Chemical Changes of Snow Cover by Melting

Transcription

1 Jap. J. Limnol. 43, 2, , Chemical Changes of Snow Cover by Melting Keisuke SUZUKI Abstract Chemical changes of snow cover were studied. The concentration of chemical constituents, ph and water equivalent of snow cover were observed together with other related factors in Sapporo during the winter of During the melting periods, the concentration of chemical constituents of the total snow cover decreased and ph increased. During the temporary snowmelts and the early phase of the snowmelt season, only the surface snow layer melted, and showed a decreased concentration of chemical constituents. In these phases the meltwater flowed downwards in the form of a water channel flow. When the meltwater began flowing downwards in the form of a water-film flow as the snowmelt progressed, the snow particles changed into large granular snow and the concentration of the chemical constituents of the inner layers began to decrease. The earlier meltwater was considered to contain extremely large amounts of chemical constituents and had a low ph. The mechanism of the decrease in the concentration of chemical constituents in the snow cover and that of the increased concentration in the meltwater were discussed. 1. Introduction In a snowy region, the snowmelt has a considerable influence on the quality of surface waters. For example, the meltwater causes sharp drops in the ph of surface waters which lead to physiological stress in fish and other aquatic organisms (BELL, 1971; HAGEN and LANGELAND, 1973; LEIVESTAD and MUNIZ, 1976). In regard to these drops in the surface water ph, it is considered that acid chemical constituents which have accumulated in the snow cover are released during a short period of the snowmelt. Laboratory and field studies show that the first fractions of meltwater contain 2-5 times higher concentration of chemical constituents than snow cover (JOHANNESSEN and HENRIKSEN, 1978). In the northeastern region of Japan, high chloride concentration was observed in the stream water in the initial period of snowmelt (KATO and IIZAWA, 1976; SUZUKI, 1979). However, the mechanisms of this low ph and the high concentration of chemical constituents in the meltwater have never been clarified. The purpose of the present study is to obtain information on the chemical mechanisms of snowmelt through observation of the chemical characteristics of the snow cover. 2. Methods 2-1. Observation Site The observations were made at the Agricultural Experiment Farms of Hokkaido University, Sapporo (Fig. 1) during the period from January 7 to April 9, The observation site was an open area of about 500 m 500 m with a smooth and level surface which made for an even accumulation of snow. In Sapporo, the mean annual precipitation for the period was 1158 mm, of which more than one-third fell as snow. The mean maximum depth of snow cover was 102 cm Sampling Procedures A trench, approximately 2 m 2 m, was dug through the snow cover to the ground surface. The wall facing north was made vertical and smooth, following which the snow stratigraphy was observed, and the thickness and density of the snow layers were measured. Water equivalents of the snow layers were obtained by multiplying thickness by density. Snow samples were taken from each layer with a clear plastic scoop. The samples were put into polypropylene boxes which were sealed and the snow was allowed to melt. Several snow

2 Suzuxi 103 Fig. 1. Location of observation site. cores were removed from a side of the trench using a 58 mm diameter clear plastic pipe. The cores were put into a polypropylene box and allowed to melt. The meltwater weight was measured, and the water equivalent of the total snow cover was obtained by dividing the weight of one core by the cross-sectional area of the plastic pipe. Everyday a new trench was used approximately 2 m away from the previous one Analytical Procedures Immediately after melting, each meltwater sample was filtered through a 1.0 micron pore size membrane filter (Togo Roshi Co., TM-100) which was previously washed with 300 ml distilled water. Conductivity and ph were measured potentiometrically at 25 C. Sodium, potassium, magnesium and calcium were determined by atomic-absorption spectrophotometry (SEIKO, SAS-721). Chloride and sulfate were determined by spectrophotometry (JIS, 1966) Measurements of Soil and Snow Temperatures The soil temperature at 0.5 m depth and the snow temperatures at 0, 0. 2, 0. 4, 0.6 and 0.8 m above the ground surface were recorded with platinum resistance thermometers which were installed before winter. When the thermometer was covered with more than 5 cm of snow, the snow temperature was measured. 3. Results 3-1. Snowmelts during the Study Period During the study period, three temporary Snowmelts occurred. Air temperatures rose on January 27 and 28, resulting in densification of each snow layer and increased snow temperatures (Figs. 2 and 3). At this time, the first snowmelt occurred (henceforth referred to as melt-i). At the end of February, the second snowmelt was noticed (henceforth melt-ii). The snow temperatures on the ground surface and at 0.2 m above the ground registered 0 C from February 29 and from March 2, respectively (Fig. 3). In addition, the daily maximum snow temperatures registered 0 C at each level on March 10 (henceforth melt-ill). Daily maximum snow temperatures also regis-

3 104 Chemical Changes of Snow Cover by Melting Fig. 2. Depth of snow cover and densification curves of each snow layer at the observation site. Daily maximum and minimum air temperatures and daily precipitation for the period JST at the Sapporo District Meteorological Observatory. Fig. 3. Daily maximum soil and snow temperatures for the period JST at the observation site. Symbols: Q, soil temperature;., Om; ~, 0.2m; /, 0.4m; Q, 0.6m and /, 0.8m above the ground surface (snow temperatures).

4 SUZUKI 105 tered 0 C at each level from March 23. The snow cover then began the main spring melt, and completely disappeared on April Chemical Changes of the Total Snow Cover The results of total snow cover analyses are shown in Fig. 4. Both the equivalent concentration of cations (lea, K, Mg and Ca) and that of anions (Cl and S04) fluctuated in the same way (correlation coefficient, 0.98). As shown in Fig. 4, the water equivalent of total snow cover increased gradually until the beginning of the snowmelt season. The concentration of chemical constituents remained approximately constant except for the three periods of temporary snowmelts. During the temporary snowmelts, the concentration of chemical constituents decreased and ph increased. The decrease of all the chemical constituents and the increase of ph were observed in the snowmelt season. However, the concentration of chemical constituents did not decrease indefinitely. The lower limit of the major ion concentration of the total snow cover was about 3 mg/l (Fig. 4). Fig. 4. Water equivalent (Hw), conductivity (22b), ph, major ion concentration and major ion load of the total snow cover.

5 106 Chemical Changes of Snow Cover by Melting During the snowmelt season, the ion concentration started to decrease prior to the decrease in the water equivalent. The water equivalent decreased from March 24, when the daily maximum snow temperatures began to reach 0 C at all levels. The ion concentration decreased from March 19. The major ion load which was obtained by multiplying the major ion concentration by water equivalent gradually increased until the snowmelt season, except when the three temporary snowmelts occurred. With the beginning of the snowmelt season it decreased to zero (Fig. 4). The chemical constituents accumulated in the snow cover in winter and were released during the snowmelt season Chemical Changes of the Internal Snow Layers In order to show the chemical changes of the internal snow layers, the snow cover was divided into seven layers in terms of the physicochemical conditions, as shown in Fig. 5. Layer-b and layer-f were constituted by one fall unit which had extremely low concentrations of cations in comparison with the other layers (Table 1). Figure 6 shows the changes in the water equivalent for each layer. In layer-a, the water equivalent decreased during melt-i. After that it remained constant for a time. Snow temperatures on the ground surface and 0.2 m above it registered 0 C since melt-ii, as seen in Fig. 2. Thus, layer-a did not refreeze, so its water equivalent gradually decreased with the snowmelt at the bottom of the snow cover. Although the water equivalent of layer-c showed a slight fluctuation, it suddenly began to decrease when layer-c was exposed (April 6). Layer-d absorbed meltwater of the Fig. 5. Division of snow layers. Table 1. Water equivalent (Hw), density (G) and major cation concentrations of each layer on the initial day.

6 SuzuKi 107 on occasions. Figure 7 shows the fluctuations in the concentration of the major cations in each layer. The equivalent concentration of cations and anions for the total snow cover fluctuated in the same way, as mentioned Fig. 6. Water equivalent of each layer. upper layers during melt-ii and changed into granular snow and ice sheet through refreezing of the meltwater. Consequently, the density and water equivalent of layer-d increased from 0.27 g/cm3 and 7.2 g/cm2 on February 27 to g/cm3 and 9.9 g/cm2 on March 6, respectively. After that the water equivalent of layer-d remained constant, but decreased rapidly as soon as it was exposed (March 31). The water equivalent of layer-e decreased during melt- III when it became the surface layer. Layer-g was always the surface layer in the snowmelt season, so the water equivalent decreased rapidly. As mentioned above, it is clear that the melting begins from the snow surface. The water equivalent of the internal layers displayed a slight fluctuation, but that of the surface layer decreased under the snowmelt conditions. The exception was the secondary upper layer, which absorbed the meltwater and changed its texture; its water equivalent was observed to increase Fig. 7. Major cation concentration of each layer. in section 3-2. This suggests that the equivalent concentration of cations and anions in each layer may also fluctuate in the same way. During melt-iii, the concentration of major cations of surface layer-e decreased, whereas that in the other lower layers fluctuated only slightly. It was also noted that the concentration of major cations in layer-g rapidly decreased in the early phase of the snowmelt season; nevertheless, the decrease in the lower layers began on March 25. The decrease in the concentration of chemical constituents in the total snow cover, which began on March 19 (Fig. 4), was caused by their decreasing concentration in the

7 108 Chemical Changes of Snow Cover by Melting surface layer. The beginning of the decrease in the concentration of the chemical constituents in the lower layers coincided with the decrease in the water equivalent of the total snow cover. During the temporary snowmelts and the early phase of th snowmelt season, only the surface snow layer is considered to show a decreased concentration of chemical constituents. Figure 8 presents the fluctuations of the Fig. 8. Major cation load of each layer. major cation load for each layer. If the meltwater flows down uniformly, the loss of the chemical constituents of the surface layer must induce some changes in the lower layers. During melt-iii, the loss of the major cation load of layer-e induced only slight changes in the lower layers. In the early phase of the snowmelt season, the greater part of the major cation load loss in layer-g was not compensated by the lower layers either. These results suggest that the meltwater flows downwards in the form of a water channel flow during the temporary snowmelts and the early phase of the snowmelt season. As noted above, during the snowmelt season, the water equivalent began to decrease as layer was exposed in contrast to the ion load which began to decrease even while layer was unexposed, as shown in Fig. 8. The fluctuations of the ion load were clearly different from those of water equivalent Concentration of Chemical Constituents and ph of the Meltwater The meltwater chemistry was estimated using the results of the analyses of the total snow cover. The index of the snowmelt due to sensible heat transfer, which is obtained by multiplying air temperature by wind speed (NARUSE et al., 1970), was calculated with data from the Sapporo District Meteorological Observatory. When this index has a positive value, the snowmelt is caused by the sensible heat transfer from air to snow. From March 16 to 18, this index displayed negative values, but positive values were always obtained from March 19. In addition, the ion load of the total snow cover definitely decreased from March 19 (Fig. 4). From these results it was determined that the snowmelt season began on March 19. Water equivalent, H+ load (calculated from ph) and major ion load of the total snow cover during the snowmelt season are shown in Fig. 9. The results of regression analyses of the plots in Fig. 9 are given in Table 2. It is assumed that the reductions of water equivalent and ion load of the total snow cover are equivalent to the outflow of meltwater containing chemical constituents from the snow cover. Based on this assumption, the flow rates of the meltwater, H+ and major ions were calculated by using the regression equations. Subsequently, the ion concentration of the meltwater was calculated by the flow rate of ions divided by the flow rate of the meltwater. H+ concentration was converted into ph for the meltwater. The results of this estimation are shown in Fig. 10. Daily values of the major ion concentration

8 SUZUKI 109 Fig. 10. ph and major ion concentration of meltwater during the snowmelt season. Fig. 9. Water equivalent (Hw), H~ load and major ion load of the total snow cover during the snowmelt season. and ph of the meltwater are plotted. The major ion concentration and ph of the total snow cover were 13.0 mg/l and 5. 5, respectively on March 19. However, the major ion concentration and ph of the out-flowing meltwater were 300 mg/l and 4. 2, respectively from March 19 to 20. The meltwater on the first day of the snowmelt season contained a concentration of major ions approximately 20 times as high as the total snow cover. The concentration of chemical constituents in the meltwater rapidly decreased as the snowmelt progressed, and the ph of the meltwater gradually increased. In the early phase of the snowmelt, the acid meltwater containing extremely large amounts of chemical constituents flowed out from the snow cover. 4. Discussion 4-1. Effects of Synoptic Weather Conditions on the Coneentration of Chemical Constituents in Snow The concentration of chemical constituents differed from one snow layer to the other (Table 1). Layer-b and layer-f had extremely low concentrations of major cations. This difference was thought tobe caused by the process of snow formation Table 2. Regression analyses for the total snow cover during the snowmelt season. D : days A : significant at the level B : significant at the 0.05 level

9 110 Chemical Changes of Snow Cover by Melting which was controlled by weather conditions. The synoptic weather conditions of snowfalls were examined. Layer-b and layer-f were formed from January 30 to 31 and on March 10, respectively. On these days there were cyclones southwest of Hokkaido. The radar echoes obtained by the Sapporo District Meteorological Observatory showed stratiform clouds. When a cyclone is located southwest of Hokkaido, the snow is produced from stratiform clouds (KIKUCHI et al., 1975). The above results suggest that the snow produced from stratiform clouds has lower concentrations of chemical constituents. The same conclusion was derived from the result observed at the cloud base. The concentration of chemical constituents in the snow produced from convective clouds were higher than those from stratiform clouds (KIMURA, 1981). In observing the snow produced by the winter-monsoon, TSUNOOAI et al. (1975) noted much larger amount of chemical constituents in the snow after the cold front had passed than before it. From these results it is reasonable to conclude that the snow with extremely low concentrations of chemical constituents is produced from stratiform clouds Mechanisms of Chemical Changes in Snow Cover Based on the following physicochemical processes, the possible mechanisms involved in the decrease in the concentration of chemical constituents in the snow cover and their increase in meltwater are discussed. The snow begins to metamorphose itself as soon as it accumulates on the ground. The physicochemical characteristics of the snow cover are changed with the progress of the metamorphism. Especially during the snowmelt, granular snow is formed by the repeated melting and refreezing of the newly fallen snow. In this refreezing process, the chemical constituents, collected by the snow crystals under the rainout and the washout processes, segregate from granular snow particles, because chemical constituents in ice segregate along the grain boundaries, at the cusps of boundaries and at those points where three grain boundaries meet in ice forming at a moderate velocity (MIzuNo and KuRoIWA, 1969). The meltwater produced at or near the surface of the snow cover permeates the snow layers while scavenging the chemical constituents segregated from the snow particles. But the permeation of the meltwater does not always occur uniformly in the downward direction. The meltwater often moves laterally within the snow cover and flows downwards making water channels at certain points (WAKAHAMA, 1963). Based on these processes the following explanation of the mechanisms of the chemical changes of the snow is proposed. In the early phase of the snowmelt season, the texture of layer-g changed to large granular snow, and the concentration of chemical constituents in layer-g rapidly decreased as a result of the segregation process. The meltwater produced at layerg permeated layer-f and layer-e, and was refrozen at layer-e. Consequently, the water equivalent of layer-e increased. However, because the chemical constituents are difficult to refreeze, the concentration of chemical constituents in layer-e did not increase so much. The chemical constituents flowed downwards. It is assumed that the meltwater flowing down was blocked by boundaries between layer-e and layer-d, and flowed downwards making water channels at certain points. The meltwater flowing down through the water channels was refrozen at the lower layers, and the concentration of chemical constituents in the meltwater gradually increased. The outflow of concentrated meltwater caused a decrease in the concentration of chemical constituents in the total snow cover prior to a decrease of the water equivalent in the early phase of the snowmelt season. As the snowmelt progressed, all of the snow cover became wet, and the meltwater covering the ice grains of snow flowed

10 SUZUKI 111 down slowly in the form of a water-film flow. These flowing water-films are reported to be from several to about 20 microns thick (WAKAHAMA, 1968). The texture of snow soaked by meltwater permeating the snow cover changes from fine to coarse (WAKAHAMA, 1965). During the snowmelt season, the texture of the inner layer changed to large granular snow, and the chemical constituents which segregated along the surface of snow particles were scavenged by flowing meltwater. The decrease in the concentration of chemical constituents of the inner layers and the wetting of the entire snow cover began at about the same time, as shown in Figs. 3 and 7. This result tends to corroborate the above discussion. 5. Conclusions The chemical constituents were retained in the snow cover during the cold period. However, the concentration of chemical constituents in the snow cover decreased rapidly as soon as the snowmelt began. During the temporary snowmelts and the early phase of the snowmelt season, only the surface snow layer melted. As a result of the decrease in the concentration of chemical constituents in the surface snow layer, the concentration of chemical constituents in the total snow cover decreased. During the snowmelt season all of the snow cover was soaked by the meltwater and the concentration of chemical constituents in the total snow cover decreased as the snowmelt progressed. It was estimated that the earlier meltwater contained extremely large amounts of chemical constituents and had a low ph. The segregation process was considered to be the principal mechanism of the decrease in the concentration of chemical constituents in the snow cover by the snowmelt. Acknowledgements The author expresses heartfelt thanks to Prof. H. KADOMURA, Graduate School of Environmental Science, Hokkaido University, for his helpful suggestions and reading of the manuscript. Gratitude is also expressed to Dr. D. KOBAYASHI, Institute of Low Temperature Science, Hokkaido University, for his discussion and reading of the manuscript, and to Dr. M. UZIIE, Faculty of Agriculture, Hokkaido University, for the use of the atomic-absorption spectrophotometer. Thanks are owed to the Agricultural Experiment Farms of Hokkaido University for allowing use of the observation site and to the members of the Laboratory of Fundamental Research, Hokkaido University for the useful discussions. References BELL, H. L. (1971) : Effect of low ph on the survival and emergence of aquatic insects. W at. Resour. Res., 5: HAGEN, A. and A. LANGELAND (1973) : Polluted snow in southern Norway and the effect of the meltwater on freshwater and aquatic organisms. Envir. Pollut., 5:

11 112 Chemical Changes of Snow Cover by Melting JIS (1966) : Japanese Industrial Standard, K 0101: and (in Japanese) JOHANNESSEN, M. and A. HENRIKSEN (1978) : Chemistry of snow meltwater: changes in concentration during melting. Wat. Resour. Res., 14: KATO, T. and T. IIZAWA (1976) : Chemical studies on melt water in No. 1 Brook of Kamabuchi Experimental Forest, Yamagata Prefecture. Jap. J. Limnol., 37: (in Japanese with English abstract) KIKUCHI, K., T. ISHIKAWA, K. NANASAWA and T. YOSHIDA (1975) : Heavy snowfall. In: Cooperative Research Group of Natural Disasters, Report No. A-50-8: (in Japanese) KIMURA, T. (1981) : A study of scavenging effect of precipitation. Master's thesis, Hokkaido Univ., 61pp. (in Japanese) LEIVESTAD, H. anc I. P. MUNIZ (1976) : Fish kill at low ph in a Norwegian river. Nature, 259: MIZUNO, Y. and D. KUROIWA (1969) : Solute segregation in ice observed by autoradiography. Low Temp. Sci., Ser. A, 27: (in Japanese with English abstract) NARUSE, R., H.OURA and K. KOJIMA (1970) Field studies on snow melt due to sensible heat transfer from the atmosphere. Low Temp. Sci., Ser. A, 28: (in Japanese with English abstract) SUZUKI, K. (1979) : Variations of water qualities in a small stream during snowmelt season. Prep. Geogr., 17: (in Japanese) TSUNOGAI, S., K. FUKUDA and S. NAKAYA (1975) A chemical study of snow formation in the winter-monsoon season: the contribution of aerosols and water vapor from the continent. J. Meteorol. Soc. Japan, 53: WAKAHAMA, G. (1963) : The infiltration of melt water into snow cover - I. Low Temp. Sci., Ser. A, 21: (in Japanese with English abstract) WAKAHAMA, G. (1965) : Metamorphisms of wet snow. Low Temp. Sci., Ser. A, 23: (in Japanese with English abstract) WAKAHAMA, G. (1968) : Infiltration of melt water into snow cover - III. Low Temp. Sci., Ser. A, 26: (in Japanese with English abstract) Laboratory of Fundamental Research, Graduate School of Environmental Science, Hokkaido University, Kita-ku, Sapporo 060 ; Present address: Department of Geography, Fuculty of Science, Tokyo Metropolitan University, Fukazawa, Setagaya-ku, Tokyo 158} Accepted: 20 January 1982