Modeling reactive solute transport in permafrost-affected groundwater systems

Abstract:

Permafrost thaw leads to enhanced groundwater circulation and alters subsurface hydrology in the cryosphere. An important consequence of this fundamental shift in hydrogeological regime across cold regions is a change in the timing, magnitude, and chemical composition of groundwater discharge. Understanding the links between permafrost dynamics and its feedback on groundwater flow and associated solute transport is imperative to quantify terrestrial feedbacks to climate change, and for understanding the fate of anthropogenic contamination in permafrost regions. Meaningful prediction of such changes is challenging due to the inherent interactions and coupling between mass and energy transport processes in the subsurface. To this end, we developed a numerical model that considers coupled groundwater flow, heat transfer including water-ice phase change, and reactive solute transport. We present simulations evaluating groundwater flow and transport scenarios under various permafrost thaw scenarios, considering the production and mobilization of dissolved organic carbon released by permafrost degradation, as well as potential groundwater contamination from wastewater lagoons underlain by permafrost. Results reveal the importance of transport mechanisms controlling the behavior of solutes in permafrost landscapes, including the effects of active-layer freeze-thaw on the temporary immobilization of water and solutes, and the hydrogeologic factors affecting the time scales on which advective-dispersive transport interacts with temperature-dependent biogeochemical processes. Such ongoing model development and improvement will be critical for further elucidating the thermo-hydrogeologic process controlling the fate and transport of dissolved carbon, nutrients, and aqueous contaminants in warming permafrost environments.