Composition Characteristics and Source Analysis of Major Ions in Four Small Lake-watersheds on the Tibetan Plateau,China

To investigate the ionic compositions of small lake-watersheds on the Tibetan Plateau,water samples from the brackish lakes( Pung Co( lake),Angrenjin Co and Dajia Co),the freshwater lake( Daggyaima Co),their inflowing rivers and the hot spring( Dagejia Geothermal Field),were collected during July-August 2013. The results showed that the major anions and cations of the brackish lakes were HCO-3,SO2-4and Na+,respectively,and the hydrochemical types were HCO3-SO4-Na and HCO3-Na. The major anions and cations of the inflowing rivers and the freshwater lake were HCO-3,SO2-4and Ca2 +,Mg2 +,respectively,and the hydrochemical types were HCO3-Ca,HCO3-Ca-Mg,HCO3-Mg-Ca,HCO3-SO4-Ca and SO4-HCO3-Ca. The major anions and cations of the hot spring were HCO-3and Na+,respectively,and the hydrochemical type was HCO3-Na. Water chemistry in the brackish lakes was primarily dominated by evaporation-crystallization processes,while the inflowing rivers and the freshwater lake were mainly influenced by carbonate weathering,and the hot spring was mainly controlled by hot water-granite interaction. Ca2 +was preferentially removed over Mg2 +from the water when carbonate minerals precipitation occured,which resulted in the high Mg2 +/ Ca2 +molar ratios of the brackish lakes. In the contribution of cation compositions,the largest contribution was carbonate weathering( 54%-79%),followed by silicate weathering( 13%-29%) and evaperite dissolution( 4%-23%),and the smallest was atmospheric input( 3%-7%).