IMPACTS OF HYDRAULICS AND CHANNEL AMENDMENTS ON ARSENIC CONCENTRATIONS IN FLOWING IRRIGATION WATER

Across Southern Asia, dry-season irrigation with groundwater has enabled the expansion of rice production, greatly improving food security and economic opportunity for farm households. Extensive use of arsenic-contaminated groundwater for irrigation during the dry season threatens these benefits. Following years of irrigation with groundwater, soil arsenic concentrations have risen, and arsenic is now transferring into rice at concentrations sufficient to decrease yields and create dangerous levels of arsenic in rice grains. Due to the large volumes of irrigation water required, as well as the cost of highly technical treatment options, there are currently no practical methods for large-scale removal of arsenic from these systems. The overall goal of our research is to develop low-cost methods for treating arsenic-contaminated irrigation water and improve the sustainability of irrigated rice production. To this end, we have conducted controlled experiments in Bangladesh to define and manipulate the hydrogeochemical processes governing arsenic concentrations over space and time in water flowing through distribution channels. Arsenic may be naturally removed from solution via sorption to soil and suspended solids, as well as co-precipitation with iron oxides formed in the water column, but removal is limited by high phosphate concentrations in irrigation water. Within our experiments, channel designs and amendments which increased treatment surface area and/or residence times decreased dissolved As concentrations in irrigation water by up to 47%. Lengthening and enhancing the tortuosity of channels most greatly increased arsenic removal from solution. Additionally, structures made of jute mesh placed within channels also enhanced arsenic removal by creating multiple hydraulic regimes within channels that allowed for longer flow paths and promoted particle settling. Collectively, these results suggest that mitigation of arsenic loading to rice fields may be promoted with low-cost irrigation management strategies, and future work could optimize and establish the season-long sustainability of such approaches.