This article published in Current Science discusses the nature of arsenic pollutants in the groundwater of Bengal basin. The problem of arsenic contamination in groundwater in large areas of West Bengal and Bangladesh has been receiving wide attention because groundwater is the major source of drinking water in this part of the world. This vast area appears to be contaminated and also locally polluted and a large number of people in this densely populated region are likely to be affected by the menace.
The article informs that arsenic pollution of groundwater in this part of Bengal basin, is a geological problem. The shallow subsurface sediments are rich in arsenic and the shallow groundwater shows arsenic pollution at several small patches. There are safe zones of groundwater in the shallow aquifer. Such safe zones always have a thick clay cap, which should be used as deciding criterion for locating future shallow tube wells in these areas.
There are reports of new incidence of high arsenic in groundwater from adjacent areas of this basin. This is not because the arsenic contamination is spreading rapidly, but because of emergence of new data, increased awareness and more wells being tested.
Arsenic was present in the sediments of geological past and it continues to be present. The arsenic pollutants are identified to be the iron-rich clastic grains and the authigenic iron-rich carbonate (siderite). Release of arsenic from these pollutants into groundwater is incidental and possibly related to microbial degradation of organic matter. Since the microbes activate more at the air–water interface, the sediments having higher porosity in the top of the sedimentary column, will be the ideal sites for their proliferation. Such activities will eventually release arsenic faster than the sedimentary column starting with an impervious layer. This is the reason why the groundwater below a thick clay cap is safe.
The article informs that excessive withdrawal of groundwater results in gradual lowering down of the water table, which exposes more sediment surfaces for microbial reduction. This, in effect, results in release of more arsenic into the groundwater and accentuates the problem of arsenic pollution.
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