RENZENG LaMu, LUO Zhen, CHEN Hu-lin,HUANG Xiang
(School of Sciences, Tibet University, Laboratory of environmental chemistry in plateau basin, Lhasa 850000, China)

Abstract: The Nyangchu River basin is called "The granary of Tibet" and plays a decisive role in regional economic development. Report on systematic studies of water chemistry in the area was few. In order to reveal the water chemical characteristics and its controlling factors of the river basin, water samples were collected in November 2018 (Level period) from the river main stream and its major tributaries for a hydrochemical characteristics evaluation. Collected water samples were mainly measured for its physiochemical parameters, including major ions and trace elements. The results showed that water from the Nyangchu River basin was slightly alkaline, with pH range from 8.10 to 9.41. A similar spatial trend, i.e. increased from upstream to the downstream, appeared for total dissolved solids (TDS), electrical conductivity (EC) and salinity (Sal.). Average value of TDS, EC and Sal. were 275.64 mg/L, 389.06 μS/cm and 172.71 mg/L, respectively. Among which, Ca2+ was dominated cation, accounting for 71.45% of the total cations, while the anions was dominated by HCO3-, accounting for 63.21% of the total anions. The concentration of major ions in the river showed a certain difference in terms of spatial variation. Over all, these ions increased gradually from the upper to the middle catchments and, stabilized on the river confluence, where it runs to the Yarlung Tsangpo. Among the three major source areas (i.e. Tsokarpu lake, Chumbyumco lake and Sawang lake), water hydrochemical of Tsokarpu lake, Chumbyumco lake and its downstream outlet were mainly HCO3-Ca type, however, Sawang lake, upper to middle reaches of the Nyangchu river main stream as well as the estuary of the Yarlung Tsangpo River were HCO3？SO4 - Ca？Mg type. Water hydrochemical types for the downstream areas, including its tributaries (i.e. ChongBa YongQu and Longma River), were mainly HCO3？SO4 - Ca. Weathering of carbonate and silicate rocks in the river basin was the major controlling factor of the water chemical characteristics. Nevertheless, the controlling intensity was gradual transited from carbonate to silicate weathering along the river course. In addition, the potential impact of agricultural runoff in the river basin cannot be ignored.

Key word: Spatial distribution characteristics; Controlling factors of hydrochemistry; Lithologic end member; The Nyangchu River basin

EARTH AND ENVIRONMENT Vol.49, No.4, Tot No.342, 2021, Page 358-366