NUMERICAL SIMULATION OF GROUNDWATER FLOW WITH FREEZE-THAW IN DYNAMIC PERMAFROST SYSTEMS

Rapid warming of continuous and discontinuous permafrost regions is changing cold regions hydrology, but the effect of these changes on groundwater hydrology is complex and difficult to observe and quantify. Subsurface freezing and thawing involves complex feedbacks between the coupled subsurface ice and groundwater flow systems. We present results of numerical groundwater simulation that elucidate some of these processes. We have developed a new coupled groundwater-energy-transport model, SUTRA-ICE, based on SUTRA, a U.S. Geological Survey model for coupled groundwater flow and heat transport. SUTRA-ICE simulates freezing and melting of groundwater and includes proportional heat capacity and thermal conductivity of water and ice, decreasing matrix permeability due to ice formation, and latent heat. The model was verified by correctly simulating an analytical solution for ice formation in a porous medium with a mixed ice-water zone. We present two- and three-dimensional hillslope benchmark problems available for model comparison. Results demonstrate the potential for freezing to alter the groundwater flow regime. For example, in the two dimensional benchmark, ‘winter' freezing effectively isolates the regional groundwater system from surface recharge, and induces seasonal horizontal flow reversals. To further demonstrate the influence of permafrost distribution on regional hydrology, we present MODFLOW results from the Yukon Flats Basin of Alaska, using time-varying low permeability zones to represent frozen regions. Simulating various stages of permafrost expansion and thaw shows that the existence of permafrost exerts a significant control on recharge and discharge patterns. The results illustrate the potential impact of future climate warming and permafrost thaw on groundwater flow systems.