Release of meltwater and ionic solute from melting snow

Abstract

The release of ionic solute from melting seasonal snow produces an influx of ion laden water into hydrologic systems at the start of spring snowmelt. The spatial and temporal variability of meltwater and solute release from melting snow was investigated at different spatial scales to assess the magnitude and variability of this process. Four laboratory experiments were performed where an 0.4 m³ volume of snow was placed in a plexiglass box and melted from above. NaCl and dye tracer experiments revealed contemporaneous areas of concentrated dye and dilute meltwater in flow fingers, indicating that meltwater in preferential flow paths is diluted by low concentration water from the top of the snowpack. Meltwater discharge and meltwater electrical conductivity were measured in snow lysimeters, and snow accumulation and electrical conductivity of samples from snowpits were measured over four snowmelt seasons at an alpine field site. Peak snow-water equivalent ranged from 0.57 to 2.92 m, and lysimeter discharges ranged from 20 to 205% of the mean flow; however mean lysimeter flow was representative of snow ablation observed in snow pits. The electrical conductivity in snowpit samples and lysimeter meltwater averaged 2-3 μS cm⁻¹. Peak meltwater electrical conductivity ranged from 6 to 14 times that of the bulk premelt snowpack. The highest conductivities were observed during the first few days following the onset of flow, and the lysimeters that began flowing earliest tended to have the highest conductivities at the onset of flow. A mathematical model for solute transport in snow was developed that includes the effects of mass transfer between mobile and immobile liquid phases, advection, hydrodynamic dispersion, and melt—freeze episodes. The ability of the model to accurately simulate solute movement and release depends on the validity of the assumption of one—dimensional flow and on the accuracy of modeling the snowpack energy balance. This model is preferable to the empirical models of solute elution currently in use for investigations of watershed hydrogeochemical response because it has the ability to respond directly to changes in snow accumulation or meteorlogical conditions.