Preferential elution of ionic solutes in melting snowpacks: Improving process understanding through field observations and modeling in the Rocky Mountains

The preferential elution of ions from melting snowpacks is a complex problem that has been linked to temporary acidification of water bodies. However, the understanding of these processes in snowpacks around the world, including the polar regions that are experiencing unprecedented warming and melting, remains limited despite being instrumental in supporting climate change adaptation.

In this study, data collected from a snowmelt lysimeter and snowpits at meadow and forest-gap sites in a high elevation watershed in Colorado were combined with the PULSE multi-phase snowpack chemistry model to investigate the controls of meltwater chemistry and preferential elution. The snowdepth at the meadow site was 64% of that at the forest-gap site, and the snowmelt rate was greater there (meadow snowpit) due to higher solar irradiance. Cations such as Ca²⁺ and NH4+">4+ were deposited mostly within the upper layers of both the meadow and forest-gap snowpacks, and acid anions such as NO3−">3− and SO42−">42− were more evenly distributed. The snow ion concentrations were generally greater at the forest-gap snowpit, except for NH4+">4+, which indicates that wind erosion of wet and dry deposited ions from the meadow may have reduced concentrations of residual snow. Furthermore, at the forest-gap site, snow interception and scavenging processes such as sublimation, ventilation, and throughfall led to particular ion enrichment of Ca²⁺, Mg²⁺, K⁺, Cl⁻, SO42−">42− and NO3−">3−. Model simulations and observations highlight that preferential elution is enhanced by low snowmelt rates, with the model indicating that this is due to lower dilution rates and increased contact time and area between the percolating meltwater and the snow. Results suggest that low snowmelt rates can cause multiple early meltwater ionic pulses for ions subject to lower ion exclusion. Ion exclusion rates at the grain-size level have been estimated for the first time.