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Transport and biogeochemical reaction of metals in a physically and chemically heterogeneous aquifer

¹ Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA
² University of Wisconsin, Department of Geology and Geophysics, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA
³ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 90-1116, Berkeley, California 94720, USA
⁴ Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA
⁵ Oak Ridge National Laboratory, P.O. Box 2008, MS 6038, Oak Ridge, Tennessee 37831-6038, USA
⁶ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 90-1116, Berkeley, California 94720, USA

Biologically mediated reductive dissolution and precipitation of metals and radionuclides play key roles in their subsurface transport. Physical and chemical properties of natural aquifer systems, such as reactive iron-oxide surface area and hydraulic conductivity, are often highly heterogeneous in complex ways that can exert significant control on transport, natural attenuation, and active remediation processes. Typically, however, few data on the detailed distribution of these properties are available for incorporation into predictive models. In this study, we integrate field-scale geophysical, hydrologic, and geochemical data from a well-characterized site with the results of laboratory batch-reaction studies to formulate two-dimensional numerical models of reactive transport in a heterogeneous granular aquifer. The models incorporate several levels of coupling, including effects of ferrous iron sorption onto (and associated reduction of reactive surface area of) ferric iron surfaces, microbial growth and transport dynamics, and cross-correlation between hydraulic conductivity and initial ferric iron surface area. These models are then used to evaluate the impacts of physical and chemical heterogeneity on transport of trace levels of uranium under natural conditions, as well as the effectiveness of uranium reduction and immobilization upon introduction of a soluble electron donor (a potential biostimulation remedial strategy).

Keywords: reactive transport • heterogeneity • modeling • groundwater • uranium