Groundwater Geochemistry, Microbiology, and Mineralogy in Two Arsenic-Bearing Holocene Alluvial Aquifers from the United States

摘要

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 (FeCO_3) and rhodochrosite (MnCO_3). Locally, conditions suitable for metabolism of sulfate-reducing bacteria may prevail, leading to precipitation of As-bearing biogenic pyrite enriched in ~(32)S. Precipitation of As-poor siderite and As-bearing pyrite will affect earlier dissolved As/Fe ratios of groundwaters, perhaps masking the original effect of concomitant As and Fe release by bacterial-mediated reductive dissolution of As-bearing hydrous ferric oxide. Comparison of the results of this study to data from Bangladesh and India suggest that these groundwater geochemical and microbiologic processes may be universal to Holocene alluvial aquifers in many places around the world. If so, then release of arsenic to these young river flood-plain sediments is an expected consequence of common biogeochemical processes.