大同盆地地下水中砷的分布及成因研究摘 要 随着全球人口的增长，人们赖以生存的水资源却日益枯竭，这已引起了全世界的高度关注，水资源枯竭的大部分原因直接来自水资源的污染，由环境地质因素造成的大面积地下水水质恶化是水资源污染的一个重要来源，该污染会引起典型的环境问题。地下水高砷造成的饮水型地方病是典型环境问题，也是世界上普遍存在的地方病之一，该病主要是由环境地质因素引起地下水含砷量较高所致。大同盆地最典型的、也是最突出的环境地质问题是地下水砷含量异常，由于饮用高砷地下水，该地区局部地砷病的发病率非常高，根据砷中毒主要分布的朔州、应县及山阴县的调查，砷中毒率达12.05％。地砷病严重的影响了当地居民的身体健康，也限制了当地的经济发展。因此，研究该地区地下水中砷的来源，查明地下水中砷的空间分布特征及其成因，对防砷改水勘探及地砷病防治具有重要的意义。本论文先结合大同盆地地质条件与盆地演化历史，在分析岩石、土壤及地下水数据的基础上研究了盆地地下水区域分布规律、地下水中砷主要来源及高砷地下水成因；然后结合岩石与土壤的室内实验，定量考察了高砷区周边岩石及沉积物中砷对地下水的影响，并总结了砷在土壤中的吸附行为及迁移规律。结论如下：1、盆地高砷地下水分布具有明显的区域分布特征。盆地高砷地下水主要在地势低洼的黄水河朔州－山阴县－应县段呈连续分布，其中，以地势最低的黄水河山阴县段地下水中砷含量最高。就深度而言，深度小于20m的潜水含水层组地下水中砷浓度基本没有超标现象（浓度小于0.01mg/L），地下水砷超标主要出现在深度范围为20～50m的浅层半承压含水层，其中，以深度为20～30m处地下水中砷含量最高。2、大同盆地风化基岩，尤其是含砷量较高的灰岩、砂页岩等是地下水中砷的原生来源。盆地演化历史是导致地下水中砷富集直接因素，砷的富集促使了次生富砷土壤形成。3、大同盆地封闭、低洼的地形地貌特征及地球化学环境是导致次生富砷介质中砷释放至地下水的直接原因。封闭、低洼的环境造成地表水水流不畅，土壤长期处于饱和状态而形成还原环境。还原环境促使富砷含水介质中铁锰含水氧化物，尤其是铁含水氧化物的还原溶解，直接导致了被结合的砷释放到水中；另外，在长期的还原环境下被含铝氧化物等吸附的As5+转化为As3+，导致了不易被吸附的As3+释放到地下水中。另外，水环境中磷酸根、碳酸氢根等阴离子，尤其是磷酸根的存在与地下水中砷形成竞争吸附，对地下水高砷有一定的促进作用。4、室内450d的钙质石灰岩溶滤实验表明，岩石中砷及铝溶出量逐渐增加，不同pH条件下砷的溶出量不同，pH为6.0时砷溶出量最大，最大值为0.01μg/g，其溶出率为0.8‰，在当地降雨pH条件下（pH＝7.0），砷最大溶出量为0.006μg/g，其溶出率为0.48‰。在两种不同性质土壤中，亚砂土总砷含量（18.0mg/kg）小于亚黏土总砷含量（26.8 mg/kg），但是，在与地下水的接触过程中，亚砂土中砷释放量（1.1μg/g）远大于亚黏土的砷释放量（0.4μg/g），两者地下水中砷释放量远远大于岩石中砷的释放量（0.006μg/g），说明地下水中砷主要由次生富砷介质释放而致。5、在吸附实验中，温度对亚黏土吸附砷的影响不大，对亚砂土略有影响，25℃时亚砂土吸附砷量较大；在砷浓度较低时，土水比对土壤吸附砷量影响不明显，而在砷浓度较高时，对土壤吸附砷量影响较显著，砷吸附量随土水比减小而减小。pH对土壤吸附砷（Ⅴ）量有较大的影响，在pH＜9.0时，土壤的砷吸附量变化较小；当pH＞9.0时，土壤对砷吸附量降低，特别是pH＞10.0时，土壤吸附砷量大幅度下降；亚砂土和亚黏土对砷（Ⅴ）吸附的行为符合Freundlich模式。6、砷在土壤中具有一定的迁移能力。在淋滤初期，由于土壤对砷的吸附、化学反应等作用导致渗出液中砷含量较低，随着淋滤液量的增加，渗出液中砷浓度逐渐增加，直至达到淋滤液中砷的初始浓度；砷（Ⅴ）在亚砂土中迁移能力明显大于亚黏土，且亚粘土对砷的吸附量大于亚砂土；在渗出液中砷浓度达到淋滤液中砷浓度时，不同深度土壤中砷含量并不相同，砷在土壤中迁移存在累积现象，这与砷在迁移过程中发生的物理化学作用有关。关键词：砷 地下水 释放 迁移 大同盆地

Arsenic Distribution and Origin in Groundwater in Datong Basin, Shanxi Province ABSTRACTWith the increase of population in the world, water resources becomes indispensable to people and dry up day by day. This is causing more and more attention from all over the world. This is great partially caused by water pollution in which large-scale groundwater quality deterioration resulted by environmental geology is a main part. This kind of pollution usually brings to typical environmental problems including endemic arseniasis existing in many countries and regions all over the world.Typical and prominent environmental problem of Datong basin is arsenic concentration abnormity in groundwater, the incidence of disease caused by arsenic is high in some regions in the basin because of drinking high-arsenic-contained groundwater with disease rate of 12.05% according to the investigation of arseniasis area of Shuo canton, Shanyin and Ying counties. Endemic arseniasis bring on a serious health problem for local inhabitant and restrict the development of local economy. Therefore, it is meaningful for strategies making to control arsenic contamination and prevent endemic arseniasis prevention to study arsenic sources of groundwater, find out the characteristics of arsenic distribution and the origin of arsenic.Based on analyzing the characteristics of rock, soil and groundwater combining with geological conditions and the evolvement process of Datong basin, this thesis revealed the way arsenic distributing，arsenic sources in groundwater and the reason of high-contained arsenic in groundwater; Then analyzed the influence of arsenic in rocks and soil for groundwater and summarized the adsorption behavior and migration rule of arsenate in soil combining with laboratory experiment of rocks and sediments. By this research, the following conclusions can be drawn:1. Arsenic in groundwater presents obvious regional distribution characteristics in Datong basin. Groundwater with high-contained arsenic distributes continuously in the area of Shuo canton-Shanyin county –Ying county along Huang river with low－lying landforms. Thereinto, groundwater in Shanyin county with the lowest hypsography have the highest arsenic concentration; For the depth, arsenic concentration is less than 0.01mg/L in groundwater at the sits above the depth of 20m belonging unconfined aquifer, and more than 0.01mg/L in the depth between 20m and 50m in the semi-confined aquifer. That means groundwater between 20m and 30m have the highest arsenic concentration.2. Arsenic in weathering rocks in Datong basin is the direct sources of arsenic in groundwater. And the evolvement process of this basin is the direct factor of arsenic enrichment in some areas, which results in the form of soils with high arsenic.3. Closed and low-lying landforms and geochemical environment in Datong basin are the direct reason of arsenic releasing from secondary medium with high-contained arsenic. The closed and low-lying landforms lead the runoff of surface water and groundwater becoming slower, and soils in a saturated state exist in reducing environment, leading to the reductive dissolution of Fe and Mn oxides, also leading to arsenic release into groundwater. At the same time, this reductive environment leads to arsenic adsorbed on mineral oxide, such as Al oxide releasing into groundwater because of arsenate being transformed into arsenite. Also, any other anion such as phosphate, bicarbonate can enhance the desorption of As because of competition for adsorption sites, which has certain impact on the form of high-contained arsenic groundwater.4. Limestone soaking experiment last 450days in the laboratory show that the quantity of arsenic from rock increase, and the quantity of arsenic releasing from rock is various in different pH conditions. At pH 6.0, the highest arsenic content with concentration of 0.01μg/g (0.8‰) is observed. At pH 7.0, similar to the pH of local rainfall, this value decreases to 0.006μg/g (0.48‰). For two different soil samples, the total arsenic content in fine sand (18.0mg/kg) is less than that in clay (26.8mg/kg). However, the quantity of arsenic releasing from fine sand(1.1μg/g) is more than that from clay(0.4μg/g), meaning a much more releasing ability than that of rock, which demonstrates that the main reason of high content arsenic in groundwater is arsenic released from secondary medium with high arsenic.5. Temperature has little impact on the adsorption process of arsenic on clay and fine sand. The adsorbing ability of fine sand to arsenic is high at 25℃. Ratio of solid to liquid influent little on arsenic adsorption to soil at low-arsenic-concentration, however, arsenic adsorption ability of soil to arsenic decreases with the ratio of reducing. Value of pH has little influence on arsenic adsorption on soil when pH is less than 9.0, and arsenic adsorption quantity decreases with at the range of pH>9.0, especially when pH is above 10.0. Freundlich equation is suitable to describe isotherm adsorption of As, and the adsorption coefficient of arsenic on clay is 1.34 times larger than that on fine sand. 6. Arsenic has certain ability of migration in soil. At the beginning of leaching experiment, arsenic concentration in leaching liquid is low because of arsenic sorption and chemical reaction in soil, and then increases with the leaching process and reach to the initial arsenic concentration. The capacity of arsenic migration in fine sand is much better than that in clay, and the adsorption quantity of arsenic in clay is higher than that in fine sand. Arsenic content at break-through point in the column experiments using sediments from different depths is different, demonstrating that accumulation phenomenon happens with migration of arsenic in soil.KEY WORDS: arsenic, groundwater, release, migration, Datong basin