Groundwater flow and solute transport in a permafrost hillslope under seasonal thaw and climate warming

Groundwater flow in hillslopes in permafrost environments is strongly controlled by seasonal freeze-thaw dynamics. The seasonally thawed active layer acts as a transient groundwater system perched above permafrost, where coupled thermal and hydrological processes control subsurface connectivity, solute residence times and export to surface water recipients. Understanding these processes is critical for predicting hydrological responses to climate warming in permafrost regions.

Here we investigate groundwater flow and solute transport in a high-Arctic hillslope setting in Endalen Valley, Svalbard, underlain by continuous permafrost, using a physics-based numerical thermal-hydrological flow model with solute transport. Breakthrough curves are obtained for tracers released at different depths in the subsurface under present-day climatic conditions and under a set of warming scenarios. Results show that solute transport behaves very differently depending on release depth. Solutes originating near the ground surface are transported slowly, reflecting predominantly unsaturated flow conditions with seasonal thaw, producing long residence times. In contrast, solutes released at depth, near the permafrost table, experience rapid lateral groundwater transport following thaw, driven by water saturated conditions and the development of laterally connected subsurface flow paths above the permafrost.

Furthermore, solute mobilisation from newly thawed permafrost under climate warming is highly sensitive to the rate and mode of warming. Gradual warming promotes limited annual mobilisation dominated by vertical transport through percolation and cryosuction, whereas abrupt thaw associated with anomalously warm years leads to more rapid lateral transport comparable to that observed within the active layer.

Finally, we demonstrate how groundwater saturation and temperature conditions influence in situ solute transformation, showing that rapid transport under highly saturated conditions coincides with low potential mineralisation prior to export. These results highlight the central role of seasonal groundwater flow regimes in controlling subsurface transport in permafrost hillslopes and their response to climate change.