HYDROLOGICAL PROCESSES, VOL. 7, 193-203 (1993)

MELT WATER CHEMISTRY AND ITS IMPACT ON STREAM WATER QUALITY

ALAN JENKINS Institute of Hydrology, Crowmarsh Gifford, Wallinsford, Oxon OX10 888, U K

ROBERT FERRIER Macaulay Land Use Research Institute, Aberdeen, UK

AND DAVID WATERS

Institute of Hydrology, Crowmarsh Giford, Wallinsford, Oxon OX10 8BB, U K

ABSTRACT

Samples of snowpack leachate were collected over a 60 day period of the spring melt season in 1988 and 1989 at a 10 km2 upland catchment in the Cairngorm mountains of Scotland. These were analysed for major ions to assess snowpack chemistry dynamics through the spring and to assess the melt water influence on stream water chemistry. The data clearly show preferential elution of sulphate and nitrate over chloride and hydrogen over the other cations during the early melt of 1988. Following the addition of ions to the snow surface, either as snow or later in the season as rain, the elution sequence is reproduced. Comparison of leachate chemistry with stream chemistry samples taken at the basin outlet indicate that snow pack melt water contributes directly to stream water. The stream water chemistry signal is, however, noisy and the stream concentrations are considerably damped relative to the snowpack leachate. This is thought to be a consequence of differential melting within the catchment as the snowpack at lower altitudes is at a more advanced stage of melt and so holds fewer solutes and mixing with groundwater contributions. Temperature observations at different altitudes within the catchment support this interpretation.

KEY WORDS Snowmelt chemistry Preferential elution Stream water chemistry

INTRODUCTION

Snowpack development represents the accumulation of both wet and dry deposition during the winter months (Bales et al., 1989; Ferrier et al., 1989). When climatic conditions are suitable for melting, pollutants accumulated into the snowpack may be released. This release of chemical impurities has been shown to cause a rapid change in the chemical composition of lakes and streams (Maule and Stein, 1990), which can have serious detrimental effects on aquatic biota (Jacks et al., 1986; Melack and Stoddard, 1990). Field lysimeter studies have shown that the first 20-30% of snowpack melt water has concentrations several times higher than the bulk concentration in the remaining volume, and this phenomenon has been termed ‘differential’ or ‘preferential elution’ (Johannessen and Henriksen, 1978; Tranter et al., 1986; Bales et al., 1990). The effect of this ionic pulse on the water quality of streams is dependent on a number of factors (Williams and Melack, 1991). The effect will be enhanced if transfer routes from the pack to the lake or stream are direct, for example, over ice lenses within the pack, over frozen ground, or through preferential pathways within the soil and snowpack. Should snowpack melt water infiltrate the soil and displace stored water of higher pH, or react with the soil matrix itself, then partial buffering can take place, neutralizing the effect of an acid melt water release (Bottomley et al., 1984).

It is also important to consider these processes occurring within a particular watershed as potentially highly spatially and temporally variable. Although elution, hydrological pathway and dilution are the predominant factors, differences in snow accumulation, altitude, slope, density and aspect will all combine to

0885-6087/93/020193-11$10.50 0 1993 by John Wiley & Sons, Ltd.

Received 10 October 1991 Accepted 6 January 1992

194 A. JENKINS ET AL.

affect local responses. The relationship between altitude and temperature could result in an extremely variable pattern of melt, introducing dilution and mixing within the catchment, which may act to suppress the effect of any snowpack contaminant release (Ferrier et al., 1989).

This paper reports spring melt phenomenon measured in the Allt a’Mharcaidh catchment, Cairngorms, north-east Scotland. The objective of the study was to identify snowpack chemistry dynamics during the main melt season and to assess the effect of melt water chemistry on stream water quality.

METHODS AND MATERIALS

Study site The study catchment lies on the western edge of the Cairngorm mountains, draining an area of 10 km2 into

the river Feshie, which is a tributary of the river Spey. The mean annual rainfall in the area is lo00 mm (Harriman et al., 1990), of which approximately 307; falls as snow during the winter months (Jenkins, 1989). The main stream (Allt a’Mharcaidh) leaves the catchment at a height of 320 m (Figure 1) and the highest point in the catchment (1 11 1 m) is the summit of Sgoran Dubh Mor (NH905002). The catchment broadly consists of three geomorphological units: (i) the valley floor, consisting predominantly of peat soils with northern blanket bog vegetation; (ii) the valley sides, with podzolic soils and lichen-rich boreal heather

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Figure 1 . Allt a Mharcaidh catchment and location of snowmelt lysimeters (A and B), weather stations (AWS and MWS) and stream gauges (G1 and G3)

EFFECT OF MELT WATER ON STREAMS 195

moor; and (iii) the upland plateau, consisting of alpine soils and alpine azalea-lichen heath (Ferrier and Harriman, 1990). The valley floor extends from 320 to around 550 m, with steep slopes on the valley sides rising to approximately 700 m, topped by a gently rising plateau area.

Sampling regime and methodology The location of all sampling and monitoring stations are shown in Figure 1. The air temperature was

monitored just below the catchment summit (MWS) at a height of 11 11 m and in the valley bottom at the centre of the catchment (AWS, 550 m). The stream stage was recorded at 20 minute intervals at two sites, the basin outflow (Gl) and the major subcatchment outflow (G3), using Druck pressure transducers installed in a stilling well. Stage is related to discharge through a rating equation constructed by dilution gauging techniques. Stream water sampling for chemical analysis at G1 and G3 was carried out twice weekly between 1985 and 1990 as part of a wider monitoring programme. For a detailed summary of the analytical methodology see Harriman et al. (1990).

Snowpack leachate, or melt water, was collected at two locations within the catchment, site A (950 m) and site B (550 m). The lysimeters consist of a cylindrical base and shallow funnel top with a 12 cm perimeter ring and are constructed from uPVC. Each lysimeter contains 60 60-ml polypropylene bottles which are fed from a central spout. A controller/logger shifts the central spout to a fresh bottle every 24 hours at midnight. Any faults in the positioning of the spout which might lead to contamination of the samples are recorded by the logger. The lysimeter, filled with clean bottles, is installed in the ground in late autumn and is activated by the logger/controller which is attached without disturbing the snowpack around or over the lysimeter at the start of the melt season. Each 60-ml sample represents a collection of the snowpack leachate over the lysimeter through a 24-hour period. As the bottles fill relatively quickly during a vigorous melt it is unlikely that further additions to the full bottle will mix completely throughout the day and so during these periods the sample will represent the first 60 ml of leachate collected at the start of the period. The comparability between the first 60 ml sample and the total daily melt cannot be ascertained from these data and as such the lysimeter data must be regarded as spot measurements. All overflowing leachate water runs to waste and dummy bottles, which the spout does not reach, are included to ensure that the entire lysimeter has not flooded.

The lysimeter at site B was situated in a deep bowl-shaped feature at the head of a gully. At site A the lysimeter was placed on a straight hillslope draining directly into the stream above G3. The samplers were activated to cover two periods, 19 January to 15 March 1988 and 1 March to 30 April 1989; however, lysimeter B failed to operate during the latter period.

Laboratory analysis After the melt period, samples collected in the lysimeters were transported to the laboratory and

subsequently filtered through 0.45 pm filters. Analyses were carried out in duplicate, along with control standards to ensure high quality data with a standard deviation of less than 5 % of the mean. Nitrate nitrogen and chloride were determined spectrophotometrically. Sodium, calcium, magnesium and sulphate were determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES). Stream water samples were analysed at the Freshwater Fisheries Laboratory, Pitlochry using similar procedures.

RESULTS

Snow accumulation and melt during the two study periods The first snow of the 1987-88 season fell in September and October 1987, but these were light and quickly

melted to leave the catchment free of snowpack, even in the deep gullies. The snowpack melt water iysimeters were installed in early November and were covered by snow following heavy falls later that month and in early December. By early January, however, much melting had occurred such that snowpack remained only in the gullies and sheltered corries, although both lysimeters remained covered. From early January 1988, a more complete snowpack built up in the catchment and the lysimeter at site A was activated on 19 January. At site B, the snow cover was so deep that the lysimeter box could not be located until later in the season and so was not activated until 16 February. Light snow fell throughout the latter part of January but

196 A. JENKINS ET AL.

temperatures remained well below freezing at MWS and mainly below freezing at AWS allowing little melting and, consequently, stream flow remained low throughout the period.

The main melt in the catchment began in mid-February (Figure 2a, b, c) and three periods when temperatures at AWS were well above freezing occurred in quick succession, each causing a flow peak in the stream. The three peaks are also identified at MWS, and although these do not rise above O'C, the lysimeter is about 150 m lower and so the melt was apparently in progress over nearly all of the catchment. One further heavy fall of snow occurred at the beginning of March, whereafter the snow cover receded rapidly from the catchment slopes and by mid-March snow cover remained only in the gullies.

The snowpack in 1988-89 was much more transient than in the previous year and the winter was characterized by a number of snow accumulation periods followed by rapid and almost complete melts, from October 1988. Indeed, the snow samplers remained visible until mid-February 1989 when heavy snowfall occurred between 12 and 26 February (Figure 2d, e, f). During early March, temperatures at all levels were well above freezing and a high flow event occurred on the 5-7 March in response to a rapid thaw. The snow lysimeter at site A was activated on 5 March. More fresh snow fell on 20-23 March and this melted almost

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EFFECT OF MELT WATER ON STREAMS 197

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198 A. JENKINS ET AL.

immediately to cause another high flow event in the stream. Isolated showers of fresh snow occurred throughout early April, but generally below freezing temperatures at high levels prevented melting and the snowpack had already disappeared from below about 600 m in the catchment and so there was little effect on stream discharge.

Snowmelt chemistry The major ion chemistry of the snowmelt samples collected in the lysimeters at sites A and B during the

winter and spring of 1988 are shown in Figure 3. The early melt water was high in solutes and concentrations of sulphate (about 400 pEq. 1 -I) and nitrate (about 650 pEq. 1 -') represent extreme inputs at this site (Ferrier et al., 1990). As the melt progresses, the concentrations of all ions decreased rapidly until 18 February, when the concentration of ions increased sharply to be followed by a similarly rapid decline. This increase in concentration reflects an input of fresh snow to the surface, bringing more ions, during the period 9-16 February (Figure 2a). This fresh snow built up in the catchment and did not contribute to the snowpack leachate during this period due to below freezing air temperatures. The lysimeter at this time was presumably still collecting leachate from the much older snowpack underneath the fresh snow. On 16 February, a rapid increase in air temperature allowed the ions in the fresh snow to leach rapidly through the snowpack and caused the observed increase in melt water concentrations.

The chemistry of the melt water at site B mirrors this second 'pulse' seen at site A, indicating that the melt processes within the snowpack were similar and that the melt was occurring over the whole catchment (Figure 3). Initial peak SO:- concentrations of about 180pEq 1 - ' are similar to the maximum concentration observed at the second peak at site A of about 150 pEq. 1-'.

The snowpack leachate chemistry at site A during the spring of 1989 is shown in Figure 4. The lysimeter was started rather late during the melt season due to problems in relocating it. As a consequence of the late start, the initial concentrations of ions are relatively low and the temperature record before the start of sampling indicates that considerable melt had already occurred and the initial pulse of ions from the snowpack had been missed. As a result of a fresh input of snow during the period 2-5 April (Figure 2d), however, ion concentrations increased sharply on 8 April in response to an increase in temperature at MWS, followed by the characteristic, rapid decrease in concentrations (Figure 4).

The chemical evolution of the snowpack through time, reflected in the changing ionic composition of the melt water, follows a similar pattern at both sites and in both years. The relative contribution of sulphate and nitrate, calculated as a proportion of the total negative charge of the melt water leachate, decreases with time while chloride increases (Figure 5). Following a further input of ions during the period 9-16 February, the pattern is repeated. The same pattern is observed following fresh input of ions in 1989 (Figure 5). This supports the wealth of evidence from other areas suggesting preferential elution of sulphate and nitrate from the snowpack (Brimblecombe et al., 1985; Tranter et al., 1986). The cations show a similar preferential elution (Figure 5) as sodium becomes relatively more dominant in the leachate while the hydrogen contribution decreases rapidly. The proportion of calcium tends to gradually increase, although it has only a small contribution to total charge whereas magnesium remains constant.

Contribution of melt water to stream water chemistry The influence of melt water chemistry on stream water chemistry is by no means clear during 1988 (Figure

6). Concentrations of non-catchment derived ions (sulphate and chloride) are generally very damped in the stream relative to the lysimeter. The same is true for sodium, which behaves essentially as a non-catchment derived ion as the high concentrations are attributable to an input of sea salt. Calcium, on the other hand, shows comparable concentrations between the stream and lysimeter samples. Despite this damped response, both sulphate and chloride signals in the stream apparently reflect the pattern of change in the lysimeter chemistry, decreasing initially until around 15 February and then increasing again at the same time as the melt water concentrations increase. Furthermore, throughout the early part of the melt, concentrations of sulphate and chloride in the stream are well above the long term mean stream concentration at G1 (54, 123 and 91 pEq. 1 for sulphate, sodium and chloride, respectively). This picture is not as clear for sodium and there is apparently no relationship between the lysimeter and the stream chemistry for calcium.

EFFECT OF MELT WATER ON STREAMS 199

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The links between lysimeter and stream chemistry observed in 1988 are not evident in 1989 (Figure 7). Melt water concentrations of sulphate are very low and even the high peak in mid-April causes no stream response. Lysimeter calcium concentrations are very low with some ‘spikes’ probably representing dust contamination to the snowpack. Both sodium and chloride, however, show high concentrations in the stream coinciding with the two early peak concentrations observed in the lysimeter.

200 A. JENKINS ET AL.

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DISCUSSION AND CONCLUSIONS

The snow lysimeter chemistry data indicate that preferential elution of ions from the snowpack occurs at all altitudes within the catchment and occurs over periods of many days following fresh input and at the start of melt periods. This elution process is identified in both years. Sulphate and nitrate form a major component of the anion concentration and hydrogen and sodium are the dominant cations in the early melt water. Chloride dominates the anion charge in the later stages of the melt and, as the hydrogen concentrations recede during the melt, sodium maintains its large contribution whereas the other base cations remain relatively constant. The dominance of chloride and sodium ions in the snowpack leachate reflects the strong maritime influence on precipitation chemistry in this area. Initial very high concentrations in the melt water are apparently associated with 'snowpack ripening' in the absence of pronounced melting and during prolonged cold spells. As a consequence, ion fluxes from the snowpack at these times are small as melt water volumes are low, indicated by the continued baseflow conditions in the basin outflow stream. This also goes

EFFECT OF MELT WATER ON STREAMS 201

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some way to explaining the rather poor relationship between ion concentations in the melt water and the stream.

The influence of melt water on stream chemistry revolves around the question of whether catchment stored water or melt water reaches the stream during melt-induced high flow periods. The similarities in concentration trends in the lysimeters and stream water during 1988 imply that melt water does reach the stream quickly and dictates stream chemistry. The signal matches most closely for sulphate, chloride and sodium, but no match is observed for hydrogen or other base cations, and nitrate concentrations in the stream are near zero throughout the year. Neither does stream chemistry reflect the very large concentrations seen in the snow lysimeters; the response in the stream is considerably damped. This damped response and the difficulty in identifying the link between melt water and stream water chemistry are not surprising results and can be easily explained. The dominant source of calcium and magnesium in the stream is through contributions from a groundwater store which maintains streamflow when the catchment receives little input from precipitation or snowmelt, that is, periods when air temperatures are dominantly below freezing. An input of melt water with comparable concentrations of base cations will not, therefore, influence stream chemistry. On the other hand, concentrations of chloride, sodium and sulphate in groundwater will approximate to the long term mean for the stream and so an input of melt water containing relatively high concentrations of these ions will increase concentrations in the stream.

The damped stream response and the apparent lack of a more consistent relationship between snowmelt and stream chemistry is also expected and can be largely attributed to differential melting within the catchment caused by the large altitudinal range. Clearly, the snowpack will 'ripen' at a different rate dependent on altitude and exposure and so the elution process may be more or less advanced when a more

202 A. JENKINS ET AL.

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general melt occurs. The stream will then reflect a mix of water from lower, already leached snow and the still relatively ion enriched snow at higher levels. In the same way it is easy to envisage that melt water volumes will differ with altitude and the chemistry signal is damped and attenuated by the time the water reaches the catchment outflow.

On a basin scale, melt and pre-melt conditions have to be conducive to producing an acidic pulse at the catchment outflow; the melt must occur rapidly over the entire catchment, the snowpack must cover a substantial area of the catchment and the melt history of the snowpack must be similar at all altitudes. Even then, if the ground was not frozen before the build up of snowpack, it is possible that damping of the melt water concentrations will occur as the melt water mixes with catchment stored groundwater and soil water during transit to the stream channel. Nevertheless, quick flow mechanisms must exist to transport melt water to the stream at the onset of the melt. These pathways, however, do not transport the melt water to the stream chemically unchanged. Nitrate concentrations, in particular, are high during the early part of the melt, but nitrate peaks are not seen in the stream, indicating that biological activity (uptake, exchange across plant surfaces or biological transformation) occurs to remove the nitrate from the drainage water. Future studies must account for differential spatial and temporal 'ripening' using not only temporal melt water chemistry but also spatial surveys of snowpack composition.

ACKNOWLEDGEMENTS

We thank Ron Harriman (FFL Pitlochry) for stream water chemistry data, Chris Smith and the staff of the chemistry laboratories at the Institute of Hydrology for analyses of melt water samples, Roger Wyatt and the

EFFECT OF MELT WATER ON STREAMS 203

Instruments Section at the Institute of Hydrology for equipment design and development and Jo and Mollie Porter for help in the field under difficult conditions.

REFERENCES

Bales, R. C., Davis, R. E., and Stanley, D. A. 1989. ‘Ion elution through shallow homogeneous snow‘, Waf. Resour. Res., 25,1869-1877. Bales, R. C., Sommerfeld, R. A,, and Kebler, D. G. 1990. ‘Ionic tracer movement through a Wyoming snowpack’, Atmos. Enuiron., 24,

Bottomley, D. J., Craig, D., and Johnston, L. M. 1984. ‘Neutralisation of acid runoff by groundwater discharge to streams in Canadian Pre-Cambrian shield watersheds’, J. Hydrol., 75, 1.

Brimblecombe, P., Tranter, M., Abrahams, P. W., Blackwood, I., Davies, T. D., and Vincent, C. E., 1985. ‘Relocation and preferential elution of acidic solutes through the snowpack of a small, remote, high altitude Scottish catchment’, Ann. Glaciol., 7, 141-147.

Ferrier, R. C. and Harriman, R. 1990. ‘Pristine, transitional, and acidified catchments studies in Scotland’, in Mason, B. J. (Ed.), The Surface Waters Acidijication Programme, Cambridge University Press, Cambridge, pp. 9- 18.

Ferrier, R. C., Anderson, J. A., Miller, J. D., and Christophersen, N. 1989. ‘Changes in soil and stream hydrochemistry during periods of spring snowmelt at a pristine site in mid-Norway’, Waf. Air Soil Pollut., 44, 321-337

Harriman, R., Ferrier, R. C., Jenkins, A., and Miller, J. D. 1990. ‘Long and short-term hydrochemical budgets in Scottish catchments’, in Mason, B. J. (Ed.), The Surface Waters Acidification Programme, Cambridge University Press, Cambridge, pp. 3 1-45.

Harriman, R., Gillespie, E., King, D., Watt, A. W., Christie, A. E. G., Cowan, A. A,, and Edwards, T. 1990. ‘Short term ionic responses as indicators of hydrochemical processes in the Allt a Mharcaidh catchment, western Cairngorms, Scotland’, J. Hydrol., 116, 267-285.

Jacks, G., Olofsson, E., and Werme, G. 1986. ‘An acid surge in a well-buffered stream’, Ambio, 15,282-285. Jenkins, A. 1989 ‘Storm period hydrochemical response in an unforested Scottish catchment’, Hydrol. Sci. J., 34, 393-404. Johannessen, M. and Henriksen, A. 1978. ‘Chemistry of snowmelt: changes in concentration during melting’, War. Resour. Rex, 14,

Maule, C. P. and Stein, J. 1990. ‘Hydrological flow path definition and partitioning of spring meltwater’, War. Resour. Res., 26,

Melack, J. M. and Stoddard, J. L. 1990. ‘Acidic deposition and aquatic ecosystems: Sierra Nevada, California’, in Charles, D. F. (Ed.),

Tranter, M., Brimblecombe, P., Davies, T. D., Vincent, C. E., Abrahams, P. W., and Blackwood, I. 1986. ‘The composition of snowfall,

Williams, M. W. and Melack, M. 1991. ‘Solute chemistry of snowmelt and runoff in an alpine basin, Sierra Nevada’, War. Resour. Res.,

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