Digging tips to avoid arsenic contamination

Children near a tube-well that has been disconnected due to its high arsenic content Copyright: WHO

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Three simple tips concerning where to dig wells in arsenic affected areas and pump water for irrigation could go a long way in reducing exposure to unsafe levels of the element in affected areas of South and South-East Asia, a review suggests.

These are: wells should be dug as deep as possible in arsenic affected areas; into deep orange sands rather than grey sands or shallow orange sands; and every effort should be made to prevent pumping water for irrigation from deeper, low-arsenic aquifers.

The conclusions of the review by a team of US geologists based on a decade of field research in Asia were published last week (28 May) in Science. Over 100 million rural Asians are exposed to unsafe arsenic levels through untreated and contaminated drinking water.

The affected areas are mostly flat plains drained by major Himalayan rivers such as the Ganges, Brahmaputra and Mekong. Eroding coal seams and rocks containing sulphide minerals are transported by the rivers. The arsenic binds with iron oxides in the sediments and is later released in the water through complex natural chemical reactions within rocks, aided by microbes inside.
High arsenic exposure increases risks of cancers of the liver, bladder and lung; and heart disease in adults and stunts mental growth in children.
Local communities install tube wells to extract deeper underground water to avoid risk of diarrhoeal infections from contaminated surface water; and more than half of them do not meet the World Health Organization guidelines of 10 micrograms (a microgram is one-billionth of a gram) of arsenic per litre.
"Low arsenic aquifers — especially deep ones and to some extent shallow — have and will continue to provide the most reliable sources of safe drinking water," one of the U.S. geologists, Alexander van Geen, told SciDev.Net .
The average well depth varies across the affected countries or regions. For example, it is 350 metres in Bangladesh and neighbouring West Bengal state in India; but only 100 metres in Cambodia, Nepal and Vietnam. The depth at which arsenic occurs in each well also varies.
Aquifers with grey sands have high levels of arsenic, while those with orange sands, indicating high iron content, have low arsenic, the review said.
Areas with intense groundwater pumping for irrigation, which alters natural water flow patterns, contain high arsenic. It is, therefore, preferable to pump water from low, high-arsenic levels rather than from deep, low-arsenic levels. The latter case makes room for arsenic to migrate downwards, mixing up hitherto low-contaminated waters with high-arsenic ones.
The review also suggested that governments and international organisations should restart periodic well-testing and monitoring of arsenic levels; and make better use of geological data and well test results to target low-arsenic zones for installing tube wells.
Van Geen said that a simple field kit is sufficient for testing but suggested that it would be useful to "explore ways of setting up a network of regulated but still commercial testers".