snow grains have similarsolute concentrations (represented by shading). As uwter vapor moves from warmerto colder surfaces, solutes within the shrinking grains (time t2) become concen-trated. At time t3, some grains have completely sublimed, leaving their soluteload upon the surface of the larger grains.

10

- CI0 000•

00*

. "• Figure 19. Hypothetical distribution of S04 and Cl within a mature, aged snow0 . .grain that has grown by vapor deposition at the bottom. The selective exclusionfreten-

0 tion of chemical species during metamorphosis results in snow grains with enriched surface* * o concentrations, especially of insoluble ions such as S 02-

* * * 0

The extent of this exclusion, or conversely inclu- Table 4. Averagenormalizedsion within the growing ice grain matrix, depends ionic ratios for initial melt-upon the chemical species or ion, i.e.; it is preferen- water eluate aliquots.tial. The process is analogous to the exclusion of ionsfrom a freezing salt solution: less soluble salts, such Normalized A7 e (days)ratio 7 14 28 56as sulfates, are more efficiently excluded than moresoluble chlorides (Gross 1968; Gross et al. 1975). Ac- S04-/NO3 1.8 2.4 2.7 2.8

cordingly, a mature or well metamorphosed snow 2-

grain (Fig. 19) would have higher sulfate concentra- S04 /CI- 17 2.4 2.5 21

tions and S042 -/ ClF ratios on its surface than with- N0 3 /Cl- 0.9 1.0 0.8 0.8

in its interior. The more efficiently excluded ions aremore highly enriched on the snow grain surfaces andthus appear sooner and in higher concentrations in for 7 days to 2.8 for the same snow aged for 56 days,the elution water. indicating that sulfate is excluded more efficiently

Preferential exclusion should also result in chang- than nitrate. The average normalized SO4-/C l- ra-ing eluate ionic ratios with time. This is manifested tio increases from 1.7 to 2.5 between 7 and 28 daysmost dearly for the SO-/ NO3 ratio (Fig. 20), which but then drops to 2.1 for the 56-day-old snow. Thisgenerally increases progressively as the snow age indicates that at least initially, sulfate is excludedincreases. more efficiently than chloride. But why the drop in

Average ratios of thc three anions for the first five the SO 4 -/C1 ratio between 28 and 56 days? It is noteluate aliquots are given in Table 4. The normalized caused by higher chloride levels because concentra-SO--/ N03 ratio increases from 1.8 for snow aged tions of this ion in the eluate aliquots (Fig. 17d) de-

crease. Also the average normalized N03/Cl- ratio

3.5 1remains constant between 28 and 56 days and in fact3.0 Idoes not deviate appreciably from unity for the four

6 3.0 Daging periods. A closer look at the ionic concentra-

V ' "tion curves in Figure 17 reveals that the degree ofS . 56 Days ; . fractionation does not increase with sample age forZ2.5-- ...... ""

o, " ... all ions. For the initial aliquots, sulfate concentra-Cn2 , tions increase from 7 to 14 days, show little changeS2.0 - '-

N14 between 14 and 28 days, and are lower at 56 days.

E 1.5So the lower SO4 -/C I- ratio at 56 days could be at-7 tributed to lower sulfate concentrations. Now the

. question becomes: why do sulfate concentrations

2 4 6 8 10 decrease at 56 days and, further, why do they re-Aliquot main stable instead of increasing between 14 and 28

Figure 20. Individual normalized (to bulk snow) days? An influential factor, we believe, is the type

S042-/N03 ratio for initial elution aliquots. This ratio ane shape of the snow crystals themselves. At 28increases with snow age because of more efficient exclusion of anti 56 days (Fig. 14 and 15) the snow crystals areSO than N03 (or conversely less efficient inclusion of S042- not just larger but highly faceted and sintered as well.than NO;) within the ice crystal lattice during snow crystal It is reasonable to expect that percolating watergrowth. would rinse surface impurities from highly struc-

11

tured, sintered, depth hoar-like crystals less efficient- taneous physical and chemical changes taking placely than from rounded individual crystals. Thus, in during snow metamorphism.cases where a snowpack is dominated by depth-hoar-like crystals, solute release is retarded and ini- LERATURECITEDtial chemical concentrations are lower.

These results demonstrate the effects of metamor- Bales, R.C. (1991) Modeling in-pack chemical trans-phism upon solute release from snow that is initial- formations. In Seasonal Snowpacks: Processes of Com-ly chemically homogeneous. The magnitude of the positional Change (T.D. Davies, M. Tranter and H.G.ionic pulse can also be influenced by vertical vari- Jones, Ed.). Berlin, Germany: Springer-Verlag, p. 1 39 -ability in solute distribution (Davies et al. 1985, Bales 163.et al. 1989, Bales 1991), diurnal melting and refreez- Bales, R.C., R.E. Davis and D.A. Stanley (1989) Ioning (Colbeck 1981, Suzuki 1991), biological activity elution through shallow homogeneous snow. Water(Jones and Deblois 1987, Jones, 1991), rainfall on Resources Research, 25: 1869-1877.snow (Tranter et al. 1992), as well as the thermal Brimblecombe, P., S.L. Clegg, T.D. Davies, D. Shoot-regime during snowmelt. Much of the past work er and M. Tranter (1987) Observation of the preferen-has focused upon these processes which occur dur- tial loss of major ions from melting snow and labora-ing snowmelt. Our studies show that dry snow pro- tory ice. Water Resources, 21: 1279-1286.cesses occurring weeks and even months prior to Brimblecombe, P., S.L. Clegg, T.D. Davies, D. Shoot-snowmelt can also exert strong influences upon er and M. Tranter (1988) The loss of halide and sul-snow meltwater chemistry. fate ions from melting ice. Water Resources, 22: 693-

700.

CONCLUSIONS Colbeck, S.C. (1981) A simulation of the enrichmentof pollutants in snowcover runoff. Water Resources

The initial fractions of meltwater from natural Research, 17: 1383-1388.snow have ionic concentrations that are two to five Colbeck, S.C. (1987) Snow metamorphism and clas-times higher than that of the bulk snow from which sification. In Seasonal Snowcovers: Physics, Chemistrythey originate. Initial fractions of meltwater from and Hydrology (H.G. Jones and W.J. Orville-Thomas,frozen solution droplets also contain higher concen- Ed.). Dordrecht: D. Reidel, p. 1-35.trations than the bulk solution from which they are Davies T.D., C.E. Vincent and P. Brimblecombe (1982)prepared, but these fractions are two to three times Preferential elution of strong acids from a Norwe-less concentrated than comparable fractions from gian ice cap. Nature, 300: 161-1 63.natural snowmelt. During controlled laboratory ex- Davies T.D., P. Brimblecombe, M. Tranter, S. Tsiour-periments ice grains did not exhibit any chromato- is, C.E. Vincent, P.W. Abrahams and I.L. Blackwoodgraphic sorption for chloride, nitrate or sulfate, in- (1987) The removal of soluble ions from meltingdicating that the preferential chemical elution ob- snowpacks. In Seasonal Snowcovers: Physics, Chemis-served in snowpack meltwater is not du. o ,n ion try, Hydrology (H.G. Jones and W.J. Orville-Thomas,chromatographic process. Ed.). Dordrecht: D. Reidel, p. 337-392.

Preferential chemical elution during snowpack Gross, G.W. (1968) Some effects of trace inorganicsmelting is strongly influenced by preferential chem- on the ice/water interface. In Trace Inorganics in Wa-ical exclusion during snow crystal growth. Less sol- ter, 153rd ACS Symposium, Miami, Florida. Americanuble chemical impurities, such as sulfates, are ex- Chemical Society, 73:27-97.cluded more efficiently and therefore appear sooner Gross, G.W., C. Wu, L. Bryant and C. McKee (1975)and in higher concentrations in the eluate than more Concentration dependent solute redistribution at thesoluble species, such as chloride. Snow crystal hab- ice/water phase boundary. II. Experimental investi-it also influences fractionation and preferential elu- gation, Journal of Chemical Physics, 62: 3085-3092.tion both directly through different chemical release Hewitt, A.D, J.H. Cragin and S.C. Colbeck (1989) Doesrates from crystals with different geometries and snow have ion chromatographic properties? In Pro-surface area volume ratios and indirectly by affect- -eedings of the Forty-Sixth Annual Eastern Snow Confer-ing meltwater flow rate downward through the a-rih, Quebec City, Quebec, p. 165-1 71.snow. Fractionation and preferential chemical elu- Hewitt, A.D., J.H. Cragin and S.C. Colbeck (1991) Ef-tion can result solely from dry snow metamorphism fects of crystal metamorphosis on the elution fromindependent of other processes such as melt freeze chemical species from snow. In Proceedings of the For-cycles. Future efforts toward a better understand- ty-Eighth Eastern Snow Conference, Guelph, Ontario, p.ing of these processes should focus upon the simul- 1-10.

12

Johannessen, M., T. Dale, E.T. Gjessing, A. Henrik- Tranter, M., P. Brimblecombe, T.D. Davies, CE. Vin-sen and R.F. Wright (1975) Acid precipitation in Nor- cent, P.W. Abrahams and I. Blackwood (1986) Theway: The regional distribution of contaminants in composition of snowfall, snowpacks and meltwatersnow and chemical processes during snowmelt. In in the Scottish highlands: Evidence for preferentialProceedings of Isotopes and Impurities in Snow and Ice, elution. Atmospheric Environment, 20:517-525.Grenoble. International Association of Sci. Hydrology Tranter, Nt, S. Tsiouris, T.D. Davies and H.G. JonesPublication, 118:116-120. (1992) A laboratory investigation of the leaching ofJohannessen, M. and A. Henriksen (1978) Chemistry solute from snowpack by rainfall. Hydrological Pro-of snow meltwater: Changes in concentration dur- cesses, 6: 169-178.ing melting. Water Res urces Research, 14:615-619. Tsiouris, S., C.E. Vincent, T.D. Davies and P. Brim-Jones, H.G. and C. Deblois (1987) Chemical dynam- blecombe (1985) The elution of ions through fieldics of N-containing ionic species in a boreal forest and laboratory snowpacks. Annals of Glaciology, 7:snowcover during the spring melt period. Hydrolog- 196-201.ical Processes, 1: 271- 282. Valdez, M.P., R.C Bales, D.A. Stanley and G.A. Daw-Jones, H.G. (1991 ) Snow chemistry and biological son (1987) Gaseous deposition to snow: 1. Experi-activity: A particular perspective on nutrient cycling, mental study of SO 2 and NO2 deposition. Jounial ofIn Seasonal Snowpacks: Processes of Compositional Geophysical Research, 92.9779-9787.Change (T.D. Davies, M. Tranter and H.G. Jones, Ed.). Williams, M.W. and J.M. Melack (1991) Solute chem-Berlin, Germany: Springer-Verlag, p. 173--228. istry of snowmelt and runoff in an alpine basin, Si-Suzuki, K. (1991) Diurnal variation of the chemical erra Nevada. Water Resources Research, 27:1575-1588.characteristics of meltwater. Seppyo, 53: 21-31.

13

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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Elution of Ions from Melting Snow: DA: 4A161102AT24Chromatographic Versus Metamorphic Mechanisms TA: SS

6. AUTHORS WU: 002

James H. Cragin, Alan D. Hewitt and Samuel C. Colbeck

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U.S. Army Cold Regions Research and Engineering Laboratory72 Lyme Road CRREL Report 93-8Hanover, New Hampshire 03755-1290

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13. ABSTRACT (Maximum 200 words)

Columns of natural and laboratory-aged snow grains and frozen water droplets were washed with deionized distilled waterand with synthetic precipitation solutions to investigate both snowpack chemical fractionation and preferential ion elution.

2Concentrations of CI-, NO3 and SO2- in sequential fractions of the column's eluate showed no chromatographic effects,indicating that ice crystals do not possess selective affinity for inorganic anions. Consequently, preferential chemicalelution previously observed in melting snow is not caused by snow acting as a chromatographic column. Additional columnexperiments involving elution from frozen solution drops and from aged snow showed that both fractionation and preferen-tial elution were strongly influenced by ion exclusion during the snow crystal growth.

14. SUBJECT TERMS 15. NUMBER OF PAGESAcidification Fractionation Preferential elution 20Chemical analysis Ion exclusion Snowmelt 16. PRICE CODE

Crystal growth Meltwater Snow metamorphosis17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

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