South Asian arsenic update

Skin lesions from arsenic poisoning in Bangladesh

The first signs of chronic arsenic poisoning: skin keratoses. Image by waterdotorg via Flickr

That groundwater in West Bengal, India was polluted with arsenic to such levels that symptoms of poisoning had become endemic was reported by Depankar Chakraborti in 1983, leading to his being branded a ‘panic monger’ by the Indian authorities. The news broke internationally in 1993 as the now infamous tragedy in neighbouring Bangladesh emerged. Means of mitigating the effects – lesions or keratoses and skin discoloration, and later increases in incidence of several forms of cancer – and ideas of how the pollution had occurred had to await proper geochemical analyses of well waters and logging of the mainly alluvial sediments from which water was being withdrawn; another 8 years went by. Reports of arsenicosis began to emerge from other areas of alluvial sediments in SE Asia, revealing by far the worst mass poisoning in history and the likelihood that the lives of millions would be blighted by what Bangladeshis dubbed ‘the Black  Rain’ from the resemblance of the characteristic skin lesions to drops of black water.

Thanks principally to the work of water engineer Peter Ravenscroft with other geochemists, the source of arsenic in groundwater was narrowed down to the effect of reducing conditions in grey, carbonaceous sandstones and peats on the mineral goethite, an iron oxy-hydroxide that forms the main colorant in oxidised sediments and whose loose structure normally encourages the mopping-up by surface adsorption of a wide spectrum of dissolved ions, including those of arsenic. Goethite readily breaks down under reducing conditions, and when that happens all the adsorbed material is released into solution. The upper parts of the alluvial and deltaic sediments in the lower reaches of the Ganges and Brahmaputra rivers contain abundant organic remains picked up when vegetation burgeoned during the Holocene, which mixed with goethite-coated sand grains derived from erosion in the Himalayan stretches of the rivers. Purely natural sedimentary and hydrogeological processes created the dreadful plight of villagers. The terrible irony was that before the 1980s there were no signs of arsenicosis, yet mortality, especially of under-fives, was very high due to water-borne pathogens in surface water supplies. Indian and Bangladeshi authorities and UN agencies waged a campaign to sink shallow wells for drinking water rather than relying on river and pond supplies. At first rural people resisted the change since they regarded water from wells as the ‘Devil’s water’, but as infant mortality began to fall, the resistance turned to rapid construction nationwide of wells, both public and private. A few years later came the ‘Black Rain’.

In the attempts to mitigate the arsenicosis plague, filters containing adsorptive materials, including goethite, were installed on pumps. However, the geochemists showed that in the deeper wells there were consistently low concentrations of arsenic in sediments that were brown-coloured due to prevailing oxidising conditions and the presence of goethite. Although arsenic was present in the sediments it was safely locked in the goethite coatings of sand grains. Steadily major public supplies were transferred to deep, high-yield wells. Alluvial and deltaic deposits are generally highly permeable, so it was feared that as the deeper wells were pumped arsenic-rich water from the reduced shallow sediments would replace the safe groundwater. Thankfully, it seems that is not likely to be a problem (Radloff, K.A. and 12 others 2011. Arsenic migration to deep groundwater in Bangladesh influenced by adsorption and water demand. Nature Geoscience, v. 4, p. 793-798). The study injected As-bearing groundwater into a deep aquifer and monitored its arsenic concentration over time, once in place. Within a day, the concentration of dissolved arsenic fell by 70% and by 5 days had fallen below recommended maximum levels for drinking water; a dramatic demonstration of the clean-up power of even minute films of goethite in sediments, for that seems the only explanation for the fall. The US-Bangladeshi team verified this by testing samples of the deeper sediments from drill cuttings. They mixed highly contaminated groundwater with the cuttings, to find that arsenic sorption over  about a week was extremely high (~40mg kg-1).

Rather than just publishing their reassuring findings, the team input them to hydrogeological models of the Bengal Basin, varying hypothetical pumping rates to assess the changes in deep-groundwater chemistry over time due to downward migration of the highly polluted near-surface waters. Sure enough, the As-rich waters would end up in the deep aquifer eventually to overwhelm the sorptive capacity of its goethite content; arsenic would once again enter well supplies. However, if deep extraction was limited to drinking water by limiting pumping for irrigation to intermediate depths, safe limits could be sustained theoretically for a thousand years or more, except in some areas especially prone downward intrusion of polluted shallow groundwater. (Use of highly contaminated shallow groundwater for irrigation would simply transfer the problem to crops.) Clearly, monitoring is obligatory, but one hopes this important study does resolve the horrifying plight faced by so many people in catchments fed by Himalayan waters.

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