title: Sampling methods to determine the spatial gradients and flux of arsenic at a groundwater seepage zone.
abstract: Sampling techniques with centimeter-scale spatial resolution were applied to investigate biogeochemical processes controlling groundwater arsenic fate across the groundwater-surface water interface at a site characterized by fine sediments (40% sand, 46% silt, 14% clay). Freeze-core sediment collection gave more detailed and depth-accurate arsenic and iron contaminant and microbial distributions than could be obtained with the use of a 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. Microbial enrichments indicated that iron-oxidizing bacteria (IOB) were up to 1% of the total bacterial abundance, whereas iron-reducing bacteria (IRB) were about two orders of magnitude less abundant than IOB. The abundance of IRB mirrored the IOB depth profile. Push-point pore-water sampling captured large amounts of sediment fines, even with controlled (20 ml/min) water withdrawal, thereby necessitating filtration before water quality analysis. Bead columns containing glass media enabled short-term (29 d) 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.
authors.affiliation: Environmental Engineering Program, Department of Civil and Environmental Engineering, University of Connecticut, Storrs, Connecticut 06269-2037, USA.
authors.forename: Ping
authors.initials: P
authors.lastname: Gan
authors.forename: Ran
authors.initials: R
authors.lastname: Yu
authors.forename: Barth F
authors.initials: BF
authors.lastname: Smets
authors.forename: Allison A
authors.initials: AA
authors.lastname: MacKay
mesh_terms: D001151:Arsenic / Q000032:analysis*; D019015:Geologic Sediments / Q000737:chemistry; D014871:Water Microbiology; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014874:Water Pollutants, Chemical; D001151:Arsenic