Source and release mechanism of arsenic in aquifers of the Mekong Delta, Vietnam

In order to elucidate the arsenic source and its release mechanism into groundwater in the Mekong Delta, Vietnam, groundwater samples were collected from wells at different depths (20 to 440 m) and core samples (from 20 to 265 m depth) were analyzed. Based on the analytical results for groundwater and core samples, the As source in groundwater is considered to be pyrite (FeS2) in acid sulfate soil (ASS) under oxidizing conditions and hydrous ferric oxide (Fe(OH)3) under reducing conditions. Geochemical modeling demonstrated that As(III) is the dominant species and the presence of As-bearing sulfides, Fe-bearing sulfides and oxides phases may locally act as potential sinks for As. From variation between Fe and As concentrations in groundwater samples, the release mechanism of As is: dissolution of Fe(OH)3 containing As under reducing conditions and oxidative decomposition of FeS2 containing As under oxidizing conditions.