Groundwater flow and storage in aquifers on top of permafrost are very sensitive to climate conditions. These supra-permafrost aquifers (SPAs) exhibit high concentrations of dissolved organic carbon (DOC) due to rapid leaching from peat, the main material comprising SPAs. Thus, SPAs have an important role in carbon cycling. The biogeochemical transformation of the DOC into carbon dioxide and methane and DOC transport into aquatic water bodies depend on hydrologic conditions. How climate variability, such as variability in extreme weather events and freeze-thaw cycles, affects the coupled hydro-bio-geochemical processes within SPAs and the connectivity of SPAs with surface water bodies and the atmosphere are still unknown. This study addresses this gap through combining observations with modeling. First, we developed an ensemble of steady-state groundwater flow models which combined information on active layer hydro-stratigraphy, high resolution topography, and DOC concentrations from a first-order stream watershed, the Imnavait Creek in Alaska. The model ensemble showed that the range of groundwater discharge going into Imnavait Creek is the same as the range of summer streamflow. However, this approach does not explicitly represent SPA hydro-bio-geochemical dynamics. To address this, this study will be applying a multiphysics model, the Advanced Terrestrial Simulator (ATS), which integrates coupled surface and subsurface permafrost hydrology, energy transport, and biogeochemical reactions, to investigate the impact of different climate forcing. We will develop a 2D ATS model to first assess the impact of climate variability. The 2D domain is a 100 m-long and 40 m-thick transect beginning from the foot of a hillslope, through the riparian zone, and ending at Imnavait. Transect-specific observations determine the model geometry, flow and transport parameters, and boundary conditions. High-resolution hydro-stratigraphy and temperature observations (water table and ice table elevations) will be used for model assessment. The modeling will ultimately address how future climate, variable freeze-thaw cycles, and extreme weather affect lateral groundwater flow and overall biogeochemistry along the terrestrial-aquatic continuum.

Seasonally warm summers in the Arctic produce supra-permafrost aquifers within the active layer. However, the magnitude of groundwater flow, the amount of dissolved carbon and nutrients, and the solute flow paths are largely unknown, but critical to quantifying downgradient contributions to surface waters (lakes and rivers). To develop approachable methods to quantify groundwater inputs in continuous permafrost watersheds, we selected Imnavait Creek watershed on the North Slope of Alaska as a representative headwater drainage. We conducted 1000 groundwater flow simulations based on topography of the watershed and varying aquifer hydraulic conductivity and saturated thickness values. We fitted a lognormal distribution to the resulting 1000 model outputs, and we derived n=1e6 possible discharge values based on Monte Carlo random sampling on the model outputs. The groundwater discharge values integrated across the watershed generally agree with observed streamflow in Imnavait Creek over 2 months.  When groundwater discharge estimates were combined with in-situ measurements of groundwater-dissolved organic carbon and nitrogen concentrations, we found that Imnavait Creek’s organic matter load is also dominantly sourced from groundwater. Thus, riverine and lacustrine ecological and biogeochemical processes relate strongly to groundwater phenomena in these continuous permafrost settings. As the Arctic warms and the active layer deepens, it will become more important to understand and predict supra-permafrost aquifer dynamics.