原生高砷地下水的类型、化学特征及成因

Types,chemical characteristics and genesis of geogenic high-arsenic groundwater in the world

由于分布广、危害大,原生高砷地下水严重威胁全球内数亿居民的身体健康。研究原生高砷地下水的分布、化学特征及成因有助于进一步理解地下水中砷的迁移转化规律,并确保高砷区地下水的可持续利用。在查阅大量文献资料的基础上,结合近10年的高砷地下水研究经验,把原生高砷地下水分为还原性-中性高砷地下水(Ⅰ-1型)和还原性-弱碱性高砷地下水(Ⅰ-2型)、氧化性-弱碱性高砷地下水(Ⅱ型)和氧化性-弱酸性高砷地下水(Ⅲ型)。Ⅰ-1型高砷地下水主要分布于河流三角洲地区,Ⅰ-2型分布于干旱-半干旱封闭内陆盆地,Ⅱ型主要分布于干旱-半干旱平原-盆地,Ⅲ型主要分布于富含黄铁矿或硫化物矿物的基岩地区。Ⅰ-1型高砷地下水处于还原环境,pH呈中性,Fe/Mn氧化物矿物的还原性溶解是造成As富集的主要原因。Ⅰ-2型高砷地下水处于还原环境,pH呈弱碱性,除了Fe/Mn氧化物矿物的还原性溶解外,As的解吸附是含水层中砷释放的重要原因。Ⅱ型高砷地下水处于氧化-弱氧化环境,pH呈弱碱性,As的解吸附是含水层中砷释放的主要原因。Ⅲ型高砷地下水处于氧化环境,pH呈弱酸性,黄铁矿及其他硫化物矿物的氧化溶解导致了含水层中砷的释放。对于Ⅰ-2型高砷地下水,需要深入研究Fe/Mn氧化物矿物的还原性溶解以及As的解吸附对地下水砷富集的相对贡献量。

Because of its wide distribution and high toxicity,the geogenic high-arsenic groundwater has posed negative impacts on thousands of million residents.Investigation on distribution,chemical characteristics and genesis of high-arsenic groundwater would help in better understanding arsenic transfer and transportation in groundwater systems and ensuring sustainable usage of groundwater resources in As-affected areas.On the basis of recently-published literatures and ten-year experiences on high-As groundwater study,we divide the geogenic high-As groundwater into four types：reducing-neutral pH high As groundwater（Type Ⅰ-1）,reducing-weak alkaline high As groundwater（Type Ⅰ-2）,oxic-weak alkaline high As groundwater（TypeⅡ）,and oxic-weak acid high As groundwater（TypeⅢ）.TypeⅠ-1is mainly distributed in river delta areas,while TypeⅠ-2in arid-semiarid closed inland basins.TypeⅡ has been mainly found in arid-semiarid plainbasin areas,and Type Ⅲ in bedrock areas containing substantial pyrite and sulfide minerals.Type Ⅰ-1groundwater occurs in reducing conditions,with neutral pH.The major mechanism for As mobilization is the reductive dissolution of Fe/Mn oxides.TypeⅠ-2groundwater has been observed in reducing conditions with weak alkaline pH,where both the reductive dissolution of Fe/Mn oxides and desorption of As lead to As release from sediments.TypeⅡgroundwater is present in oxic-weak oxic conditions with weak alkaline pH.The desorption of arsenic under high pH conditions has been believed to cause As enrichment in groundwaters.In TypeⅢ groundwater with weak acid pH,oxic conditions result in oxidation of pyrite and other sulfide minerals,subsequently releasing As into groundwater.Concerning TypeⅠ-2groundwater,It is important to deeply study the relative contribution of the reductive dissolution of Fe/Mn oxides and of the desorption of As to the As enrichment in groundwaters.