Wilson, Al膩na M.听1听;听Williams, Mark W.听2
1听Geography & INSTAAR, CU Boulder
2听Geography & INSTAAR, CU Boulder
New results from tritium (H3) analysis of waters from the Langtang Valley, Nepal offer insight into the relative ages of different source waters in the glacierized catchment. Since tritium in the environment is primarily attributed to nuclear fallout in the 1950s and 1960s and its half-life is around 12 years, the presence of tritium in surface waters provides age-constraints on when it arrived as precipitation. The ambient value in new snow in the Langtang Valley, Nepal was measured at 2.2 Tritium Units (TU) in November of 2013. High-elevation groundwater in the catchment had a tritium value of 4.55 TU, and lower elevation springs had vaues between 3.4 and 3.8 TU, while geothermally-influenced hotspring waters had values of 2.0 and 2.5 TU. Ice at the termini of three glaciers was found to be tritium-dead meaning the ice formation pre-dates the nuclear testing era, however surface waters from the Langtang River had values of 4 TU or more in November 2013 and 3 TU or more in January 2014. This decline in tritium values between November and January is indicative of a shift towards baseflow that may be more strongly controlled by tritium-dead glacier melt than the November river water.
Concentrations of the oxygen-18 isotope (未18O) in precipitation are dependent on temperature and distance traveled from the moisture source. Snow during the pre-monsoon season of 2012, arriving from the south, had 未18O concentrations of -5? to -9? while rain during the same time period had concentrations of -3?. Post-monsoon snow in 2013, likely delivered by the Westerlies, had an 未18O concentration of -27?. Values of 未18O measured in glacier ice varied between -16? and -20?, suggesting a monsoon season source that is intermediate to the pre- and post-monsoon precipitation samples that were measured.
Isotopes of hydrogen and oxygen offer insight into age and origin of surface waters and their sources. This information is explored further in order to estimate the contribution of different source waters to river streamflow.