Seasonal hydrology of oxygen-18 in the Allt a' Mharcaidh, Scotland : implications for water movement and residence times

The seasonal variation of oxygen-18 has been observed in precipitation, groundwater and streamwater in the Allt a' Mharcaidh catchment, Cairngorm mountains, Scotland. Precipitation showed strong seasonal variation in its isotopic signature over the 1995-1998 study period. As anticipated, such variation was substantially damped in groundwater and surface water. Nevertheless, 18O proved a useful tracer, indicating the influence of spring snowmelt events and summer rainfall on stream waters. Detailed examination of the seasonal variation in [dgr ]18O levels in various catchment waters provided an insight into mixing processes and a first approximation of mean residence times. Preliminary estimates for the latter are <180 days for (inferred) near-surface soil water and storm runoff, 2.5 and >5 years for shallow and deeper groundwater respectively. These longer-term data sets demonstrate the ability of the catchment to effectively mix new precipitation with resident soil and groundwater over the hydrological year. This implies that the influence of groundwater on the hydrology and hydrochemistry upland catchments has been underestimat- ed.