A study of solute redistribution and transport in seasonal snowpack using natural and artificial tracers

Solute releases from a seasonal snowpack are important features of spring runoff in temperate climate zones because meltwater accounts for a significant proportion of contaminant loading to catchments. Here we report a study that traces the movement of natural and artificial solutes in snow through the processes of snow deposition, snowpack metamorphism, and snowmelt at the Central Sierra Snow Laboratory (CSSL), Soda Springs, California, USA. Rare earth elements (REEs) were added to the snowpack to mark individual snow layers. New snow, snowpack profiles and snowmelt were sampled and analyzed for REE and anion (SO₄^{2 -}, Cl^- and NO₃^-) concentrations. Correlations between NO₃^- and SO₄^{2 -} concentrations increased from new snow to snowpack profiles and to snow melt, suggesting significant redistributions of natural solutes in the snowpack. The maximum concentrations of NO₃^- and SO₄^{2 -} were both five times greater in the snow melt than in the new snow. Solute release by the snowpack was strongly controlled by weather conditions; high concentrations of natural tracers were observed at the onset of intensive melting episodes separated by relatively cold days of little melting. This weather pattern resulted in three ionic pulses rather than one. Natural solutes also showed a diurnal variation that was negatively correlated with the diurnal melting cycle. This pattern can be simulated by the standard water percolation equation linked with a mobile-immobile solute transport model (MIM). The diurnal variations of REE tracers showed positive association with the discharge, and cannot be satisfactorily simulated by the same model, suggesting that additional physical processes, such as dual or multiple flow regimes, may be needed to accurately describe solute transport in snow.