RESEARCH ON THE HYDROCHEMICAL CHARACTERISTICS OF HIGH-ARSENIC GROUNDWATER IN THE SHALLOW AQUIFER OF NORTHERN YINCHUAN PLAIN
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摘要: 宁夏银川平原是继河套平原之后，在黄河流域发现的又一个高砷地下水分布区.为了总结其高砷地下水的水化学特征，并探索水化学因素对地下水砷释放和富集的影响机制，本文以银川平原北部（银北平原）作为典型研究区，采取野外水文地质调查、水样采集与测试、砷与水化学组分散点图相关分析及水文地球化学方法进行了综合研究.结果表明，银北平原地下水砷含量在0.2~177 μg/L之间；高砷地下水（大于50 μg/L） pH值多在7.5~8.5，水化学类型主要为HCO3-Na·Ca、Cl·HCO3-Na及Cl·HCO3-Na·Ca型，Eh多在-200~-100 mV.银北平原砷含量较高的地下水中COD、NH4+、HCO3-含量相应也较高，而NO3-和SO42-含量较低.高砷富有机质的冲-湖积含水层经过长期演化，形成偏碱性的中强还原性地下水环境和特殊的水化学特征，也具备极大的砷释放能力.较高的pH导致砷从铁锰氧化物或氢氧化物等水合物或黏土矿物表面解吸.其次部分铁锰氧化物在高pH、低Eh条件下可被还原为低价态可溶性铁锰，从而使与其结合的砷也得以释放进入地下水中.此外重碳酸根与砷酸根、亚砷酸根的竞争吸附行为促使含水层砷的解吸.Abstract: The Yinchuan Plain in Ningxia Region is another high-arsenic groundwater area which was found in the Yellow River basin after the Hetao Plain. In order to ascertain the hydrochemical characteristics of the high-As groundwater and their influence mechanism to arsenic release and enrichment in the aquifer, this article chooses the Northern Yinchuan Plain as a typical study area for comprehensive research, including hydrogeology field survey, water sampling and test, correlation analysis of As and hydrochemical components, as well as hydrogeochemistry. The results show that the As content in groundwater of Northern Yinchuan Plain ranges from 0.2 to 177 μg/L. The pH value of high-As (more than 50 μg/L) groundwater is 7.5-8.5. The hydrochemical types are mainly HCO3-Na·Ca, Cl·HCO3-Na and Cl·HCO3-Na·Ca. The Eh value is -200 to -100 mV. High contents of COD, NH4 and HCO3- occur in high-As groundwater in the Northern Yinchuan Plain, while the contents of NO3- and SO42- are low. After a long period of evolution, the alluvial and lacustrine aquifer in Northern Yinchuan Plain with high arsenic and rich organic matter formed alkaline-reducing groundwater environment, with special hydrochemical characteristics and a great potential of arsenic releasing. The high pH value causes arsenic desorption and release from the clay mineral surface and hydrates such as iron and manganese oxide or hydroxide. The arsenic combined with iron/manganese oxides is released into the groundwater, as part of iron and manganese oxides can be reduced into low state and more soluble compounds under the condition of high pH and low Eh. The competitive adsorption behavior between bicarbonate and arsenate/arsenite prompts the arsenic desorption of aquifer.
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