Modelling of groundwater age under variable permafrost and environmental conditions

Groundwater discharge age is a useful metric for retracing flowpaths, and is essential to estimate an aquifer’s renewability and vulnerability. As permafrost thaws in cold regions, supra-permafrost aquifers will expand, which will cause new pathways to develop and potentially alter the spatiotemporal distribution, quantity, and age of groundwater discharge. While numerous modelling studies have analysed the shift in groundwater discharge magnitude and patterns in permafrost regions, the associated changes to groundwater age have been largely overlooked. Using heat-transfer and solute-transport numerical models for cold regions, we recreated various archetypical conceptual models of permafrost-groundwater distributions. The object is to use different environmental conditions to evaluate which setting could result in a pronounced shift in groundwater discharge age and to identify the most significant parameters driving alterations to groundwater discharge age. In general, the results show that groundwater discharge is expected to become gradually older with permafrost thaw. Continuous permafrost settings exhibit very small changes in groundwater age until the lowering permafrost table allows for the formation of a supra-permafrost talik. The biggest shift occurs when taliks evolve from closed to open by connecting supra- and sub-permafrost aquifers. These insights are useful to determine the potential vulnerability and renewability of newly formed aquifers in permafrost settings.