Modeling Reactive Solute Transport in Permafrost-Affected Groundwater Systems

Understanding the interactions between ground freeze-thaw, groundwater flow, and solute transport is imperative for evaluating the fate of contaminants in permafrost regions. However, predicting solute migration in permafrost-affected groundwater systems is challenging due to the inherent interactions and coupling between subsurface mass and energy transport processes. To this end, we developed a numerical model that considers coupled groundwater flow, subsurface heat transfer, and solute transport, including water-ice phase change, solute-dependent porewater freezing, and temperature-dependent solute reaction rates. As an illustrative example, we present simulations to investigate the potential for contamination from a municipal wastewater lagoon in the Canadian sub-arctic. Two-dimensional groundwater models assuming varying permafrost conditions were developed to evaluate possible contaminant migration scenarios associated with groundwater flow from the lagoon to a river, including the transport of conservative, degradable, and sorbing solutes. Model results reveal important transport mechanisms controlling the behavior of aqueous contaminants in permafrost landscapes as well as the hydrogeologic factors affecting reactive transport in cold regions. Seasonal freeze-thaw episodically restricts connectivity of transport pathways, which attenuates both transport and reaction rates. However, elevated solute concentrations can depress the freezing temperature of porewater and produce thaw-induced solute transport. Both thermally driven and solute-enhanced thaw can decrease ice content in permafrost, which can have significant implications for solute migration. Therefore, thermo-hydrogeologic process controlling reactive transport in cold-region groundwater systems must be considered when quantitatively assessing the impact of changing environmental conditions on contaminant hydrogeology.