COMMENTS ON THE USE OF CHEMICALLY BASED MIXING MODELS IN GLACIER HYDROLOGY

The assumptions involved in the use of chemically based mixing models for analysis of flow routing of meltwaters in glacierized basins are critically evaluated. The assumption that glacial drainage systems consist of only two primary flow components is arbitrary and must be supported by independent evidence. Recent studies of the processes by which meltwaters acquire solute indicate that the assumption that flow components have unique and constant chemical compositions is unlikely to be correct. Source-water composition and weathering potential will vary over the course of a melt season, and the extent of subglacial weathering is strongly dependent upon such factors as meltwater residence time and the availability of reactive sediment, both of which are known to vary on diurnal to seasonal time-scales. Mixing of flow components does not appear to be confined to the terminal regions of glaciers and is therefore unlikely to be conservative as assumed. A multi-parameter mixing model is applied to the analysis of data on the chemistry of waters sampled From boreholes drilled through Haut Glacier d'Arolla, Switzerland, to demonstrate the range of dissolved species for which the assumption of conservative mixing is violated. The consequences of this violation for quantitative hydrograph separation are shown to be highly significant. The utility of mixing models as a tool for the investigation of glacier hydrological systems is questionable and the results of previous studies are unreliable.