Hydrogeochemistry of montane springs and their influence on streams in theCairngorm mountains, Scotland

Springs are important groundwater discharge points on the high altitude (>8 00m) plateaux of the Cairngorm mountains, Scotland and form important wetla nd habitats within what is often a dry, sub-arctic landscape. The hydrogeoc hemistry of a typical spring in the Allt a' Mharcaidh catchment was examine d between 1995-98 in order to characterise its chemical composition, identi fy the dominant controls on its chemical evolution and estimate groundwater residence time using O-18 isotopes. Spring water, sustained by groundwater flow in shallow drift deposits and fractured bedrock, was moderately acidi c (mean pH 5.89), with a very low alkalinity (mean 18 mu eq l(-1)) and the ionic composition was dominated by sea-salts derived from atmospheric sourc es. Geochemical modelling using NETPATH, predicted that the dissolution of plagioclase mainly controls the release of Si, non-marine Na, Ca, K and Al into springs waters. Hydrological conditions influenced seasonal variations in spring chemistry, with snowmelt associated with more rapid groundwater flows and lower weathering rates than summer discharges. Downstream of the spring, the chemistry of surface waters was fundamentally different as a re sult of drainage from larger catchment areas, with increased soil and drift cover, and higher evaporation rates. Thus, the hydrogeochemical influence of springs on surface waters appears to be localized. Mean delta(18)O value s in spring water were lower and more damped than those in precipitation. N evertheless, a sinusoidal seasonal pattern was observed and used to estimat e mean residence times of groundwater of around 2 years. Thus, in the high altitude plateau of the Cairngorms, shallow, coarse drift deposits form sig nificant aquifers. At lower altitudes, deeper drift deposits, combined with larger catchment areas, increase mean groundwater residence times to >5 ye ars. At high altitudes, the shallow, permeable nature of the drifts dictate s that groundwater is vulnerable to impacts of environmental changes that c ould be usefully monitored at spring sites.