Geochemistry and source waters of rock glacier outflow, Colorado Front Range

We characterize the seasonal variation in the geochemical and isotopic content of the outflow of the Green Lake 5 rock glacier (RG5), located in the Green Lakes Valley of the Colorado Front Range, USA. Between June and August, the geochemical content of rock glacier outflow does not appear to differ substantially from that of other surface waters in the Green Lakes Valley. Thus, for this alpine ecosystem at this time of year there does not appear to be large differences in water quality among rock glacier outflow, glacier and blockslope discharge, and discharge from small alpine catchments. However, in September concentrations of Mg2+ in the outflow of the rock glacier increased to more than 900 mu eq L-1 compared to values of less than 40 mu eq L-1 at all the other sites, concentrations of Ca2+ were greater than 4,000 mu eq L-1 compared to maximum values of less than 200 mu eq L-1 at all other sites, and concentrations of SO42- reached 7,000 mu eq L-1, compared to maximum concentrations below 120 mu eq L-1 at the other sites. Inverse geochemical modelling suggests that dissolution of pyrite, epidote, chlorite and minor calcite as well as the precipitation of silica and goethite best explain these elevated concentrations of solutes in the outflow of the rock glacier. Three component hydrograph separation using end-member mixing analysis shows that melted snow comprised an average of 30% of RG5 outflow, soil water 32%, and base flow 38%. Snow was the dominant source water in June, soil water was the dominant water source in July, and base flow was the dominant source in September. Enrichment of 6180 from -10 parts per thousand in the outflow of the rock glacier compared to -20 parts per thousand. in snow and enrichment of deuterium excess from + 17.5 parts per thousand. in rock glacier outflow compared to +11 parts per thousand in snow, suggests that melt of internal ice that had undergone multiple melt/freeze episodes was the dominant source of base flow. Copyright (C) 2005 John Wiley & Sons, Ltd.