Melt water chemistry and its impact on stream water quality

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.