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. In the high Arctic valley of Adventdalen, Svalbard, sub-permafrostgroundwater feeds several pingo springs distributed along the valley axis.The driving mechanism for groundwater discharge and associated pingoformation is enigmatic because wet-based glaciers are not present in theadjacent highlands and the presence of continuous permafrost seems topreclude recharge of the sub-permafrost groundwater system by either asubglacial source or a precipitation surplus. Since the pingo springsenable methane that has accumulated underneath the permafrost to escapedirectly to the atmosphere, our limited understanding of the groundwatersystem brings significant uncertainty to predictions of how methaneemissions will respond to changing climate. We address this problem with anew conceptual model for open-system pingo formation wherein pingo growth issustained by sub-permafrost pressure effects, as related to the expansion ofwater upon freezing, during millennial-scale basal permafrost aggradation.We test the viability of this mechanism for generating groundwater flow withdecoupled heat (one-dimensional transient) and groundwater (three-dimensional steady state) transportmodelling experiments. Our results suggest that the conceptual modelrepresents a feasible mechanism for the formation of open-system pingos inlower Adventdalen and elsewhere. We also explore the potential foradditional pressurisation and find that methane production and methaneclathrate formation and dissolution deserve particular attention on accountof their likely effects upon the hydraulic pressure. Our model simulationsalso suggest that the generally low-permeability hydrogeological units causegroundwater residence times to exceed the duration of the Holocene. Thelikelihood of such pre-Holocene groundwater ages is supported by thegeochemistry of the pingo springs which demonstrates an unexpected seawardfreshening of groundwater potentially caused by a palaeo-subglacial meltwater “wedge” from the Weichselian. Whereas permafrost thickness (and age)progressively increases inland, accordingly, the sub-permafrost meltwaterwedge thins, and less unfrozen freshwater is available for mixing. Ourobservations imply that millennial-scale permafrost aggradation deservesmore attention as a possible driver of sustained flow of sub-permafrostgroundwater and methane to the surface because, although the hydrologicalsystem in Adventdalen at first appears unusual, it is likely that similarsystems have developed in other uplifted valleys throughout the Arctic.

Numerical modelling of permafrost spring discharge and open-system pingo formation induced by basal permafrost aggradation