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

Abstract:

The preferential elution of ions from melting snowpacks is a complex problem that has been linked to acidification of water bodies. However, the understanding of these processes in snowpacks around the world, including high mountain 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 catchment in Colorado were combined with the PULSE multi-phase snowpack chemistry model to diagnose the processes controlling chemical accumulation, meltwater chemistry and preferential elution. It was found that 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⁺ were deposited mostly within the upper layers of both the meadow and forest-gap snowpacks, and acid anions such as NO₃⁻ and SO₄²⁻ were more evenly distributed. The snow ion concentrations were generally greater at the forest-gap snowpit, except for NH₄⁺, which indicates that wind erosion of wet and dry deposited ions from the meadow may have reduced concentrations of residual snow. Snow interception and scavenging processes such as sublimation, ventilation, and throughfall led to ion enrichment of Ca²⁺, Mg²⁺, K⁺, Cl⁻, SO₄²⁻ and NO₃⁻. Model simulations and observations highlight that preferential elution is enhanced as snowmelt rates decline, 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. Ion exclusion rates at the grain-size level have been estimated for the first time. Both model and field observational results suggest that low snowmelt rates can cause multiple early meltwater ionic pulses for ions subject to less ion exclusion.