Field Observations and Numerical Modeling of the Thermal Effects of Groundwater Flow Through a Subarctic Fen

Abstract:

Field observations and numerical modeling of ground temperatures are the main tools for understanding current and projecting future permafrost changes in the rapidly warming Arctic. Traditionally, most studies have focused on vertical fluxes of heat through the ground. Groundwater can transport heat in both lateral and vertical directions but its influence on ground temperatures at local scales in permafrost environments is not well understood. In this study field observations from a subarctic fen located within the sporadic permafrost zone are combined with numerical simulations for investigating coupled water and thermal fluxes. Ground temperature profiles and groundwater levels were observed in boreholes at the Tavvavuoma study site in northern Sweden. Based on these observations, one- and two-dimensional simulations down to 2 m depth across a gradient of permafrost conditions both within and surrounding the fen, were set up. To quantify the influence of groundwater flows on the ground temperature, two-dimensional scenarios representing the fen under various groundwater fluxes were developed. The observations suggest that lateral groundwater flows significantly affect ground temperatures. This is corroborated by modeling results that show seasonal ground ice melts 1 month earlier when a lateral groundwater flux is present. Further, although the thermal regime may be dominated by vertically conducted heat fluxes during most of the year, isolated high groundwater flow events can be potentially important for ground temperatures. Sporadic permafrost environments contain substantial portions of unfrozen ground, often with active groundwater flow paths such as fens. Knowledge of this heat transport mechanism is therefore important for understanding permafrost dynamics in these environments.