Subsurface Biogeochemical Heterogeneity (Field-scale removal of U(VI) from groundwater in an alluvial aquifer by electron donor amendment)

Determine if biostimulation of alluvial aquifers by electron donor amendment can effectively remove U(VI) from groundwater at the field scale. Uranium contamination in groundwater is a significant problem at several DOE sites. In this project, the possibility of accelerating bioreduction of U(VI) to U(IV) as a means of decreasing U(VI) concentrations in groundwater is directly addressed by conducting a series of field-scale experiments. Scientific goals include demonstrating the quantitative linkage between microbial activity and U loss from groundwater and relating the dominant terminal electron accepting processes to the rate of U loss. The project is currently focused on understanding the mechanisms for unexpected long-term ({approx}2 years) removal of U after stopping electron donor amendment. Results obtained in the project successfully position DOE and others to apply biostimulation broadly to U contamination in alluvial aquifers

A Generic Reaction-Based Biogeochemical Simulator

This paper presents a generic biogeochemical simulator, BIOGEOCHEM. The simulator can read a thermodynamic database based on the EQ3/EQ6 database. It can also read user-specified equilibrium and kinetic reactions (reactions not defined in the format of that in EQ3/EQ6 database) symbolically. BIOGEOCHEM is developed with a general paradigm. It overcomes the requirement in most available reaction-based models that reactions and rate laws be specified in a limited number of canonical forms. The simulator interprets the reactions and rate laws of virtually any type for input to the MAPLE symbolic mathematical software package. MAPLE then generates Fortran code for the analytical Jacobian matrix used in the Newton-Raphson technique, which is copiled and linked into the BIOGEOCHEM executable. With this feature, the users are exempted from recoding the simulator to accept new equilibrium expressions or kinetic rate laws. Two examples are used to demonstrate the new features of the simulator. © 2004 Elsevier B.V

Oxidative dissolution of biogenic uraninite in groundwater at Old Rifle, CO

Reductive bioremediation is currently being explored as a possible strategy for uranium-contaminated aquifers such as the Old Rifle site (Colorado). The stability of U(IV) phases under oxidizing conditions is key to the performance of this procedure. An in situ method was developed to study oxidative dissolution of biogenic uraninite (UO2), a desirable U(VI) bioreduction product, in the Old Rifle, CO, aquifer under different variable oxygen conditions. Overall uranium loss rates were 50–100 times slower than laboratory rates. After accounting for molecular diffusion through the sample holders, a reactive transport model using laboratory dissolution rates was able to predict overall uranium loss. The presence of biomass further retarded diffusion and oxidation rates. These results confirm the importance of diffusion in controlling in-aquifer U(IV) oxidation rates. Upon retrieval, uraninite was found to be free of U(VI), indicating dissolution occurred via oxidation and removal of surface atoms. Interaction of groundwater solutes such as Ca2+ or silicate with uraninite surfaces also may retard in-aquifer U loss rates. These results indicate that the prolonged stability of U(IV) species in aquifers is strongly influenced by permeability, the presence of bacterial cells and cell exudates, and groundwater geochemistry

Reactive transport codes for subsurface environmental simulation

International audienceA general description of the mathematical and numerical formulations used in modern numerical reactive transport codes relevant for subsurface environmental simu- lations is presented. The formulations are followed by short descriptions of commonly used and available subsurface simulators that consider continuum representations of flow, transport, and reactions in porous media. These formula- tions are applicable to most of the subsurface environmental benchmark problems included in this special issue. The list of codes described briefly here includes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, Crunch- Flow, MIN3P, and PFLOTRAN. The descriptions include a high-level list of capabilities for each of the codes, along with a selective list of applications that highlight their capabilities and historical development

Correction to Oxidative Dissolution of Biogenic Uraninite in Groundwater at Old Rifle, CO

Correction to Oxidative Dissolution of Biogenic Uraninite in Groundwater at Old Rifle, C