贵州喀斯特地区河流的研究——碳酸盐岩溶解控制的水文地球化学特征

HYDROGEOCHEMISTRY OF RIVERS IN GUIZHOU PROVINCE, CHINA: CONSTRAINTS ON CRUSTAL WEATHERING IN KARST TERRAIN

测量了喀斯特地区乌江、沅江两大水系的河流枯水期的主元素、Sr2+离子浓度和Sr同位素比值。这些河流的化学组成代表了流经碳酸盐岩地层的河水的化学组成。这些河流及其支流有高的溶解盐,TZ+变化范围为: 2. 1~6. 3meq/L,高于全球河流的平均值(TZ+ =0. 725meq/L)。河水含有较高的溶质浓度,河水水化学组成以Ca2+和HCO-3 为主,其次为Mg2+和SO2-4 ,Na+ +K+和Cl- +Si分别只占阳离子和阴离子组成的5% ~10%。这些河流的化学和同位素组成主要受其自流盆地的地质特征控制。流经碳酸盐岩地层的乌江水系河流具有较高的Sr浓度(1. 1~9. 70mol/L)和较低的87Sr/86Sr比值(0. 7077 ~0. 7110),与流经碎屑岩地层的沅江水系的清水江河流中较高的87Sr/86Sr比值(0. 7090 ~0. 7145)及较低的Sr浓度(0. 28~1. 32mol/L)形成鲜明的对比。流域盆地的地理岩性控制了河水的化学组成和同位素组成。对河水的化学计量分析表明河水化学组成受碳酸盐岩溶解控制,而碳酸盐岩主要受碳酸和硫酸作用而溶解。乌江流域受硫酸作用特别明显,表明硫酸主要来源于燃煤或流域盆地硫化物矿物氧化而形成的大气输入。化学元素和同位素比值之间的相互关系表明3个主要来源为:石灰岩、白云岩和硅酸盐岩的风化。同时估计了碳酸盐岩和硅酸盐岩的化学风化速率,结果?

Major ions and Sr^(2+) concentrations and Sr isotopic compositions of two major river systems were measured in Guizhou karst region. The chemical composition of these rivers in the karst region represents that of river water from the typical carbonate areas. Its hydrogeochemical characteristics are different from those of global major rivers: The river and its tributaries have high total dissolved solid concentrations, with Ca^(2+)and HCO^-_(3) being dominant, Ma^(2+) and SO^(2-)_4 coming next. Both Na^(+)+K^(+) and Cl^(-)+Si account for 5%~10% of the cations and anions, respectively. These rivers have high total concentration (TZ^(+)) ranging from 2.0 to 5.5 meq/L, which are significantly higher than those of the global river averge (TZ^(+)=0.725meq/L). The chemical and isotopic compositions of these rivers are largely under controll by the geological signatures of their drainage areas. The Wujiang River system draining carbonate strata show high Sr^(2+) concentrations(1.07^(9.70)) and lower ^(87)Sr/^(86)Sr ratios of 0.7077~0.7110, as compared with the Yuanjiang River system (^(87)Sr/^(86)Sr=0.7090^(0.7145,) Sr^(2+)=0.39~1.32) draining detrital strata. The end-member identification by the intercorrelations between chemical and isotopic ratios shows three main sources, which are derived from weathering of limestone, dolomite and silicates. The Wujiang River system has characterized by low Na/Ca, Mg/Ca mole ratios ((0.05)~0.34 and 0.18~0.79, respectively), showing a strong influence of limestone, about 96% of solutes derived from the weathering of carbonate. On the other hand, the Yuanjiang river system show high Na/Ca mole ratio from 0.06~1.80, Mg/Ca from 0.23~1.30, suggestive of a strong influence of limestone and dolomite weathering, about 90% of solutes derived from the weathering of dolomite in the Wuyanghe river system and about 54% of solutes derived from the weathering of limestone in the Qingshuijiang river system.