A Conceptual Cryohydrogeological Model of a Degrading Permafrost Mound Near Umiujaq (Nunavik, Canada)

Abstract:

Numerical modeling studies of permafrost degradation generally conceptualize heat transfer as occurring exclusively by conduction. However, recent work has shown that advective heat transport by flowing groundwater can effectively control permafrost dynamics. Thus, in order to understand the impact of groundwater flow on permafrost degradation, the contribution of heat advection on thawing rates must be determined. This study characterizes the thermal and hydrogeological regimes of a permafrost mound located in the discontinuous permafrost zone near Umiujaq (Nunavik, Canada) in order to create a conceptual cryohydrogeological model. Previous work at the study site documented the spatial extent of the permafrost and deployed an extensive network of monitoring instrumentation. This instrumentation includes thermistor cables, groundwater monitoring wells, temperature and water content sensors in the active layer, heat flux plates, and a snow cover monitoring system.

Data indicate that ice-rich permafrost is restricted to a 15-m-thick unit of marine silt, which is overlain by a 4-m-thick layer of sand. The active layer encompasses the full extent of the surficial sand layer, as well as the top 2 m of the silt unit. Depending on climatic and surficial conditions, the furthest extent of the freezing front in a given year may not reach the permafrost table, resulting in the formation of a sporadic talik between the active layer and the permafrost table. Data further indicate that the surficial sand layer acts as an unconfined aquifer, and that infiltrating precipitation flows downward to the permafrost table before flowing radially towards the sides of the mound. A downward hydraulic gradient was observed across the low hydraulic conductivity silt unit. From these data, a conceptual cryohydrogeological model was created to provide insight into the physical processes that govern local permafrost dynamics and degradation at the site, and to guide future field campaigns and modeling projects.