Chemical and Physical Controls On Arsenic Removal From Flowing Irrigation Water In Bangladesh.

Low-dose arsenic poisoning from consumption of well water is known to affect millions of people across Bangladesh, but recent studies indicate contaminated rice can now contribute over half of the daily dietary arsenic intake. Irrigating rice fields with arsenic-contaminated well water can lead to a buildup of arsenic in soils and subsequent translocation of arsenic into rice. Due to the quantities of water involved in irrigation and the resources required for complex technological treatments, there are currently no widely used practical methods for large scale removal of arsenic from irrigation water.

Distribution systems made of native soils can remove arsenic from flowing water, as indicated by decreases in arsenic concentrations along flow channels and across rice fields. However, the coupled chemical and physical processes governing arsenic in these systems remain unresolved, limiting our ability to manage and maximize arsenic removal from irrigation water prior to field application. We have established an experimental field area in Bangladesh where, by varying channel geometries and making measurements over space and time, we have investigated arsenic transport, oxidation, sorption, and precipitation during irrigation flow. Dissolved arsenic concentrations along channel wetting fronts can decrease to less than 20% of well water concentrations, but gradually increase as irrigation flow reaches its full hydraulic profile. Additionally, we observe a 30% decrease in arsenic concentrations with a five-fold increase in channel length. Arsenic concentrations are correlated with dissolved iron, phosphorus, and silicon. Decreases in iron concentrations along channels are likely due to oxidative precipitation, possibly removing arsenic from solution through co-precipitation. Phosphate and silicate compete with arsenate and arsenite for sorption sites on soil mineral surfaces, and decreases in these species indicate potential sites that could be occupied by arsenic. These results suggest modifications to irrigation channel designs could help manage arsenic loading to rice fields.