How a lack of oxygen is releasing arsenic into wells around the Brahmaputra River Basin
Research Matters Staff Writer(s) Kharagpur 4 Aug 2026 Researchers have found why groundwater across one of the world’s most populated river basins is frequently contaminated with toxic elements. A new study of the Brahmaputra River Basin in Northeast India has identified how the physical arrangement of sand and clay deep underground dictates whether a village’s well water is life-sustaining or…
A new study conducted by researchers from the Indian Institute of Technology Kharagpur, CSIR-National Institute of Oceanography, and Bhabha Atomic Research Centre has revealed how a lack of oxygen in groundwater across the Brahmaputra River Basin in Northeast India is releasing toxic elements such as arsenic and nickel into wells.
The study explains that the distribution of these contaminants is determined by the underlying geological formations, specifically the presence of thick clay layers in certain areas. When these clay layers act as a biogeochemical sieve, they prevent new oxygen from entering the groundwater, creating reducing environments where specialized bacteria strip the oxygen from minerals and release arsenic and nickel directly into the water.
The research team analyzed the sediment composition along a 90-kilometer stretch of the river, sampling 114 wells and drilling ten boreholes. They found that wells on the northern bank, dominated by sandy alluvial fans from the Himalayas, generally have lower contamination levels compared to wells on the southern bank, which is dominated by thick, ancient clay sequences.
In some cases, arsenic concentrations in the southern clay-rich wells reached 400 micrograms per liter, 40 times the safe limit set by the World Health Organization. The study suggests that the natural geological layout of the Brahmaputra basin, rather than modern water pumping practices, is the primary driver of groundwater contamination.
By identifying these localized variations in the distribution of clay layers, the researchers have developed a well-to-well prediction model that could help local authorities make informed decisions about where to drill for safe drinking water. The findings highlight the need for more detailed sedimentological mapping to improve the accuracy of predictive models.
With over 70 percent of India's population relying on groundwater for their daily needs, this research provides critical evidence for aquifer mapping, allowing authorities to prioritize safer, sandier depths for new wells and avoid the hazardous reducing zones created by clay-heavy geology.
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