THE CHARACTER AND CAUSES OF A PRONOUNCED SNOWMELT-INDUCED ACIDIC EPISODE IN A STREAM IN A SCOTTISH SUB-ARCTIC CATCHMENT

Specific objectives were: (1) to characterize the major 'episode' of the year in a catchment in a region which is regarded as being sensitive to pollutant deposition; (2) to examine downstream compositional gradients and to explain them in terms of downstream changes in flowpath regime and differences in soil depth or type; (3) to test the hypothesis that both direct pollution inputs to the stream and the sea-salt effect are reflected in streamwater composition. Streamwater was collected, during two distinct snowcover melt events at two points, 400 m apart, with high time resolution. Snowcover and soilwater samples were also collected. There were distinct differences in the response of the stream at the two sites to the melt events; this was a consequence of spatially variable snowmelt regimes as well as of differences in soil and sub-snowcover hydrology. There were a number of unusual features apparent. There was no evidence of the almost ubiquitous base cation dilution associated with acidic episodes. There was also a most pronounced direct effect of pollution input at the uppermost stream sampling site because of the accumulation of pollution on top of a deep snowcover, the melt from which followed rapid flowpaths to the stream. The direct effect was not apparent at the lower sampling site, although evidence for the sea-salt mechanism was very clear, unlike the case for the top site. The evidence for an ion exchange control on streamwater Al3+ was strong at the lower site, but weak at the top site. The observations are interpreted via a schematic flowpath model.