Seasonal evolution of active-layer thaw depth and streamflow chemistry in a permafrost catchment

In permafrost environments, hillslope and catchment scale hydrological and biogeochemical dynamics remain difficult to predict due to uncertainties about the spatiotemporal patterns of active layer thaw. The extent of active-layer thaw varies spatially and temporally in response to atmospheric and environmental conditions and as a function of the physical characteristics of the surface and shallow subsurface. Here we present the results of a hillslope and subcatchment scale study aimed at advancing our understanding of permafrost hillslope drainage dynamics and its influence on streamflow hydrochemistry. We instrumented a hillslope-stream sequence located in the headwaters of the Niaqunguk River watershed, Nunavut, Canada (63°N, 68°W), and combined high spatial resolution field measurements of water and frost tables across the hillslope with semi-weekly measurements of groundwater and streamflow chemistry to track the evolution of surface and subsurface water chemistry during active layer thaw. Interestingly, localized differential thaw patterns emerged under near saturation conditions across the instrumented hillslope; the result of historically high summer rainfall. Hillslope structure and uneven active layer thaw created two distinct fill-and-spill domains. A subsurface-domain defined by frost table microtopography and a surface-domain defined by surface topography. Immediately downstream of the hillslope we observed a seasonal shift in streamflow chemistry with an increased influence of water flowing through the underlying mineral soils as that active layer thawed. As thaw progressed streamflow chemistry began to most closely match that of the riparian groundwater; a mixture of hillslope surface and subsurface water. Hillslope-stream surface connections were punctual and occurred when rainfall, and saturation conditions across the lower portion of the hillslope, were sufficient for water to spill out of mid-slope surface depressions and across a saturated riparian zone and into the stream. Our research shows how hillslope structure and thaw processes influence hillslope-stream connectivity and streamflow chemistry in permafrost environments.