Stable isotope (delta-hydrogen-2 and delta-oxygen-18) characterization of the South Platte River, Colorado

Stable isotope compositions (delta2H and delta 18O) were examined at 13 sites along the South Platte River, Colorado during the water year 2009-2010. As a natural part of the water molecule, stable isotope (delta2H and delta18O) compositions are used as indicators to trace flow of water in the hydrologic cycle, allowing the identification of sources of water and evaporation processes. A strong positive correlation was determined between delta2H and delta 18O compositions along the South Platte River, from Henderson to Julesburg, CO. All isotope compositions plotted below the global and regional meteoric water lines, indicating these were enriched in the heavy isotope forms relative to precipitation. Isotopic compositions were observed to vary spatially and temporally and between low and high flows. Mean isotope compositions became enriched with distance downstream from Henderson (-110.2 ‰ delta 2H and -14.4 ‰ delta18O) to Julesburg (-94.5‰ delta2H and -12.0 ‰ delta18O). Mean isotope compositions also varied during different seasonal periods, with enriched compositions in the fall (September and October) (-98.9 ‰ delta2H and -12.5 ‰ delta18O) and winter (January and February) (-100.7 ‰ delta2H and -12.5 ‰ delta18O) seasons of 2009-2010 relative to those during the summer (June and July) (-104.8 ‰ delta2 H and -13.5 ‰ delta18O) of 2009 and the spring (April and May) (-102.8 ‰ delta2H and -13.6 ‰ delta18O) and summer (June) (-114.1 ‰ delta 2H and -15.2 ‰ delta18O) seasons of 2010. Furthermore, mean stable isotope compositions were generally enriched during the low flow (-99.5 ‰ delta2H and -12.8 ‰ delta 18O) period from July 2009 to April 2010 relative to the high flow (-110.1 ‰ delta2H and -14.3 ‰ delta 18O) period in June 2009, May and June 2010. The observed enrichment and changes in isotope compositions could have been due to: 1) the contribution to river discharge of different sources of water characterized by particular isotope signatures 2) evaporation processes where lighter isotope forms are favorably transformed to the vapor phase, concentrating the heavier isotope forms in the liquid phase 3) a change of origin of vapor sources with varying isotope signatures during different seasons and 4) a combination of the above.