Shallow groundwater/surface-water interface (GSI) as a realistic zone for natural attenuation of groundwater arsenic

Assessing the ability of the Groundwater/Surface-water Interface (GSI) sediment to attenuate contaminant loads is critical to understanding the fate of groundwater-borne contaminants. Sampling techniques with centimeter-scale spatial resolution were applied to investigate the sequestration of arsenic and iron in the sediments of the GSI at a site characterized by fine sediments. Freeze-core sediment collection gave more detailed and depth-accurate arsenic and iron contaminant distributions than could be obtained with the use of a standard hand auger. Selective chemical extractions indicated that greater than 90% of the arsenic was strongly sorbed to very amorphous iron oxyhydroxides. These solids accounted for more than 80% of the total iron in the sediments. Bead columns containing glass media enabled short-term characterization of pore water-to-sediment transfer of arsenic and iron. Bead columns indicated quantitative capture of groundwater arsenic and iron during 2003, suggesting that freeze-core inventories corresponded to 2 to 20 years of accumulation, depending on location. ^ Effects of seasonal and spatial variation on sequestration of arsenic in sediments was investigated through field monitoring and lab experiments. Bead column studies indicated that the iron oxides continued to form during the flooded season, although with lowered rates compared to intermediate and flooded conditions. Iron and arsenic deposition seemed to be related to the proximity to plant roots. Accumulation of iron and arsenic in the shallow sediment seemed to be less abundant at low-lying, close-to-water locations than at other locations. Under anaerobic conditions in the sediment, that may occur in flooded conditions, reductive dissolution of iron oxides occurred. Microbial iron oxide reduction was not limited by the abundance of iron-reducing bacteria or by the availability of electron donors. The concurrent release of arsenic was limited and reduction process reached a plateau where about 60% of the original oxides had been reduced. The high arsenic sorption capacity of shallow sediments suggested that arsenic released from reduced iron oxides was sorbed to other available sites in the sediment. Thus, the GSI sediment at the seasonally flooded site can effectively sequester groundwater arsenic under both dry and flooded conditions.