Isotope hydrology of the Allt a' Mharcaidh catchment, Cairngorms, Scotland: implications for hydrological pathways and residence times

The hydrology of oxygen-18 (O-18) isotopes was monitored between 1995 and 1 998 in the Allt a' Mharcaidh catchment in the Cairngorm Mountains, Scotland . Precipitation (mean delta(18)O = -7.69 parts per thousand) exhibited stro ng seasonal variation in delta(18)O values over the study period, ranging f rom -2.47 parts per thousand in the summer to -20.93 parts per thousand in the winter months. As expected, such variation was substantially damped in stream waters, which had a mean and range of delta(18)O of -9.56 parts per thousand and -8.45 to -10.44 parts per thousand, respectively. Despite this , oxygen-is proved a useful tracer and streamwater delta(18)O variations co uld be explained in terms of a two-component mixing model, involving a seas onally variable delta(18)O signature in storm runoff, mixing with groundwat er characterized by relatively stable delta(18)O levels. Variations in soil water delta(18)O implied the routing of depleted spring snowmelt and enric hed summer rainfall into streamwaters, probably by near-surface hydrologica l pathways in peaty soils. The relatively stable isotope composition of bas eflows is consistent with effective mixing processes in shallow aquifers at the catchment scale. Examination of the seasonal variation in delta(18)O l evels in various catchment waters provided a first approximation of mean re sidence times in the major hydrological stores. Preliminary estimates are 0 .2-0.8 years for near-surface soil water that contributes to storm runoff a nd 2 and > 5 years for shallow and deeper groundwater, respectively. These O-18 data sets provide further evidence that the influence of groundwater o n the hydrology and hydrochemistry of upland catchments has been underestim ated. Copyright (C) 2000 John Wiley & Sons, Ltd.