High levels of arsenic in the groundwater in Bangladesh and West Bengal, India, are strongly correlated with levels of iron, methane, and ammonia, report researchers from the University of Rochester and the International Atomic Energy Agency in Austria. The findings support the hypothesis that arsenic is being released from the sediments into the groundwater by microbial breakdown of iron oxyhydroxides.

Carolyn Dowling and colleagues analyzed 68 groundwater samples taken from wells throughout the Bengal Basin. They sampled wells from multiple depths (9–335 m) and examined a sediment core taken from a location where the groundwater had high arsenic levels.

For the most part, the researchers found that the groundwater samples contained high levels of calcium carbonate, dissolved iron, methane, and ammonia, some sodium chloride, undetectable dissolved oxygen and sulfate, and low levels of all trace metals except strontium, barium, iron, manganese, and arsenic. Importantly, they found high levels of arsenic only in the shallow groundwater wells (<60 m), which suggests that drilling deeper wells could help arsenic mitigation efforts in the region. Recent activities, such as the excessive use of groundwater for irrigation and the use of phosphorus-containing fertilizers, were ruled out as the cause of the arsenic problem because groundwater residence times in some areas were greater than 60 years. The researchers observed a strong arsenic–methane and arsenic–ammonia correlation, indicating that methanogens, as well as other bacteria, are involved in the release of the arsenic.

On the basis of their analyses, the researchers believe the ultimate source of the arsenic is the weathering of micas, which are intrinsically high in arsenic. They suggest that the arsenic is being released from the micas into the water, where it is then scavenged by iron oxyhydroxides. Eventually, through bacterially mediated sediment desorption reactions, the arsenic is then converted from As(V) to the more toxic As(III) and rereleased into the groundwater. (Water Resources Res. 2002, 38, 10.1029/2001WR000968)