What Governs the Rate of Arsenic Release From Mekong River Delta Sediments?.

Arsenic-contaminated groundwater used for drinking has lead to the chronic poisoning of tens of millions of people in the Bengal Basin, Red River Delta, and Mekong Delta. Under the anaerobic conditions of these deltaic sediments, microbially driven oxidation of organic carbon coupled to the dissimilatory reductive dissolution of arsenic-bearing iron (hydr)oxides causes the transfer of arsenic from the solid to the aqueous phase. The reactivity and quantity of organic carbon and arsenic-bearing iron (hydr)oxides dictate the rate of arsenic release from soil/sediment to pore-water. Methods exist for quantifying the reactivity of iron (hydr)oxides, although they have been applied scarcely for predicting arsenic release; however, no widely-accepted method exists for quantifying organic carbon reactivity. Moreover, methods are lacking for measuring the reactivity of organic carbon and arsenic in the field. Here we developed proxies for organic carbon and arsenic reactivity in Mekong Delta sediments in Cambodia. Incubations of fresh sediment – from depths of 0.1 and 2.5 m – with sterile, anoxic groundwater medium (pH 7.1) were initiated in the field immediately upon sediment collection under nitrogen atmosphere. Prior to sediment addition, seven different treatment additions to the groundwater medium were performed: 0.1, 1, and 10 mM glucose (organic carbon), 0.03, 0.3, and 3 mM Fe (as arsenic-loaded goethite), and no amendment. Each of the seven treatments had an abiotic counterpart achieved by antibiotic addition. The 1 and 10 mM glucose additions were the only treatments to exhibit arsenic and Fe(II) release, suggesting organic carbon reactivity was limiting the rate of arsenic release in the upper 2.5 m of the sediment profile.  Ultimately, we hope these assays for organic carbon and arsenic reactivity will contribute to a spatial and temporal model of arsenic fate and transport that is globally applicable.