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

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 (Type II), and oxic-weak acid high As groundwater (Type III). Type I-1 is mainly distributed in river delta areas, while Type I-2 in arid-semiarid closed inland basins. Type II has been mainly found in arid-semiarid plain-basin areas, and Type III in bedrock areas containing substantial pyrite and sulfide minerals. Type I-1 groundwater occurs in reducing conditions, with neutral pH. The major mechanism for As mobilization is the reductive dissolution of Fe/Mn oxides. Type I-2 groundwater 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. Type II groundwater 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 Type III groundwater with weak acid pH, oxic conditions result in oxidation of pyrite and other sulfide minerals, subsequently releasing As into groundwater. Concerning Type I-2 groundwater, 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.