Chemistry and isotopic composition of precipitation and surface waters in Khumbu valley (Nepal Himalaya): N dynamics of high elevation basins

Abstract

We monitored the chemical and isotopic compositions of wet depositions, at the Pyramid International Laboratory (5050ma.s.l.), and surrounding surface waters, in the Khumbu basin, to understand precipitation chemistry and to obtain insights regarding ecosystem responses to atmospheric inputs.The major cations in the precipitation were NH4+ and Ca2+, whereas the main anion was HCO3−, which constituted approximately 69% of the anions, followed by NO3−, SO42− and Cl−. Data analysis suggested that Na+, Cl− and K+ were derived from the long-range transport of marine aerosols. Ca2+, Mg2+ and HCO3− were related to rock and soil dust contributions and the NO3− and SO42− concentrations were derived from anthropogenic sources. Furthermore, NH4+ was derived from gaseous NH3 scavenging. The isotopic composition of weekly precipitation ranged from −1.9 to −23.2‰ in δ18O, and from −0.8 to −174‰ in δ2H, with depleted values characterizing the central part of the monsoon period. The chemical composition of the stream water was dominated by calcite and/or gypsum dissolution. However, the isotopic composition of the stream water did not fully reflect the composition of the monsoon precipitation, which suggested that other water sources contributed to the stream flow.Precipitation contents for all ions were the lowest ones among those measured in high elevation sites around the world. During the monsoon periods the depositions were not substantially influenced by anthropogenic inputs, while in pre- and post-monsoon seasons the Himalayas could not represent an effective barrier for airborne pollution. In the late monsoon phase, the increase of ionic contents in precipitation could also be due to a change in the moisture source.The calculated atmospheric N load (0.30kgha−1y−1) was considerably lower than the levels that were measured in other high-altitude environments. Nevertheless, the NO3− concentrations in the surface waters (from 2 to 17μeqL−1) were greater than expected based on the low N inputs from wet deposition.