西藏拉萨河流域河水主要离子化学特征及来源

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

为掌握拉萨河流域水化学的时空变化特征、来源以及主要控制因子,于2014年8月~2015年7月在拉萨河拉萨水文站断面定点采集水样,并对其主要的化学离子进行分析.结果表明:HCO_3^-是主要的阴离子,占离子总量的68.73%,SO_4^(2-)含量其次,Ca^(2+)是主要的阳离子,占离子总量的67.75%,其次为Mg^(2+),拉萨河流域四季的pH值介于8.31~8.90,平均值为8.59,整体偏碱性,其中夏季的pH平均值最高,主要是因为水生植物光合作用以及浮游植物生长的影响.EC值介于155.0~257.0μS·cm-1之间,平均值为210.5μS·cm-1,TDS均值为181.35 mg·L-1,高于世界河流平均值.这是因为高原构造抬升活动频繁,造成岩石机械风化加强,从而加速岩石溶解.拉萨河流域的主要离子浓度大小表现为冬季>春季>秋季>夏季的变化规律.离子来源分析表明,HCO_3^-、Ca^(2+)和Mg^(2+)主要来源于碳酸盐岩的风化,Cl-、SO_4^(2-)、NO_3^-离子主要来源于大气和岩石风化.此外,由于降雨和冰川融水的稀释作用,河流中的主要离子浓度与河流径流量呈现负相关的关系.

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 SO4^2-. Ca^2＋is the dominant cation,which accounts for 67. 75% of the cations,followed by Mg^2＋. 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 between155. 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 Ca^2＋,Mg^2＋,and HCO3^- are mainly derived from chemical weathering of carbonate minerals,Cl-,SO4^2-,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.