Groundwater geochemistry, microbiology, and mineralogy in two arsenic-bearing Holocene alluvial aquifers from the United States

Groundwaters in two Holocene alluvial aquifers containing elevated dissolved arsenic in the USA have similar geochemical characteristics and microbiology. These include: 1) near-neutral pH and moderately reducing redox state where reactive organic matter is present; 2) significant dissolved iron and manganese; 3) presence of both iron- and sulfate-reducing bacteria of the genera Geobacter and Desulfovibrio, respectively; and 4) evidence that precipitation of authigenic minerals (e.g., carbonates and sulfides) limits concentrations of dissolved metal(loid)s. This study indicates that heterotrophic anaerobic bacteria directly mediate both the dissolution of detrital iron and manganese minerals in the aquifers, which leads to release of As and other trace elements, and also indirectly leads to supersaturation with respect to authigenic minerals such as siderite (FeCO3) and rhodochrosite (MnCO3). Locally, conditions suitable for metabolism of sulfate-reducing bacteria may prevail, leading to precipitation of As-bearing biogenic pyrite enriched in S-32.