Hydrological disconnection of deep groundwater in Krycklan
Résumé
Water residence times are useful descriptors of hydrological connectivity. Hydrological connectivity and disconnection within or between compartments of a catchment has been demonstrated to determine temporal and spatial variations in runoff processes as well as the export of weathering products or other solutes. In Krycklan, a 67 km2 forested catchment with long-term monitoring in northern Sweden, water stable isotopes and modeling revealed an active flow zone in the upper 1-2 m getting temporally connected/disconnected to the stream. Residence times (mean residence time = 12 y) and water storage vary between seasons that are well-marked like in other northern catchments. Hydraulic conductivity decreases quickly with depth creating this active flow zone and disconnecting the deeper groundwater under certain climatic conditions. The deep groundwater has been found to contribute to streamflow and to affect stream water chemistry without having further information about timescales of deep groundwater flow or any measure for the extend of the hydrological connectivity between these two groundwater compartments.
We compile findings of previous studies regarding water residence times and water storage of the upper active flow zone while providing new data about the deeper groundwater. We measured chlorofluorocarbons (CFCs) at several locations distributed within the Krycklan catchment. All samples were taken in glacial till and sampling depths varied between 2 and 18 m. We found relatively old CFC based groundwater ages of 28 years at just 2 m depth increasing up to 55 years at 18 m depth. Groundwater age stratification follows a logarithmic increase with depth showing a remarkable shift of 28 years. This shift of CFC based groundwater ages represents a hydrological disconnection of the deeper groundwater to the upper active flow zone. The analysis of groundwater age stratification provides the basis for further investigations of relative contributions of the two zones to the overall water storage. The observed hydrological disconnection might be also relevant in other forested catchments with low groundwater recharge rates or other northern catchments with similar climatic conditions.