title: Distribution of chromium contamination and microbial activity in soil aggregates.
abstract: Biogeochemical transformations of redox-sensitive chemicals in soils can be strongly transport-controlled and localized. This was tested through experiments on chromium diffusion and reduction in soil aggregates that were exposed to chromate solutions. Reduction of soluble Cr(VI) to insoluble Cr(II) occurred only within the surface layer of aggregates with higher available organic carbon and higher microbial respiration. Sharply terminated Cr diffusion fronts develop when the reduction rate increases rapidly with depth. The final state of such aggregates consists of a Cr-contaminated exterior, and an uncontaminated core, each having different microbial community compositions and activity. Microbial activity was significantly higher in the more reducing soils, while total microbial biomass was similar in all of the soils. The small fraction of Cr(VI) remaining unreduced resides along external surfaces of aggregates, leaving it potentially available to future transport down the soil profile. Using the Thiele modulus, Cr(VI) reduction in soil aggregates is shown to be diffusion rate- and reaction rate-limited in anaerobic and aerobic aggregates, respectively. Thus, spatially resolved chemical and microbiological measurements are necessary within anaerobic soil aggregates to characterize and predict the fate of Cr contamination. Typical methods of soil sampling and analyses that average over redox gradients within aggregates can erase important biogeochemical spatial relations necessary for understanding these environments.
authors.affiliation: Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. tktokunaga@lbl.gov
authors.forename: Tetsu K
authors.initials: TK
authors.lastname: Tokunaga
authors.forename: Jiamin
authors.initials: J
authors.lastname: Wan
authors.forename: Terry C
authors.initials: TC
authors.lastname: Hazen
authors.forename: Egbert
authors.initials: E
authors.lastname: Schwartz
authors.forename: Mary K
authors.initials: MK
authors.lastname: Firestone
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Sutton
authors.forename: Matthew
authors.initials: M
authors.lastname: Newville
authors.forename: Keith R
authors.initials: KR
authors.lastname: Olson
authors.forename: Antonio
authors.initials: A
authors.lastname: Lanzirotti
authors.forename: William
authors.initials: W
authors.lastname: Rao
mesh_terms: D001420:Bacteria, Aerobic / Q000502:physiology; D001421:Bacteria, Anaerobic / Q000502:physiology; D002274:Carcinogens, Environmental / Q000737:chemistry / Q000378:metabolism*; D002857:Chromium / Q000737:chemistry / Q000378:metabolism*; D004058:Diffusion; D004784:Environmental Monitoring; D010084:Oxidation-Reduction; D012988:Soil Microbiology
chemical_list: D002274:Carcinogens, Environmental; D002857:Chromium; C074702:chromium hexavalent ion