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

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