CONTROLS ON ARCTIC GROUNDWATER FLOW AND ITS IMPACTS ON CARBON PROCESSING

Vast reservoirs of organic matter become accessible each summer when Arctic soils thaw. Groundwater transports this organic matter into streams, where it can be respired into CO2. However, we know little about how groundwater moves through the shallow and temporary aquifer exposed by thawing soils. Further understanding of these flowpaths is required to accurately understand the Arctic terrestrial carbon budget.

Here we present findings from field data and numerical groundwater flow models that help discern where and how flowpaths develop in the Arctic shallow subsurface. We monitored temperature, specific conductance, soil hydraulic conductivity, and thaw during Summer 2015. This was a low precipitation year and thus allowed us to observe hillslope groundwater dynamics without the added input of precipitation. Radon observations highlight a unique groundwater signature within the hillslope and flow path connections between the hillslope and stream. Head and thaw observations show that groundwater flow paths mimic land surface topography on the hillslope scale, yet often depart from surface topography, following ice topography instead, at a finer scale. Hydraulic conductivity decreases with depth, but appears to be uniform laterally. Time-series and spatial snapshots of specific conductance suggest that it could be a useful tracer of preferential flowpaths within this shallow aquifer system.

These data will be critical for the further development of flow models that accurately represent aquifer properties, preferential flowpaths, and residence times in this and other Arctic watersheds. With an accurate understanding of water movement, we will be better able to understand the constituent profile that is advected and the reactions that occur during that process. This allows us to mechanistically understand the role of Arctic hillslopes as the primary suppliers of DOM to streams.