Scientists have discovered widespread uranium pollution in drinking water supplies in India.
The study, led by researchers from Duke University in the US, believes the main source of the contamination found in wells and aquifers is natural, although human factors are believed to be making it worse.
Studies have shown that uranium-contaminated drinking water can cause severe health problems such as chronic kidney disease.
"Nearly a third of all water wells we tested in one state, Rajasthan, contained uranium levels that exceed the World Health Organization and US Environmental Protection Agency's safe drinking water standards," said Professor Avner Vengosh.
Professor Vengosh, who researches geochemistry and water quality, said: "By analysing previous water quality studies, we also identified aquifers contaminated with similarly high levels of uranium in 26 other districts in northwestern India and nine districts in southern or southeastern India."
"The results of this study strongly suggest there is a need to revise current water-quality monitoring programs in India and re-evaluate human health risks in areas of high uranium prevalence," said Professor Vengosh said.
"Developing effective remediation technologies and preventive management practices should also be a priority."
According to the WHO, water is safe to drink if there are less than 30 micrograms – one millionth of a gram – of uranium per litre.
Despite this, the Bureau of Indian Standards' drinking water specificatons does not list or measure for uranium contamination.
Professor Vengosh and his colleagues sampled water from 324 wells in Rajasthan and Gujarat in India.
They measured the isotope ratios in a number of the samples and compared them to data from 68 previous studies of groundwater geochemistry in 16 Indian states.
"Our analysis showed that the occurrence of uranium in these groundwater sources depends on several factors," said Rachel Coyte, a PhD student in Professor Vengosh's lab, and lead author of the study.
Part of that was natural and included how much uranium occurred in the rocks of the aquifers, wich environmental factors can make increasingly soluble and likely to enter the water chain.
However human activities, especially groundwater being over-used for agricultural irrigation, may be contributing to the problem, said Ms Coyte.
India's acquifers are mostly composed of clay, silt and gravel which have been carried down from the Himalayas by streams and uranium-rich granitic rocks.
When these acquifers are over-pumped, their water levels decline causing the minerals to oxidise and effectively enrich the uranium in the shallow groundwater that's left behind.
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"One of the takeaways of this study is that human activities can make a bad situation worse, but we could also make it better," Professor Vengosh said.
"Including a uranium standard in the Bureau of Indian Standards' drinking water specification based on uranium's kidney-harming effects, establishing monitoring systems to identify at-risk areas, and exploring new ways to prevent or treat uranium contamination will help ensure access to safe drinking water for tens of millions in India," he said.
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