HYDROGEOCHEMISTRY OF SHALLOW GROUNDWATER IN AN UPLAND SCOTTISH CATCHMENT

The hydrogeochemistry of shallow groundwater has been characterized in the Allt a' Mharcaidh catchment in the Scottish Cairngorms in order t o: (i) assess the spatial and temporal variation in groundwater chemis try; (ii) identify the hydrogeochemical processes regulating its evolu tion; and (iii) examine the influence of groundwater on the quality an d quantity of stream flow. Shallow groundwater in superficial drift de posits is circumneutral (pH similar to 7.1) and base cation concentrat ions are enriched compared with precipitation and drainage water from overlying podzolic soils. Modelling with NETPATH suggests that the dom inant geochemical processes that account for this are the dissolution of plagioclase, K-feldspar and biotite. Groundwater emerging as spring s from weathered granite underlying high altitude (> 900 m) alpine soi ls shows similar characteristics, though weathering rates are lower, p robably as a result of reduced residence times and lower temperatures. Chemical hydrograph separation techniques using acid neutralizing cap acity (ANC) and Si as tracers show that groundwater is the dominant so urce of baseflow in the catchment and also buffers the chemistry of st ream water at high flows: groundwater may account for as much as 50-60 % of annual runoff in the catchment. Climate and land use in the Cairn gorms are vulnerable to future changes, which may have major implicati ons for hydrogeological processes in the area. (C) 1998 John Wiley & S ons, Ltd.