内蒙古河套平原高砷地下水赋存环境特征

Geochemical characteristics of the As-bearing aquifer in the Hetao plain,Inner Mongolia

内蒙古河套平原是世界地方性砷中毒较为严重的地区之一。笔者以平原西部的重病区杭锦后旗为研究区,对高砷地下水赋存环境进行了调查研究。研究表明:高砷区沉积物中As的含量为7.7～34.6mg/kg,其中粘性土和亚粘性土中As含量相对较高。高砷地下水的pH值为7.0～8.3,平均Eh值为-155.1mV,平均矿化度为1.58g/L,主要的水化学类型包括:Cl-HCO3-Na型、Cl-Na型、HCO3-Cl-Na型,As的含量为15.5～1093μg/L,且主要以As(Ⅲ)形态存在,水中DOC(0.73～35.76mg/L)、HCO3-(283.75～1290.48mg/L)、NH4+(0.27～10.48mg/L)的浓度较高,硝酸盐和硫酸盐含量较低。研究区的氟中毒现象也较严重,高氟地下水中氟含量为1.11～6.01mg/L。绝大多数高氟水中砷含量也超标,出现了一种高砷水与高氟水并存的现象。综合判断,河套平原的高砷地下水赋存环境处于还原性环境。还原条件下,高砷区沉积物中的铁/锰氧化物的还原溶解易使吸附的砷释放到地下水中。这是研究区高砷地下水形成的主要原因。

The Hetao plain of Inner Mongolia is one of the serious endemic arsenic poisoning areas in the world. The authors took Hanggin Rear Banner in western Hetao plain as a typical study area, where endemic arsenic poisoning constitutes a very severe problem. Studies show that As concentration is 7.7-34.6mg/kg in sediments of the high As groundwater area and As concentrations are relatively high in clay and mild clay. According to the results of the authors’ investigation, As is incorporated in amorphous iron oxides and is associated with sulfides and targeting acid volatile sulfurs, carbonates constitute the main association of sedimentary As, accounting for over 50% of total As. pH values of high As groundwater are 7.0-8.3, average Eh value is -155.1mV, average TDS value is 1.58 g/L, main hydrochemical types are Cl -HCO3-Na, Cl -Na and HCO3-Cl -Na, and As concentration ranges from 15.5 to 1093μg/L and is mainly represented by As （Ⅲ）, high level DOC （0.73- 35.76mg/L）, HCO3-（283.75-1290.48mg/L）, NH4＋（0.27-10.48 mg/L） and minor nitrates and sulfate. Hanggin Rear Banner is also one of the serious endemic fluorosis areas, with fluoride content being 1.11-6.01mg/L in high F groundwater. Due to the presence of excessive As in most high F groundwater areas, there exists a coexistence phenomenon of high F groundwater and high As groundwater. A comprehensive analysis shows that high As groundwater was formed in a reductive environment in Hetao plain. Under reducing conditions, the reductive dissolution of Fe/Mn oxides （or hydroxide） from sediments in high As areas can release arsenic into groundwater. This is the main factor responsible for the formation of high As groundwater in the study area.