Monsoon flooding flushes arsenic from rice paddies

Sojeeb Chowdhury from a local village helps to sample floodwater for arsenic

Linda Roberts

[CHENNAI] Monsoon flooding removes arsenic from rice-paddy soils in Bangladesh, scientists have found.

The groundwater used to irrigate rice paddies often contains high levels of arsenic, and researchers have previously found that arsenic levels increase with irrigation and decrease with seasonal flooding.

Research published this month (13 December) in Nature Geoscience shows that flooding removes up to two-thirds of the arsenic from soil.

Bangladeshi and Swiss researchers assessed arsenic levels in rice paddies in Bangladesh's Munshiganj district, which is subject to extreme flooding between June and October. They found that flooding removes 13–62 per cent of the arsenic. The arsenic-laden floodwater then flows into the Bay of Bengal, where it remains at safe concentrations.

Arsenic in soil is normally bound to iron particles. Floods produce anaerobic conditions so submerged microorganisms, deprived of air, start breaking down other substances, such as iron, releasing arsenic into the water.

This has implications for targeting arsenic removal, said Linda Roberts of the Swiss Federal Institute of Aquatic Science and Technology, lead author of the research.

"The main message of our work is that arsenic accumulation is likely to be significantly more severe in areas that are not affected by monsoon flooding," she said. "Efforts to mitigate arsenic in soils should, therefore, be focused with priority on unflooded areas."

Mitigation techniques include irrigating land with arsenic-free surface water or running groundwater over a 'spare' field first to leach the arsenic into soil that won't be used to grow crops.

Arsenic poisoning from drinking water or eating contaminated crops is a huge problem, affecting millions of people across Asia. High arsenic levels can also compromise rice yields .

After flooding, it takes longer for arsenic to reach levels that can adversely affect rice yield and quality and human health, said M. Ashraf Ali of the Bangladesh University of Engineering and Technology, which jointly conducted the study.

"Now we know that floodwater removes accumulated arsenic, this should be an important part of flood management for arsenic-affected areas," he told SciDev.Net.

But the relationship between flooding and arsenic levels in rice crops is complicated, said Roberts. Flooding caused by over-irrigation with arsenic-contaminated groundwater, rather than arsenic-free floodwater, can increase the levels of arsenic in rice (see Flooded rice stores more arsenic, research finds).