VARIABILITY IN THE CHEMICAL-COMPOSITION OF IN-SITU SUBGLACIAL MELTWATERS

Meltwaters collected from boreholes drilled to the base of the Haut Gl acier d'Arolla, Switzerland have chemical compositions that can be cla ssified into three main groups. The first group is dilute, whereas the second group is similar to, though generally less concentrated in maj or ions, than contemporaneous bulk glacial runoff. The third group is more concentrated than any observed bulk runoff, including periods of flow recession. Waters of the first group are believed to represent su praglacial meltwater and ice melted during drilling. Limited solutes m ay be derived from interactions with debris in the borehole. The spati al pattern of borehole water levels and borehole water column stratifi cation, combined with the chemical composition of the different groups , suggest that the second group represent samples of subglacial waters that exchange with channel water on a diurnal basis, and that the thi rd group represent samples of water draining through a 'distributed' s ubglacial hydraulic system. High NO3- concentrations in the third grou p suggest that snowmelt may provide a significant proportion of the wa ters and that the residence time of the waters at the bed in this part icular section of the distributed system is of the order of a few mont hs. The high NO3- concentrations also suggest that some snowmelt is ro uted along different subglacial flowpaths to those used by icemelt. Th e average SO42-: (HCO3- + SO42-) ratio of the third group of meltwater s is 0.3, suggesting that sulphide oxidation and carbonate dissolution (which gives rise to a ratio of 0.5) cannot provide all the HCO3- to solution. Hence, carbonate hydrolysis may be occurring before sulphide oxidation, or there may be subglacial sources of CO2, perhaps arising from microbial oxidation of organic C in bedrock, air bubbles in glac ier ice or pockets of air trapped in subglacial cavities. The channel marginal zone is identified as an area that may influence the composit ion of bulk meltwater during periods of recession flow and low diurnal discharge regimes. (C) 1997 by John Wiley & Sons, Ltd.