Comparing chloride and water isotopes as hydrological tracers in two Scottish catchments

Time series of chloride concentrations and oxygen-18 isotopic ratios are widely used for tracing catchment storage and mixing processes and for inferring catchment travel-time distributions. However, neither chloride nor oxygen-18 is an ideal hydrologic tracer: chloride concentrations in streamwater can be affected by dry deposition, evapoconcentration and biogeochemical cycling, and water isotopes can be fractionated by evaporation. One way to test the reliability of these tracers is to determine whether, despite artifacts such as these, both tracers lead to similar inferences when measured in the same catchment. Here, we compare chloride and oxygen-18 time series in the Girnock and Allt’ a Mharcaidh catchments in Scotland. Semivariograms and power spectra for both tracers exhibit similar patterns of fluctuation damping, implying that the travel-time distributions of the two tracers have similar shapes. Fluctuations of both tracers are strongly damped in streamflow compared to precipitation, implying that these catchments integrate tracer signatures over many different storm events. Streamflow fluctuations of both tracers are more strongly damped at the Allt’ a Mharcaidh catchment, implying longer storage and greater mixing of waters of different ages, compared to the Girnock catchment. At both sites, streamflow fluctuations of oxygen-18 are more strongly damped, relative to precipitation, than those of chloride, leading to estimates of mean travel times that are longer, by a factor of 2–3, for oxygen-18 than for chloride. The greater variability of chloride compared to oxygen-18 may arise from spatially and temporally varying occult deposition and evapoconcentration. Nonetheless, the similarities in the behaviour of the two tracers imply that the strong tracer damping and long travel times that have been observed in many catchment studies are not artifacts, but instead reflect storage and mixing of waters over long time scales. Copyright  2010 John Wiley & Sons, Ltd.