Ion enrichment of snowmelt runoff water caused by basal ice formation

Once meltwater reaches the base of a snowpack it can infiltrate the underlying stratum, runoff, or refreeze and form a basal ice layer. Basal ice formation is most common early in melt over saturated or very cold frozen soils. Initial meltwater becomes enriched in ion concentrations compared to the parent snow due to ion fractionation during thaw and percolation through the snowpack. If ion exclusion occurs during basal ice formation, further enrichment of initial runoff water ion concentrations might occur. The influence of basal ice formation on runoff water chemistry was examined by comparing ion concentrations in runoff water that had sustained basal ice contact, to meltwater before basal ice contact. A series of experiments, involving melting a snowpack in a large insulated box over a cold impermeable substrate in a temperature-controlled room, were carried out. A cooling system at the chemically inert base ensured formation of basal ice during snowmelt. Meltwater samples were collected throughout melt from within the snowpack using an extraction tube; runoff water was collected at the base. All samples were analysed for major anions and cations. Results showed that formation of basal ice layers can sometimes enrich the initial runoff water compared to meltwater before basal ice contact. Ion concentrations in basal ice contact runoff water were up to sixteen times greater than those in no-contact meltwater; however, on average, basal ice contact runoff water showed 1·5 times the ion concentrations of the no-contact meltwater. Enrichment was greatest with the rapid formation of a thick basal ice layer. The implications are that basal ice formation alters both meltwater ion pathway and concentration. When no basal ice is present, enhanced infiltration of meltwater ion load can cause relatively dilute runoff water. When basal ice is present all meltwater runs off and further ion-concentration enrichment occurs.