WATER QUALITY IN THE SOUTHERN TIBETAN PLATEAU: CHEMICAL EVALUATION OF THE YARLUNG TSANGPO (BRAHMAPUTRA)

Yarlung Tsangpo (Brahmaputra) is the largest river system draining the northern slopes of the Himalayan ranges on the southern Tibetan Plateau. It remains one of only two large non-regulated rivers in China. In this paper the chemical composition of Yarlung Tsangpo and its major tributaries (Raga Tsangpo, Nyangchu and Lhasa River) are studied.Water samples (55) were collected and measured for major ions, trace elements and nutrients in order to: (1) define the present chemical quality of this water course; (2) address possible mechanisms governing the water chemical compositions, and (3) identify potential sources for contaminants. Multivariable analysis shows that geology and climate are the major explanatory variables for the spatial variation in water chemistry in this river system. In general, water chemistry is mainly controlled by carbonate weathering, with Ca2 and HCO3 being the dominant ions. In addition, runoff from brackish/saline lakes and geothermal waters, enriched in Na, Cl, SO24 , Mg2 and Li, are major contributors of elevated concentrations of these solutes in the headwater regions resulting in a relatively high loading of total dissolved solids (TDS, 146–397 mg L-1). Levels of most heavy metals and total dissolved nutrients were generally found to be low. However, elevated As concentration (avg. 95mgL-1) in the headwaters and additions from untreated wastewater were evident at some locations.