CFC stream water mean transit times reveal subsurface flow processes across scales at Krycklan

Abstract

Catchment transit time distributions span the range from minutes to hours (e.g. overland flow) to years, decades and longer. As time-variant catchment descriptors they are useful indicators for flow and transport processes. Stable water isotopes are established tracers to inform about young water components in stream water, but are less well-suited to defining ages for the older components of the transit time distribution. To infer slow flow water components, tracers that are able to date water over longer timescales are needed. Here, we used atmospheric tracers (i.e. chlorofluorocarbons (CFCs)) that are able to cover the timescale of subsurface flow over decades to determine mean transit times of stream water. CFCs are well established tracers for dating groundwater, but their use is limited in surface waters as they might partially reequilibrate with ambient atmospheric concentrations of CFCs within a few hours. We measured CFCs at different subcatchment outlets of the Krycklan catchment basin under different flow conditions (49 samples in total). Krycklan is a boreal research catchment in northern Sweden at which stable water isotopes are extensively used to understand hydrological processes. The CFC results show that stream water mean transit times vary between 32 years and 59 years. These ages are similar to those observed for groundwater in the aquifer. This, and the patterns for individual CFCs suggest limited reequilibration with contemporary atmospheric CFC concentrations. Mean transit times across scales are independent of catchment size suggesting local groundwater contributions to streams. Furthermore, mean transit times negatively correlate with specific discharge supporting findings of increasing young water components during high flow conditions.