Identifying streamflow sources during spring snowmelt using water chemistry and isotopic composition in semi-arid mountain streams

Understanding streamflow generation using natural tracers in semi-arid, seasonally snow-covered mountain streams is essential for water resources management, water quality study and evaluation of impacts from climate change. This study reports temporal variations in stable isotopic ratios and concentrations of major dissolved ions of streamwater and precipitation between October, 2005 and May, 2007 in Red Canyon Creek and its tributary, Cherry Creek, draining carbonate-rich catchments on the southeastern flank of Wind River Range (Wyoming, USA). Although the isotopic ratios of oxygen and hydrogen in precipitation increased from approximately -33‰ to -13‰ and -260‰ to -110‰, respectively, during winters of 2006 and 2007, the oxygen and hydrogen isotopic compositions of streamwater at all sites remained unchanged throughout the year at -18.6±0.3‰ (n=88) and -142±1.6‰ (n=40) for δ ¹⁸O and δ ²H, respectively. The isotopic values for the streamwater were identical to that found in groundwater, which had the values of -18.6±0.2‰ (n=26) and -142±1.1‰ (n=26) for δ ¹⁸O and δ ²H, respectively. On the other hand, the temporal pattern of streamwater chemistry differed in space. In upper Red Canyon Creek, major dissolved ion concentrations in water varied little throughout the year. Nearly constant isotopic and chemical composition of streamwater at upper Red Canyon Creek indicated the dominance of the groundwater contribution throughout the year. In contrast, Cherry Creek had clear dilution of base metal and sulfate concentrations during increasing discharge at snowmelt, which is a clear indication of " new" water coming from fresh snowmelt. The contrasting behavior of stable isotopes and dissolved solutes during snowmelt at Cherry Creek suggests the isotopic tracers traditionally used in hydrograph separation failed to indicate different water sources at Cherry Creek. Combining isotopes and geochemical tracers indicates that streamwater at Cherry Creek during snowmelt is primarily a mixture of snowmelt and groundwater which have similar isotopic compositions but different chemical concentrations. The snowmelt is well mixed during temporary storage in a headwater wetland before reaching the sampling site. Such mixing plays an important role in reducing temporal variability of stable isotope values of fresh snowmelt water. We suggest that development of direct tracer experiments might help address the hydrodynamics of these kinds of watersheds in future research.