Preferential meltwater flowpaths as a driver of preferential elution of chemicals from melting snowpacks.

Seasonal snowcovers release nutrients accumulated over the winter during spring snowmelt and this can be an important part of the annual biogeochemical cycling of chemicals and their loading to soils and water bodies. The characteristics of this load are controlled by snowmelt dynamics and the physical and chemical properties of the snowpack, which are affected by overwinter and snowmelt metamorphism, refreezing of meltwater, and ion exclusion from snow crystals. Rain-on-snow (ROS) events can accelerate and modify the snowpack discharge process. The interplay of these processes can cause microscale flow heterogeneity and preferential flow pathways (PFP). Previous experimental work has examined PFP and ion elution processes in snowpacks, but their combined effect on the spatial and temporal characteristics of snowmelt ion elution remains uncertain. In this research, two controlled laboratory experiments were performed to investigate the role of PFP and ROS in controlling snow ion release to runoff. These involved the high frequency monitoring of flow and meltwater concentrations during snowmelt induced by radiation-convection (RC) processes and rain-on-snow (ROS). Results without ROS show that PFP was responsible for the transport of 68-75%, 73-75%, and 55% of the total NO3 , PO4 and SO4 load discharged during the experiment. Lateral flow caused the contact zones to extend beyond the leading edges of the PFP, increasing meltwater concentrations particularly in the RC experiment where the snow microscale flow channels were better connected in early snowmelt. The results suggest that preferential elution in early melt resulting from snow ion exclusion may be more pronounced than previously thought, with the first 1.5% of melt transporting more than 20% of the total snowpack NO3 and PO4 load. The expected increase in the frequency of ROS events with climate change will likely reduce the magnitude of early ionic pulses in snowmelt runoff, but also make their occurrence more frequent.