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 Mg2þ in the outflow of the rock glacier increased to more than 900 meq L 1 compared to values of less than 40 meq L 1 at all the other sites, concentrations of Ca2þ were greater than 4,000 meq L 1 compared to maximum values of less than 200 meq L 1 at all other sites, and concentrations of SO 4 reached 7,000 meq L , compared to maximum concentrations below 120 meq 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 O from 10% in the outflow of the rock glacier compared to 20% in snow and enrichment of deuterium excess fromþ17.5% in rock glacier outflow compared toþ11% in snow, suggests that melt of internal ice that had undergone multiple melt/freeze episodes was the dominant source of base flow. Copyright # 2005 John Wiley & Sons, Ltd.