Arsenic removal from flowing irrigation water in bangladesh: impacts of channel properties.

Across Bangladesh, dry-season irrigation with arsenic-contaminated well water is loading arsenic onto rice paddies, leading to increased arsenic concentrations in plants, diminished crop yields, and increased human health risks. As irrigation water flows through conveyance channels between wells and rice fields, arsenic concentrations change over space and time, indicating that channels may provide a location for removing arsenic from solution. However, few studies have systematically evaluated the processes controlling arsenic concentrations in irrigation channels, limiting the ability to manipulate these systems and enhance arsenic removal from solution. The central goal of this study was to quantify how channel design affected removal of dissolved arsenic from flowing irrigation water. Field experiments were conducted in Bangladesh using a chemically constant source of arsenic-contaminated irrigation water and an array of constructed channels with varying geometries. The resulting hydraulic conditions affected the quantity of arsenic removed from solution within the channels by promoting known hydrogeochemical processes. Channels three times the width of control channels removed ∼3 times the mass of arsenic over 32 min of flowing conditions, whereas negligible arsenic removal was observed in tarp-lined channels, which prevented soil-water contact. Arsenic removal from solution was ∼7 times higher in a winding, 200-m-long channel than in the straight, 45-m-long control channels. Arsenic concentrations were governed by oxidative iron-arsenic coprecipitation within the water column, sorption to soils, and phosphate competition. Collectively, these results suggest that better design and management of irrigation channels may play a part in arsenic mitigation strategies for rice fields in Southern Asia.