Transient effect on groundwater age distribution – age measurements disprove steady state assumption

Abstract

Groundwater ages are key indicators for flow and transport processes as well anthropogenic and geogenic solutes that impact groundwater quality. Commonly, lumped parameter models (LPMs) are applied to interpret environmental tracers, like chlorofluorocarbons (CFCs) or tritium. LPMs require a steady state assumption and they are less complex, computational demanding and data intensive compared to transient numerical models. But when steady-state assumptions are valid for groundwater age simulations is questionable.An initial sampling campaign of CFCs measured in 9 wells at different depths on a 0.47 km2 subcatchment of the Krycklan catchment in 2017 revealed a groundwater age stratification with depth that was representative for the area1. Mean groundwater ages at the water table (2-6 meters depth) in the unconsolidated till overburden were already 30 years and increased with depth to the deepest sampling at 30 meters. These results indicate a lag of rejuvenation caused by a subsurface discharge zone that evolves between two soils types with different hydrogeological properties. The comparison of the steady-state numerical simulation and LPM has proven that the LPM yields an overall recharge rate and estimation of the extent of the subsurface discharge zone.  Seasonal changes of recharge were not expected to impact the age stratification. But repeated sampling in 2021 and 2022 has shown a clear shift of the groundwater age stratification. Numerical modeling is used to understand that transient effect.