Arsenic Pollution

Arsenic Sources

Arsenic is present as a natural constituent of the Earth's crust and can be released through volcanic eruptions, weathering, or some industrial processes. As such, it can be found in bedrock and soil and, to some extent, in water and air. Different mechanisms of action have been proposed to explain how arsenic can migrate from bedrock and become bioavailable through drinking water. These explanations rely on solid-solution interactions in which arsenic bonded to minerals in the soil, sediments, and rocks interacts with the flow of groundwater in the presence of oxygen or other reactive agents and optimal pH conditions.

In Bangladesh, arsenic-contaminated bedrock was derived from the natural, historical shifting and sifting of sediment from the Himalaya Mountains into the Delta region. Sediment traveled down the channels of the major regional rivers and was unable to be swept away efficiently by relatively weak ocean currents in the Bay of Bengal and thus accumulated. However, prior to drilling, this arsenic had remained separated from aquifers by a layer of clay or silt that acted as a cap, preventing water and oxygen from interacting with the bedrock. The drilling of tube wells disrupted this cap and, in combination with other human-industrial activities, prompted the widespread contamination witnessed today.

One theory regarding the mechanism of pollution involves pyrite oxidation, in which drilling or irrigation extraction forces air to interact with previously undisrupted land. The oxygen within air causes a decomposition of the bonds that keep arsenic immobile, and thus it is released into the water. Another theory involves oxyhydroxide reduction. This latter theory supports the idea that arsenic has a high affinity to other particles that are present in the soils and sediments of the floodplains of Bangladesh, such as iron or manganese oxyhydroxides.

While attached to these agents, arsenic is contained and unable to contaminate ground-water, but when conditions catalyze the dissociation of arsenic from this interaction—through drilling, mining, or excessive flooding—then arsenic is released from bondage and comes to contaminate the groundwater. In its organic form, arsenic is essentially nontoxic because it is attached to carbon and hydrogen and is thus contained; however, when it reacts with oxygen, chlorine, or sulfur, it enters into an inorganic form and can have dire health consequences. In the past, the inorganic form of arsenic has been used to treat wood and, additionally, to combat parasitic agents in veterinary settings.

...