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 a t the Central Sierra Snow Laboratory, California indicate that (1) tracers applied to the snow surface immediately prior to the storm quickly appear a t the bottom of the pack, with the tracer traversing the pack faster when t he snowpack is wetter; (2) unlike most previous studies in which low solute concentrations were observed at high flow in diurnal cycles, the concentra tions 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 decreas e was greater at high flow than at low flow. These observations can be qual itatively simulated by partitioning liquid water in the snowpack into mobil e and immobile phases. Transport of the mobile water phase is governed by t he advection-dispersion equations, while the immobile water only moves by e xchanging with the mobile water, The rate of exchange between mobile and im mobile waters follows first-order kinetics, Unlike previous mobile-immobile models for snow, the exchange rate coefficient is assumed to increase expo nentially with the effective water saturation. The model successfully simul ates the positive concentration dependency on input water flux, However, it remains unclear how the exchange rate coefficient varies with the nature o f the medium and with hydrological conditions. These observations suggest t hat tracer concentrations in the outflow are largely dominated by solute tr ansport via fast flow channels. This surprising result implies that a spati ally averaged flow rate may not be adequate for modeling solute transport p roperties in unsaturated media.