GEOLOGIC, BIOLOGIC, AND HYDROLOGIC CONTROLS ON NATURAL ARSENIC CONTAMINATION OF GROUNDWATER IN HOLOCENE RIVER FLOODPLAINS

Anthropogenic releases and weathering of As-bearing sulfide minerals are two common ways arsenic impacts nearby groundwater. However, it is natural arsenic contamination of groundwater in a distal setting that currently poses the greatest threat to human health, particularly in some developing nations where surface water sources have been contaminated. This groundwater contains arsenic released from minerals transported large distances by the hydrologic system and deposited in young (Holocene) river floodplains. If reactive organic matter is also deposited with these As-bearing minerals, then anaerobic (reducing) groundwater conditions are established destabilizing minerals such as Fe-Mn oxyhydroxides that may have sorbed As and other trace elements from surface waters. In particular, it is heterotrophic dissimilatory Fe-, Mn-, and sulfate-reducing bacteria that control mineral dissolution and precipitation reactions involving metals, and metalloids such as As. For example, Fe-and Mn-reducing bacteria mediate the thermodynamically spontaneous reductive dissolution of Fe-Mn oxyhydroxides, releasing Fe, Mn, and trace elements such as As to groundwater. This process can lead to As concentrations 1-2 orders of magnitude higher than the US and WHO standard of 10 ppb total arsenic in groundwater. Further, the low-relief river floodplains, and resulting low hydraulic gradients, keep alluvial aquifers from being “flushed” of their arsenic. Resulting As-rich waters typically contain elevated Fe and Mn, and bicarbonate is the major anion, the latter of which is also a metabolic product of anaerobic bacterial metabolism. Thus siderite (FeCO₃) and rhodochrosite (MnCO₃) are possible authigenic minerals in this setting. Similarly, secondary Fe-sulfide phases can also form due to the action of sulfate-reducing bacteria, locally removing some As that coprecipitates in he FeS phases. However, these terrestrial waters are limited in their original sulfate concentrations, and thus bacterial sulfate reduction only proceeds to a limited extent, leaving water still rich in As and devoid of sulfate.