Physicochemical Heterogeneity Controls on Uranium Bioreduction Rates at the Field Scale

Abstract

It has been demonstrated in laboratory systems that U(VI) can be reduced to immobile U(IV) by bacteria in natural environments. The ultimate efficacy of bioreduction at the field scale, however, is often challenging to quantify and depends on site characteristics. In this work, uranium bioreduction rates at the field scale are quantified, for the first time, using an integrated approach. The approach combines field data, inverse and forward hydrological and reactive transport modeling, and quantification of reduction rates at different spatial scales. The approach is used to explore the impact of local scale (tens of centimeters) parameters and processes on field scale (tens of meters) system responses to biostimulation treatments and the controls of physicochemical heterogeneity on bioreduction rates. Using the biostimulation experiments at the Department of Energy Old Rifle site, our results show that the spatial distribution of hydraulic conductivity and solid phase mineral (Fe(III)) play a critical role in determining the field-scale bioreduction rates. Due to the dependence on Fe-reducing bacteria, field-scale U(VI) bioreduction rates were found to be largely controlled by the abundance of Fe(III) minerals at the vicinity of the injection wells and by the presence of preferential flow paths connecting injection wells to down gradient Fe(III) abundant areas.

Biogeochemical aspects of uranium mineralization, mining, milling, and remediation

“…Sensitivity analyses show that the model is most sensitive to groundwater flow velocity, as well as U and SO 4 2À bioreduction rates. In addition, RTMs have been used to test the impact of local heterogeneity (on the order of tens of centimeters) on simulations of field-scale U bioreduction rates, determining that local hydraulic conductivity and Fe(III) sediment content have large effects on the field-scale behavior of dissolved U(VI) (Li et al, 2011). The interactions of microbial communities, biomass sorption, and subtleties in rate expressions for U and SO 4 2À reduction are key areas where, in general, RTMs can be improved.…”

Modeling the evolution of complex conductivity during calcite precipitation on glass beads

“…Calcite precipitation in porous media can be observed in situ by conventional monitoring approaches using geochemical analyses of borehole samples (Scheibe et al 2006;Li et al 2009Li et al , 2011. Geochemical borehole data give accurate measurements of the water chemical composition associated with calcite precipitation but they are time consuming, not continuous in time, invasive and spatially limited to the location of the boreholes.…”

CHROTRAN 1.0: A mathematical and computational model for in situ heavy metal remediation in heterogeneous aquifers

“…For U(VI), field-scale modeling studies have been performed on bio-reduction under anaerobic conditions at the Old Rifle Site in Colorado (Li et al, 2010(Li et al, , 2011Yabusaki et al, 2011). These conceptions treat the contaminant as the sole electron acceptor, with an externally applied electron donor, and the implied equations have a similar form to those devised by Chiang et al (1991): linear in biomass, Monod in contaminant, and Monod in electron donor.…”

“…Recently, Molins et al (2015) have published a numerical study of a column experiment with multiple species, all of whose dynamics are of the above form, but including an extra chemical inhibition factor. The models of Li et al (2010Li et al ( , 2011 were implemented at field scale in CrunchFlow (Steefel et al, 2015), using its capability to represent single and multiple Monod formulations. Systems of governing reactive transport equations for enzymatic microbial Cr(VI) reduction have been presented by Alam (2004), and by Shashidhar et al (2007).…”

“…Systems of governing reactive transport equations for enzymatic microbial Cr(VI) reduction have been presented by Alam (2004), and by Shashidhar et al (2007). Shashidhar et al (2007) described the Cr(VI) degradation reaction slightly differently from Li et al (2011):…”