PRELIMINARY ANALYSIS OF HYDROGEN AND OXYGEN ISOTOPES INDICATING THAT GROUNDWATER SEEPAGE FROM VEGAS DOMINATES STREAM FLOW IN FOUR CENTRAL ANDEAN WATERSHEDS

The purpose of this investigation is to evaluate the contribution of melt water from rock glaciers to water supply in four alpine watersheds in the central Andes, 31^* S. To do so, we have examined the el Altar, la Pantanosa, Rio Atutia, and Rio Salinas watersheds, Calingasta County, San Juan Province, Argentina. We report analyses of stable H and O isotopes indicating that surface waters strongly reflect groundwater contributions from vegas (springs) and that the discharge of meltwater from perennial ice is minimal at most. The isotopic signatures of ice from rock glaciers for d¹⁸O vs SMOW was -14.55 to -14.60 and for d²H vs SMOW was -107.06 to -107.31 for la Pantanosa and, respectively, -13.85 to -14.51 and -99.87 to -104.54 for Rio Atutia. Groundwater from monitor wells or vegas in closest proximity to these rock glaciers displayed d¹⁸O vs SMOW of -15.64 to -16.53 and for d²H vs SMOW was -113.35 to -121.03 for la Pantanosa and, respectively, -15.27 to -17.96 and -112.53 to -113.67 for Rio Atutia. These isotopic ratios reveal that the seeps in closest proximity to rock glaciers do not display isotopic ratios particularly indicative of those measured for perennial ice extracted by excavating and sampling the rock glaciers. The isotopic signatures of streams for d¹⁸O vs SMOW was -14.55 to -14.60 and for d²H vs SMOW was -107.06 to -107.31 for la Pantanosa and, respectively, -13.85 to -14.51 and -99.87 to -104.54 for Rio Atutia. Indeed, the isotopic ratios of stream samples most closely compare to the signatures of baseflow seeping from vegas. Discharge of vegas and streams, in concert with field measurement of pH, temp, and conductivity, along with concentrations of major ions, suggest that stream flow is dominated by groundwater contributions, which accumulate down valley, as stream flow rises by a couple orders of magnitude along several kilometers of stream reach. The isotopic signatures of stream waters, overall, reveal substantial contributions from the flow of vegas, while the contributions of melt water from rock glaciers are difficult to decipher, again suggesting that perennial ice is not a significant contribution to alpine water supply.