Nitrogen dynamics at high elevation basins in the Khumbu Valley (Nepal Himalaya), combining isotopic and geochemical tracers

Among the alpine environments of the world, the region of Mount Everest, is a unique ecosystem with a degree of biodiversity among the highest existing, but characterized by a recognized fragility and low resilience. There is a growing concern about the potential effect due to the global warming and other large scale phenomenon such as the atmospheric brown cloud on aquatic undisturbed systems in the Himalayan region. A yearly sampling campaign was conducted in the Khumbu Valley, within the Sagarmatha National Park, during the monsoon season in 2008 to collect surface water samples at high elevation from 4300 to 5500 m asl. In addition during 2007 and 2008 the daily sampling of wet deposition was carried on at 5050 m asl at the Nepal Climate Observatory - Pyramid ABC site located in the same area. We determined the main hydro-chemical species and the stable isotopes of oxygen (?18O) and deuterium (?2H) on both rain and surface water samples Nitrate was the dominant nitrogen species in running waters, with a median concentration equal to 7 ?eq l-1, while ammonia and organic nitrogen were below the detection limit. For comparison, nitrate concentration doubled the value measured in rain. The yearly N deposition load, equal to 0.31 kg ha-1 y-1, was remarkably lower than that measured at high elevation areas in Europe and North America. The ?18O and ?2H compositions of the precipitation showed a large variability, ranging from -1.9 ?? to -23.2 ??, and -0.79 ?? to -174 ??, respectively. Also the deuterium excess was highly variable, ranging from 7.4 ?? to 19.8 ?? with an average value of 12.2 ??. Based on weekly precipitation values, we defined the equation of the local meteoric water line (LMWL), which is remarkably close in slope to the global meteoric water line (GMWL), but with a higher intercept. The isotopic composition of precipitation is strongly influenced by the amount of precipitation: depleted values, associated to large precipitation amounts, characterize the central monsoon period. In comparison, the isotopic composition of stream water only show minor seasonal fluctuations, of about 2 ? in ?18O. Stream water isotopic composition falls on the previously defined LMWL, indicating a minor influence of evaporation. Stable isotopes and geochemical tracers (e.g. electrical conductivity and ion concentrations) were used to infer the streamflow components, and derive information on runoff processes and flowpaths.