Hydrochemistry and evolution of high arsenic groundwater in the plain along the Changjiang River (Anqing Reach)

Anqing, an important node in the Changjiang River Economic Zone, has been found in recent years to have high arsenic concentrations in groundwater, but nothing is known about the reasons of such high-arsenic levels．Chemical evolution of groundwater along typical groundwater flow path was analyzed here to reveal the cause of high arsenic．Arsenic concentration in un-confined water was found to be significantly higher than in confined water in the transition zone of alluvial-flood and alluvial-lake．Content of available arsenic in the medium in high-arsenic area was found to be significantly higher than in the low-arsenic area, as iron and manganese oxide or in carbonate adsorption state．From a typical groundwater flow path it was obvious that from the recharge area to the discharge area, groundwater gradually evolved into a weakly alkaline reducing environment．Release of arsenic adsorbed on surface of aqueous medium or crystal lattice due to reductive dissolution of iron oxides is confirmed to be a major reason for high arsenic concentrations in groundwater．