Development of a three-dimensional geological model, based on Quaternary chronology, geological mapping, and geophysical investigation, of a watershed in the discontinuous permafrost zone near Umiujaq (Nunavik, Canada)

Abstract

Among the few positive impacts of climate warming in cold regions, permafrost degradation can increase the availability of groundwater as a potential source of drinking water for northern communities. Near the Inuit community of Umiujaq in Nunavik, Canada, a watershed in a valley in the discontinuous permafrost zone was instrumented to monitor the impacts of climate change on permafrost and groundwater, and assess the groundwater availability and quality. Based on Quaternary chronology, knowledge of periglacial processes, and an investigation carried out in the valley (including mapping of Quaternary deposits and icerich permafrost distribution, drilling and sampling of deposits, and geophysical surveys), a three-dimensional (3D) geological model of the watershed was built into GoCAD to assess the hydrogeological context in this degrading permafrost environment. In total, six units were identified within the watershed including an upper aquifer in marine sediments, a lower aquifer at depth in glaciofluvial and glacial sediments, and the bedrock acting as a low-permeability boundary. An aquitard, made of frostsusceptible silty sand and discontinuously invaded by ice-rich permafrost, confines the lower aquifer. This 3D geological model clarifies the local stratigraphic architecture and geometries of Quaternary deposits, especially the stratigraphic relationship between the two aquifers, aquitard, and bedrock, and the extent of ice-rich permafrost within the watershed. It is the cornerstone to understand the groundwater dynamics within the watershed and to carry out numerical modelling of coupled groundwater flow and heat transfer processes to predict the impacts of climate change on groundwater resources in this degrading permafrost environment.

Guidelines for cold‐regions groundwater numerical modeling

“…For site‐specific models, initial conditions can be based on field data. This approach requires extensive datasets of hydraulic heads, permafrost distributions, and ground temperatures based on geophysical measurements, observation wells, and temperature sensors (e.g., Fortier et al, 2020; Figure 6a). These point measurements can yield interpolated field data to generate the initial (present‐day) distribution of temperature and hydraulic head (e.g., Sjöberg et al, 2016).…”

Numerical modelling of permafrost dynamics under climate change and evolving ground surface conditions: application to an instrumented permafrost mound at Umiujaq, Nunavik (Québec), Canada

“…The study site is located in a 2 km 2 watershed in the Tasiapik Valley near the Inuit community of Umiujaq along the east coast of Hudson Bay in Nunavik (Québec), Canada (Figure 2), which lies within the discontinuous permafrost zone (Allard and Seguin 1987). Following the retreat of the Laurentide Ice Sheet and marine transgression around 8000 cal years BP (Hilaire-Marcel 1976;Lavoie et al 2012), different types of Quaternary sediments were deposited in the valley floor such as coarse-grained glacial and fluvio-glacial sediments, fine-grained offshore sediments, and intertidal and littoral sediments (Fortier et al 2020). Due to isostatic rebound after deglaciation, the Quaternary deposits in the valley gradually emerged and came in contact with the subarctic cold climate, which induced permafrost aggradation and formation of permafrost mounds (Figures 1 and 2c) (Lafortune et al 2006;Lavoie et al 2012;Fortier et al 2017Fortier et al , 2020.…”

“…Following the retreat of the Laurentide Ice Sheet and marine transgression around 8000 cal years BP (Hilaire-Marcel 1976;Lavoie et al 2012), different types of Quaternary sediments were deposited in the valley floor such as coarse-grained glacial and fluvio-glacial sediments, fine-grained offshore sediments, and intertidal and littoral sediments (Fortier et al 2020). Due to isostatic rebound after deglaciation, the Quaternary deposits in the valley gradually emerged and came in contact with the subarctic cold climate, which induced permafrost aggradation and formation of permafrost mounds (Figures 1 and 2c) (Lafortune et al 2006;Lavoie et al 2012;Fortier et al 2017Fortier et al , 2020. Evidence of permafrost degradation including thaw settlement, formation of thermokarst ponds, pellicular slope movement, and shrubification is already seen in the Tasiapik Valley due to climate warming.…”

“…The domain is divided into four distinct stratigraphic layers reflecting the local stratigraphy (Fortier et al 2017(Fortier et al , 2020 which is defined from top to bottom as: (1) a shallow sand aquifer, (2) a frost-susceptible silt aquitard, (3) a fluvio-glacial sand and gravel confined aquifer, and (4) impermeable quartzite bedrock (Figure 6). The initial body of ice-rich permafrost in the frozen silt layer extends horizontally from 60 to 100 m and from 22 to 45 m in elevation above sea level (elevation asl).…”

“…A conceptual model of a characteristic permafrost environment in the discontinuous permafrost zone in Nunavik (Québec), Canada, is shown in Figure 1 which highlights the physical processes controlling permafrost dynamics and degradation. This conceptual model is based on field observations of permafrost mounds in the Tasiapik Valley which drains into Lake Tasiujaq (formerly Lake Guillaume-Delisle) near the Inuit community of Umiujaq in Nunavik (Buteau et al 2004;Fortier and Aubé-Maurice 2008;Fortier et al , 2020Pelletier et al 2018). Formed during frost heave in a unit of frostsusceptible silty sediments around 2000 cal years BP (calibrated years before present), the permafrost mounds in this valley appear as raised periglacial landforms with dome-shaped surfaces known as lithalsas (Pissart 2002).…”