Catchment-scale groundwater age stratification reveals groundwater recharge and discharge processes

The distribution of groundwater ages with depth provides information about subsurface structures and flow dynamics. Upslope measured groundwater age stratifications are commonly used to estimate groundwater recharge rates, whereas downslope measured age stratifications are influenced by recharge conditions, the aquifer structure and interactions between groundwater and surface water. In our study we address the question whether downslope measured groundwater ages from different locations can provide spatial and temporal information about catchment-scale groundwater dynamics and the relationship between groundwater recharge and discharge.
We derived an overall groundwater age stratification, representative for the Svartberget subcatchment (0.47 km²) located within the Krycklan study site, by measuring CFCs from 9 different sampling locations with depths of 2 m to 18 m. All sampling locations were downslope and located in basal till which is overlain by ablation till. 
The CFC-based groundwater age stratification reveals an unexpected pattern, with groundwater ages of already 30 years immediately below the water table. Groundwater ages increase then with depth. We could reproduce the observed groundwater age stratification by using a groundwater flow model and show that the lag of rejuvenation, noticeable in groundwater ages of 30 years at the water table, derives from return flow of groundwater at a subsurface discharge zone that evolves at the interface between the two soil types (basal and ablation till). Furthermore, we demonstrate by varying the infiltration rate how the extent of the discharge zone and the partitioning of the infiltration amount to the two layers change, i.e. young runoff in the upper layer (ablation till) and old groundwater circulation through the deeper layer (basal till).
By providing a simple analytical approximations of the observed groundwater age stratification, we show that the extent of the subsurface discharge zone is a powerful indicator of the relation between the recharge and discharge zone, while the vertical gradient of the age-depth relationship provides information about the overall aquifer recharge.