Findings on water quality in Upper Mustang (Nepal) from a preliminary geochemical and geological survey

The major ion and trace element chemistry of water samples, including springs, rivers and irrigation ditches, collected during a survey on August 2016 in the Upper Mustang region of the Mustang District of Dhawalagiri Zone (Nepal) has been investigated. The Upper Mustang region, a cold desert, represents a hot-spot for climate change: indeed, violent hailstorms and rainstorms have been recently observed, consequently exposing land to erosion. Results of this study indicate that waters in the region belong to the Ca–HCO3, Ca–Mg–Cl– SO4 and Na–K–Cl-types, reflecting different hydrochemical regimes. Uranium is widespread in waters, with concentrations up to 19 lg/L recorded in a potable water supply. Locally, anoxic conditions affect uranium mobility due to the low solubility of U(IV) minerals. Highly toxic thallium was detected in a thermal spring at an elevated concentration (45 lg/L Tl). The association of thallium with high concentrations of iron suggests that these elements are derived from pyrite oxidation. Detectable levels of thallium were also measured in the water of an irrigation ditch. Lithium concentrations ranged from 7 lg/L to 12 mg/L in the thermal water and showed a strong association with chloride ions. Arsenic concentrations up to