Major ion chemistry of water and its controlling factors in the Yamzhog Yumco Basin, South Tibet

Abstract

水化学主离子特征是流域湖泊的一个重要特征,对气候以及河流所经地区的环境具有指示作用.本文对藏南羊卓雍错流域水化学主离子组成特征及其控制因素进行分析,结果显示流域内不同水体(湖水、河水、地下水)之间的主离子组成以及水化学类型差异显著.其中,羊卓雍错的水化学类型为SO₄^2--HCO₃^--Mg²⁺-Na⁺,巴纠错为SO₄^2--Mg²⁺-Na⁺,沉错为SO₄^2--Na⁺-Mg²⁺-Ca²⁺,普莫雍错为HCO₃^--SO₄^2--Mg²⁺-Ca²⁺,空姆错为HCO₃^--SO₄^2--Ca²⁺;流域河水中主要阴离子为HCO₃^-和SO₄^2-,Ca²⁺为绝对优势阳离子;流域地下水化学类型则为HCO₃^--Ca²⁺.究其原因,流域水体化学组成主要受岩石风化作用控制;除此,羊卓雍错、巴纠错和沉错水化学组成亦受自身蒸发-结晶作用的影响.就入湖河水而言,羊卓雍错入湖河水整体受碳酸盐岩石风化的影响较大,蒸发岩溶解的影响次之;沉错和空姆错入湖河流(卡鲁雄曲)的蒸发岩来源则略大于碳酸盐岩来源;而硅酸盐对流域内河水的水化学性质影响较小.与入湖河水相比,羊卓雍错和沉错湖水的Mg²⁺、Na⁺和SO₄^2-含量较高,而Ca²⁺和HCO₃^-含量较低.这应该与湖水蒸发强烈使得湖水中Ca²⁺和HCO₃^-析出并沉积到湖底有关.而空姆错由于湖泊面积小、入湖河水流量大,致使其湖水与入湖河水的主离子组成差异不显著.;Water chemical composition of a lake and its inflow rivers is an important lake characteristic, which can reflect the climatic and environmental background of the basin where the lake is located and the river flows through. Water samples were collected and their major ions were measured from the major lakes (i. e., Yamzhog Yumco, Bajiu Co, Chen Co, Kongmu Co and Pumoyong Co), inflow rivers, and wells in the Yamzhog Yumco Basin. Possible controlling factors on the major ion compositions from different waterbody were analyzed. Results revealed that the major ion compositions (e. g. hydrochemical types) of the water in the basin were significantly different as follows. The major ion compositions of lake water in Yamzhog Yumco, Bajiu Co, Chen Co, Pumoyong Co and Kongmu Co were SO₄^2--HCO₃^--Mg²⁺-Na⁺, SO₄^2--Mg²⁺-Na⁺, SO₄^2--Na⁺-Mg²⁺-Ca²⁺, HCO₃^--SO₄^2--Mg²⁺-Ca²⁺, and HCO₃^--SO₄^2--Ca²⁺, respectively. For the inflow river water, the major anions were HCO₃^- and SO₄^2- and the preponderant cation was Ca²⁺. Comparing with the water chemical compositions of inflow rivers, the contents of Mg²⁺, Na⁺ and SO₄^2- in Yamzhog Yumco and Chen Co were much higher with a relative low contents of Ca²⁺ and HCO₃^-. On the contrary, there was little difference for the Kongmu Co and its inflow rivers. In addition, the major ion compositions of well waters were characterized by HCO₃^--Ca²⁺. Further analysis indicated that the water chemical compositions were mainly controlled by rock weathering in the whole basin, and also by evaporation and crystallization for Yamzhog Yumco, Bajiu Co and Chen Co. For the inflow river waters in the basin, the main controlling factors of chemical compositions were carbonate weathering and evaporite weathering, while the effect of the silicate weathering on the river water chemistry was minor. Thereinto, the dominant controlling processes were carbonate weathering for inflow river waters flowing into Yamzhog Yumco, and the evaporite weathering for the inflow river waters flowing into Chen Co and Kongmu Co. As for the different water chemical characteristics between the inflow river water and lake water of Yamzhog Yumco and Chen Co, the possible reason should own to the strong evaporation of two lakes which may intensify calcium phosphate precipitation. The small lake area of Kongmu Co and the vast amounts of inflowing river water can explain the little difference of water chemical compositions between the lake and its inflow rivers. The study also showed that the controlling processes of water chemical compositions among inflow rivers were different because the different rock-stratigraphic area they flowed was dominated by physical weathering/erosion. With global warming, the style and intensity of rock weathering would be changed. So it is indispensable to further investigate the speed of rock weathering in the basin and its possible response to climate change.