Seasonal variability in the surface-groundwater interactions in an interior Alaskan stream

The interaction between surface and ground water in stream ecosystems influences the transport and biogeochemical processing of carbon and nutrients, thus altering the flux of organic carbon and nutrients from terrestrial ecosystems to the ocean. In high latitude streams surface-groundwater exchanges are likely controlled by the seasonal thaw of surface soil, changes in hydrology, and presence of permafrost. Our research investigated surface-groundwater exchanges using an end-member mixing model in a headwater stream to understand how near-stream groundwater flowpaths are altered by the seasonal thaw of surface soil and changes in stream discharge. We conducted this work in the Caribou-Poker Creeks Research Watershed in interior Alaska located within the zone of discontinuous permafrost. Four transects of groundwater wells were installed in the valley bottom, perpendicular to the stream channel, in regions with hydrologically neutral and gaining reaches. Transects consisted of five slotted wells spaced 2 m apart installed to a maximum depth of approximately 1 m. Stream and well water were sampled during the summers of 2009 and 2010 to capture variation of thaw depth and discharge. Water samples were analyzed for anions, cations, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and total dissolved organic nitrogen (TDN). Well water chemistry was explained using a two end-member mixing model with chemistry resembling a shallow soil and deep flowpath. The fraction of soil water contributing to near stream wells decreased from 70% to 50% as soil thawed throughout the summer. The relationship between day of year and fraction of soil water was not significant in wells furthest from the stream suggesting surface-groundwater exchange is greater in wells 2 to 6 m from the stream channel, while being limited between 8 and 10 m. We found a weak negative correlation between the day of year and DOC:TDN in wells nearest the stream; however, in most wells day of year did not predict DOC:TDN suggesting other factors are important to the transport and cycling of biogeochemical constituents. We found that surface-groundwater interactions are seasonally variable indicating the sources of carbon and nitrogen to the stream will vary through the active season and that stream-riparian zone connections were most important to a distance of 6 m from the channel. Near stream exchanges are vulnerable to climate change because longer term shifts in precipitation and temperature regimes would alter patterns in stream discharge and seasonal thaw of surface soil likely altering stream-riparian flowpaths. Moreover, shifts in riparian flowpaths may result in variation in hydrologic residence time and redox conditions, which will have important implications for the flux of carbon and nutrients to oceans.