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 1998 in the Allt a' Mharcaidh catchment in the Cairngorm Mountains, Scotland. Precipitation (mean delta(18)O = -7.69 parts per thousand) exhibited strong seasonal variation in delta(18)O values over the study period, ranging from -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 could be explained in terms of a two-component mixing model, involving a seasonally variable delta(18)O signature in storm runoff, mixing with groundwater characterized by relatively stable delta(18)O levels. Variations in soil water delta(18)O implied the routing of depleted spring snowmelt and enriched summer rainfall into streamwaters, probably by near-surface hydrological pathways in peaty soils. The relatively stable isotope composition of baseflows is consistent with effective mixing processes in shallow aquifers at the catchment scale. Examination of the seasonal variation in delta(18)O levels in various catchment waters provided a first approximation of mean residence times in the major hydrological stores. Preliminary estimates are 0.2-0.8 years for near-surface soil water that contributes to storm runoff and 2 and > 5 years for shallow and deeper groundwater, respectively. These O-18 data sets provide further evidence that the influence of groundwater on the hydrology and hydrochemistry of upland catchments has been underestimated. Copyright (C) 2000 John Wiley & Sons, Ltd.