The chemical composition of river water collected from the main stream of the Naqu and its tributaries was analyzed to reveal its hydrochemical characteristics and to evaluate the water quality for irrigation purposes. Based on 39 samples, the results revealed mildly alkaline pH values and total dissolved solids (TDS) values ranging from 115 to 676 mg/L, averaging 271 mg/L. Major ion concentrations based on mean values (mg/L) were in the order of
for cations and
for anions. Most hydrochemical types were of the Ca–HCO3
(~74.36%) type. Cluster analysis (CA) suggested that the hydrochemical characteristics upstream of the main stream of the Naqu were obviously different from those from the middle and downstream of the main stream and its tributaries. The analysis shows that the Sangqu, Basuoqu, Mumuqu, Zongqingqu, Mugequ basin tributary, and the Gongqu basin tributary were mainly affected by carbonate weathering. Carbonate and silicate weathering commonly controlled the hydrochemistry upstream and downstream of the main Naqu, Chengqu, and Mugequ streams. The middle of the main stream of the Naqu was mainly affected by silicate weathering, and anhydrite/gypsum dissolution mainly affected the hydrochemistry of the main Gongqu stream. The quality of water samples was suitable for irrigation purposes, except for one sample from the main stream of the Mugequ.
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