Reducing arsenic levels in rice through improved irrigation practices

19 December 2007, Rome – High levels of arsenic in rice could be reduced by applying improved irrigation management practices in Asia, FAO said today in a new report entitled Remediation of Arsenic for Agriculture Sustainability, Food Security and Health in Bangladesh.

Studies have shown that high concentrations of arsenic in soil and irrigation water often lead to high levels of arsenic in crops and are posing an increased food safety risk. At present, twelve countries in Asia have reported high arsenic levels in their groundwater resources.

“The problem of high arsenic levels in crops, particularly rice, needs to be urgently addressed by promoting better irrigation and agricultural practices that could reduce arsenic contamination significantly,” said Sasha Koo-Oshima, FAO water quality and environment officer.

“Arsenic-contaminated rice could aggravate human health when consumed with arsenic-laden drinking water. The widespread addition of arsenic to soils, for example in Bangladesh, is degrading soil quality and causing toxicity to rice. Arsenic contamination is threatening food production, food security and food quality,” she noted.

Entering the food chain

Arsenic enters the food chain mainly through crops absorbing contaminated irrigation water. Millions of shallow tube wells have been installed throughout Asia over the last three decades pumping water from contaminated shallow groundwater aquifers.

Contamination originates in arsenic-rich sediments of the Ganges and Brahmaputra river that filters into groundwater water pumped to the surface through millions of tube wells.

Bangladesh has the highest percentage of contaminated shallow tube wells and an estimated 30 million people are dependent on those wells for drinking water and irrigation. Of the four million hectares under irrigation, 2.4 million ha are irrigated with approximately 900 000 shallow tube wells. Approximately 95 percent of the groundwater extracted is for irrigation.

It has been estimated that water pumping from shallow aquifers for irrigation adds one million kilogram of arsenic per year to the arable soil in Bangladesh, mainly in the paddy fields. Rice is the staple food in Bangladesh and it is consumed in large quantities.

Raised beds

Planting rice in raised beds around 15 centimetres above the ground and not in conventional flooded fields counteracted yield losses and resulted in lower arsenic levels in crops and in the soil, a pilot field study in Bangladesh conducted by FAO and Cornell University revealed. In addition, the raised bed rice buffers against floods and drought and serves as a measure in climate adaptation.

“Raised beds are significantly reducing the exposure of rice plants to contaminated irrigation water and are producing higher yields,” Sasha Koo-Oshima said.

A related Cornell University project proved that between 30 and 40 percent less irrigation water is needed in raised-bed- system. Fertilizers are also captured better – with the effect that farmers will need less fertilizers. The raised-bed-system represents a major shift in rice production but tests show that farmers prefer the new approach due to visibly higher yields, water savings, lower tillage and labour costs and production of a safer crop.

Identifying and targeting rural areas that are worst affected by arsenic contamination and further developing and extending the raised-bed-system should become a main priority in order to reduce the risk of arsenic contamination to human health, secure food safety and production, Koo-Oshima said.