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The composition of snowfall, snowpack and meltwater in the Scottish highlands-evidence for preferential elution
THE COMPOSITION MELTWATER
M.
GRANTER,
OF SNOWFALL., SNOWPACK AND
IN THE SCOTTISH HIGHLANDS-EVIDENCE FOR PREFERENTIAL ELUTION
P. BRIMBLECOMBE, T. D. DAVIES, C. E. VINCENT, P. W. ABRAHAMS~~~ I. BLACKWOOD
School of Environmental Sciences.University of East Anglia. Norwich, Norfolk NR4 711, U.K. {First receiued 10 Januclry 1985 and receivedfor publication2 September 1985) Abstract-Acidic snows in a small, remote, high-altitude snowpack in the Cairngorms, Scotland. give rise to meltwaters which are proportionally rich in sulphate and nitrate. As a consequence,the within-pack snows become proportionally rich in chloride. even though depleted in solute. Preferential elution appears to be a major proms in the chemical evolution of snowfall and snowpack. Key word index: Snow, preferential elution, meltwater, acid deposition, triangular diagrams.
INTRODUCTION Up to 80 % of the total solute load of snowpacks can be lost with the first 20 % of meltwater (Jo~nnassen er al. 1975; Johannassen and Henriksen, 1977; Johannes et of., 1981; Cadle et al., 1984a; Tsiouris et al., 1985). Since the first meltwaters are often acidic, interest in snowpack chemistry has centred around the so-called “acid flush”, which may lead to short-term acidification of str~mwater (Oden and Aht, 1970; Haapla et al., 1975; Hultberg, 1976; Gjessing et ct., 1976; Henriksen and Wright, 1977; Jeffries et al., 19?9; Skartveit and Gjessing. 1979; Cadle et al., 1984b; Henriksen et al., 1984). Downstream acidification may be exacerbated by: slow melting (Marsh and Webb. 1979); theaccumuIation of a large q~ntity of acidic snow; the deposition of very acidic snows (e.g. Davies er al., 1984); or accumulation of easily leached solute near the base of the pack (Colbeck, 1981). If the bedrock or soil has little buffering capacity there may be important ecological consequences, such as fish kill (Leivestad and Muniz, 1976; National Research Council of Canada, 198t). Whereas the fractionation of acidic solute into meltwater is well established in the literature, preferential elution is as yet poorly understood. Some ions are removed more quickly by meltwaters than others: this process is defined as preferential elution (Davies er ol., 1982). In particular, SO:- and NO; appear to be preferentially Juted with respect to Cl(Brimblecombe et 01.. 1985; Tsiouris et al., 1985). The mechanism for preferential removal of ions is unartain, although there are two obvious possibilities: (a) Melting of snow or ia crystals which contain inhomo~n~us distributions of solute will produce me&waters of different composition over time. For example, sodium chloride may have a more homogeneous distribution throughout the snow crystal, in contrast to acidic solute, which could be found pre-
dominantly on the surface (Wolff and Paren, 19841, having been incorporated onto the crystal from supercooled cloud droplets to produce rimed snow crystals (e.g. ~hemenauer er al., 1981) or scavenged during descent (Knutson et (I!., 1976; Raynor and Haynes, 1982). Previous work has demonstrated that different solutes have different solubilities in ice (e.g. the maximum solubility of HF is 2000 pmol /-I in comparison to 200 /unol I-’ for HCI), fhat the solubility will depend on the rate of freezing (Gross et al., 1975, 1977; Seip, 1980) and that the distribution of solute within ice is heterogeneous (Mizuno and Kuroiwa, 1970). (b) There may be chromatographic effects during percolation of meltwater through the snowpack if the snow or ice crystal surfaces have different affinities for the individual ions. This paper examines the major ion composition of the snowpack and its meltwaters to determine whether preferential elution of sulphate and nitrate is observed in a complex system which shows both temporal and spatial variability in com~s~tion. The study area was remote so difficulties in sampling during harsh weather conditions meant it was rarely possible to sample at times which would have been experimentally ideal. However, such problems are common with snow studies and one of the most encouraging aspects of this work is that it is possible to draw useful condusions about the processes occurring within snowpacks despite these limitations. The work represents the first attempt to monitor concurrently the composition of snowfall. snowpack and meltwater and to examine their evolution through time.
Fieldworkarea The semi-permanent snowpack is located within a small. remote. high-altitude catchment in the Cairngorm
518
M.
TRAh
Mountains, Scotland. The catchment, Ciste Mhearad (57‘06’N. 03’38’W). is located to the NE of the Caimgorm peak, has an area of 0.4 km” and an altitude of around 1080m (see Fig. I). The catchment experiences strong winds on occasions, and there is much local redistribution of surface snows. There are no woodland influences on the catchment (cf. Jones, 1985; Jones and Sochanska. 1985). The main characteristics of snowcover in this region are: (a) surface layers of wind-blown snow forming hard or soft siab, (b) basal layers of coarse, granular snow with grain diameters usually > 2mm intercalated with ice layers, some of which are multiple-bedded or up to 5cm thick (Ward, 1980,Ward et (II., 1985). Equi-temperature and melt metamorphism dominate the evolution of the snowcover for much of the season. Ice layers are formed by repeated melting and refreezing of surface snow, possibly by diurnal cycles, or when surface meltwaters penetrate the pack and refreeze at a temperature inversion a few cm below the surface. Slush layers may form during spring (Ward et al., 198s).
Silmp/ingmerh4kfalogy Three sets ofsamples, (i) newly fallen snows. (ii)snows from within the snowpack, and (iii) meltwaters, were collected during January through to June 1984. Snow samples were transferred to pre-cleaned polyethylene bottles by PTFEcoated scoops. Newly fallen snows were sampled whenever possible throughout the season. and usually within 6 h of snowfall. Within-pack snows were sampled from randomly located snowpits, which were dug with a stainless steel spade and faced by a PTFE-coated plastic scoop. A number of snow samples were taken from each snowpit, Typically nine samples of a~roxi~teiy iOcm thickness were taken from a 3m snowpit. Each major snow layer, distinguished by differences in crystal type, grain size and hardness, was sampled. At least one snowpit was sampled every week from January through to March. and thereafter at least two snowpits were sampled each month. Two snowpits were resampled a number of times. The initial snowpit was partially ref&d, and subsequent resampiing involved redig-
’
ITER et al.
ging and refacing the sampling surface by at least 30 cm. Titis procedure was repeated for up to five times over two weeks, Snowmeit samples were collected 3m below the snow surface and 2m from the base of the snowpack by an automatic snowmeit sampler (designed by the Institute of Hydrology), which had a sampling period of one day. The instrument was installed and operating for a month before the firs1 sample was collected. The automatic sampler was installed to ensure that the first snowmelt fractions of a melt event were captured, irrespective of whether or not such a melt had been forecast and to allow for the possibility that no workers would be on site during the melt. Snowmelt samples were collected on 6.7.8 and 10 March, and are representative of the second major melt event of the season (Morris and Thomas, i985a). A further composite sample was collected over several days due to instrument failure, and is ignored in the following calculations. During a period of snowmeit in May, near-surface meitwaters were collected at half huuriy intervals from a ledge in the side of a snowpit (see Tsiouris et a/., 1985, for fuller details), 0.5 m and 1.0 m below the snow surface to examine the detailed temporal variations in meltwater composition. Other meltwaters were collected simultaneously from the pipfeed of the automatic snowmelt sampler. Pretreatment and chemical analysis The snow samples were transferred to the field laboratory within 4 h of collection and showed little sign of melting. They were melted by immersing the bottles into hot water. Before the snowmeit reached IO’C (and typically YC). they were suction filtered through 0.45pm Oxoid filter papers (Cook and Miles, 1980). Thereafter, ail samples were stored at approximately 4’C until analysis (Galloway and Likens, 1976). An aliquot of the filtered sample was taken for immediate pH determination by an Orion research grade electrode and Orion 501 portable pH meter, calibrated with BDH buifers of 4.00 and 7.00. Care was taken to ensure that buffer and sample temperatures (typically 7-12°C) did not diiier by more than 3°C. Measurement of the pH of acidic solutions with low ionic strength is difficult (Galloway et ai.,
!
,
’ li90m
.-.
_
.’ / Enn
i
i,..
I
*l-inn...
Fig. I. Location map of Ciste Mhearad. The sampling area is enclosed by the dotted lines. Other dotted lines arc footpaths.
519
Evidence for preferential elution in Scottish snows 1979; Covington et al., 1983; &Quaker et al., 1$83; Tyree, 1981; Neale and Thomas, 1985), and errors of at krst f 0.2 pH units may be common. Measurements of the pH of dilute acid standards in our laboratory show that iield pH is systematically underestimated by 02 units between pH 3.7 and 4.3. The pH data presented in this work are uncorrected for errors in measurement. Analysis for the major anions (SO:-, NO; and Cl-) was performed by ion chromatography on a Dionex Model 12 Ion Analyser. Detection limits were 0.2 m I-’ and precision was better than 5%. Major metal ion concentrations fNa+ and Mg”*) were determined by AAS on a Pyc Unicam SP9 Spectrophotometer. Precision was: Na‘, f 2 m /-‘; Mg’+, f 1 peq I-‘. Detection limits were an order of magnitude less. RESULTS
The composition of newly fallen snow varies during the season. In all, at least one sample of 19 different fresh snowfalls were collected, and the range of pH and major ion concentrations is shown in Table la. Approximately 50% of all snowfaits were sampled during fanuary through to May. No rainfall onto the snowpack was observed during the sampling periods until late afternoon on 2 May, when the sampling of near-surface meltwaters was terminated. On the basis of nearby synoptic observations it was unlikely that there was rainfall in the catchment before May. The fresh snows are all acidic fpH 3.1-5.4). H,SO, and HNO, are usually dominant, but HCl was also found to be of importance in the most acidic snowfall of the season, where the molar non-sea salt chloride: non-sea salt sulphate: nitrate ratios approached 1: 1: 1.8 (Davies et al., 1984). However, the predominance of a sea salt source of Cl- for the fresh snows over the season is indicated by the high correlation between Cl and Na+, in comparison with the low correlation between Cl- and H+ (see Table la). Minor quantities
Table la. The com~sitions of fresh snows
H+ Na+ Mg;_” %; cl-
Mean
Max
SD*
CV$( %)
4.00 1.91 0.00 5.63 1.77 2.26
137 76.4 18.0 55.3 40.5 111
777 227 55.8
186 80.4 19.3 67.6 59.6 109
136 105 107 122 147 98
Units are caq /- I. * SD = Standard deviation. $CV = Coeflkicnt of variation. Correlation coefEcients H’ Na* Mg’+ ;+<
Na+
Mg2*
so:-
NO;
Cl-
0.136
0.107 0.991
0.880 0.448 0.423
0.705 0.350 0.300 0.756
0.259 0.892 0.880 0.764 0.402
Critical 95% value (N = 19); 0.456.
Min
Mean
Max
SD
cv (%)
Na+
1.30 0.87
34.4 54.1
316 252
39.2 54.9
114 101
!$;
0.33 0.00
18.6 11.9
64.1 279
29.7 12.9
160 108
z15 -
0.08 3.95
81.3 9.37
316 232
74.6 23.9
25s 92
H+
Units are peq f- I.
Correlation coefficients
H’ Nat Mg’, sotNOi
Na*
Mg**
SO:-
NO,
Cl-
0.193
0.275 0.989
0.844 0.373 0.456
0.861 0.112 0.201 0.916
0.123 0.974 0.955 0.340 0.088
Critical 95% value (N = 154); O.lS2.
Table lc. The composition of meltwaters Min
Mean
Max
SD
CV (%I
H+ Na+ Mg2+
3.12 1.74 0.00
53.5 51.8 13.0
708 748
138 139 31.9
258 268 245
;o& Cl-’
;: 8.18
40.2 588 71:2
447 z 1071
141 89.2 177
240 222 249
Units are peq /- ‘.
Na’ H’ Na+ 2+
0.989
Correlation coefficients Mg2’ so:NO; 0.980 0.989
0.983 0.993 0.998
Nd; s”o”2-
Min
;: 341
Table lb. The composition of within-pack snows
Cl-
0.977 0.983 0.999
0.981 0.991 0.977
0.996
0.973 0.976
Critical 95 % value (N = 61); 0.252.
of organic anions, such as acetate and methane sulphonic acid (Fernandes, 1984) have also been detected. The composition of a newly fallen snow surface shows considerable spatial variability, as seen in Table 2. These snows were collected every 50 m along a 700 m transect of the catchment. and represent the top 2-5~1 of snowfall (and snowpack). The coefEcient of variation, defined as (standard dev~tion/m~n) x lOO%, of NO;, Cl- and SO:- for this transect are 41”/, 41% and 14 %, respectively. The nange of pH is j: 0.05 units and there is no systematic variation of pH or anion concentration along the traverse. The major ions are all well correlated. suggesting internal consistency of the data set. Similar variability is found within other fresh snow surveys from the catchment, although less variability is found in urban snows from
520
M. TRANTER er of. in the surfacecomposition ofnewlyfatten snow along a transect of the catchment. Date 29.3.84.
Table 2. Variation
Base of catchment (1OlOm)
Head of catchment (t16Om)
H+
Na’
Mg*+
281 316 316 252 316 316 282 282 224 251 25I 251 281 316 251
8.26 13.5 7.83 5.22 16.1 22.6 8.26 10.9 4.78 9.57 7.82 5.65 11.7 13.9 12.2
1.67 3.33 I .67 0.83 4.17 5.83
1.67 2.50 0.83 2.50
1.67 I .67 2.50 3.33
1.67
so: -
NO;
IS6 121 125 77.1 125 121 91.7 106 91.7 96.5 104 86.9 96.5 116 98.3
13.4 24.1 17.4 15.8 42.6 37.7 17.9 37.7 15.3 21.9 18.5 16.3 22.6 33.1 17.6
11.8 17.8 9.87 7.05 19.7 23.7 9.87 13.8 5.08 13.0 II.0 7.90 13.0 18.9 9.59
23.4 9.54 41
12.8 5.19 41
Units are peq / -
Mean SD cv ( 7;)
219 31.4 11
Na+ H” Na’
10.6 4.72 4s Correiation Mg”’
0.692
0.717 0.957
Mg” so:NO; Critical
2.39 1.33 56 coefficients SO:0.827 0.638 0.648
Cl‘
’
105 15.1 14
NO;
ci-
0.619 0.788 0.830 0.545
0.793 0.934 0.965 0.124 0.872
95 % value (N = IS); 0.514.
Norwich (Tranter et al., in prep. a). The composition of aged surface snows also shows large spatial variability (Brimbl~om~ et al., 1985), which persists throughout the season, even after the extensive leaching (Tranter et ol., in prep. a). The composition of snows from within the snowpack varies (see Table lb), mainly because of the difference in composition between individual snowfalls, and leaching and redistribution of sotute. Redistribution and leaching of solute is IikeIy to occur during diurnal melt-freeze cycles and during major melt events when increased solute levels are observed in the stream (Morris and Thomas, 198Sb). Hence, the composition of within-pack snows is likely to undergo rehtively rapid modification, although this modification is not considered in detait here. Therefore, no attempt has been made to distinguish between pre- and post-melt within-pack snows. Meltwater compositions are summarized in Table lc. Apart from Ii’, the maximum conccntrations were found in samples colkcted by the automatic snowmeft sampIer, 2 m from the base of the snowpack. They are 1.5-2.0 times more concentrated than the maximum snow concentrations fsee Table la). All major ions are well correlated, in contrast to the poor correlation which exists between some species
(e.g. Cl- and NO;) in the fresh and within-pack snows. The average meltwater composition is less con~ntrated than the average fresh snow com~sition because 50 of the 64 samples were collected at the end of the season, during the detailed examination of nearsurface meltwater composition in May. The end of season snows were relatively leached and consequently the meltwaters were more dilute than earlier in the season. A tip gauge within the snowmelt sampler monitored the quantity of snowmelt passing through a 200cm2 aperture each day. The values recorded when samples were collected (6, 7, 8 and 10 March) were 110, 70, 30 and 20 cm3, respectively, which correspond to rates of ablation ofS.5,3.5,1.5 and l.Omm (H,O)day- ‘. Using 30% as the average water equivalent per unit depth (from McKay and Gray, 1981), the water equivalent above the snowmelt sampler would have been 90cm; therefore the meltwater collected represents approximately 1% of the water equivalent above. Because of bad weather conditions prior to the melt event, we have no estimate of the tota1 solute load of the snowpack prior to the melt. Near-surface meltwater samples were collected on 1 and 2 May. From visual observation, the snowpack at the sampling location had experienced approximately
Evidence
for preferential
elution
6Ocm (H,O) of ablation over the previous two weeks. Hence the surface snows were well leached. The rate of ablation during the sampling period was estimated as I-2cm (H,O) day-‘, and the snowpack comprised > 6Ocm (H,O). Principal components analysis of the data sets
Preferential elution results in complementary changes in the composition of meltwaters and withinpack snows. For example, if SO:- and NO; are preferentially eluted with respect to Cl-, the meltwaters should be proportionally rich in SOi- and NO;, while the within-pack snows should become proportionally rich in Cl-. This is difficult to demonstrate within a catchment when there is large variability in each data set, due to spatial inhomogeneities and continuous temporal alteration. However, principal components analysis and graphical representation on triangular diagrams considerably reduces this difficulty (Brimblecombe et al., 1985) and allows the apparently complex data set to be interpreted in terms of several simple processes. In the following discussion, it should be noted that the magnitude of the loading indicates the contribution each variable makes to the principal component. Positive and negative loadings indicate positive and negative correlation between the variable and the principal component, respectively (SmeyersVerbeke et al., 1984). Furthermore, if all ions were perfectly correlated (e.g. all ions derived from the same source), the second component would be redundant. Interpretation of the principal components is usually difficult (e.g. Chatfield and Collins, 1980), but in the case of the components presented in Table 3, a relatively straightforward interpretation can be made. The first component, which accounts for much of the total variance, allocates a similar loading to each ion in all three cases and represents the overall effect of concentration and its high variability. The second component differentiates between the two major sources of solute, sea salt (Na+, Mg2+ and Cl-)and acid (H *, SO:- and NO;) ions, to the fresh snows and the within-pack snows. The second component of the meltwaters explains too little of the total variance to be of
521
in Scottish snows
significance. The physical interpretation of the second component, and particularly the sign reversal in the loadings, is that acidic solute is associated with nonmaritime air masses, i.e. polluted continental air. Principal components analysis of aged surface snows and acidic precipitation gives rise to first and second components similar to those for the fresh snows (Brimblecombe et al., 1985; Brimblecombe et al., in prep.) Representation of the proportional anion composition of acidic snows and meltwaters on triangular diagrams
Triangular diagrams remove the effect of highly variable concentration (i.e. much of the effect of the first principal component) and reveal the underlying trends in the data sets. By representing the data in this way, up to 99% of the total variance has been eliminated. Furthermore, simple fractionation of solute into meltwater will not alter the position of within-pack samples on the triangular diagram, since there will be no change in the relative proportion of ions. The percentage of each anion is plotted such that the proportion of each anion is represented by the proximity to each apex. For example, samples rich in Cl- lie near to the Cl- apex. Figure 2a shows the proportional anionic composition of all the fresh snows and demonstrates the considerable variation in composition. The composition of each snow frequently reflects the air trajectory associated with the precipitation event (Brimblecombe et al., in prep), even though only the top 2-5 cm of the total snowfall has been collected. All snows collected from the same snowfall lie in the same area of the triangle, although the absolute concentrations of the ions may vary considerably (Tranter er al., in prep. a). Figure 2b presents the proportional anionic compositions of all within-pack snows. No attempt has been made to distinguish between pre- and post-melt within-pack snows. There is less scatter in the data in comparison with the fresh snows. The within-pack snows cluster towards the Cl- apex, near to the sea salt Cl-:SO:-ratiooflO:l.TherearenoSO:--orNO;rich within-pack snows. Unfortunately, the contri-
Table 3. Principal components analysis of the major ions of fresh snows, within-pack snows and meltwaters Component
H*
Na’
0.319 0.556
0.434 -0.395
:
0.447 0.355
1
0.407 0.354
Mg*+
NO,
so: -
0.422 -0.419
0.371 0.384
0.435 0.361
0.452 -0.284
62.3 91.5
-0.385 0.434
-0.328 0.459
0.508 0.337
0.361 0.429
0.422 -0.396
60.0 95.4
0.409 0.246
0.409 -0.394
0.578 - 0.499
0.581 -0.279
0.574 0.577
98.8 99.5
Cl-
Total variance (%)
Fresh snows
1 2 Within-pack
snows
Meltwaters 2
522
M.
TRANTER
et al.
G
.oo
Evidence for preferential elution in Scottish snows bution that each snowfall made to the snowpack is unknown. However, since each major snow layer was sampled, the tighter distribution of within-pack snows in comparison to the fresh snows suggests that either: (a) acidic solute has been redistributed within or removed from the snowpack, leaving behind leached snows which are proportionally rich in Cl-. (b) only snowfalls rich in chloride made significant contributions of solute to the snowpack, or (c) dry deposition of sea salt and its subsequent redistribution throughout the pack has a major influence on snowpack chemistry. Figure 2c presents the proportional anionic compositions of all meltwaters. All data cluster in a narrow band leading towards the Cl- apex, even though some samples are up to two orders of magnitude more concentrated than others. The tight grouping of the meltwaters is unexpected in view of the disparate nature of the fresh snows. Despite the fact that meltwaters were collected (a) at two different sites, (b) during two different melt events, and (c) using two different sampling techniques, they tend to be enhanced in SOi- and NO; in comparison to the within-pack snows (see Fig. 2b).
DISCUSSION
A disadvantage of working in a remote, mountainous catchment is that it is seldom possible to sample snowfall, snowpack and meltwaters in a systematic fashion. There is much redistribution of snow by the wind, with the result that the thickness of individual snowfalls is not constant throughout the catchment. Difficulties in obtaining a volume-weighted contribution of each snowfall to the snowpack were compounded by the variable nature of the solute load. Attempts to collect representative, volume-weighted samples of snowfall and snowpack therefore proved impracticable. Instead, samples were collected on a random basis. Comparison of the ionic ratios of fresh and within-pack snows (see below) suggests that this approach has been successful. However, the limitations of the undefined nature of the sampling schedule should be remembered during the following discussion. Meltwaters appear to remove SO:- and NO; in preference to Cl- (see Fig. 2). This is supported by variations in the composition of meltwaters collected near the surface of the snowpack at 30 min intervals on 2 May. Figure 3 shows a clear example of the evolution in meltwater composition towards a Cl--rich, though solute-depleted, system as melting and leaching progress, and the source of SO:- and NO; is depleted. The jump in composition between the two groups of points suggests some change in snowpack hydrology. Similar examples of meltwater evolution towards a Cl--rich. solute depleted system are shown by Brimblecombe er al. (1985) and Tranter et al. (in prep. b). In addition, laboratory experiments on the melting
523
Fig. 3. The compositional variation ormeltwaters collected 1 m below the surface of the snowpack on 2 May, 1984. Samples were collected at intervals of 38 min. commencing at 1200. The length of the solid line is proportional to the total anion concentration of the adjacent meltwater, which decreased monotonicly during the day. The maximum and minimum total anion concentrations were 39 and 24 peq / - ‘, respectively.
of natural snows, where dry deposition is excluded, demonstrate that SO:- and NO; are eluted preferentially to Cl- (Tsiouris et al., 1985). Preferential elution of H+ and SO:- with respect to Na’ and Cl- (and possibly Ca*+) has been observed in snowpack at the Storgama area, Norway (Seip er al., 1980). The effect of preferential elution on the composition of the snowpack can be seen in Fig. 4. Preferential elution of SO:- and NO; will make Cl- the dominant anion in older, leached snows, in contrast to the fresh snows, where all three ions may make significant contributions to the total anion concentration. Hence the Cl-: NO; and Cl-: SO:- ratios of within-pack snows should become greater than those of the parent fresh snows as melting and leaching occurs. Figure 4 presents these ratios plotted against the total anion load (on logarithmic axes to encompass the large ranges exhibited). In comparison to the fresh snows, the within-pack snows lie above and to the left of the fresh snows. Six leached snows collected from a snowpit in the remnant snowpack in July lie further above and to the left of the fresh snows. This suggests that preferential elution of NO; or SOi- is an important process in the chemical evolution of snowfall and snowpack.
CONCLUSIONS
The composition of fresh snowfall in a small, remote, high altitude catchment in the Cairngorms is acidic and shows considerable spatial variability. The composition of snows within the pack is also very variable. The spatial variability of the composition of
M. TRANTERet al.
524
Cl/N03 1
-t
standard deviation
leached
r
100 /
100 Leached 5”ows
SCOWL
10 Wtthin-DaCk
snows
’
---VT
Fresh snows
-
i
Fig. 4. A comparison of the equivalent Cl-:SO$- and Cl-:NO; ratios of fresh snows and ~thin-~ck snows as a function of total anion ~~ent~tion. Six snows collected from a snowpit in the leached, remnant snowpack during July are also shown.
snowfall and snowpack means that representative volume-weighted sampling is difficult. Principal components analysis of fresh snows, within-pack snows and m&waters demonstrates that much of the variability is due to variations in the overall solute load. Therefore, variability in the proportional anionic composition of acidic snows and meltwoters is best represented on triangular diagrams, since the dominant e&t of variation in concentration is removed
from the data. In general, meltwaters appear to remove NO; and SOi- in preference to Cl- from the snowpack. As a consequence the snowpack becomes proportionally rich in Cl- as the solute is depleted. The evolution of meltwaters is also towards a Cl--rich, solute depleted system. Plots of Cl- : NO; and Cl-: SO:- ratios against total anion load for within-pack and fresh snows broadly confirm this hypothesis, since relative to the parent fresh snows, within-pack snows which are depleted in solute have higher ratios. Preferential clution is therefore an impo~nt process in the chemical evolution of snowfall and snowpack.
wish to thank M. Burrows, R. Bryant, A. Femyhough, B. Howes, E. M. Morris, L. Reynolds, F. Robinson, A. G. Thomas and S. Tsiouris for logistic, operational and technical support. Acknowledgements-We
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Brimblccombe P., Tranter M.. Abrahams P. W., Btackwood I., Davies T. D. and Vincent C. E. (1985a) Rehxation and preferential elution of acidic solute through the snowpack of a small, remote, high-altitude Scottish catchment. Ann. Glacioi. 7, i41-147. Brimbiecombe P., Davies T. D. and Tranter M. (in prep.) Air
mass evolution and the composition of precipitation in the Scottish Highlands. Chatfield C. and Collins A. J, (1980~ ~~zr~~cz~~n lo ~u~ri~ar~re Analysis. Science Paperbacks, Chapman & Hall, London. Cadle S. H., Dasch 3. M. and Grossnickle N. E. (1984a) Northern Michigan snowpack-a study of acid srability and release. Atmospheric Environment 18,807-816. Cadle S. H., Dasch J. M., and Grossnickle N. E., (1984b) Retention and release of chemical species by a northern Maim snowpack. Water Air Soil Pollur. 22, 303-319. Colbeck S. C. (1981) A simulation of enrichment of atmospheric pollutants ‘in snow cover runoff, Wurer Resources Res. 17, 1383-1388. Cook J. M. and Miles D. L. (1980) Methods for thechemical analysis of groundwater. Institute of Geological Sciences Report, SO/S. Covington A. K., WhaRey P. D. and Davison W. (1983) Procedures for the measurement of pH in low ionic strength solutions including freshwater. AnolJxsr 108, 1528-l 532. Davies T. D., Abrahams P. W.. Tranter M., Blackwood 1.. Brimblecombe P. and Vincent C. E. (1984) Black acid snow in the remote Scottish Highlands. Nature 312, 58-61. Davies T. D.. Vincent C. E. and Brimbhxombe P. (1982) Preferential elution of strong acids from a Norwegi& ice cap. Nature 300, 161-163. _ Femandes A.. (19841Organic acids in snow and rain in Great Britain. Unpublished- MSc. thesis, School of Chemical Sciences, UEA. Norwich. Cialtoway J. N.. Cosby B. J. and Likens G. E. (1979) Acid precipitation: measurement of pH and acidity. timnol. 0rconagr. 24, iHit-1165. Galloway J. N. and Liens G. E. (1976) ~lib~tion of colhxtion procedures for the determination of preeipitation chemistry. Water Air Soil Pollur. 6, 241-258. Gjessing E. T., Henriksen M., Johannessen M. and Wright R. F. (1976) Effects of acid precipitation on freshwater chemistrv. In Immxl ol mid precipiralion on fores1 and f&&v&r ecosysiernr in Nor&y (&Ii&d by B&kke F.). Sur N~b~rs Yirknina Pu SIim_ oil - Fisk Res. Rep., 5,%. 64-85. Gross G. W.. Wu C., Bryant L. and McKee C. (19151
525
Evidence for preferential elution in Scottish snows Concentration dependent solute redistribution at the ice/water phase boundary-II. Experimental investigation. J. &em. Phys. 62, 3085-3092. Gross G. W., Wong P. M. and Humes K. (1977) Concentration dependent redistribution at the ice.-water phase boundary-III. Spontaneous convection. Chloride solutions. J. them. Phys. 67, 52645274. Haapla H., Sepponen P. and Meskus E. (1975) Effects of spring floods on water acidity in the Kiiminkijoki area, Finland. Oilios 26, 26-31. Henriksen A., Skogheim 0. K. and Rosseland B. 0. (1984) Episodic changes in pH and aluminium speciation kill fish in a Norwegian salmon river. Farlen 40, 255-260. Henriksen A. and Wri8ht R. F. (1977) Effects of acid precipitation on a small acid lake in southern Norway. Nordic Hydrol. 8, I-IO. Hultberg H. (1976) Thermally stratified acid water in late winter-a key factor inducing self-accelerating processes which increase acidification. Water Air Soif Polfuf. 7, 279-294. Jeffries D. S., Cox C. M. and Dillon P. J. (1979) Depression of pH in lakesand streams in central Ontario. J. Fish. Res. Ed. Can. 36.640-646.
Johannes A. H., Galloway J. N. and Troutman D. E. (1981) Snowpack storage and ion release. In Integrated LakeWatershed Stud;, EPRl report, EA-1256-L Johannessen M., Dale T., Gjessing E. T., Henriksen A. and Wright R. F. (1975) Acid precipitation in Norway: the regional distribution of contaminants in snow and the concentration processes during snowmelt. Proc. Isoropes und Impurities in Snow and Ice, Symposium, Grenoble 1975. Int. Ass. H&of. Sci. Pub!. 118. 116-110, Johannessen M. and Henriksen Ai (1977) Chemistry of snow meltwater: changes during melting. SNSF-Project, Norway, FR 1l/77. Jones H. G. (1985) The chemistry of snow and meltwaters within the mesostructure ofa boreal forest snowcover. Ann. Glacial. 7, 161-166. Jones H. G. and Soehanska W. (1985) The chemical characteristics ofsnowcover in a northern boreal forest during the spring runoff period. Ann. Giueiol. 7, 167-174. Knutson E. O.,Sood S. K. and Stockholm J. D. (1976)Aerosol collection by snow and ice crystals. Atmospheric Encironmenr 10, 395402. Leivestad H. and Muniz 1. P. (1976) Fish kill at low QH in a Norwegian river. Narure 259, 391-392. Marsh A. R. W. and Webb A. H. 419791Phvsicothemical aspects of snow-melt. CEGB Reports RD/L/N, 60/79. McKay G. A. and Gray D. M. (1981) The distribution of snowcover. In Handbook ofSnow (edited by Gray D. M. and Male D. H.). Per8amon Press, Oxford. McQuaker N. R., Kluckner P. D. and Sandberg D. K. (1983) Chemical analysis of acidic precipitation: pH and acidity determinations. Envir. Sci. Technol. 17,431-439. Morris E. M. and Thomas A. G. (l985a) Hydrcchemieal processes in a small subarctic catchment. Part I. Hydrology. In IHP Workshop on ~ydro~ogi~o~ nnd
Hydrochemicul Model Approaches to rhe Aridifiation 01 U~~sak. Sweden. Seotember 1984. NHP Rep&s 10; 25128. Morris E. M. and Thomas A. G. (19855) Preferential discharge of pollutants during snowmelt in Scotland. J. Glacial. 31, 190-193. Mizuno Y. and Kuroiwa D. (1970) Solute segregation in ice observed by auto radiography. J. Ghriol. 9, I 17-l 24. National Research Council of Canada 1981. Acidification in the Canadian aquatic environment: scientific criteria for assessing the effects of acidic deposition on aquatic ecosystems. NRCC Publication 18475196. Neal C. and Thomas A. G. (1985) Field and laboratory measurements of pH in low conductivity natural waters. J. Hydrol. 79, 319-322. Oden S. and Ahl T. (1970) The acidification of Scandinavian waters. YMER, Arsbok. 1970, 103-122. Raynor G. S. and Hayes J. V. (1982) Variation in chemical wet deposition with meteorological conditions. Atmospheric Ecosvstems.
Ensironmenr
16, 1647-l 656.
Seip H. M. (1980) Acid snow-snowpack chemistry and snowmelt. In Eflerts 01 Arid Preripitorion on Terrestrial Ecosystems (edited by Hutchinson T. C. and Havas M.) pp. 77-94. Plenum, London. Seip H. M., Abrahamsen G., Christophersen N.. Gjessing E. T. and Stuanes A. 0. (1980) Snow and meltwater chemistry in mini-catchments. SNSF Project, IR53/80. Skartveit A. and Gjessing Y. T. (1979) Chemical budgets and chemical quality of snow and runoff during spring snowmelt. Nordic Hydrol. 10, 141-154. Smeyers-Verbeke J., Den Hartog J. C., Dekker W. H., Coomans D., Buydens L. and Massart D. L. (1984)The use of principal components analysis for the investigation of an organic air pollutants data set. Atmospheric Environment 18, 247 l-2478. Schemenauer R. S., Berry M. 0. and Maxwell J, B. (1981) Snowfall formation. In Handbook ofSnow (edited by Gray D. M. and Male D. H.). Pergamon Press, Oxford. Tranter M., Brimblecombe P., DaviesT. D., Abrahams P. W., Blackwood I. and Vincent C. E. (in prep a) The spatial variability of the composition of surface snows at a remote and urban locality. Tranter M., Brimblecombe P., Davies T. D, and Vincent C. E. (in prep b) Meltwater evolution and preferential elution. Tsiouris S., Vincent C. E.. Davies T. D. and Brimblecombe P. (1985) The elution of ions through field and laboratory snowpacks. Ann. Gfuciol. 7. Tyree S. Y. (1981) Rainwater acidity m~surement problems, hnosofre=rir Ent;ironmPnt5. 57-b& Ward R.‘D. W. (1980) Avalanche hazard in the Cairngorrns Mountains, Scotland. J. Glacial. 26. 31-41.
Ward R. G. W., Langmuir E. D. G. and Beattie B. (1985) Snow profiles and avalanche activity in the Cairngorm Mountains, Scotland. J. Gkiol. 31, 18-27. Wolff E. W. and Paren J. G. (19841 A two-ohase model of
electrical conduction in polar ice sheets. Bll, 94339438.
J. g~oph~~s. Res.
89,
M.
GRANTER,
OF SNOWFALL., SNOWPACK AND
IN THE SCOTTISH HIGHLANDS-EVIDENCE FOR PREFERENTIAL ELUTION
P. BRIMBLECOMBE, T. D. DAVIES, C. E. VINCENT, P. W. ABRAHAMS~~~ I. BLACKWOOD
School of Environmental Sciences.University of East Anglia. Norwich, Norfolk NR4 711, U.K. {First receiued 10 Januclry 1985 and receivedfor publication2 September 1985) Abstract-Acidic snows in a small, remote, high-altitude snowpack in the Cairngorms, Scotland. give rise to meltwaters which are proportionally rich in sulphate and nitrate. As a consequence,the within-pack snows become proportionally rich in chloride. even though depleted in solute. Preferential elution appears to be a major proms in the chemical evolution of snowfall and snowpack. Key word index: Snow, preferential elution, meltwater, acid deposition, triangular diagrams.
INTRODUCTION Up to 80 % of the total solute load of snowpacks can be lost with the first 20 % of meltwater (Jo~nnassen er al. 1975; Johannassen and Henriksen, 1977; Johannes et of., 1981; Cadle et al., 1984a; Tsiouris et al., 1985). Since the first meltwaters are often acidic, interest in snowpack chemistry has centred around the so-called “acid flush”, which may lead to short-term acidification of str~mwater (Oden and Aht, 1970; Haapla et al., 1975; Hultberg, 1976; Gjessing et ct., 1976; Henriksen and Wright, 1977; Jeffries et al., 19?9; Skartveit and Gjessing. 1979; Cadle et al., 1984b; Henriksen et al., 1984). Downstream acidification may be exacerbated by: slow melting (Marsh and Webb. 1979); theaccumuIation of a large q~ntity of acidic snow; the deposition of very acidic snows (e.g. Davies er al., 1984); or accumulation of easily leached solute near the base of the pack (Colbeck, 1981). If the bedrock or soil has little buffering capacity there may be important ecological consequences, such as fish kill (Leivestad and Muniz, 1976; National Research Council of Canada, 198t). Whereas the fractionation of acidic solute into meltwater is well established in the literature, preferential elution is as yet poorly understood. Some ions are removed more quickly by meltwaters than others: this process is defined as preferential elution (Davies er ol., 1982). In particular, SO:- and NO; appear to be preferentially Juted with respect to Cl(Brimblecombe et 01.. 1985; Tsiouris et al., 1985). The mechanism for preferential removal of ions is unartain, although there are two obvious possibilities: (a) Melting of snow or ia crystals which contain inhomo~n~us distributions of solute will produce me&waters of different composition over time. For example, sodium chloride may have a more homogeneous distribution throughout the snow crystal, in contrast to acidic solute, which could be found pre-
dominantly on the surface (Wolff and Paren, 19841, having been incorporated onto the crystal from supercooled cloud droplets to produce rimed snow crystals (e.g. ~hemenauer er al., 1981) or scavenged during descent (Knutson et (I!., 1976; Raynor and Haynes, 1982). Previous work has demonstrated that different solutes have different solubilities in ice (e.g. the maximum solubility of HF is 2000 pmol /-I in comparison to 200 /unol I-’ for HCI), fhat the solubility will depend on the rate of freezing (Gross et al., 1975, 1977; Seip, 1980) and that the distribution of solute within ice is heterogeneous (Mizuno and Kuroiwa, 1970). (b) There may be chromatographic effects during percolation of meltwater through the snowpack if the snow or ice crystal surfaces have different affinities for the individual ions. This paper examines the major ion composition of the snowpack and its meltwaters to determine whether preferential elution of sulphate and nitrate is observed in a complex system which shows both temporal and spatial variability in com~s~tion. The study area was remote so difficulties in sampling during harsh weather conditions meant it was rarely possible to sample at times which would have been experimentally ideal. However, such problems are common with snow studies and one of the most encouraging aspects of this work is that it is possible to draw useful condusions about the processes occurring within snowpacks despite these limitations. The work represents the first attempt to monitor concurrently the composition of snowfall. snowpack and meltwater and to examine their evolution through time.
Fieldworkarea The semi-permanent snowpack is located within a small. remote. high-altitude catchment in the Cairngorm
518
M.
TRAh
Mountains, Scotland. The catchment, Ciste Mhearad (57‘06’N. 03’38’W). is located to the NE of the Caimgorm peak, has an area of 0.4 km” and an altitude of around 1080m (see Fig. I). The catchment experiences strong winds on occasions, and there is much local redistribution of surface snows. There are no woodland influences on the catchment (cf. Jones, 1985; Jones and Sochanska. 1985). The main characteristics of snowcover in this region are: (a) surface layers of wind-blown snow forming hard or soft siab, (b) basal layers of coarse, granular snow with grain diameters usually > 2mm intercalated with ice layers, some of which are multiple-bedded or up to 5cm thick (Ward, 1980,Ward et (II., 1985). Equi-temperature and melt metamorphism dominate the evolution of the snowcover for much of the season. Ice layers are formed by repeated melting and refreezing of surface snow, possibly by diurnal cycles, or when surface meltwaters penetrate the pack and refreeze at a temperature inversion a few cm below the surface. Slush layers may form during spring (Ward et al., 198s).
Silmp/ingmerh4kfalogy Three sets ofsamples, (i) newly fallen snows. (ii)snows from within the snowpack, and (iii) meltwaters, were collected during January through to June 1984. Snow samples were transferred to pre-cleaned polyethylene bottles by PTFEcoated scoops. Newly fallen snows were sampled whenever possible throughout the season. and usually within 6 h of snowfall. Within-pack snows were sampled from randomly located snowpits, which were dug with a stainless steel spade and faced by a PTFE-coated plastic scoop. A number of snow samples were taken from each snowpit, Typically nine samples of a~roxi~teiy iOcm thickness were taken from a 3m snowpit. Each major snow layer, distinguished by differences in crystal type, grain size and hardness, was sampled. At least one snowpit was sampled every week from January through to March. and thereafter at least two snowpits were sampled each month. Two snowpits were resampled a number of times. The initial snowpit was partially ref&d, and subsequent resampiing involved redig-
’
ITER et al.
ging and refacing the sampling surface by at least 30 cm. Titis procedure was repeated for up to five times over two weeks, Snowmeit samples were collected 3m below the snow surface and 2m from the base of the snowpack by an automatic snowmeit sampler (designed by the Institute of Hydrology), which had a sampling period of one day. The instrument was installed and operating for a month before the firs1 sample was collected. The automatic sampler was installed to ensure that the first snowmelt fractions of a melt event were captured, irrespective of whether or not such a melt had been forecast and to allow for the possibility that no workers would be on site during the melt. Snowmelt samples were collected on 6.7.8 and 10 March, and are representative of the second major melt event of the season (Morris and Thomas, i985a). A further composite sample was collected over several days due to instrument failure, and is ignored in the following calculations. During a period of snowmeit in May, near-surface meitwaters were collected at half huuriy intervals from a ledge in the side of a snowpit (see Tsiouris et a/., 1985, for fuller details), 0.5 m and 1.0 m below the snow surface to examine the detailed temporal variations in meltwater composition. Other meltwaters were collected simultaneously from the pipfeed of the automatic snowmelt sampler. Pretreatment and chemical analysis The snow samples were transferred to the field laboratory within 4 h of collection and showed little sign of melting. They were melted by immersing the bottles into hot water. Before the snowmeit reached IO’C (and typically YC). they were suction filtered through 0.45pm Oxoid filter papers (Cook and Miles, 1980). Thereafter, ail samples were stored at approximately 4’C until analysis (Galloway and Likens, 1976). An aliquot of the filtered sample was taken for immediate pH determination by an Orion research grade electrode and Orion 501 portable pH meter, calibrated with BDH buifers of 4.00 and 7.00. Care was taken to ensure that buffer and sample temperatures (typically 7-12°C) did not diiier by more than 3°C. Measurement of the pH of acidic solutions with low ionic strength is difficult (Galloway et ai.,
!
,
’ li90m
.-.
_
.’ / Enn
i
i,..
I
*l-inn...
Fig. I. Location map of Ciste Mhearad. The sampling area is enclosed by the dotted lines. Other dotted lines arc footpaths.
519
Evidence for preferential elution in Scottish snows 1979; Covington et al., 1983; &Quaker et al., 1$83; Tyree, 1981; Neale and Thomas, 1985), and errors of at krst f 0.2 pH units may be common. Measurements of the pH of dilute acid standards in our laboratory show that iield pH is systematically underestimated by 02 units between pH 3.7 and 4.3. The pH data presented in this work are uncorrected for errors in measurement. Analysis for the major anions (SO:-, NO; and Cl-) was performed by ion chromatography on a Dionex Model 12 Ion Analyser. Detection limits were 0.2 m I-’ and precision was better than 5%. Major metal ion concentrations fNa+ and Mg”*) were determined by AAS on a Pyc Unicam SP9 Spectrophotometer. Precision was: Na‘, f 2 m /-‘; Mg’+, f 1 peq I-‘. Detection limits were an order of magnitude less. RESULTS
The composition of newly fallen snow varies during the season. In all, at least one sample of 19 different fresh snowfalls were collected, and the range of pH and major ion concentrations is shown in Table la. Approximately 50% of all snowfaits were sampled during fanuary through to May. No rainfall onto the snowpack was observed during the sampling periods until late afternoon on 2 May, when the sampling of near-surface meltwaters was terminated. On the basis of nearby synoptic observations it was unlikely that there was rainfall in the catchment before May. The fresh snows are all acidic fpH 3.1-5.4). H,SO, and HNO, are usually dominant, but HCl was also found to be of importance in the most acidic snowfall of the season, where the molar non-sea salt chloride: non-sea salt sulphate: nitrate ratios approached 1: 1: 1.8 (Davies et al., 1984). However, the predominance of a sea salt source of Cl- for the fresh snows over the season is indicated by the high correlation between Cl and Na+, in comparison with the low correlation between Cl- and H+ (see Table la). Minor quantities
Table la. The com~sitions of fresh snows
H+ Na+ Mg;_” %; cl-
Mean
Max
SD*
CV$( %)
4.00 1.91 0.00 5.63 1.77 2.26
137 76.4 18.0 55.3 40.5 111
777 227 55.8
186 80.4 19.3 67.6 59.6 109
136 105 107 122 147 98
Units are caq /- I. * SD = Standard deviation. $CV = Coeflkicnt of variation. Correlation coefEcients H’ Na* Mg’+ ;+<
Na+
Mg2*
so:-
NO;
Cl-
0.136
0.107 0.991
0.880 0.448 0.423
0.705 0.350 0.300 0.756
0.259 0.892 0.880 0.764 0.402
Critical 95% value (N = 19); 0.456.
Min
Mean
Max
SD
cv (%)
Na+
1.30 0.87
34.4 54.1
316 252
39.2 54.9
114 101
!$;
0.33 0.00
18.6 11.9
64.1 279
29.7 12.9
160 108
z15 -
0.08 3.95
81.3 9.37
316 232
74.6 23.9
25s 92
H+
Units are peq f- I.
Correlation coefficients
H’ Nat Mg’, sotNOi
Na*
Mg**
SO:-
NO,
Cl-
0.193
0.275 0.989
0.844 0.373 0.456
0.861 0.112 0.201 0.916
0.123 0.974 0.955 0.340 0.088
Critical 95% value (N = 154); O.lS2.
Table lc. The composition of meltwaters Min
Mean
Max
SD
CV (%I
H+ Na+ Mg2+
3.12 1.74 0.00
53.5 51.8 13.0
708 748
138 139 31.9
258 268 245
;o& Cl-’
;: 8.18
40.2 588 71:2
447 z 1071
141 89.2 177
240 222 249
Units are peq /- ‘.
Na’ H’ Na+ 2+
0.989
Correlation coefficients Mg2’ so:NO; 0.980 0.989
0.983 0.993 0.998
Nd; s”o”2-
Min
;: 341
Table lb. The composition of within-pack snows
Cl-
0.977 0.983 0.999
0.981 0.991 0.977
0.996
0.973 0.976
Critical 95 % value (N = 61); 0.252.
of organic anions, such as acetate and methane sulphonic acid (Fernandes, 1984) have also been detected. The composition of a newly fallen snow surface shows considerable spatial variability, as seen in Table 2. These snows were collected every 50 m along a 700 m transect of the catchment. and represent the top 2-5~1 of snowfall (and snowpack). The coefEcient of variation, defined as (standard dev~tion/m~n) x lOO%, of NO;, Cl- and SO:- for this transect are 41”/, 41% and 14 %, respectively. The nange of pH is j: 0.05 units and there is no systematic variation of pH or anion concentration along the traverse. The major ions are all well correlated. suggesting internal consistency of the data set. Similar variability is found within other fresh snow surveys from the catchment, although less variability is found in urban snows from
520
M. TRANTER er of. in the surfacecomposition ofnewlyfatten snow along a transect of the catchment. Date 29.3.84.
Table 2. Variation
Base of catchment (1OlOm)
Head of catchment (t16Om)
H+
Na’
Mg*+
281 316 316 252 316 316 282 282 224 251 25I 251 281 316 251
8.26 13.5 7.83 5.22 16.1 22.6 8.26 10.9 4.78 9.57 7.82 5.65 11.7 13.9 12.2
1.67 3.33 I .67 0.83 4.17 5.83
1.67 2.50 0.83 2.50
1.67 I .67 2.50 3.33
1.67
so: -
NO;
IS6 121 125 77.1 125 121 91.7 106 91.7 96.5 104 86.9 96.5 116 98.3
13.4 24.1 17.4 15.8 42.6 37.7 17.9 37.7 15.3 21.9 18.5 16.3 22.6 33.1 17.6
11.8 17.8 9.87 7.05 19.7 23.7 9.87 13.8 5.08 13.0 II.0 7.90 13.0 18.9 9.59
23.4 9.54 41
12.8 5.19 41
Units are peq / -
Mean SD cv ( 7;)
219 31.4 11
Na+ H” Na’
10.6 4.72 4s Correiation Mg”’
0.692
0.717 0.957
Mg” so:NO; Critical
2.39 1.33 56 coefficients SO:0.827 0.638 0.648
Cl‘
’
105 15.1 14
NO;
ci-
0.619 0.788 0.830 0.545
0.793 0.934 0.965 0.124 0.872
95 % value (N = IS); 0.514.
Norwich (Tranter et al., in prep. a). The composition of aged surface snows also shows large spatial variability (Brimbl~om~ et al., 1985), which persists throughout the season, even after the extensive leaching (Tranter et ol., in prep. a). The composition of snows from within the snowpack varies (see Table lb), mainly because of the difference in composition between individual snowfalls, and leaching and redistribution of sotute. Redistribution and leaching of solute is IikeIy to occur during diurnal melt-freeze cycles and during major melt events when increased solute levels are observed in the stream (Morris and Thomas, 198Sb). Hence, the composition of within-pack snows is likely to undergo rehtively rapid modification, although this modification is not considered in detait here. Therefore, no attempt has been made to distinguish between pre- and post-melt within-pack snows. Meltwater compositions are summarized in Table lc. Apart from Ii’, the maximum conccntrations were found in samples colkcted by the automatic snowmeft sampIer, 2 m from the base of the snowpack. They are 1.5-2.0 times more concentrated than the maximum snow concentrations fsee Table la). All major ions are well correlated, in contrast to the poor correlation which exists between some species
(e.g. Cl- and NO;) in the fresh and within-pack snows. The average meltwater composition is less con~ntrated than the average fresh snow com~sition because 50 of the 64 samples were collected at the end of the season, during the detailed examination of nearsurface meltwater composition in May. The end of season snows were relatively leached and consequently the meltwaters were more dilute than earlier in the season. A tip gauge within the snowmelt sampler monitored the quantity of snowmelt passing through a 200cm2 aperture each day. The values recorded when samples were collected (6, 7, 8 and 10 March) were 110, 70, 30 and 20 cm3, respectively, which correspond to rates of ablation ofS.5,3.5,1.5 and l.Omm (H,O)day- ‘. Using 30% as the average water equivalent per unit depth (from McKay and Gray, 1981), the water equivalent above the snowmelt sampler would have been 90cm; therefore the meltwater collected represents approximately 1% of the water equivalent above. Because of bad weather conditions prior to the melt event, we have no estimate of the tota1 solute load of the snowpack prior to the melt. Near-surface meltwater samples were collected on 1 and 2 May. From visual observation, the snowpack at the sampling location had experienced approximately
Evidence
for preferential
elution
6Ocm (H,O) of ablation over the previous two weeks. Hence the surface snows were well leached. The rate of ablation during the sampling period was estimated as I-2cm (H,O) day-‘, and the snowpack comprised > 6Ocm (H,O). Principal components analysis of the data sets
Preferential elution results in complementary changes in the composition of meltwaters and withinpack snows. For example, if SO:- and NO; are preferentially eluted with respect to Cl-, the meltwaters should be proportionally rich in SOi- and NO;, while the within-pack snows should become proportionally rich in Cl-. This is difficult to demonstrate within a catchment when there is large variability in each data set, due to spatial inhomogeneities and continuous temporal alteration. However, principal components analysis and graphical representation on triangular diagrams considerably reduces this difficulty (Brimblecombe et al., 1985) and allows the apparently complex data set to be interpreted in terms of several simple processes. In the following discussion, it should be noted that the magnitude of the loading indicates the contribution each variable makes to the principal component. Positive and negative loadings indicate positive and negative correlation between the variable and the principal component, respectively (SmeyersVerbeke et al., 1984). Furthermore, if all ions were perfectly correlated (e.g. all ions derived from the same source), the second component would be redundant. Interpretation of the principal components is usually difficult (e.g. Chatfield and Collins, 1980), but in the case of the components presented in Table 3, a relatively straightforward interpretation can be made. The first component, which accounts for much of the total variance, allocates a similar loading to each ion in all three cases and represents the overall effect of concentration and its high variability. The second component differentiates between the two major sources of solute, sea salt (Na+, Mg2+ and Cl-)and acid (H *, SO:- and NO;) ions, to the fresh snows and the within-pack snows. The second component of the meltwaters explains too little of the total variance to be of
521
in Scottish snows
significance. The physical interpretation of the second component, and particularly the sign reversal in the loadings, is that acidic solute is associated with nonmaritime air masses, i.e. polluted continental air. Principal components analysis of aged surface snows and acidic precipitation gives rise to first and second components similar to those for the fresh snows (Brimblecombe et al., 1985; Brimblecombe et al., in prep.) Representation of the proportional anion composition of acidic snows and meltwaters on triangular diagrams
Triangular diagrams remove the effect of highly variable concentration (i.e. much of the effect of the first principal component) and reveal the underlying trends in the data sets. By representing the data in this way, up to 99% of the total variance has been eliminated. Furthermore, simple fractionation of solute into meltwater will not alter the position of within-pack samples on the triangular diagram, since there will be no change in the relative proportion of ions. The percentage of each anion is plotted such that the proportion of each anion is represented by the proximity to each apex. For example, samples rich in Cl- lie near to the Cl- apex. Figure 2a shows the proportional anionic composition of all the fresh snows and demonstrates the considerable variation in composition. The composition of each snow frequently reflects the air trajectory associated with the precipitation event (Brimblecombe et al., in prep), even though only the top 2-5 cm of the total snowfall has been collected. All snows collected from the same snowfall lie in the same area of the triangle, although the absolute concentrations of the ions may vary considerably (Tranter er al., in prep. a). Figure 2b presents the proportional anionic compositions of all within-pack snows. No attempt has been made to distinguish between pre- and post-melt within-pack snows. There is less scatter in the data in comparison with the fresh snows. The within-pack snows cluster towards the Cl- apex, near to the sea salt Cl-:SO:-ratiooflO:l.TherearenoSO:--orNO;rich within-pack snows. Unfortunately, the contri-
Table 3. Principal components analysis of the major ions of fresh snows, within-pack snows and meltwaters Component
H*
Na’
0.319 0.556
0.434 -0.395
:
0.447 0.355
1
0.407 0.354
Mg*+
NO,
so: -
0.422 -0.419
0.371 0.384
0.435 0.361
0.452 -0.284
62.3 91.5
-0.385 0.434
-0.328 0.459
0.508 0.337
0.361 0.429
0.422 -0.396
60.0 95.4
0.409 0.246
0.409 -0.394
0.578 - 0.499
0.581 -0.279
0.574 0.577
98.8 99.5
Cl-
Total variance (%)
Fresh snows
1 2 Within-pack
snows
Meltwaters 2
522
M.
TRANTER
et al.
G
.oo
Evidence for preferential elution in Scottish snows bution that each snowfall made to the snowpack is unknown. However, since each major snow layer was sampled, the tighter distribution of within-pack snows in comparison to the fresh snows suggests that either: (a) acidic solute has been redistributed within or removed from the snowpack, leaving behind leached snows which are proportionally rich in Cl-. (b) only snowfalls rich in chloride made significant contributions of solute to the snowpack, or (c) dry deposition of sea salt and its subsequent redistribution throughout the pack has a major influence on snowpack chemistry. Figure 2c presents the proportional anionic compositions of all meltwaters. All data cluster in a narrow band leading towards the Cl- apex, even though some samples are up to two orders of magnitude more concentrated than others. The tight grouping of the meltwaters is unexpected in view of the disparate nature of the fresh snows. Despite the fact that meltwaters were collected (a) at two different sites, (b) during two different melt events, and (c) using two different sampling techniques, they tend to be enhanced in SOi- and NO; in comparison to the within-pack snows (see Fig. 2b).
DISCUSSION
A disadvantage of working in a remote, mountainous catchment is that it is seldom possible to sample snowfall, snowpack and meltwaters in a systematic fashion. There is much redistribution of snow by the wind, with the result that the thickness of individual snowfalls is not constant throughout the catchment. Difficulties in obtaining a volume-weighted contribution of each snowfall to the snowpack were compounded by the variable nature of the solute load. Attempts to collect representative, volume-weighted samples of snowfall and snowpack therefore proved impracticable. Instead, samples were collected on a random basis. Comparison of the ionic ratios of fresh and within-pack snows (see below) suggests that this approach has been successful. However, the limitations of the undefined nature of the sampling schedule should be remembered during the following discussion. Meltwaters appear to remove SO:- and NO; in preference to Cl- (see Fig. 2). This is supported by variations in the composition of meltwaters collected near the surface of the snowpack at 30 min intervals on 2 May. Figure 3 shows a clear example of the evolution in meltwater composition towards a Cl--rich, though solute-depleted, system as melting and leaching progress, and the source of SO:- and NO; is depleted. The jump in composition between the two groups of points suggests some change in snowpack hydrology. Similar examples of meltwater evolution towards a Cl--rich. solute depleted system are shown by Brimblecombe er al. (1985) and Tranter et al. (in prep. b). In addition, laboratory experiments on the melting
523
Fig. 3. The compositional variation ormeltwaters collected 1 m below the surface of the snowpack on 2 May, 1984. Samples were collected at intervals of 38 min. commencing at 1200. The length of the solid line is proportional to the total anion concentration of the adjacent meltwater, which decreased monotonicly during the day. The maximum and minimum total anion concentrations were 39 and 24 peq / - ‘, respectively.
of natural snows, where dry deposition is excluded, demonstrate that SO:- and NO; are eluted preferentially to Cl- (Tsiouris et al., 1985). Preferential elution of H+ and SO:- with respect to Na’ and Cl- (and possibly Ca*+) has been observed in snowpack at the Storgama area, Norway (Seip er al., 1980). The effect of preferential elution on the composition of the snowpack can be seen in Fig. 4. Preferential elution of SO:- and NO; will make Cl- the dominant anion in older, leached snows, in contrast to the fresh snows, where all three ions may make significant contributions to the total anion concentration. Hence the Cl-: NO; and Cl-: SO:- ratios of within-pack snows should become greater than those of the parent fresh snows as melting and leaching occurs. Figure 4 presents these ratios plotted against the total anion load (on logarithmic axes to encompass the large ranges exhibited). In comparison to the fresh snows, the within-pack snows lie above and to the left of the fresh snows. Six leached snows collected from a snowpit in the remnant snowpack in July lie further above and to the left of the fresh snows. This suggests that preferential elution of NO; or SOi- is an important process in the chemical evolution of snowfall and snowpack.
CONCLUSIONS
The composition of fresh snowfall in a small, remote, high altitude catchment in the Cairngorms is acidic and shows considerable spatial variability. The composition of snows within the pack is also very variable. The spatial variability of the composition of
M. TRANTERet al.
524
Cl/N03 1
-t
standard deviation
leached
r
100 /
100 Leached 5”ows
SCOWL
10 Wtthin-DaCk
snows
’
---VT
Fresh snows
-
i
Fig. 4. A comparison of the equivalent Cl-:SO$- and Cl-:NO; ratios of fresh snows and ~thin-~ck snows as a function of total anion ~~ent~tion. Six snows collected from a snowpit in the leached, remnant snowpack during July are also shown.
snowfall and snowpack means that representative volume-weighted sampling is difficult. Principal components analysis of fresh snows, within-pack snows and m&waters demonstrates that much of the variability is due to variations in the overall solute load. Therefore, variability in the proportional anionic composition of acidic snows and meltwoters is best represented on triangular diagrams, since the dominant e&t of variation in concentration is removed
from the data. In general, meltwaters appear to remove NO; and SOi- in preference to Cl- from the snowpack. As a consequence the snowpack becomes proportionally rich in Cl- as the solute is depleted. The evolution of meltwaters is also towards a Cl--rich, solute depleted system. Plots of Cl- : NO; and Cl-: SO:- ratios against total anion load for within-pack and fresh snows broadly confirm this hypothesis, since relative to the parent fresh snows, within-pack snows which are depleted in solute have higher ratios. Preferential clution is therefore an impo~nt process in the chemical evolution of snowfall and snowpack.
wish to thank M. Burrows, R. Bryant, A. Femyhough, B. Howes, E. M. Morris, L. Reynolds, F. Robinson, A. G. Thomas and S. Tsiouris for logistic, operational and technical support. Acknowledgements-We
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