浑善达克沙地东部地区天然水体的水化学组成及其控制因素

浑善达克沙地东部地区位于内蒙古高原,其天然水体为淡水。利用原始型Durov三线图对其天然水体进行水化学分类,结果表明,地表水水化学类型属于碳酸氢盐组,而地下水属于碳酸氢盐组、混合组和硫酸盐组多种类型。天然水体样品在Gibbs图中的分布表明,岩性控制是形成该地区天然水体水化学组成的主要因素,这种情况不同于我国西部和中部沙漠由于蒸发强烈而受控于蒸发-结晶作用的天然水体。天然水体的离子比例关系表明,地下水中蒸发盐岩和硅酸盐岩的风化程度较高,而碳酸盐岩风化程度低,这可能是由于地下水的水-岩作用时间长、二氧化碳供给不充分引起的;相反,由于水-岩作用时间短、二氧化碳供给充分,地表水中蒸发盐岩和硅酸盐岩的风化程度较低,而碳酸盐岩风化程度高。

Hydrochemical Composition of Natural Waters and Its Affecting Factors in the East Hunshandak Sandy Land

The East Hunshandak Sandy Land is located in the Inner Mongolia Plateau. Compared with other deserts in West China, water resources in Hunshandak Sandy Land are relatively abundant, even some rivers originate from the sandy land. In this study 24 water samples including 11 groundwater samples, 6 lake water samples, 5 river water samples and 2 spring water samples were collected from 2011 to 2012. Physical parameters including the pH value, oxidation-reduction potential, electrical conductivity, total dissolved solids and salinity were measured on site with a portable instrument. Major anions (Cl-, NO3- and SO42-) and cations (Na+, K+, Mg2+, Ca2+ and NH4+) were determined by electrochemical detectors of ion chromatography at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The concentration of HCO3- was measured by titration with HCl following Gran Method. The results showed that total dissolved solids (TDS) of water samples ranged from 67 mg·L-1 to 660 mg·L-1, which revealed that freshwater was dominant in the natural waters in Hunshandak Sandy Land. Moreover, the concentration of calcium was the highest in cations in almost all water samples, while that of bicarbonate was the highest in anions except for some groundwater and spring water samples. The pH values of groundwater were commonly slightly lower than 7, while those of others, such as lake water, river water and spring water, were slightly higher than 7. In addition, almost all concentrations of cations and anions in groundwater were the highest in natural waters. The hydrochemical types were determined by the original Durov Diagram. The results showed that surface water was dominated by bicarbonate, but groundwater was dominated by bicarbonate, mixed type or sulfate. Gibbs Diagram was used to determine the factors affecting the hydrochemical composition of natural waters. Different from the deserts in the western and central parts of China, where the evaporation was so strong that evaporation-crystallization affected mainly the hydrochemical composition of the natural waters, and the water-rock interaction was the main factor affecting the natural waters in East Hunshandak Sandy Land. Ion ratios in samples indicated that the rock weathering in waters was different. The weathering degree of evaporative salt rock and silicate rock in groundwater is higher, while that of carbonate rock is lower, which may be caused by the insufficient supply of carbon dioxide due to the long water-rock interaction time in groundwater. On the contrary, due to the short water-rock interaction time and sufficient supply of carbon dioxide, the degree of weathering of evaporative and silicate rocks in surface water is low, while that of carbonate rocks is high.