大同盆地地下水砷异常及其成因研究

The genesis of high arsenic groundwater:a case study in Datong basin

大同盆地是中国典型的原生高砷地下水分布区。笔者对大同盆地高砷地下水的分布特征、水化学演化过程、砷的来源以及控制高砷地下水形成的地球化学过程等近期研究成果进行了总结。盆地周边石炭—二叠纪煤系地层是盆地高砷环境的主要原生物源,含水层系统中铁磁性矿物为砷的主要载体,盆地内富含有机质的湖相沉积物是次生富砷介质。在盆地中心,地下水径流受阻,蒸发成为主要的排泄方式,浓缩作用使得地下水中TDS含量增大。在富含有机质的地层中,有机质在细菌或微生物作用下不断发生分解,使得地下水环境呈还原性。在高pH、低Eh条件下,由于铁锰氧化物或氢氧化物等水合物或粘土矿物对砷的吸附性降低,一部分被吸附的砷从这些矿物表面解吸;同时部分铁锰氧化物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。在还原条件下,水中的SO24-和有机碳可被还原成H2S和CH4等低价态化合物,尽管生成的硫化物达到一定浓度时可与水中的亚铁离子和砷反应生成FeAsS沉淀,降低了地下水中砷含量,但由于地下水中Fe和硫酸盐含量有限,Fe普遍含量较低,硫酸盐耗尽后,CH4生成细菌就会成为主导力量,砷就会继续在地下水中积聚。此外,由于pH的升高,还可引起其他不同的酸根离子的解吸,如磷酸根、钒酸根、铀酰和钼酸根等也趋向于在溶液中积累,这些被吸附的阴离子以竞争吸附方式,进一步促进砷的解吸。

Geogenic high arsenic groundwater occurs in Datong basin, northern China. This paper reports the results of recent studies of high arsenic groundwater in Datong, which include spatial distribution of high arsenic groundwater, hydrochemical evolution, source of arsenic, and geochemical processes controlling the mobilization of arsenic. Carboniferous-Permian coal-bearing clastic rocks around the basin may constitute the primary sources for arsenic in groundwater. Iron-bearing magnetic minerals are the main carrier of arsenic in the aquifer system and organic matter -enriched lacustrine sediments is the secondary source for arsenic in groundwater. In the central part of the basin, groundwater permeation is very slow and evaporation is the major way of groundwater discharge. Intensive evaporation results in the increase of TDS content. Decomposition or oxidation of organic carbon in aquifer sediments is catalyzed by bacteria and/or microorganisms, resulting in prevalence of anoxic conditions. Under high pH and low Eh conditions, due to the adsorption of arsenic by iron or/and manganese oxyhydroxides and the decrease of clay minerals, part of arsenic absorbed on the surface of these minerals tends to be released from sediments during this process. On the other hand, iron and manganese oxyhydroxides with strong adsorbing capacity for arsenic are reduced to low valence states which are soluble under low Eh conditions, so arsenic adsorbed on their surface is released to groundwater. Under anoxic conditions, dissolved SO 24- and organic carbon can be reduced to produce H2S and CH4. Although dissolved arsenic can be removed from groundwater via the reaction with pyrite due to FeAsS precipitation, the low concentration of dissolved iron and sulfate in groundwater may limit this reaction. When sulfate is used up, methanogens will prevail the biogeochemical processes and further promote the enrichment of arsenic in groundwater. In addition, pH increase induces desorption of a wide variety of oxyanions, such as phosphate, vanadate, uranyl and molybdate, which tend to accumulate in the groundwater. Competitive adsorption of these oxyanions with arsenic can result in the release of arsenic from sediments into groundwater