HR: 10:50h
AN: H32D-03    [Abstracts]
TI: Biogeochemical Reaction Kinetics Associated With Uranium Bioremediation at Multiple Scales
AU: * Li, L
EM: lili@lbl.gov
AF: Lawrence Berkeley National Laboratory, Earth Sciences Division, 1 Cyclotron Road, MS 90- 1116, Berkeley, CA 94720, United States
AU: Steefel, C I
EM: cisteefel@lbl.gov
AF: Lawrence Berkeley National Laboratory, Earth Sciences Division, 1 Cyclotron Road, MS 90- 1116, Berkeley, CA 94720, United States
AU: Kowalsky, M B
EM: MBKowalsky@lbl.gov
AF: Lawrence Berkeley National Laboratory, Earth Sciences Division, 1 Cyclotron Road, MS 90- 1116, Berkeley, CA 94720, United States
AB: Effective bioremediation requires understanding and quantification of biogeochemical reaction kinetics in natural porous media. Although the intrinsic rates of biogeochemical reactions have been measured in well-mixed laboratory systems, extrapolation of those rates to natural porous media remains challenging, partly due to the fact that natural systems are rarely well-mixed. In this work, we examine the reaction kinetics of iron and sulfate reductions involved in a bioremediation field experiment at the Old Rifle UMTRA site in Western Colorado where acetate as an electron donor was injected into the subsurface to reduce mobile U(VI) to relatively immobile U(IV). We examine at multiple spatial scales how the biogeochemical reaction rates are affected by the extent of mixing and how this in turn affects the efficiency of bioremediation and the evolution of physical and chemical properties of bioremediation sites over the long term. The rates of iron and sulfate reduction were examined at the pore scale and the field scale. At the pore scale (tens to thousands of microns), numerical experiments show that transport processes are fast enough to homogenize the concentration and that the intrinsic rate measured under well-mixed conditions can be directly used. However, at the field scale (tens of meters), dispersion/diffusion processes are not fast enough to homogenize the concentration and the local reaction rates depend on local, spatially variable concentrations. As such, the overall reaction rates at the field scale depend largely on the extent of mixing, which is controlled in large part by the physical and chemical heterogeneities present in the subsurface. Focusing on dissimilatory Fe reduction and assuming the same average flow velocity and the same average total solid iron content, we compare three different cases: 1) a homogeneous permeability distribution with a homogeneous Fe distribution; 2) a heterogeneous permeability distribution, determined from the inverse modeling of field-scale tracer breakthrough data, with a homogeneous Fe distribution; and 3) the same heterogeneous permeability distribution as in the second case, but with a heterogeneous Fe distribution based on an assumed negative correlation between permeability and iron content. Reactive transport modeling results were compared to field data to determine the overall volume-averaged reaction rates. The first case represents the largest extent of mixing and therefore leads to the largest overall Fe reduction rates, while the second and third cases, respectively, represent progressively lower extents of mixing and therefore lead to slower reduction rates. Differences in the overall Fe reduction rates result in different amounts and spatial patterns of precipitated secondary minerals. In the homogeneous case, secondary minerals precipitate evenly across the transverse distance, while in the heterogeneous cases, secondary minerals precipitate primarily where the contact between the electron donor acetate and solid iron is greatest. As such, both physical and chemical heterogeneity may dramatically affect reductions in permeability over the long term.
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 0418 Bioremediation
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2007 Fall Meeting