Fate and Origin of Major Ions in River Water in the Lhasa River Basin, Tibet

Abstract

In order to understand the temporal and spatial variations of major ions in water and their sources in the Lhasa River Basin, water samples were collected monthly at the hydrological station in the Lhasa River from August 2014 to July. The results show that HCO3- is the dominant anion in the water of the Lhasa River, which accounts for 68.73% of the anions, followed by SO42-. Ca2+ is the dominant cation, which accounts for 67.75% of the cations, followed by Mg2+. The pH values of the river water range between 8.31 and 8.90, with a mean of 8.59 throughout the year, generally showing alkaline water. The highest pH values occur in summer, which is probably due to the photosynthesis of aquatic plants and the growth of phytoplankton. Electrical conductivity (EC) varies between 155.0 and 257.0 μS·cm-1, with a mean of 210.5 μS·cm-1. Because of the frequent uplift of the Tibetan Plateau that enhanced the mechanical weathering of rocks and mineral dissolution, the total dissolved solid (TDS) concentration, at an average of 181.35 mg·L-1, is significantly higher than the average value of rivers around the world. The Lhasa River is recharged by surface runoff, so the concentrations of major ions in water are higher during winter, but lower in summer. An ion source analysis indicates that Ca2+, Mg2+, and HCO3- are mainly derived from chemical weathering of carbonate minerals, Cl-, SO42-, and NO3- are mainly affected by precipitation and rock weathering. Furthermore, the concentrations of major ions in the water have a negative correlation with the river discharge rate, which suggests there might be a dilution effect occurring during the rainy season.