ABSTRACT
ABSTRACT: Groundwaters in Holocene alluvial aquifers in the USA contain elevated dissolved arsenic (As) have similar geochemical characteristics and microbiology. These include: (1) near-neutral pH and moderately reducing redox state; (2) significant dissolved iron and manganese; (3) presence of both iron-and sulfate-reducing bacteria; and (4) evidence that precipitation of authigenic carbonates and sulfides limits metal (loid) solubility. Our research indicates that anaerobic bacteria directly mediate dissolution of iron and manganese minerals in the alluvial aquifers leading to release of As and other trace elements, and also indirectly cause the precipitation of authigenic minerals such as siderite and rhodochrosite. Genetic sequencing indicated that the genus Geobacter are responsible for As release from Fe-oxyhydroxide. We propose a global-scale Geo-Bio-Hydro (GBH) arsenic cycle that directly invokes Pleistocene glaciers as important in physically weathering rocks setting the stage for Holocene chemical weathering that initiated a major release of As to the hydrosphere.
