In the 1980s grim news began to emerge from the Indian State of West Bengal and a decade later from neighbouring Bangladesh. Villagers from the low-lying delta plains of the Ganges and Brahmaputra river systems at the head of the Bay of Bengal began to present at clinics with disfiguring skin lesions or keratoses on hands and feet, loss of feeling in fingers and toes and dark skin patches on their torsos. The latter were colloquially known as ‘black rain’. The victims were often stigmatised, as their neighbours believed they were suffering from leprosy. These symptoms were followed a few years later by increased incidences of lung, liver, kidney and bladder cancers. The first medical practitioner to recognise these typical signs of chronic arsenic poisoning in 1983, Dr Depankar Chakraborti of Kolkata, was branded as a ‘panic monger’ by local authorities. His warnings, backed by evidence published by the World Health Organisation (WHO) in 1988 that there was a connection with high arsenic levels in West Bengal drinking water supplies from new tubewells, went largely unheeded for a decade. Tragically, as it turned out, thousands of tubewells had been sunk in the Bengali delta plains from the 1970s onwards, aimed at reducing the risk of disease from pathogens in the previously used surface water from ponds and streams. After a conference on the perceived problem, organized in Kolkata by Dr Chakraborti in 1995, the WHO declared the situation in Bangladesh to be a ‘Major Public Health Issue’, and the world’s press took up the story. Clearly, millions of Bengali villagers were at risk or were already suffering from chronic arsenic poisoning. By the late 1990s thousands of samples of tubewell waters from the delta plains had been analysed, many of which revealed arsenic levels far above the 10 μg l-1 safe threshold. In 2002, 400 Bangladeshi victims sued the British Geological Survey (BGS) for negligence. The BGS had analysed 150 water samples from the Bangladesh delta plains in 1992 and had not reported any risks, but arsenic was not among the elements being analysed. The civil action eventually failed.
Almost two decades after the arsenic scandal on the eastern side of the subcontinent well-water analyses showing high arsenic values have been published from the Indus plains of Pakistan (Podorski, J.E. et al. 2017. Extensive arsenic contamination in high-pH unconfined aquifers in the Indus Valley. Science Advances, v. 3,; doi:10.1126/wsciadv.1700935). The Indus catchment having a similar Himalayan source and being at a similar latitude it has long been considered to be at potential risk from arsenic derived from its thick alluvial sediments. The Swiss-Pakistani-Chinese team have produced geochemical data from 1200 tubewells throughout the catchment within Pakistan. A swath from Lahore to Karachi, with the country’s greatest population density, is at high risk of water with arsenic concentrations above the WHO guideline safe concentration, suggesting some 50 to 60 million people being subject to its hazard.
Although the geological setting is similar to that in the Bengal plains, a different natural chemical process causes the high concentrations ultimately from the iron hydroxide veneer on sediment grains which selectively absorbs several trace elements, including arsenic, from river water. In Bangladesh arsenic is released from sediments as a result of highly reducing conditions due to organic matter buried in some layers of alluvium, by a process known as reductive dissolution – when insoluble ferric iron (Fe3+) hydroxide (goethite) is exposed to a ready supply of electrons the iron is reduced to the soluble ferrous (Fe2+) form and the mineral breaks down to release its absorbed trace elements. Most of the alluvium in the Indus plain contains little organic carbon, so another mechanism is implicated. The high arsenic levels correlate with high pH in the groundwater and therefore seem most likely to be released from goethite grain coatings by alkaline water. That, in turn, is often a product of high evaporation and salinisation from the massive irrigation using groundwater in semi-arid southern Pakistan. The alkaline water then returns to the underlying groundwater in the highly permeable Indus alluvium; i.e. it is a consequence of irrigated agriculture rather than of a natural geochemical process as in more humid Bengal.
Whereas a remedy in Bangladesh and West Bengal has been to sink new tubewells into oxidising alluvial strata (red coloured rather than the reducing grey sediments) that yield water with safe arsenic levels, the risky areas in Pakistan may need expensive use of absorbent filters on a large scale to remove the hazard. Because irrigation using groundwater is on such a large scale on the Indus plain there is also a definite risk of ingesting arsenic from crops produced there, principally rice but also unwashed leaf vegetables