Hydrochemical characteristics and element contents of natural waters in Tibet, China

Sixty water samples （35 groundwater samples, 22 surface water samples and three hot-spring water samples） were collected at 36 points from villages and towns in Lhasa city, Nagchu （Nagqu） prefecture, Ali （Ngari） prefecture and Shigatse （Xigaze） prefecture （Tibet） in 2013 to study the hydrochemical characteristics and element contents of natural waters. The concentrations of elements were determined in the water samples and compared with the concentrations in water samples from other regions, such as southeast Qinghai, south Xinjiang, east Sichuan and west Tibet. The hydrochemical species in different areas were also studied. Water in most parts of Tibet reaches the requirements of the Chinese national standard and the World Health Organization international standard. The pH values of the water samples ranged from 6.75 to 8.21 and the value for the mean total dissolved solids was 225.54 mg/L. The concentration of arsenic in water from Ali prefecture exceeded the limit of both the Chinese national standard and the international standard and the concentration of fluoride in water from Shuanghu exceeded the limit of both the Chinese national standard and the international standard. The main hydrochemical species in water of Tibet is Ca （HCO3）2. From south to north, the main cation in water changes from Ca^2＋ to Na^＋, whereas the main anions in water change from HCO3^- to Cl^- and SO4^2-. The chemistry of river water and melt water from ice and snow is dominated by the rocks present at their source, whereas the chemistry of groundwater is affected by many factors. Tectonic divisions determine the con- centrations of the main elements in water and also affect the hydrochemical species present.