Beyond the Geological Origin of Sediment Arsenic in Groundwater Systems:Arsenic Redux by Redox

Geogenic arsenic(As)in sediments of numerous Holocene aqui-fers has been recognized as the culprit for elevated As(＞10 μg/L)levels detected in groundwater of many countries,threatening the health of an estimated 94 to 220 million people,mostly in Asian countries downgradient of the Tibetan Plateau[1,2].Much progress has been made on elucidating the coupled hydrogeologi-cal and biogeochemical processes involved in mobilizing sediment As to groundwater[3],yet little is known regarding the exact geo-logical origin of such sediment As.