西江干流河水主要离子及锶同位素地球化学组成特征

Geochemical characteristics of main ion and Sr isotope in the main channel of Xijiang River,South China

水环境中锶(Sr)含量及其同位素组成特征的研究有助于认识区域水文地球化学特征及流域盆地岩石风化速率等地球化学行为,因此河流环境中Sr的地球化学行为是地球化学研究的重要课题之一。对中国南方西江干流及其支流河水主要阴阳离子及Sr同位素组成进行了分析,发现其水化学组成主要以Ca2+、Mg2+、HCO3-和SO42-离子为主,分别占阴阳离子组成的80%以上。郁江河水具有较高的Sr同位素比值(87Sr/86Sr=0.71049)和较低的Sr含量(0.0263mg/L),而西江河水具有较低的Sr同位素比值以及较高的Sr含量,其Sr含量变化范围在0.0667～0.187mg/L之间,平均含量为0.136mg/L;Sr同位素比值变化范围在0.70856～0.70936之间,平均比值为0.70894。与世界主要河流水体中Sr含量平均值(0.078mg/L)及Sr同位素比值(87Sr/86Sr=0.7119)相比,西江干流水体具有较高的Sr含量及较低的Sr同位素比值。两条河流河水中主要离子及Sr的来源存在显著差异,西江干流河水中的主要溶解离子及Sr同位素主要源于碳酸盐岩(石灰岩和白云岩)的风化作用,主要受黔江及其上游红水河等化学组成所控制,而郁江则主要受碎屑岩风化作用的影响;同时,中上游地区的酸沉降及人类活动的输入可能导致西江干流水体受到一定程度的污染,而郁江受污染程度则相对较小。

Strontium geochemistry in surface waters helps shed light on regional hydrogeochemical characteristics and rock weathering rates in draining basins, as well as on anthropogenic influences to the environment. The analysis of Sr isotope ratios in rivers, therefore, has long been an interesting subject to geochemists. In this study, the ^87Sr/^86Sr ratios and the concentration of dissolved Sr and major anions and cations were measured in 19 water samples from the main channel of Xijiang and its tributaries in south China. The data indicates that the chemical ion composition of the river water is characterized by dominance of Ca^2＋, Mg^2＋, HCO3^-, and SO4^2-, which account for more than 80% of total anions and cations concentration, respectively. The sample water collected from the main tributary of Xijiang river has lower Sr concentration and higher Sr isotope ratios. By contrast, most of the water samples have higher Sr concentration and lower Sr isotope ratios, ranging from 0. 0667 mg/L to 0. 187 mg/L, and from 0. 70856 to 0. 70936, respectively. Compared to the world average concentration 0. 078 mg/L and isotope ratios （^87Sr/^86Sr = 0.7119）, waters in the main channel of Xijiang have higher Sr concentrations and lower ^87Sr/^86Sr ratios. Weathering of limestones in the drainage basins is responsible for the high Sr concentrations, whereas the weathering of elastic rock is responsible for the Sr geochemistry in Yujiang. Our data also indicate that acid deposition and anthropogenic activities in the upper and middle region is responsible to the pollution in the main stream of Xijiang. By contrast, Yujiang is much less polluted.