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Abstract.A numerical model that includes the effects of mass transfer between mobile and immobile liquid phases, advection, hydrodynamic dispersion, and melt-freeze episodes was developed to simulate ionic solute transport in melting snow. Model calibration using a tracer-infused laboratory snowpack experiment yielded a dispersivity of 0.05 cm and a mobile-immobile phase mass-transfer coefficient of 4 x 10 -6 S -1, but these parameter values are tentative because of the artificial nature of the experiment. The modeled concentration of meltwater flowing out the bottom of the snowpack was sensitive to residual water saturation, flow rate, dispersivity, mass-transfer rate, and the initial distribution of solute within the pack, similar to experimental observations. The model was applied to a small watershed, and it was found that the ability of the model to accurately simulate solute movement depends on the validity of the assumption of one-dimensional flow and on the accuracy of modeling the snowpack energy balance. IntroductionThe release of the chemical impurities from melting snow introduces chemicals stored in the snowpack into streams, lakes, and subsurface water. The release of a disproportionately large fraction of the ionic solute into the earliest fraction of meltwater, termed the "ionic pulse," occurs because ionic solute is segregated to the exterior of the snow grains during snow formation in the atmosphere and during snowpack metamorphism, and the solute later mixes with earliest meltwater to percolate through the snowpack.

Abstract. Meltwater discharge and electrical conductivity were measured in eight 1 x 1 m lysimeters, and snow accumulation and electrical conductivity of melted samples were measured in snow pits during four snowmelt seasons at Mammoth Mountain, California. The peak snow-water equivalent ranged from 0.57 to 2.92 m over the four melt seasons. Lysimeter discharges ranged from 20% to 205% of the mean flow; however, mean lysimeter flow was representative of snow ablation observed in snow pits. The electrical conductivity in snow pit samples and meltwater averaged 2-3 •S cm -1. Peak meltwater electrical conductivity ranged from 6 to 14 times the bulk premelt snowpack electrical conductivity. Snow depth did not affect the magnitude of the ionic pulse, and ion depletion as a function of snow ablation was similar from year to year despite interannual contrasts in melt rate and snow accumulation. Diel fluctuations in electrical conductivity were more pronounced in shallower snowpacks. The two main field observations of this study were (1) meltwater discharge at the base of the snowpack, observed using multiple snowmelt lysimctcrs, and (2) snow-water equivalent (SWE) and chemical load stored in the snowpack, observed in snow pits. Tracer tests were also conducted with dyes and ionic tracers. The observations span the spring melt seasons for four years (1992-1995).Data were collected at a research site located on Mammoth Mountain, California (37ø37'30"N, 119ø2'30"W; 2900 m above mean sea level), a ski resort located on the crest of the Sierra Nevada, California. The hydrology of the site is dominated by a deep winter snowpack deposited by wintertime frontal storms moving east from the Pacific Ocean, subsequent spring melt, and generally mild dry summers punctuated by sporadic local convective storms. The site is sparsely wooded with rolling terrain. Meltwater was collected in eight 1 x 1 m high-density polyethylene zero-tension lysimctcrs, six of which were equipped with electrical conductivity (EC) probes. The lysimctcrs were each bounded by 0.2-m vertical walls to prevent pressure-driven lateral flow from the basal saturated layer of the snowpack.Prior to snowfall, the lysimctcrs were rinsed with distilled 823

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Four experiments were performed to examine the relationship between the meltwater flow field and ion release from melting snow. A 0.4 m3 volume of snow was placed in a Plexiglas box and melted from above using a heating plate. The meltwater and solute fluxes issuing from the bottom of the snow were monitored. In experiments with NaCl tracer added to the snow, the solute concentrations were generally lower in the flow fingers than in the background wetting front. Dye tracer experiments revealed contemporaneous areas of concentrated dye and dilute meltwater in flow fingers. This suggests that the meltwater in flow fingers is diluted by low concentration water from the top of the snowpack. Flow fingers contribute more meltwater flux primarily because the flow is maintained for a longer period of time than in the non-finger areas; however, the relative contribution of flow fingers to solute flux was apparently not as great as that of the background wetting front because of dilution of solute in the flow finger areas.