A study of solute transport mechanisms using rare earth element tracers and artificial rainstorms on snow

Rare earth element (REE) tracers and three artificial rain-on-snow storms at the Central Sierra Snow Laboratory, California indicate that (1) tracers applied to the snow surface immediately prior to the storm quickly appear at the bottom of the pack, with the tracer traversing the pack faster when the snowpack is wetter; (2) unlike most previous studies in which low solute concentrations were observed at high flow in diurnal cycles, the concentrations of the REE tracers ill the outflow are positively related with input water flux, and (3) at a constant input flux the concentrations of all the REE tracers decreased exponentially with time, and the rate of this decrease was greater at high flow than at low flow. These observations can be qualitatively simulated by partitioning liquid water in the snowpack into mobile and immobile phases. Transport of the mobile water phase is governed by the advection-dispersion equations, while the immobile water only moves by exchanging with the mobile water, The rate of exchange between mobile and immobile waters follows first-order kinetics, Unlike previous mobile-immobile models for snow, the exchange rate coefficient is assumed to increase exponentially with the effective water saturation. The model successfully simulates the positive concentration dependency on input water flux, However, it remains unclear how the exchange rate coefficient varies with the nature of the medium and with hydrological conditions. These observations suggest that tracer concentrations in the outflow are largely dominated by solute transport via fast flow channels. This surprising result implies that a spatially averaged flow rate may not be adequate for modeling solute transport properties in unsaturated media.