Assessment of controlling processes for field-scale uranium reactive transport under highly transient flow conditions

Abstract

This paper presents the results of a comprehensive model-based analysis of a uranyl [U(VI)]tracer test conducted at the U.S. DOE Hanford 300 Area (300A) IFRC. Despite the highly complex field conditions the numerical three-dimensional multicomponent reactive transport model was able to capture most of the spatiotemporal variations of the observed U(VI) concentrations. A multimodel analysis was performed to interrogate the relative importance of various processes and factors for controlling field-scale reactive transport during the uranyl tracer test. The results indicate that multirate sorption/desorption, surface complexation reactions, and initial concentration distributions were the most important processes and factors controlling U(VI) migration. On the other hand, cation exchange reactions, the choice of the surface complexation model, and dual-domain mass transfer processes played less important roles under the prevailing field-test conditions. Further analysis of the modeling results demonstrates that these findings are conditioned to the relatively stable groundwater chemistry and the selected length of the field experimental duration (16 days). The model analysis also revealed the crucial role of the intraborehole flow that occurred within the long-screened monitoring wells and thus affected both field measurements and simulated U(VI) concentrations as a combined effect of aquifer heterogeneity and dynamic flow conditions. This study provides the first highly data-constrained uranium transport simulations under highly dynamic flow conditions. It illustrates the value of reactive transport modeling for elucidating the relative importance of individual processes in controlling uranium transport under specific field-scale conditions.

Model‐Based Analysis of the Effects of Dam‐Induced River Water and Groundwater Interactions on Hydro‐Biogeochemical Transformation of Redox Sensitive Contaminants in a Hyporheic Zone

“…The river bank above the RS was treated as the seepage boundary. The top boundary was specified with no flow since the precipitation is very low at the site (50 mm/year) (Ma et al, , ; Williams et al, ). No flow boundary condition was specified for the bottom since the underlying formation is almost impermeable (Johnson, ).…”

Pore-Scale Process Coupling and Effective Surface Reaction Rates in Heterogeneous Subsurface Materials

“…A numerical grid system used for simulating large scale reactive transport typically contains heterogeneity in both intraand inter-granular domains within individual grid nodes. This heterogeneity is referred to as subgrid heterogeneity, which has been attributed to 5-7 orders of magnitude decrease in the effective rate of U(VI) desorption in sediments from the molecular to fi eld scales (Liu et al , 2014aMa et al 2014), and four orders of magnitude decrease for microbial reduction of uranium from laboratory to fi eld systems (Bao et al 2014). Here the micromodel system (Fig.…”

PHT3D‐UZF: A Reactive Transport Model for Variably‐Saturated Porous Media

“…By employing PHREEQC‐2 as the reaction simulator, PHT3D‐UZF allows for the simulation of a wide range of mixed equilibrium and kinetic geochemical reactions, including aqueous speciation, dissolution and precipitation of minerals, and cation exchange. The ability to define arbitrary reaction rate expressions for kinetically controlled reactions is important as it enables the simulation of a broad range of reactions including inorganic contamination (Wallis et al , ; Ma et al ), nutrient transformations (Zhang et al ), as well as biodegradation of organic contaminants (Prommer et al ; D'Affonseca et al ) and micropollutants (Greskowiak et al ; Engelhardt et al ). With the present version of PHT3D‐UZF that includes unsaturated‐zone flow these reactions can occur in conjunction with gas dissolution‐exsolution reactions and the consumption and production of gases.…”