Arsenic hazard on a global scale

I have been following the harrowing story of how arsenic gets into domestic water supplies for 20 years (see: Earth-logs Geohazards for 2002; 2003; 2004; 2005; 2006; 2008; 2009; 2011; 2013; 2017). In my opinion, it is the greatest natural hazard in terms of the numbers at risk of poisoning. In 2006 I wrote about the emergence in Bangladesh of arsenic poisoning on a huge scale during the mid 1990s for a now defunct Open University course. If people depend for drinking water on groundwater from tube wells driven into alluvium they would not know of the risk, unless the water is rigorously analysed for levels of As greater than 10 micrograms per litre (μg l-1), the WHO recommended maximum. The sad fact is that the affected population were advised to switch from surface water supplies, which carry a high risk of biological infection, to well water. That is because during downward percolation from the surface oxidation destroys bacteria and viruses as well as parasites. Opportunities provided by a massive UN-funded drilling programme and local well digging made the choice seemingly obvious. Most people came to prefer well water as gastro-intestinal infections and child mortality fell rapidly.

Arsenic adds no taste, which is why it was once the ‘poison of choice’. How it gets into groundwater is difficult to judge, unless wells are downflow of areas riddled with metal mines. Years of research uncovered an unsuspected mechanism. The most common colorant of mineral grains, and thus sedimentary rocks, is brownish iron hydroxide (goethite), and that is able to adsorb a range of dissolved elements, including arsenic. One would think, therefore, that groundwater should be made safe by such a natural ‘filtering’ process: indeed goethite can be used in decontamination. The problem is that iron hydroxide, which contains Fe-3, is only stable in water with a high capacity for oxidation. Under reducing conditions it breaks down to soluble Fe-2 and water, thereby releasing to solution any other element that it has adsorbed. In alluvium, beds containing organic matter are prone to this ‘reductive dissolution’ of goethite. If weathering upstream has released even seemingly insignificant amounts of arsenic during the build up of alluvium, there is a potential life-threatening problem as arsenic builds up in the goethite coating of sedimentary grains to become ‘locked in’, with the potential to be released in high concentrations if subsurface chemical conditions change. The colour of the alluvial sediments penetrated by wells is a clue. If they are reddish brown, groundwater is safe, if they are greyish and goethite-free then, ‘beware’. But it is rare to examine ‘cuttings’ from a drill site aimed at groundwater, unlike those aimed at ores or oil

Since the tragedy of Bangladesh, which resulted after 5 years or so in obvious signs of arsenicosis – dark wart-line keratoses on hands and feet or black blotches on facial and torso skin – several alluvial basins in large river systems have had their well water tested. But by no means all such basins have been screened in this way, and there are many less-obvious signs of arsenic poisoning. After long exposure to the lower range of dangerous arsenic levels a variety of cancers develop in known areas of arsenic risk. There are also high levels of endemic respiratory problems, cardiovascular disease, reduced intellectual development in children and even diabetes. Geochemical monitoring of all populated and farmed river systems is a huge task that is far beyond the resources of many countries through which they run. One approach to ‘screening’ for hazard or safety is to use geological, hydrological, soil, climate and topographic data. Those from known arsenic-prone basins and those where its levels are shown to be consistently below the 10 μg l-1 danger threshold help to develop a predictive model (Podgorski, J. & Berg, M. 2020. Global threat of arsenic in groundwater. Science, v. 368, p. 845-850; DOI: 10.1126/science.aba1510).

Modelled global probability of arsenic concentration in groundwater exceeding 10 μg l-1. Click to display a larger map in a separate browser tab. (credit: Podgorski & Berg; Fig 2A, with enhanced colour)

Rather than trying to model the full range of arsenic concentrations, Joel Podgorski and Michael Berg of the Swiss Federal Institute of Aquatic Science and Technology focussed on assessing probabilities that arsenic in well water exceeds the WHO recommended maximum safe level of 10 μg l-1. Their global map highlights areas of concern for environmental health. Thankfully, huge (blue) areas are suggested to present low risk, the pale, yellow, orange and red patches signifying areas of increasing concern. No populated continent is hazard-free. What is very clear is that Asia presents the greatest worries. Most of the Asian ‘hot zones’ are spatially close to large mountain ranges and plateaus. In the case of the Indus and Ganges-Brahmaputra plains the sources for excessive arsenic in groundwater implicated by previous geochemical investigations lie in the Himalaya. The factor common to all major hot spots seems to be rapid transport of huge amounts of sediment released by weathering from areas of high topographic relief, rather than local large-scale mining operations. There are hazardous areas related to historic and active mining, such as the Andes of Bolivia, Peru and Chile and the western USA, but they are tiny by comparison with the dominance of natural arsenic mobilisation.

Despite the WHO recommended maximum of 10 μg l-1 of arsenic, many countries base their policy on levels that are five times higher, largely because of the difficulty of analysing for the lower concentration without expensive analytical facilities. Field analyses are often done using simple semi-quantitative tests based on paper impregnated with reagents that show a colour range for different concentrations, which are unreliable for those lower than 100 μg l-1. Thankfully, despite the many risky areas, most of them have population densities less than 1 per km2.

If you are interested in the geological details of the arsenic problems of Bangladesh, the course text that I produced for the Open University (Drury, S. 2006. Water and well-being: arsenic in Bangladesh. The Open University: Milton Keynes, UK. ISBN 0-7492-1435-X), the course itself (S250 Science in Context) was withdrawn some years ago.  It may be possible to arrange a PDF for private use.

See also: Zheng, Y. 2020. Global solutions to a silent poison. Science, v. 368, p. 818-819; DOI: 10.1126/science.abb9746

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).

Water well in Bangladesh. From http://www.flickr.com/photos/waterdotorg/3696304044

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.