Hydrogeochemsitry of montane springs and their influence on streams in the Cairngorm mountains, Scotland

Abstract. Springs are important groundwater discharge points on the high altitude (>800m) plateaux of the Cairngorm mountains, Scotland and form important wetland habitats within what is often a dry, sub-arctic landscape. The hydrogeochemistry of a typical spring in the Allt a'Mharcaidh catchment was examined between 1995-98 in order to characterise its chemical composition, identify the dominant controls on its chemical evolution and estimate groundwater residence time using ¹⁸O isotopes. Spring water, sustained by groundwater flow in shallow drift deposits and fractured bedrock, was moderately acidic (mean pH 5.89), with a very low alkalinity (mean 18 μeq l^-1) and the ionic composition was dominated by sea-salts derived from atmospheric sources. Geochemical modelling using NETPATH, predicted that the dissolution of plagioclase mainly controls the release of Si, non-marine Na, Ca, K and Al into spring water. 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 water was fundamentally different as a result 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 δ¹⁸O values in spring water were lower and more damped than those in precipitation. Nevertheless, a sinusoidal seasonal pattern was observed and used to estimate mean residence times of groundwater of around 2 years. Thus, in the high altitude plateau of the Cairngorms, shallow, coarse drift deposits from significant aquifers. At lower altitudes, deeper drift deposits, combined with larger catchment areas, increase mean groundwater residence times to >5 years. At high altitudes, the shallow, permeable nature of the drifts dictates that groundwater is vulnerable to impacts of environmental changes that could be usefully monitored at spring sites.