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. To this end, we have conducted controlled experiments in Bangladesh to define the physicochemical processes governing arsenic concentrations over space and time in water flowing through distribution channels. Arsenic concentrations were variable, but were impacted by channel hydraulics. Arsenic was removed from solution via sorption to soil and suspended solids, and co-precipitation with iron oxides formed in the water column. Structures made of jute mesh placed within channels enhanced these processes by creating multiple hydraulic regimes within channels that allowed for longer flow paths and promoted particle settling. Throughout 30-minute experiments, arsenic concentrations in the amended-channel outflow water were consistently 15% below both input arsenic concentrations and the outflow of a control channel with no jute. Collectively, these results suggest that mitigation of arsenic loading to rice fields may be promoted with low-cost irrigation management strategies that increase water residence times in channels, enhance oxidation of water, and trap suspended particles.