Permafrost Groundwater Dynamics : Modeling of vertical and lateral flows in the active layer across multiple scales

Abstract: Hydrological processes in the Arctic are profoundly influenced by the presence of permanently frozen ground, known as permafrost. Conversely, permafrost is greatly affected by hydrological changes resulting from climate change. Understanding and accurately representing the processes causing permafrost thaw is essential for evaluating the consequences of climate change on permafrost landscapes. In this thesis, I explore how water movements in permafrost landscapes affect the thermal state of the ground and the potential of groundwater flow to transport both heat and solutes. As groundwater is inherently difficult to observe in field experiments, the main method in this thesis is simulating permafrost dynamics with a state-of-the-art physics-based numerical model. Modeling allows investigating these dynamics in both space and time. Results show that an increase in summer rainfall and the associated vertical movement of water in the soil causes opposing effects in the ground temperature response. While enhanced summer rainfall likely leads to a warming in continental permafrost landscapes, in maritime landscapes it may cause a cooling of the ground. This is governed by the effects of rainfall on the hydrothermal properties of the soil and how efficiently it conducts and stores energy.Lateral water movement was found to substantially affect soil moisture distribution along a hillslope underlain by continuous permafrost. Soil moisture is important in the context of the hydrothermal properties within a hillslope but also for the capability of the ground to transport solutes. High soil moisture leads to higher soil hydraulic conductivity and therefore affects how fast solutes such as dissolved organic carbon can be transported with the groundwater. Depending on the vertical location of solutes within the soil, this determines the travel time of solutes in the groundwater towards surface water recipients. Additionally, depending on the rate at which air temperatures will increase in the future, permafrost carbon may experience different modes of lateral transport and residence times in the soil. This thesis highlights the complex interplay between permafrost and hydrology and why it is important to study them as a coupled system in order to fully understand the impacts of climate change on the fate of permafrost.