New Perspectives of Permafrost Distribution and its Influence on Groundwater Flow and Ecosystem Performance for the Yukon Flats Area, Northern Alaska

Permafrost is a predominant physical feature of the planet's Arctic and sub-Arctic regions and a major consideration that impacts climate processes, ecosystem function, water resources, and infrastructure engineering. The U.S. Geological Survey has conducted hydrological, geophysical, and geological studies of the Yukon Flats area within the discontinuous permafrost region of Interior Alaska. This is an area of low topographic relief that is populated with numerous streams, lakes and the braided meandering Yukon River with broad gravel-filled channels. Lake sediments and geological archives contain preserved sedimentary records of landscape changes in ecosystems, climate, and groundwater at decade-to-century and millennial time scales. The near-surface geology consists of unconsolidated Pliocene lacustrine silts and clays overlain by late Pleistocene and Holocene alluvial gravels and eolian deposits including extensive loess uplands and dunes. Remotely sensed data for the area from a variety of sensors provide information that is used for both static image interpretations and dynamic process modeling. An airborne electromagnetic survey (AEM) carried out in June of 2010 covered a large area (~1,800 line-km) with widely spaced reconnaissance lines and a 10 by 30 km block near Ft. Yukon. The AEM survey data was inverted to produce resistivity as a function of depth along the flight lines. The resistivity depth sections map the presence or absence of permafrost both laterally and with depth, as well as variations in subsurface lithology. The resulting depth images provide a unique detailed view of the subsurface that has previously only been conceptualized. One application of the remotely sensed data is to extrapolate the AEM data and interpretation between flight lines. Model development and independent test accuracies for 0-1 m electrical resistivity had training and test R2 values of 0.90 and 0.87 indicating accurate prediction models. Groundwater modeling of the Yukon Flats area for an assumed permafrost thaw sequence indicates regional groundwater flow patterns and fluxes to be highly sensitive to changes in permafrost distribution near the transition from continuous to discontinuous permafrost. AEM interpretation helps to define groundwater pathways through unfrozen zones, pathways that may control observed and future climate-change-driven changes in the state of surface waters. Modeling suggests that as permafrost degrades, the component of groundwater flow from sub-permafrost layers discharging to surface water bodies becomes increasingly important. Modeling of the hydrogeologic history of lake basins, improving groundwater models, more detailed geophysical depth section interpretation, and remote sensing analysis will improve the understanding of the relative roles of permafrost, climate, and the causal mechanisms behind widespread surface water and ecosystem changes on the landscape.