ABSTRACT
High arsenic (As) groundwater has been widely found in arid-semiarid basins, which poses negative effects on human health. Although As distribution in groundwater from flat plains of the basin is quite patchy, there are increasing trends along flow paths from alluvial fan to the plains near the mountains. Both hydrogeolocal and biogeochemical conditions control distributions of dissolved As in groundwater. In oxic conditions of alluvial fans, As is preferentially adsorbed on aquifer sediments with high contents of Fe(III) oxides. Along the flow paths, both dissolved organic matter (DOM) and sedimentary organic matter (SOM) increase, which consume dissolved oxygen (DO) and lead to reducing conditions. Labile SOM is believed to be the primary electron donor, which triggers reductive dissolution of Fe(III) oxides and/or sulfate reduction depending on availability of sulfate. In addition to reduction dissolution, sulfur disproportionation-induced Fe(III) reduction causes As mobilization.
