A uranium bioremediation reactive transport benchmark

Abstract

A reactive transport benchmark problem set has been developed based on in situ uranium bioimmobilization experiments that have been performed at a former uranium mill tailing site in Rifle, CO, USA. Acetateamended groundwater stimulates indigenous microorganisms to catalyze the reduction of U(VI) to a sparingly soluble U(IV) mineral. The interplay between the flow, acetate loading periods and rates, and microbially mediated and geochemical reactions leads to dynamic behavior in metaland sulfate-reducing bacteria, pH, alkalinity, and reactive mineral surfaces. The benchmark is based on an 8.5 m long one-dimensional model domain with constant saturated flow and uniform porosity. The 159-day simulation introduces acetate and bromide through the upgradient boundary in 14-and 85-day pulses separated by a 10 day interruption. Acetate loading is tripled during the second pulse, which is followed by a 50 day recovery period. Terminal electronaccepting processes for goethite, phyllosilicate Fe(III), U(VI), and sulfate are modeled using Monod-type rate laws. Major ion geochemistry modeled includes mineral reactions as well as aqueous and surface complexation reactions for Electronic supplementary material The online version of this article (UO 2+ 2 , Fe 2+ , and H + . In addition to the dynamics imparted by the transport of the acetate pulses, U(VI) behavior involves the interplay between bioreduction, which is dependent on acetate availability, and speciation-controlled surface complexation, which is dependent on pH, alkalinity, and available surface complexation sites. The general difficulty of this benchmark is the large number of reactions (74), multiple rate law formulations, a multisite uranium surface complexation model, and the strong interdependency and sensitivity of the reaction processes. Results are presented for three simulators: HYDROGEOCHEM, PHT3D, and PHREEQC.

Water Table Dynamics and Biogeochemical Cycling in a Shallow, Variably-Saturated Floodplain

“…Outside of the NRZs, uranium in the floodplain aquifer is specified as aqueous and surface-complexed U­(VI), consistent with observed aqueous concentrations and a three-site U­(VI) surface complexation model previously developed for the Rifle sediments. , Near the northernmost point of the floodplain, U­(VI) is specified in variably saturated sediments at an elevation [5305 ft (1617 m)] that is accessed during water table peaking, with concentrations consistent with observations in a nearby well.…”

“…Outside of the NRZs, uranium in the floodplain aquifer is specified as aqueous and surface-complexed U(VI), consistent with observed aqueous concentrations and a three-site U(VI) surface complexation model previously developed for the Rifle sediments. 18,41 been stimulated with acetate amendment in previous Rifle studies, 5,6,8,42 is not considered significant, based on metagenomic and 16S rRNA gene analyses. Fe(II) oxidation is considered to be dominantly abiotic.…”

Reactive transport benchmarks for subsurface environmental simulation

“…The three benchmarks that consider uranium fate and transport represent some of the successes in using coupled process modeling to develop a systematic understanding of uranium behavior in naturally complex subsurface materials. The first benchmark is based on a field experiment conducted at the Rifle site in western CO, USA (A uranium bioremediation reactive transport benchmark [11]) in which acetate was injected into uranium-contaminated groundwater to drive microbially mediated reduction and immobilization of uranium. The problem is simplified somewhat in terms of the hydrogeology of the site, but includes a complex biogeochemical reaction network affecting the reduction and sorption of the uranium.…”

Unification of cell-scale metabolic activity with biofilm behavior by integration of advanced flow and reactive-transport modeling and microfluidic experiments

“…Among GS modeling approaches, the flow and reactive transport model (FRTM) has been used to create spatiotemporal simulations for predicting GS bioremediation of soluble uranium (U 6+ ) to insoluble uranium (U 4+ ) minerals present in groundwater via environmental acetate loading. 7 Monod-type growth kinetics were used to represent various multi-enzyme cycles as a general black box in the FRTM, which linked the reduction rates of extracellular electron accepters (e.g., Fe 3+ and U 6+ ) to the acetate oxidation rates through universally applied electron-transfer ratios (ETR) and biomass yields (Y bio/ace ). However, the FRTM model did not account for the reality that spatiotemporal environmental conditions may influence the metabolism of the GS bacteria at different locations within the biofilm.…”