Groundwater Flow Impacts on Thawing Permafrost Systems

Abstract:

Results of numerical simulation analysis indicate that where groundwater flows in permafrost landscapes in a warming climate, advective heat transport enhances permafrost thaw rate, increasing transmissivity and the movement of warmer recharge and deep waters. Enhanced flows further increase the rate at which permafrost margins warm and thaw, resulting in positive feedback. Groundwater flow is a significant control on thaw rate and on local and regional patterns of residual permafrost in the landscape. Results indicate that residual permafrost patterns in landscapes with groundwater flow should differ from those in landscapes with little flow.

As permafrost thaws from above, a deeper seasonal active zone (the shallow subsurface layer that freezes and thaws annually) develops and more through-going thawed zones (taliks) develop that connect supra- and sub-permafrost zones. The new taliks host additional groundwater flow. Despite this potential for increasing groundwater movement in warming arctic environments, most predictive models of permafrost thaw generally have considered only subsurface heat conduction, not incorporating advective heat transport. To understand these systems and feedbacks, the USGS-SUTRA numerical groundwater code, which models coupled groundwater flow and heat transport, was modified to include freeze/thaw. When temperatures are below freezing, the modeled permeability and thermal properties are dependent on ice saturation, and latent heat of ice formation is included in the energy balance.

Simulations of groundwater flow and permafrost thaw were carried out across a hillslope cross section with undulating topography that is initially underlain by a continuous thick permafrost layer. Climate warming is applied with mean air-temperature increase of 0.5 ^°C per 100 years for 1600 years with constant temperature thereafter. This temperature evolution is superimposed on a seasonal ±10 ^°C variation that drives yearly freeze/thaw cycles in the shallow subsurface. Simulation results compare changes in permafrost distribution over a few thousand years of climate change due to (1) purely conductive heat transport (with essentially no groundwater flow) and (2) advective-conductive heat transport (with significant groundwater flow).