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.

Seasonal Arsenic Accumulation in Stream Sediments at a Groundwater Discharge Zone

“…Such coupled iron processes could arise with biological iron oxidation since iron-oxidizing bacteria are known to inhabit microaerophilic niches that are also favorable to iron-reducing bacteria 22 and we have enumerated both bacterial types to depths of 20 cm in Cohas Brook sediments. 17 We note that iron(II)-to-total iron ratios were not measured for July and October 2004 bead columns thus precluding comparisons with other hydroperiods.…”

“…and 0.3 cm slot interval, 0.064 cm slot width, 15−20 cm column length) sealed with polycarbonate plugs on each end. 17 Bead columns were covered with GoreTex 'socks' to prevent sediment entrainment during column retrieval. Low permeability of the GoreTex material may reduce flow, relative to surrounding sediments; however, we were interested in relative changes, not absolute accumulation rates that will differ from the sediments.…”

Coupling X-ray computed tomography and freeze-coring for the analysis of fine-grained low-cohesive soils

“…This extraction technique was initially developed and used for river and lake sediments analysis (Ryves et al, 2002;Stocker and Williams, 1972). It also found applications in soil science and for the analysis of the hyporheic zone, for example for the identification of the vertical distribution of microbial communities or chemical pollutants (Franchini and Zeyer, 2012;Gan et al, 2006;Moser et al, 2003). In a more recent study, Strasser et al (2015), used the freeze-cores for the geohydraulic characterization of river bed sediments.…”

Freeze-Coring Method for Characterization of Microbial Community Structure and Function in Wetland Soils at High Spatial Resolution

“…Freeze-coring methods have been developed mainly for the analysis of lake and river sediments to overcome the problems associated with compaction and water loss (3,8,21). Subsamples of frozen cores were used for cell counts, fatty acid extraction, and geochemical analyses (8,21,32).…”

“…Subsamples of frozen cores were used for cell counts, fatty acid extraction, and geochemical analyses (8,21,32). These approaches were feasible to achieve high-resolution analysis in the first 15 cm of river sediments (8,32).…”